{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "# Lemaître-Tolman solutions\n", "\n", "This Jupyter/SageMath notebook is relative to the lectures\n", "[Geometry and physics of black holes](https://relativite.obspm.fr/blackholes/).\n", "\n", "The computations make use of tools developed through the [SageManifolds project](https://sagemanifolds.obspm.fr)." ] }, { "cell_type": "code", "execution_count": 1, "metadata": {}, "outputs": [ { "data": { "text/plain": [ "'SageMath version 10.0.rc3, Release Date: 2023-05-12'" ] }, "execution_count": 1, "metadata": {}, "output_type": "execute_result" } ], "source": [ "version()" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "First we set up the notebook to display mathematical objects using LaTeX rendering:" ] }, { "cell_type": "code", "execution_count": 2, "metadata": {}, "outputs": [], "source": [ "%display latex" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Spacetime\n", "\n", "We declare the spacetime manifold $M$:" ] }, { "cell_type": "code", "execution_count": 3, "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "4-dimensional Lorentzian manifold M\n" ] } ], "source": [ "M = Manifold(4, 'M', structure='Lorentzian')\n", "print(M)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "and declare the chart of **Lemaître synchronous coordinates** on it:" ] }, { "cell_type": "code", "execution_count": 4, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\left(M,({\\tau}, {\\chi}, {\\theta}, {\\phi})\\right)\$" ], "text/latex": [ "$\\displaystyle \\left(M,({\\tau}, {\\chi}, {\\theta}, {\\phi})\\right)$" ], "text/plain": [ "Chart (M, (t, x, th, ph))" ] }, "execution_count": 4, "metadata": {}, "output_type": "execute_result" } ], "source": [ "X. = M.chart(r't:\\tau x:(0,+oo):\\chi th:(0,pi):\\theta ph:(0,2*pi):\\phi')\n", "X" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "The most general metric tensor, assuming spherical symmetry and synchronous coordinates:" ] }, { "cell_type": "code", "execution_count": 5, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle g = -\\mathrm{d} {\\tau}\\otimes \\mathrm{d} {\\tau} + a\\left({\\tau}, {\\chi}\\right)^{2} \\mathrm{d} {\\chi}\\otimes \\mathrm{d} {\\chi} + r\\left({\\tau}, {\\chi}\\right)^{2} \\mathrm{d} {\\theta}\\otimes \\mathrm{d} {\\theta} + r\\left({\\tau}, {\\chi}\\right)^{2} \\sin\\left({\\theta}\\right)^{2} \\mathrm{d} {\\phi}\\otimes \\mathrm{d} {\\phi}\$" ], "text/latex": [ "$\\displaystyle g = -\\mathrm{d} {\\tau}\\otimes \\mathrm{d} {\\tau} + a\\left({\\tau}, {\\chi}\\right)^{2} \\mathrm{d} {\\chi}\\otimes \\mathrm{d} {\\chi} + r\\left({\\tau}, {\\chi}\\right)^{2} \\mathrm{d} {\\theta}\\otimes \\mathrm{d} {\\theta} + r\\left({\\tau}, {\\chi}\\right)^{2} \\sin\\left({\\theta}\\right)^{2} \\mathrm{d} {\\phi}\\otimes \\mathrm{d} {\\phi}$" ], "text/plain": [ "g = -dt⊗dt + a(t, x)^2 dx⊗dx + r(t, x)^2 dth⊗dth + r(t, x)^2*sin(th)^2 dph⊗dph" ] }, "execution_count": 5, "metadata": {}, "output_type": "execute_result" } ], "source": [ "g = M.metric()\n", "a = function('a')\n", "r = function('r')\n", "g[0,0] = -1\n", "g[1,1] = a(t,x)^2\n", "g[2,2] = r(t,x)^2\n", "g[3,3] = (r(t,x)*sin(th))^2\n", "g.display()" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Einstein equation" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "The cosmological constant:" ] }, { "cell_type": "code", "execution_count": 6, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle {\\Lambda}\$" ], "text/latex": [ "$\\displaystyle {\\Lambda}$" ], "text/plain": [ "Lamb" ] }, "execution_count": 6, "metadata": {}, "output_type": "execute_result" } ], "source": [ "Lamb = var('Lamb', latex_name=r'\\Lambda')\n", "Lamb" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "The Ricci tensor:" ] }, { "cell_type": "code", "execution_count": 7, "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Field of symmetric bilinear forms Ric(g) on the 4-dimensional Lorentzian manifold M\n" ] } ], "source": [ "Ric = g.ricci()\n", "print(Ric)" ] }, { "cell_type": "code", "execution_count": 8, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\\(\\displaystyle \\mathrm{Ric}\\left(g\\right) = \\left( -\\frac{r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} + 2 \\, a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\right) \\mathrm{d} {\\tau}\\otimes \\mathrm{d} {\\tau} -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\mathrm{d} {\\tau}\\otimes \\mathrm{d} {\\chi} -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\mathrm{d} {\\chi}\\otimes \\mathrm{d} {\\tau} + \\left( \\frac{a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} + 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} + 2 \\, \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} - 2 \\, a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\right) \\mathrm{d} {\\chi}\\otimes \\mathrm{d} {\\chi} + \\left( \\frac{a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} + a\\left({\\tau}, {\\chi}\\right)^{3} \\frac{\\partial\\,r}{\\partial {\\tau}}^{2} + a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} + a\\left({\\tau}, {\\chi}\\right)^{3} + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} - a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,r}{\\partial {\\chi}}^{2} - a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3}} \\right) \\mathrm{d} {\\theta}\\otimes \\mathrm{d} {\\theta} + \\frac{{\\left(a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} + a\\left({\\tau}, {\\chi}\\right)^{3} \\frac{\\partial\\,r}{\\partial {\\tau}}^{2} + a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} + a\\left({\\tau}, {\\chi}\\right)^{3} + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} - a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,r}{\\partial {\\chi}}^{2} - a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}\\right)} \\sin\\left({\\theta}\\right)^{2}}{a\\left({\\tau}, {\\chi}\\right)^{3}} \\mathrm{d} {\\phi}\\otimes \\mathrm{d} {\\phi}\\)" ], "text/latex": [ "$\\displaystyle \\mathrm{Ric}\\left(g\\right) = \\left( -\\frac{r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} + 2 \\, a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\right) \\mathrm{d} {\\tau}\\otimes \\mathrm{d} {\\tau} -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\mathrm{d} {\\tau}\\otimes \\mathrm{d} {\\chi} -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\mathrm{d} {\\chi}\\otimes \\mathrm{d} {\\tau} + \\left( \\frac{a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} + 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} + 2 \\, \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} - 2 \\, a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\right) \\mathrm{d} {\\chi}\\otimes \\mathrm{d} {\\chi} + \\left( \\frac{a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} + a\\left({\\tau}, {\\chi}\\right)^{3} \\frac{\\partial\\,r}{\\partial {\\tau}}^{2} + a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} + a\\left({\\tau}, {\\chi}\\right)^{3} + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} - a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,r}{\\partial {\\chi}}^{2} - a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3}} \\right) \\mathrm{d} {\\theta}\\otimes \\mathrm{d} {\\theta} + \\frac{{\\left(a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} + a\\left({\\tau}, {\\chi}\\right)^{3} \\frac{\\partial\\,r}{\\partial {\\tau}}^{2} + a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} + a\\left({\\tau}, {\\chi}\\right)^{3} + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} - a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,r}{\\partial {\\chi}}^{2} - a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}\\right)} \\sin\\left({\\theta}\\right)^{2}}{a\\left({\\tau}, {\\chi}\\right)^{3}} \\mathrm{d} {\\phi}\\otimes \\mathrm{d} {\\phi}$" ], "text/plain": [ "Ric(g) = -(r(t, x)*d^2(a)/dt^2 + 2*a(t, x)*d^2(r)/dt^2)/(a(t, x)*r(t, x)) dt⊗dt - 2*(a(t, x)*d^2(r)/dtdx - d(a)/dt*d(r)/dx)/(a(t, x)*r(t, x)) dt⊗dx - 2*(a(t, x)*d^2(r)/dtdx - d(a)/dt*d(r)/dx)/(a(t, x)*r(t, x)) dx⊗dt + (a(t, x)^2*r(t, x)*d^2(a)/dt^2 + 