{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "# Friedmann Robertson Walker models\n", "\n", "In this section, we will take a look at some consequences of the Friedmann Robertson Walker metric that we derived for a homogeneous and isotropic Universe. \n", "\n", "## Redshifts in an expanding Universe\n", "\n", "Light travels on null geodesics, and they satisfy $ds^2=0$ . Let us consider the position of our galaxy to be at the origin as a convenience. Let us look at a galaxy whose coordinates are ($\\chi, \\theta, \\phi$). The light coming from this galaxy will travel along constant values of ($\\theta, \\phi$) on its path to us. Thus it should satisfy\n", "\n", "$ds^2=0=-c^2 dt^2 + a^2(t) d\\chi^2$\n", "\n", "which implies that\n", "\n", "\\begin{equation}\n", "\\int_{t_e}^{t_r} \\frac{c dt}{a(t)} = -\\int_{0}^{\\chi} d\\chi \n", "\\end{equation}\n", "\n", "where $t_e$ denotes the time of emission, $t_r$ denotes the time of receipt of the photon. Now let us consider the emission and reception of a consecutive crest of the light wave. Let us suppose this second crest gets emitted at $t_e + \\delta t_e$, and will be received $t_r + \\delta t_r$. This implies that for this second crest, \n", "\n", "\\begin{equation}\n", "\\int_{t_e+\\delta t_r}^{t_r + \\delta t_r} \\frac{c dt}{a(t)} = -\\int_{0}^{\\chi} d\\chi \n", "\\end{equation}\n", "\n", "Subtracting the above equations we get\n", "\n", "\\begin{equation}\n", "\\frac{c \\delta t_e}{a(