2*a(t, x)^2*d(a)/dt*d(r)/dt + 2*d(a)/dx*d(r)/dx - 2*a(t, x)*d^2(r)/dx^2)/(a(t, x)*r(t, x)) dx⊗dx + (a(t, x)^2*r(t, x)*d(a)/dt*d(r)/dt + a(t, x)^3*d(r)/dt^2 + a(t, x)^3*r(t, x)*d^2(r)/dt^2 + a(t, x)^3 + r(t, x)*d(a)/dx*d(r)/dx - a(t, x)*d(r)/dx^2 - a(t, x)*r(t, x)*d^2(r)/dx^2)/a(t, x)^3 dth⊗dth + (a(t, x)^2*r(t, x)*d(a)/dt*d(r)/dt + a(t, x)^3*d(r)/dt^2 + a(t, x)^3*r(t, x)*d^2(r)/dt^2 + a(t, x)^3 + r(t, x)*d(a)/dx*d(r)/dx - a(t, x)*d(r)/dx^2 - a(t, x)*r(t, x)*d^2(r)/dx^2)*sin(th)^2/a(t, x)^3 dph⊗dph" ] }, "execution_count": 8, "metadata": {}, "output_type": "execute_result" } ], "source": [ "Ric.display()" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "The Einstein tensor:" ] }, { "cell_type": "code", "execution_count": 9, "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Field of symmetric bilinear forms G on the 4-dimensional Lorentzian manifold M\n" ] } ], "source": [ "G = Ric - 1/2*g.ricci_scalar() * g\n", "G.set_name('G')\n", "print(G)" ] }, { "cell_type": "code", "execution_count": 10, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\\(\\displaystyle \\begin{array}{lcl} G_{ \\, {\\tau} \\, {\\tau} }^{ \\phantom{\\, {\\tau}}\\phantom{\\, {\\tau}} } & = & \\frac{2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} + a\\left({\\tau}, {\\chi}\\right)^{3} \\frac{\\partial\\,r}{\\partial {\\tau}}^{2} + a\\left({\\tau}, {\\chi}\\right)^{3} + 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} - a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,r}{\\partial {\\chi}}^{2} - 2 \\, a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2}} \\\\ G_{ \\, {\\tau} \\, {\\chi} }^{ \\phantom{\\, {\\tau}}\\phantom{\\, {\\chi}} } & = & -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\\\ G_{ \\, {\\chi} \\, {\\tau} }^{ \\phantom{\\, {\\chi}}\\phantom{\\, {\\tau}} } & = & -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\\\ G_{ \\, {\\chi} \\, {\\chi} }^{ \\phantom{\\, {\\chi}}\\phantom{\\, {\\chi}} } & = & -\\frac{a\\left({\\tau}, {\\chi}\\right)^{2} \\left(\\frac{\\partial\\,r}{\\partial {\\tau}}\\right)^{2} + 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} + a\\left({\\tau}, {\\chi}\\right)^{2} - \\left(\\frac{\\partial\\,r}{\\partial {\\chi}}\\right)^{2}}{r\\left({\\tau}, {\\chi}\\right)^{2}} \\\\ G_{ \\, {\\theta} \\, {\\theta} }^{ \\phantom{\\, {\\theta}}\\phantom{\\, {\\theta}} } & = & -\\frac{a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} + a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} + a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} - a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3}} \\\\ G_{ \\, {\\phi} \\, {\\phi} }^{ \\phantom{\\, {\\phi}}\\phantom{\\, {\\phi}} } & = & -\\frac{{\\left(a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} + a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} + a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} - a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}\\right)} \\sin\\left({\\theta}\\right)^{2}}{a\\left({\\tau}, {\\chi}\\right)^{3}} \\end{array}\\)" ], "text/latex": [ "$\\displaystyle \\begin{array}{lcl} G_{ \\, {\\tau} \\, {\\tau} }^{ \\phantom{\\, {\\tau}}\\phantom{\\, {\\tau}} } & = & \\frac{2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} + a\\left({\\tau}, {\\chi}\\right)^{3} \\frac{\\partial\\,r}{\\partial {\\tau}}^{2} + a\\left({\\tau}, {\\chi}\\right)^{3} + 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} - a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,r}{\\partial {\\chi}}^{2} - 2 \\, a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2}} \\\\ G_{ \\, {\\tau} \\, {\\chi} }^{ \\phantom{\\, {\\tau}}\\phantom{\\, {\\chi}} } & = & -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\\\ G_{ \\, {\\chi} \\, {\\tau} }^{ \\phantom{\\, {\\chi}}\\phantom{\\, {\\tau}} } & = & -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\\\ G_{ \\, {\\chi} \\, {\\chi} }^{ \\phantom{\\, {\\chi}}\\phantom{\\, {\\chi}} } & = & -\\frac{a\\left({\\tau}, {\\chi}\\right)^{2} \\left(\\frac{\\partial\\,r}{\\partial {\\tau}}\\right)^{2} + 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} + a\\left({\\tau}, {\\chi}\\right)^{2} - \\left(\\frac{\\partial\\,r}{\\partial {\\chi}}\\right)^{2}}{r\\left({\\tau}, {\\chi}\\right)^{2}} \\\\ G_{ \\, {\\theta} \\, {\\theta} }^{ \\phantom{\\, {\\theta}}\\phantom{\\, {\\theta}} } & = & -\\frac{a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} + a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} + a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} - a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3}} \\\\ G_{ \\, {\\phi} \\, {\\phi} }^{ \\phantom{\\, {\\phi}}\\phantom{\\, {\\phi}} } & = & -\\frac{{\\left(a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} + a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} + a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} - a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}\\right)} \\sin\\left({\\theta}\\right)^{2}}{a\\left({\\tau}, {\\chi}\\right)^{3}} \\end{array}$" ], "text/plain": [ "G_t,t = (2*a(t, x)^2*r(t, x)*d(a)/dt*d(r)/dt + a(t, x)^3*d(r)/dt^2 + a(t, x)^3 + 2*r(t, x)*d(a)/dx*d(r)/dx - a(t, x)*d(r)/dx^2 - 2*a(t, x)*r(t, x)*d^2(r)/dx^2)/(a(t, x)^3*r(t, x)^2) \n", "G_t,x = -2*(a(t, x)*d^2(r)/dtdx - d(a)/dt*d(r)/dx)/(a(t, x)*r(t, x)) \n", "G_x,t = -2*(a(t, x)*d^2(r)/dtdx - d(a)/dt*d(r)/dx)/(a(t, x)*r(t, x)) \n", "G_x,x = -(a(t, x)^2*(d(r)/dt)^2 + 2*a(t, x)^2*r(t, x)*d^2(r)/dt^2 + a(t, x)^2 - (d(r)/dx)^2)/r(t, x)^2 \n", "G_th,th = -(a(t, x)^2*r(t, x)^2*d^2(a)/dt^2 + a(t, x)^2*r(t, x)*d(a)/dt*d(r)/dt + a(t, x)^3*r(t, x)*d^2(r)/dt^2 + r(t, x)*d(a)/dx*d(r)/dx - a(t, x)*r(t, x)*d^2(r)/dx^2)/a(t, x)^3 \n", "G_ph,ph = -(a(t, x)^2*r(t, x)^2*d^2(a)/dt^2 + a(t, x)^2*r(t, x)*d(a)/dt*d(r)/dt + a(t, x)^3*r(t, x)*d^2(r)/dt^2 + r(t, x)*d(a)/dx*d(r)/dx - a(t, x)*r(t, x)*d^2(r)/dx^2)*sin(th)^2/a(t, x)^3 " ] }, "execution_count": 10, "metadata": {}, "output_type": "execute_result" } ], "source": [ "G.display_comp()" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "### Dust matter model\n", "\n", "Let us consider a pressureless fluid (\"dust\"). Moreover, we assume that the coordinates $(\\tau,\\chi,\\theta,\\phi)$ are **comoving**, i.e. that the fluid 4-velocity is equal to $\\partial_\\tau$:" ] }, { "cell_type": "code", "execution_count": 11, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle u = \\frac{\\partial}{\\partial {\\tau} }\$" ], "text/latex": [ "$\\displaystyle u = \\frac{\\partial}{\\partial {\\tau} }$" ], "text/plain": [ "u = ∂/∂t" ] }, "execution_count": 11, "metadata": {}, "output_type": "execute_result" } ], "source": [ "u = M.vector_field('u')\n", "u[0] = 1\n", "u.display()" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Since $(\\tau,\\chi,\\theta,\\chi)$ are synchronous, the above does define a unit timelike vector:" ] }, { "cell_type": "code", "execution_count": 12, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\begin{array}{llcl} g\\left(u,u\\right):& M & \\longrightarrow & \\mathbb{R} \\\\ & \\left({\\tau}, {\\chi}, {\\theta}, {\\phi}\\right) & \\longmapsto & -1 \\end{array}\$" ], "text/latex": [ "$\\displaystyle \\begin{array}{llcl} g\\left(u,u\\right):& M & \\longrightarrow & \\mathbb{R} \\\\ & \\left({\\tau}, {\\chi}, {\\theta}, {\\phi}\\right) & \\longmapsto & -1 \\end{array}$" ], "text/plain": [ "g(u,u): M → ℝ\n", " (t, x, th, ph) ↦ -1" ] }, "execution_count": 12, "metadata": {}, "output_type": "execute_result" } ], "source": [ "g(u,u).display()" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Let us check that $u$ is a geodesic vector field:" ] }, { "cell_type": "code", "execution_count": 13, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle 0\$" ], "text/latex": [ "$\\displaystyle 0$" ], "text/plain": [ "0" ] }, "execution_count": 13, "metadata": {}, "output_type": "execute_result" } ], "source": [ "nabla = g.connection()\n", "acc = u['^b']*nabla(u)['^a_b']\n", "acc.display()" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "The 1-form associated to the fluid 4-velocity by metric duality:" ] }, { "cell_type": "code", "execution_count": 14, "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "1-form on the 4-dimensional Lorentzian manifold M\n" ] } ], "source": [ "u_form = u.down(g)\n", "print(u_form)" ] }, { "cell_type": "code", "execution_count": 15, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle -\\mathrm{d} {\\tau}\$" ], "text/latex": [ "$\\displaystyle -\\mathrm{d} {\\tau}$" ], "text/plain": [ "-dt" ] }, "execution_count": 15, "metadata": {}, "output_type": "execute_result" } ], "source": [ "u_form.display()" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "The pressureless energy-momentum tensor:" ] }, { "cell_type": "code", "execution_count": 16, "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Field of symmetric bilinear forms T on the 4-dimensional Lorentzian manifold M\n" ] } ], "source": [ "rho = function('rho')\n", "T = rho(t,x)*(u_form * u_form)\n", "T.set_name('T')\n", "print(T)" ] }, { "cell_type": "code", "execution_count": 17, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle T = \\rho\\left({\\tau}, {\\chi}\\right) \\mathrm{d} {\\tau}\\otimes \\mathrm{d} {\\tau}\$" ], "text/latex": [ "$\\displaystyle T = \\rho\\left({\\tau}, {\\chi}\\right) \\mathrm{d} {\\tau}\\otimes \\mathrm{d} {\\tau}$" ], "text/plain": [ "T = rho(t, x) dt⊗dt" ] }, "execution_count": 17, "metadata": {}, "output_type": "execute_result" } ], "source": [ "T.display()" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "The Einstein equation:" ] }, { "cell_type": "code", "execution_count": 18, "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Field of symmetric bilinear forms E on the 4-dimensional Lorentzian manifold M\n" ] } ], "source": [ "E = G + Lamb*g - 8*pi*T \n", "E.set_name('E')\n", "print(E)" ] }, { "cell_type": "code", "execution_count": 19, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\\(\\displaystyle \\begin{array}{lcl} E_{ \\, {\\tau} \\, {\\tau} }^{ \\phantom{\\, {\\tau}}\\phantom{\\, {\\tau}} } & = & -\\frac{8 \\, \\pi a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) + {\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} - 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} - a\\left({\\tau}, {\\chi}\\right)^{3} \\frac{\\partial\\,r}{\\partial {\\tau}}^{2} - a\\left({\\tau}, {\\chi}\\right)^{3} - 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} + a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,r}{\\partial {\\chi}}^{2} + 2 \\, a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2}} \\\\ E_{ \\, {\\tau} \\, {\\chi} }^{ \\phantom{\\, {\\tau}}\\phantom{\\, {\\chi}} } & = & -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\\\ E_{ \\, {\\chi} \\, {\\tau} }^{ \\phantom{\\, {\\chi}}\\phantom{\\, {\\tau}} } & = & -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\\\ E_{ \\, {\\chi} \\, {\\chi} }^{ \\phantom{\\, {\\chi}}\\phantom{\\, {\\chi}} } & = & \\frac{{\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} - a\\left({\\tau}, {\\chi}\\right)^{2} \\left(\\frac{\\partial\\,r}{\\partial {\\tau}}\\right)^{2} - 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} - a\\left({\\tau}, {\\chi}\\right)^{2} + \\left(\\frac{\\partial\\,r}{\\partial {\\chi}}\\right)^{2}}{r\\left({\\tau}, {\\chi}\\right)^{2}} \\\\ E_{ \\, {\\theta} \\, {\\theta} }^{ \\phantom{\\, {\\theta}}\\phantom{\\, {\\theta}} } & = & \\frac{{\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} - a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} - a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} - a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} - r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} + a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3}} \\\\ E_{ \\, {\\phi} \\, {\\phi} }^{ \\phantom{\\, {\\phi}}\\phantom{\\, {\\phi}} } & = & \\frac{{\\left({\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} - a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} - a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} - a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} - r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} + a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}\\right)} \\sin\\left({\\theta}\\right)^{2}}{a\\left({\\tau}, {\\chi}\\right)^{3}} \\end{array}\\)" ], "text/latex": [ "$\\displaystyle \\begin{array}{lcl} E_{ \\, {\\tau} \\, {\\tau} }^{ \\phantom{\\, {\\tau}}\\phantom{\\, {\\tau}} } & = & -\\frac{8 \\, \\pi a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) + {\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} - 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} - a\\left({\\tau}, {\\chi}\\right)^{3} \\frac{\\partial\\,r}{\\partial {\\tau}}^{2} - a\\left({\\tau}, {\\chi}\\right)^{3} - 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} + a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,r}{\\partial {\\chi}}^{2} + 2 \\, a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2}} \\\\ E_{ \\, {\\tau} \\, {\\chi} }^{ \\phantom{\\, {\\tau}}\\phantom{\\, {\\chi}} } & = & -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\\\ E_{ \\, {\\chi} \\, {\\tau} }^{ \\phantom{\\, {\\chi}}\\phantom{\\, {\\tau}} } & = & -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)} \\\\ E_{ \\, {\\chi} \\, {\\chi} }^{ \\phantom{\\, {\\chi}}\\phantom{\\, {\\chi}} } & = & \\frac{{\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} - a\\left({\\tau}, {\\chi}\\right)^{2} \\left(\\frac{\\partial\\,r}{\\partial {\\tau}}\\right)^{2} - 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} - a\\left({\\tau}, {\\chi}\\right)^{2} + \\left(\\frac{\\partial\\,r}{\\partial {\\chi}}\\right)^{2}}{r\\left({\\tau}, {\\chi}\\right)^{2}} \\\\ E_{ \\, {\\theta} \\, {\\theta} }^{ \\phantom{\\, {\\theta}}\\phantom{\\, {\\theta}} } & = & \\frac{{\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} - a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} - a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} - a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} - r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} + a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3}} \\\\ E_{ \\, {\\phi} \\, {\\phi} }^{ \\phantom{\\, {\\phi}}\\phantom{\\, {\\phi}} } & = & \\frac{{\\left({\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} - a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} - a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} - a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} - r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} + a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}\\right)} \\sin\\left({\\theta}\\right)^{2}}{a\\left({\\tau}, {\\chi}\\right)^{3}} \\end{array}$" ], "text/plain": [ "E_t,t = -(8*pi*a(t, x)^3*r(t, x)^2*rho(t, x) + Lamb*a(t, x)^3*r(t, x)^2 - 2*a(t, x)^2*r(t, x)*d(a)/dt*d(r)/dt - a(t, x)^3*d(r)/dt^2 - a(t, x)^3 - 2*r(t, x)*d(a)/dx*d(r)/dx + a(t, x)*d(r)/dx^2 + 2*a(t, x)*r(t, x)*d^2(r)/dx^2)/(a(t, x)^3*r(t, x)^2) \n", "E_t,x = -2*(a(t, x)*d^2(r)/dtdx - d(a)/dt*d(r)/dx)/(a(t, x)*r(t, x)) \n", "E_x,t = -2*(a(t, x)*d^2(r)/dtdx - d(a)/dt*d(r)/dx)/(a(t, x)*r(t, x)) \n", "E_x,x = (Lamb*a(t, x)^2*r(t, x)^2 - a(t, x)^2*(d(r)/dt)^2 - 2*a(t, x)^2*r(t, x)*d^2(r)/dt^2 - a(t, x)^2 + (d(r)/dx)^2)/r(t, x)^2 \n", "E_th,th = (Lamb*a(t, x)^3*r(t, x)^2 - a(t, x)^2*r(t, x)^2*d^2(a)/dt^2 - a(t, x)^2*r(t, x)*d(a)/dt*d(r)/dt - a(t, x)^3*r(t, x)*d^2(r)/dt^2 - r(t, x)*d(a)/dx*d(r)/dx + a(t, x)*r(t, x)*d^2(r)/dx^2)/a(t, x)^3 \n", "E_ph,ph = (Lamb*a(t, x)^3*r(t, x)^2 - a(t, x)^2*r(t, x)^2*d^2(a)/dt^2 - a(t, x)^2*r(t, x)*d(a)/dt*d(r)/dt - a(t, x)^3*r(t, x)*d^2(r)/dt^2 - r(t, x)*d(a)/dx*d(r)/dx + a(t, x)*r(t, x)*d^2(r)/dx^2)*sin(th)^2/a(t, x)^3 " ] }, "execution_count": 19, "metadata": {}, "output_type": "execute_result" } ], "source": [ "E.display_comp()" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Solving the Einstein equation" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "### $\\tau\\chi$ component" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Let us first consider the $01 = \\tau\\chi$ component of the Einstein equation:" ] }, { "cell_type": "code", "execution_count": 20, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)}\$" ], "text/latex": [ "$\\displaystyle -\\frac{2 \\, {\\left(a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}\\right)}}{a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right)}$" ], "text/plain": [ "-2*(a(t, x)*d^2(r)/dtdx - d(a)/dt*d(r)/dx)/(a(t, x)*r(t, x))" ] }, "execution_count": 20, "metadata": {}, "output_type": "execute_result" } ], "source": [ "E[0,1]" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "A slight rearrangement of the equation:" ] }, { "cell_type": "code", "execution_count": 21, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\frac{a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}}{a\\left({\\tau}, {\\chi}\\right)^{2}}\$" ], "text/latex": [ "$\\displaystyle \\frac{a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau}\\partial {\\chi}} - \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\chi}}}{a\\left({\\tau}, {\\chi}\\right)^{2}}$" ], "text/plain": [ "(a(t, x)*d^2(r)/dtdx - d(a)/dt*d(r)/dx)/a(t, x)^2" ] }, "execution_count": 21, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq = E[0,1]*r(t,x)/(-2*a(t,x)) \n", "eq" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "We see that this equation is equivalent to \n", "$$ \\frac{\\partial}{\\partial\\tau} \\left( \\frac{1}{a}\\frac{\\partial r}{\\partial\\chi} \\right) = 0 $$\n", "since" ] }, { "cell_type": "code", "execution_count": 22, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle 0\$" ], "text/latex": [ "$\\displaystyle 0$" ], "text/plain": [ "0" ] }, "execution_count": 22, "metadata": {}, "output_type": "execute_result" } ], "source": [ "drdx = diff(r(t,x), x)\n", "eq - diff(drdx/a(t,x), t)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Hence there exists a function of $\\chi$ only, $f(\\chi)$ say, such that\n", "$\\frac{1}{a}\\frac{\\partial r}{\\partial\\chi} = f(\\chi)$.\n", "We disregard the case $f(\\chi)=0$, which would imply $\\frac{\\partial r}{\\partial\\chi}=0$ \n", "and would lead to the so-called *Datt model* (1938). Accordingly, we may write\n", "$$\n", " a(\\tau,\\chi) = \\frac{1}{f(\\chi)}\\frac{\\partial r}{\\partial\\chi} \n", "$$\n", "Let us call `af` this expression of $a$:" ] }, { "cell_type": "code", "execution_count": 23, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\frac{\\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)}{f\\left({\\chi}\\right)}\$" ], "text/latex": [ "$\\displaystyle \\frac{\\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)}{f\\left({\\chi}\\right)}$" ], "text/plain": [ "diff(r(t, x), x)/f(x)" ] }, "execution_count": 23, "metadata": {}, "output_type": "execute_result" } ], "source": [ "f = function('f')\n", "af(t,x) = drdx / f(x)\n", "af(t,x)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "We check that if we substitute $a$ by `af` in the $\\tau\\chi$ component of the Einstein equation, we get identically zero:" ] }, { "cell_type": "code", "execution_count": 24, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle 0\$" ], "text/latex": [ "$\\displaystyle 0$" ], "text/plain": [ "0" ] }, "execution_count": 24, "metadata": {}, "output_type": "execute_result" } ], "source": [ "E[0,1].expr().substitute_function(a, af)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "*NB:* `expr()` returns a Sage symbolic expression from the coordinate function `E[0,1]`, so that we may apply `substitute_function`" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Hence the first Lemaitre-Tolman equation is" ] }, { "cell_type": "code", "execution_count": 25, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle a\\left({\\tau}, {\\chi}\\right) = \\frac{\\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)}{f\\left({\\chi}\\right)}\$" ], "text/latex": [ "$\\displaystyle a\\left({\\tau}, {\\chi}\\right) = \\frac{\\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)}{f\\left({\\chi}\\right)}$" ], "text/plain": [ "a(t, x) == diff(r(t, x), x)/f(x)" ] }, "execution_count": 25, "metadata": {}, "output_type": "execute_result" } ], "source": [ "LT1 = a(t,x) == af(t,x)\n", "LT1" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "### $\\chi\\chi$ component\n", "\n", "The $11 = \\chi\\chi$ component of Einstein equation is" ] }, { "cell_type": "code", "execution_count": 26, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\frac{{\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} - a\\left({\\tau}, {\\chi}\\right)^{2} \\left(\\frac{\\partial\\,r}{\\partial {\\tau}}\\right)^{2} - 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} - a\\left({\\tau}, {\\chi}\\right)^{2} + \\left(\\frac{\\partial\\,r}{\\partial {\\chi}}\\right)^{2}}{r\\left({\\tau}, {\\chi}\\right)^{2}}\$" ], "text/latex": [ "$\\displaystyle \\frac{{\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} - a\\left({\\tau}, {\\chi}\\right)^{2} \\left(\\frac{\\partial\\,r}{\\partial {\\tau}}\\right)^{2} - 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} - a\\left({\\tau}, {\\chi}\\right)^{2} + \\left(\\frac{\\partial\\,r}{\\partial {\\chi}}\\right)^{2}}{r\\left({\\tau}, {\\chi}\\right)^{2}}$" ], "text/plain": [ "(Lamb*a(t, x)^2*r(t, x)^2 - a(t, x)^2*(d(r)/dt)^2 - 2*a(t, x)^2*r(t, x)*d^2(r)/dt^2 - a(t, x)^2 + (d(r)/dx)^2)/r(t, x)^2" ] }, "execution_count": 26, "metadata": {}, "output_type": "execute_result" } ], "source": [ "E[1,1]" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "It is equivalent to" ] }, { "cell_type": "code", "execution_count": 27, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle -{\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} + a\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} + 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right) + a\\left({\\tau}, {\\chi}\\right)^{2} - \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} = 0\$" ], "text/latex": [ "$\\displaystyle -{\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} + a\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} + 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right) + a\\left({\\tau}, {\\chi}\\right)^{2} - \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} = 0$" ], "text/plain": [ "-Lamb*a(t, x)^2*r(t, x)^2 + a(t, x)^2*diff(r(t, x), t)^2 + 2*a(t, x)^2*r(t, x)*diff(r(t, x), t, t) + a(t, x)^2 - diff(r(t, x), x)^2 == 0" ] }, "execution_count": 27, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq = (- E[1,1] * r(t,x)^2).expr() == 0\n", "eq" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Let us substitute for $a(\\tau,\\chi)$ the value found above when solving the $\\tau\\chi$ component:" ] }, { "cell_type": "code", "execution_count": 28, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle -\\frac{{\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2}}{f\\left({\\chi}\\right)^{2}} + \\frac{\\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2}}{f\\left({\\chi}\\right)^{2}} + \\frac{2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2}}{f\\left({\\chi}\\right)^{2}} - \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} + \\frac{\\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2}}{f\\left({\\chi}\\right)^{2}} = 0\$" ], "text/latex": [ "$\\displaystyle -\\frac{{\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2}}{f\\left({\\chi}\\right)^{2}} + \\frac{\\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2}}{f\\left({\\chi}\\right)^{2}} + \\frac{2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2}}{f\\left({\\chi}\\right)^{2}} - \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} + \\frac{\\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2}}{f\\left({\\chi}\\right)^{2}} = 0$" ], "text/plain": [ "-Lamb*r(t, x)^2*diff(r(t, x), x)^2/f(x)^2 + diff(r(t, x), t)^2*diff(r(t, x), x)^2/f(x)^2 + 2*r(t, x)*diff(r(t, x), t, t)*diff(r(t, x), x)^2/f(x)^2 - diff(r(t, x), x)^2 + diff(r(t, x), x)^2/f(x)^2 == 0" ] }, "execution_count": 28, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq1 = eq.substitute_function(a, af)\n", "eq1" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Some slight rearrangement and simplification:" ] }, { "cell_type": "code", "execution_count": 29, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle -{\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} - f\\left({\\chi}\\right)^{2} + \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} + 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right) + 1 = 0\$" ], "text/latex": [ "$\\displaystyle -{\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} - f\\left({\\chi}\\right)^{2} + \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} + 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right) + 1 = 0$" ], "text/plain": [ "-Lamb*r(t, x)^2 - f(x)^2 + diff(r(t, x), t)^2 + 2*r(t, x)*diff(r(t, x), t, t) + 1 == 0" ] }, "execution_count": 29, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq2 = (eq1 * f(x)^2 / diff(r(t,x), x)^2).simplify_full()\n", "eq2" ] }, { "cell_type": "code", "execution_count": 30, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle -f\\left({\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) + \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{3} + 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right) - {\\left({\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} - 1\\right)} \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) = 0\$" ], "text/latex": [ "$\\displaystyle -f\\left({\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) + \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{3} + 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right) - {\\left({\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} - 1\\right)} \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) = 0$" ], "text/plain": [ "-f(x)^2*diff(r(t, x), t) + diff(r(t, x), t)^3 + 2*r(t, x)*diff(r(t, x), t)*diff(r(t, x), t, t) - (Lamb*r(t, x)^2 - 1)*diff(r(t, x), t) == 0" ] }, "execution_count": 30, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq3 = (eq2 * diff(r(t,x),t)).simplify_full()\n", "eq3" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "We notice that the left-hand side of this equation is nothing but the partial derivative w.r.t. $\\tau$ of the following quantity:" ] }, { "cell_type": "code", "execution_count": 31, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle -\\frac{1}{3} \\, {\\left({\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + 3 \\, f\\left({\\chi}\\right)^{2} - 3 \\, \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} - 3\\right)} r\\left({\\tau}, {\\chi}\\right)\$" ], "text/latex": [ "$\\displaystyle -\\frac{1}{3} \\, {\\left({\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + 3 \\, f\\left({\\chi}\\right)^{2} - 3 \\, \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} - 3\\right)} r\\left({\\tau}, {\\chi}\\right)$" ], "text/plain": [ "-1/3*(Lamb*r(t, x)^2 + 3*f(x)^2 - 3*diff(r(t, x), t)^2 - 3)*r(t, x)" ] }, "execution_count": 31, "metadata": {}, "output_type": "execute_result" } ], "source": [ "A = (diff(r(t,x),t)^2 + 1 - f(x)^2 - (Lamb/3)*r(t,x)^2) * r(t,x) \n", "A" ] }, { "cell_type": "code", "execution_count": 32, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\mathrm{True}\$" ], "text/latex": [ "$\\displaystyle \\mathrm{True}$" ], "text/plain": [ "True" ] }, "execution_count": 32, "metadata": {}, "output_type": "execute_result" } ], "source": [ "bool(eq3.lhs() == diff(A, t))" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Hence `eq3` tells that $A$ is independent of $\\tau$, i.e. is a function of $\\chi$ only, which we call $2 m(\\chi)$:" ] }, { "cell_type": "code", "execution_count": 33, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle -\\frac{1}{3} \\, {\\left({\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + 3 \\, f\\left({\\chi}\\right)^{2} - 3 \\, \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} - 3\\right)} r\\left({\\tau}, {\\chi}\\right) - 2 \\, m\\left({\\chi}\\right) = 0\$" ], "text/latex": [ "$\\displaystyle -\\frac{1}{3} \\, {\\left({\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + 3 \\, f\\left({\\chi}\\right)^{2} - 3 \\, \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} - 3\\right)} r\\left({\\tau}, {\\chi}\\right) - 2 \\, m\\left({\\chi}\\right) = 0$" ], "text/plain": [ "-1/3*(Lamb*r(t, x)^2 + 3*f(x)^2 - 3*diff(r(t, x), t)^2 - 3)*r(t, x) - 2*m(x) == 0" ] }, "execution_count": 33, "metadata": {}, "output_type": "execute_result" } ], "source": [ "m = function('m')\n", "eq4 = A - 2*m(x) == 0\n", "eq4" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Let us solve extract $(\\partial r/\\partial\\tau)^2$ from this equation:" ] }, { "cell_type": "code", "execution_count": 34, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\left[\\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} = \\frac{{\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{3} + 3 \\, f\\left({\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) + 6 \\, m\\left({\\chi}\\right) - 3 \\, r\\left({\\tau}, {\\chi}\\right)}{3 \\, r\\left({\\tau}, {\\chi}\\right)}\\right]\$" ], "text/latex": [ "$\\displaystyle \\left[\\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} = \\frac{{\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{3} + 3 \\, f\\left({\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) + 6 \\, m\\left({\\chi}\\right) - 3 \\, r\\left({\\tau}, {\\chi}\\right)}{3 \\, r\\left({\\tau}, {\\chi}\\right)}\\right]$" ], "text/plain": [ "[diff(r(t, x), t)^2 == 1/3*(Lamb*r(t, x)^3 + 3*f(x)^2*r(t, x) + 6*m(x) - 3*r(t, x))/r(t, x)]" ] }, "execution_count": 34, "metadata": {}, "output_type": "execute_result" } ], "source": [ "drdt2_sol = solve(eq4, diff(r(t,x),t)^2)\n", "drdt2_sol" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "We thus obtain the second Lemaitre-Tolman equation:" ] }, { "cell_type": "code", "execution_count": 35, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} = \\frac{1}{3} \\, {\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + f\\left({\\chi}\\right)^{2} + \\frac{2 \\, m\\left({\\chi}\\right)}{r\\left({\\tau}, {\\chi}\\right)} - 1\$" ], "text/latex": [ "$\\displaystyle \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} = \\frac{1}{3} \\, {\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + f\\left({\\chi}\\right)^{2} + \\frac{2 \\, m\\left({\\chi}\\right)}{r\\left({\\tau}, {\\chi}\\right)} - 1$" ], "text/plain": [ "diff(r(t, x), t)^2 == 1/3*Lamb*r(t, x)^2 + f(x)^2 + 2*m(x)/r(t, x) - 1" ] }, "execution_count": 35, "metadata": {}, "output_type": "execute_result" } ], "source": [ "LT2 = drdt2_sol[0].expand()\n", "LT2" ] }, { "cell_type": "code", "execution_count": 36, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\frac{1}{3} \\, {\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + f\\left({\\chi}\\right)^{2} + \\frac{2 \\, m\\left({\\chi}\\right)}{r\\left({\\tau}, {\\chi}\\right)} - 1\$" ], "text/latex": [ "$\\displaystyle \\frac{1}{3} \\, {\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + f\\left({\\chi}\\right)^{2} + \\frac{2 \\, m\\left({\\chi}\\right)}{r\\left({\\tau}, {\\chi}\\right)} - 1$" ], "text/plain": [ "1/3*Lamb*r(t, x)^2 + f(x)^2 + 2*m(x)/r(t, x) - 1" ] }, "execution_count": 36, "metadata": {}, "output_type": "execute_result" } ], "source": [ "drdt2 = LT2.rhs()\n", "drdt2" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "### $\\tau\\tau$ component\n", "\n", "The $00 = \\tau\\tau$ component of Einstein equation is" ] }, { "cell_type": "code", "execution_count": 37, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle -\\frac{8 \\, \\pi a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) + {\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} - 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} - a\\left({\\tau}, {\\chi}\\right)^{3} \\frac{\\partial\\,r}{\\partial {\\tau}}^{2} - a\\left({\\tau}, {\\chi}\\right)^{3} - 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} + a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,r}{\\partial {\\chi}}^{2} + 2 \\, a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2}}\$" ], "text/latex": [ "$\\displaystyle -\\frac{8 \\, \\pi a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) + {\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} - 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} - a\\left({\\tau}, {\\chi}\\right)^{3} \\frac{\\partial\\,r}{\\partial {\\tau}}^{2} - a\\left({\\tau}, {\\chi}\\right)^{3} - 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} + a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,r}{\\partial {\\chi}}^{2} + 2 \\, a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2}}$" ], "text/plain": [ "-(8*pi*a(t, x)^3*r(t, x)^2*rho(t, x) + Lamb*a(t, x)^3*r(t, x)^2 - 2*a(t, x)^2*r(t, x)*d(a)/dt*d(r)/dt - a(t, x)^3*d(r)/dt^2 - a(t, x)^3 - 2*r(t, x)*d(a)/dx*d(r)/dx + a(t, x)*d(r)/dx^2 + 2*a(t, x)*r(t, x)*d^2(r)/dx^2)/(a(t, x)^3*r(t, x)^2)" ] }, "execution_count": 37, "metadata": {}, "output_type": "execute_result" } ], "source": [ "E[0,0]" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "It is equivalent to" ] }, { "cell_type": "code", "execution_count": 38, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\\(\\displaystyle 8 \\, \\pi a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) + {\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} - 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) - a\\left({\\tau}, {\\chi}\\right)^{3} \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} - a\\left({\\tau}, {\\chi}\\right)^{3} - 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) + a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} + 2 \\, a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\chi})^{2}}r\\left({\\tau}, {\\chi}\\right) = 0\\)" ], "text/latex": [ "$\\displaystyle 8 \\, \\pi a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) + {\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} - 2 \\, a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) - a\\left({\\tau}, {\\chi}\\right)^{3} \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} - a\\left({\\tau}, {\\chi}\\right)^{3} - 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) + a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} + 2 \\, a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\chi})^{2}}r\\left({\\tau}, {\\chi}\\right) = 0$" ], "text/plain": [ "8*pi*a(t, x)^3*r(t, x)^2*rho(t, x) + Lamb*a(t, x)^3*r(t, x)^2 - 2*a(t, x)^2*r(t, x)*diff(a(t, x), t)*diff(r(t, x), t) - a(t, x)^3*diff(r(t, x), t)^2 - a(t, x)^3 - 2*r(t, x)*diff(a(t, x), x)*diff(r(t, x), x) + a(t, x)*diff(r(t, x), x)^2 + 2*a(t, x)*r(t, x)*diff(r(t, x), x, x) == 0" ] }, "execution_count": 38, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq = (- E[0,0] * a(t,x)^3 * r(t,x)^2).expr() == 0\n", "eq" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "As above, we substitute for $a(\\tau,\\chi)$ the value found when solving the $\\tau\\chi$ component:" ] }, { "cell_type": "code", "execution_count": 39, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\\(\\displaystyle \\frac{8 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3}}{f\\left({\\chi}\\right)^{3}} + 2 \\, {\\left(\\frac{\\frac{\\partial}{\\partial {\\chi}}f\\left({\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)}{f\\left({\\chi}\\right)^{2}} - \\frac{\\frac{\\partial^{2}}{(\\partial {\\chi})^{2}}r\\left({\\tau}, {\\chi}\\right)}{f\\left({\\chi}\\right)}\\right)} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) + \\frac{{\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3}}{f\\left({\\chi}\\right)^{3}} - \\frac{2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{\\partial {\\tau}\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2}}{f\\left({\\chi}\\right)^{3}} + \\frac{\\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3}}{f\\left({\\chi}\\right)} - \\frac{\\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3}}{f\\left({\\chi}\\right)^{3}} + \\frac{2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\chi})^{2}}r\\left({\\tau}, {\\chi}\\right)}{f\\left({\\chi}\\right)} - \\frac{\\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3}}{f\\left({\\chi}\\right)^{3}} = 0\\)" ], "text/latex": [ "$\\displaystyle \\frac{8 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3}}{f\\left({\\chi}\\right)^{3}} + 2 \\, {\\left(\\frac{\\frac{\\partial}{\\partial {\\chi}}f\\left({\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)}{f\\left({\\chi}\\right)^{2}} - \\frac{\\frac{\\partial^{2}}{(\\partial {\\chi})^{2}}r\\left({\\tau}, {\\chi}\\right)}{f\\left({\\chi}\\right)}\\right)} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) + \\frac{{\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3}}{f\\left({\\chi}\\right)^{3}} - \\frac{2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{\\partial {\\tau}\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2}}{f\\left({\\chi}\\right)^{3}} + \\frac{\\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3}}{f\\left({\\chi}\\right)} - \\frac{\\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3}}{f\\left({\\chi}\\right)^{3}} + \\frac{2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\chi})^{2}}r\\left({\\tau}, {\\chi}\\right)}{f\\left({\\chi}\\right)} - \\frac{\\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3}}{f\\left({\\chi}\\right)^{3}} = 0$" ], "text/plain": [ "8*pi*r(t, x)^2*rho(t, x)*diff(r(t, x), x)^3/f(x)^3 + 2*(diff(f(x), x)*diff(r(t, x), x)/f(x)^2 - diff(r(t, x), x, x)/f(x))*r(t, x)*diff(r(t, x), x) + Lamb*r(t, x)^2*diff(r(t, x), x)^3/f(x)^3 - 2*r(t, x)*diff(r(t, x), t)*diff(r(t, x), t, x)*diff(r(t, x), x)^2/f(x)^3 + diff(r(t, x), x)^3/f(x) - diff(r(t, x), t)^2*diff(r(t, x), x)^3/f(x)^3 + 2*r(t, x)*diff(r(t, x), x)*diff(r(t, x), x, x)/f(x) - diff(r(t, x), x)^3/f(x)^3 == 0" ] }, "execution_count": 39, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq1 = eq.substitute_function(a, af)\n", "eq1" ] }, { "cell_type": "code", "execution_count": 40, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle 2 \\, f\\left({\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}f\\left({\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} - 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{\\partial {\\tau}\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} + f\\left({\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3} + {\\left(8 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) + {\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} - \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} - 1\\right)} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3} = 0\$" ], "text/latex": [ "$\\displaystyle 2 \\, f\\left({\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}f\\left({\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} - 2 \\, r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{\\partial {\\tau}\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} + f\\left({\\chi}\\right)^{2} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3} + {\\left(8 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) + {\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} - \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} - 1\\right)} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3} = 0$" ], "text/plain": [ "2*f(x)*r(t, x)*diff(f(x), x)*diff(r(t, x), x)^2 - 2*r(t, x)*diff(r(t, x), t)*diff(r(t, x), t, x)*diff(r(t, x), x)^2 + f(x)^2*diff(r(t, x), x)^3 + (8*pi*r(t, x)^2*rho(t, x) + Lamb*r(t, x)^2 - diff(r(t, x), t)^2 - 1)*diff(r(t, x), x)^3 == 0" ] }, "execution_count": 40, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq2 = (eq1 * f(x)^3).simplify_full()\n", "eq2" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Let us substitute for $\\partial r/\\partial \\tau$ the positive square root of the value of $(\\partial r/\\partial \\tau)^2$ found when solving the $\\chi\\chi$ component:" ] }, { "cell_type": "code", "execution_count": 41, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\sqrt{\\frac{1}{3} \\, {\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + f\\left({\\chi}\\right)^{2} + \\frac{2 \\, m\\left({\\chi}\\right)}{r\\left({\\tau}, {\\chi}\\right)} - 1}\$" ], "text/latex": [ "$\\displaystyle \\sqrt{\\frac{1}{3} \\, {\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + f\\left({\\chi}\\right)^{2} + \\frac{2 \\, m\\left({\\chi}\\right)}{r\\left({\\tau}, {\\chi}\\right)} - 1}$" ], "text/plain": [ "sqrt(1/3*Lamb*r(t, x)^2 + f(x)^2 + 2*m(x)/r(t, x) - 1)" ] }, "execution_count": 41, "metadata": {}, "output_type": "execute_result" } ], "source": [ "drdt = sqrt(drdt2)\n", "drdt" ] }, { "cell_type": "code", "execution_count": 42, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle 8 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3} - 2 \\, \\frac{\\partial}{\\partial {\\chi}}m\\left({\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} = 0\$" ], "text/latex": [ "$\\displaystyle 8 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3} - 2 \\, \\frac{\\partial}{\\partial {\\chi}}m\\left({\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} = 0$" ], "text/plain": [ "8*pi*r(t, x)^2*rho(t, x)*diff(r(t, x), x)^3 - 2*diff(m(x), x)*diff(r(t, x), x)^2 == 0" ] }, "execution_count": 42, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq3 = eq2.subs({diff(r(t,x),t): drdt, diff(r(t,x),t,x): diff(drdt, x)}).simplify_full()\n", "eq3" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "If we use the negative square root of $(\\partial r/\\partial \\tau)^2$ instead, we get the same result:" ] }, { "cell_type": "code", "execution_count": 43, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle -\\sqrt{\\frac{1}{3} \\, {\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + f\\left({\\chi}\\right)^{2} + \\frac{2 \\, m\\left({\\chi}\\right)}{r\\left({\\tau}, {\\chi}\\right)} - 1}\$" ], "text/latex": [ "$\\displaystyle -\\sqrt{\\frac{1}{3} \\, {\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + f\\left({\\chi}\\right)^{2} + \\frac{2 \\, m\\left({\\chi}\\right)}{r\\left({\\tau}, {\\chi}\\right)} - 1}$" ], "text/plain": [ "-sqrt(1/3*Lamb*r(t, x)^2 + f(x)^2 + 2*m(x)/r(t, x) - 1)" ] }, "execution_count": 43, "metadata": {}, "output_type": "execute_result" } ], "source": [ "drdt = - sqrt(drdt2)\n", "drdt" ] }, { "cell_type": "code", "execution_count": 44, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle 8 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3} - 2 \\, \\frac{\\partial}{\\partial {\\chi}}m\\left({\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} = 0\$" ], "text/latex": [ "$\\displaystyle 8 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3} - 2 \\, \\frac{\\partial}{\\partial {\\chi}}m\\left({\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} = 0$" ], "text/plain": [ "8*pi*r(t, x)^2*rho(t, x)*diff(r(t, x), x)^3 - 2*diff(m(x), x)*diff(r(t, x), x)^2 == 0" ] }, "execution_count": 44, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq3_minus = eq2.subs({diff(r(t,x),t): drdt, diff(r(t,x),t,x): diff(drdt, x)}).simplify_full()\n", "eq3_minus" ] }, { "cell_type": "code", "execution_count": 45, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\mathrm{True}\$" ], "text/latex": [ "$\\displaystyle \\mathrm{True}$" ], "text/plain": [ "True" ] }, "execution_count": 45, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq3_minus == eq3" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Thus we continue with `eq3` and rearrange it to get the third Lemaitre-Tolman equation:" ] }, { "cell_type": "code", "execution_count": 46, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle 4 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) - \\frac{\\partial}{\\partial {\\chi}}m\\left({\\chi}\\right) = 0\$" ], "text/latex": [ "$\\displaystyle 4 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) - \\frac{\\partial}{\\partial {\\chi}}m\\left({\\chi}\\right) = 0$" ], "text/plain": [ "4*pi*r(t, x)^2*rho(t, x)*diff(r(t, x), x) - diff(m(x), x) == 0" ] }, "execution_count": 46, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq4 = (eq3 / (2*diff(r(t,x),x)^2)).simplify_full()\n", "eq4" ] }, { "cell_type": "code", "execution_count": 47, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\left[\\frac{\\partial}{\\partial {\\chi}}m\\left({\\chi}\\right) = 4 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)\\right]\$" ], "text/latex": [ "$\\displaystyle \\left[\\frac{\\partial}{\\partial {\\chi}}m\\left({\\chi}\\right) = 4 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)\\right]$" ], "text/plain": [ "[diff(m(x), x) == 4*pi*r(t, x)^2*rho(t, x)*diff(r(t, x), x)]" ] }, "execution_count": 47, "metadata": {}, "output_type": "execute_result" } ], "source": [ "dmdx_sol = solve(eq4, diff(m(x),x))\n", "dmdx_sol" ] }, { "cell_type": "code", "execution_count": 48, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\frac{\\partial}{\\partial {\\chi}}m\\left({\\chi}\\right) = 4 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)\$" ], "text/latex": [ "$\\displaystyle \\frac{\\partial}{\\partial {\\chi}}m\\left({\\chi}\\right) = 4 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)$" ], "text/plain": [ "diff(m(x), x) == 4*pi*r(t, x)^2*rho(t, x)*diff(r(t, x), x)" ] }, "execution_count": 48, "metadata": {}, "output_type": "execute_result" } ], "source": [ "LT3 = dmdx_sol[0]\n", "LT3" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "### $\\theta\\theta$ and $\\phi\\phi$ components\n", "\n", "First we notice that the $\\theta\\theta$ and $\\phi\\phi$ components of the Einstein equation are equivalent:" ] }, { "cell_type": "code", "execution_count": 49, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\mathrm{True}\$" ], "text/latex": [ "$\\displaystyle \\mathrm{True}$" ], "text/plain": [ "True" ] }, "execution_count": 49, "metadata": {}, "output_type": "execute_result" } ], "source": [ "E[3,3] == E[2,2] * sin(th)^2" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Let us thus consider only the $22 = \\theta\\theta$ component:" ] }, { "cell_type": "code", "execution_count": 50, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle \\frac{{\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} - a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} - a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} - a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} - r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} + a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3}}\$" ], "text/latex": [ "$\\displaystyle \\frac{{\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} - a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^2\\,a}{\\partial {\\tau} ^ 2} - a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\tau}} \\frac{\\partial\\,r}{\\partial {\\tau}} - a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\tau} ^ 2} - r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial\\,a}{\\partial {\\chi}} \\frac{\\partial\\,r}{\\partial {\\chi}} + a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^2\\,r}{\\partial {\\chi} ^ 2}}{a\\left({\\tau}, {\\chi}\\right)^{3}}$" ], "text/plain": [ "(Lamb*a(t, x)^3*r(t, x)^2 - a(t, x)^2*r(t, x)^2*d^2(a)/dt^2 - a(t, x)^2*r(t, x)*d(a)/dt*d(r)/dt - a(t, x)^3*r(t, x)*d^2(r)/dt^2 - r(t, x)*d(a)/dx*d(r)/dx + a(t, x)*r(t, x)*d^2(r)/dx^2)/a(t, x)^3" ] }, "execution_count": 50, "metadata": {}, "output_type": "execute_result" } ], "source": [ "E[2,2]" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "It is equivalent to" ] }, { "cell_type": "code", "execution_count": 51, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle -{\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} + a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}a\\left({\\tau}, {\\chi}\\right) + a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) + a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right) + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) - a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\chi})^{2}}r\\left({\\tau}, {\\chi}\\right) = 0\$" ], "text/latex": [ "$\\displaystyle -{\\Lambda} a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right)^{2} + a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}a\\left({\\tau}, {\\chi}\\right) + a\\left({\\tau}, {\\chi}\\right)^{2} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) + a\\left({\\tau}, {\\chi}\\right)^{3} r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right) + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}a\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) - a\\left({\\tau}, {\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\chi})^{2}}r\\left({\\tau}, {\\chi}\\right) = 0$" ], "text/plain": [ "-Lamb*a(t, x)^3*r(t, x)^2 + a(t, x)^2*r(t, x)^2*diff(a(t, x), t, t) + a(t, x)^2*r(t, x)*diff(a(t, x), t)*diff(r(t, x), t) + a(t, x)^3*r(t, x)*diff(r(t, x), t, t) + r(t, x)*diff(a(t, x), x)*diff(r(t, x), x) - a(t, x)*r(t, x)*diff(r(t, x), x, x) == 0" ] }, "execution_count": 51, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq = (- E[2,2] * a(t,x)^3).expr() == 0\n", "eq" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "We substitute for $a(\\tau,\\chi)$ the value found when solving the $\\tau\\chi$ component:" ] }, { "cell_type": "code", "execution_count": 52, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle -\\frac{f\\left({\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}f\\left({\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} + {\\left({\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} - r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right)\\right)} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3} - {\\left(r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^{3}}{(\\partial {\\tau})^{2}\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{\\partial {\\tau}\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)\\right)} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2}}{f\\left({\\chi}\\right)^{3}} = 0\$" ], "text/latex": [ "$\\displaystyle -\\frac{f\\left({\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}f\\left({\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} + {\\left({\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} - r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right)\\right)} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3} - {\\left(r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^{3}}{(\\partial {\\tau})^{2}\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{\\partial {\\tau}\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)\\right)} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2}}{f\\left({\\chi}\\right)^{3}} = 0$" ], "text/plain": [ "-(f(x)*r(t, x)*diff(f(x), x)*diff(r(t, x), x)^2 + (Lamb*r(t, x)^2 - r(t, x)*diff(r(t, x), t, t))*diff(r(t, x), x)^3 - (r(t, x)^2*diff(r(t, x), t, t, x) + r(t, x)*diff(r(t, x), t)*diff(r(t, x), t, x))*diff(r(t, x), x)^2)/f(x)^3 == 0" ] }, "execution_count": 52, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq1 = eq.substitute_function(a, af).simplify_full()\n", "eq1" ] }, { "cell_type": "code", "execution_count": 53, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle -f\\left({\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}f\\left({\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} - {\\left({\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} - r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right)\\right)} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3} + {\\left(r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^{3}}{(\\partial {\\tau})^{2}\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{\\partial {\\tau}\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)\\right)} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} = 0\$" ], "text/latex": [ "$\\displaystyle -f\\left({\\chi}\\right) r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}f\\left({\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} - {\\left({\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} - r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{(\\partial {\\tau})^{2}}r\\left({\\tau}, {\\chi}\\right)\\right)} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{3} + {\\left(r\\left({\\tau}, {\\chi}\\right)^{2} \\frac{\\partial^{3}}{(\\partial {\\tau})^{2}\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right) + r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right) \\frac{\\partial^{2}}{\\partial {\\tau}\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)\\right)} \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)^{2} = 0$" ], "text/plain": [ "-f(x)*r(t, x)*diff(f(x), x)*diff(r(t, x), x)^2 - (Lamb*r(t, x)^2 - r(t, x)*diff(r(t, x), t, t))*diff(r(t, x), x)^3 + (r(t, x)^2*diff(r(t, x), t, t, x) + r(t, x)*diff(r(t, x), t)*diff(r(t, x), t, x))*diff(r(t, x), x)^2 == 0" ] }, "execution_count": 53, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq2 = (eq1 * f(x)^3).simplify_full()\n", "eq2" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Then we substitute for $\\partial r/\\partial\\tau$ the value obtained when solving the $\\tau\\tau$ component:" ] }, { "cell_type": "code", "execution_count": 54, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle 0 = 0\$" ], "text/latex": [ "$\\displaystyle 0 = 0$" ], "text/plain": [ "0 == 0" ] }, "execution_count": 54, "metadata": {}, "output_type": "execute_result" } ], "source": [ "eq3 = eq2.subs({diff(r(t,x),t,t,x): diff(drdt,t,x), diff(r(t,x),t,t): diff(drdt,t)}).simplify_full()\n", "eq4 = eq3.subs({diff(r(t,x),t): drdt, diff(r(t,x),t,x): diff(drdt,x)}).simplify_full()\n", "eq4" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "We conclude that the $\\theta\\theta$ component of Einstein equation does not add any independent equation. " ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "## Summary\n", "\n", "Let us collect the three independent equations obtained from the Einstein equation, constituting the **Lemaître-Tolman system**:" ] }, { "cell_type": "code", "execution_count": 55, "metadata": {}, "outputs": [ { "data": { "text/html": [ "\$\\displaystyle a\\left({\\tau}, {\\chi}\\right) = \\frac{\\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)}{f\\left({\\chi}\\right)}\$" ], "text/latex": [ "$\\displaystyle a\\left({\\tau}, {\\chi}\\right) = \\frac{\\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)}{f\\left({\\chi}\\right)}$" ], "text/plain": [ "a(t, x) == diff(r(t, x), x)/f(x)" ] }, "metadata": {}, "output_type": "display_data" }, { "data": { "text/html": [ "\$\\displaystyle \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} = \\frac{1}{3} \\, {\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + f\\left({\\chi}\\right)^{2} + \\frac{2 \\, m\\left({\\chi}\\right)}{r\\left({\\tau}, {\\chi}\\right)} - 1\$" ], "text/latex": [ "$\\displaystyle \\frac{\\partial}{\\partial {\\tau}}r\\left({\\tau}, {\\chi}\\right)^{2} = \\frac{1}{3} \\, {\\Lambda} r\\left({\\tau}, {\\chi}\\right)^{2} + f\\left({\\chi}\\right)^{2} + \\frac{2 \\, m\\left({\\chi}\\right)}{r\\left({\\tau}, {\\chi}\\right)} - 1$" ], "text/plain": [ "diff(r(t, x), t)^2 == 1/3*Lamb*r(t, x)^2 + f(x)^2 + 2*m(x)/r(t, x) - 1" ] }, "metadata": {}, "output_type": "display_data" }, { "data": { "text/html": [ "\$\\displaystyle \\frac{\\partial}{\\partial {\\chi}}m\\left({\\chi}\\right) = 4 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)\$" ], "text/latex": [ "$\\displaystyle \\frac{\\partial}{\\partial {\\chi}}m\\left({\\chi}\\right) = 4 \\, \\pi r\\left({\\tau}, {\\chi}\\right)^{2} \\rho\\left({\\tau}, {\\chi}\\right) \\frac{\\partial}{\\partial {\\chi}}r\\left({\\tau}, {\\chi}\\right)$" ], "text/plain": [ "diff(m(x), x) == 4*pi*r(t, x)^2*rho(t, x)*diff(r(t, x), x)" ] }, "metadata": {}, "output_type": "display_data" } ], "source": [ "for eq in [LT1, LT2, LT3]:\n", " show(eq)" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "The first equation is the unnumbered one just above Eq. (8.1) in Lemaître's article *L'univers en expansion*, Annales de la Société Scientifique de Bruxelles A **53**, 51 (1933), translated in English in [Gen. Relativ. Gravit. **29**, 641](http://dx.doi.org/10.1023/A:1018855621348) (1997). The second equation is Eq. (8.2) in Lemaître's article, while the third one is Eq. (8.3)." ] } ], "metadata": { "kernelspec": { "display_name": "SageMath 10.0.rc3", "language": "sage", "name": "sagemath" }, "language_info": { "codemirror_mode": { "name": "ipython", "version": 3 }, "file_extension": ".py", "mimetype": "text/x-python", "name": "python", "nbconvert_exporter": "python", "pygments_lexer": "ipython3", "version": "3.10.6" } }, "nbformat": 4, "nbformat_minor": 4 }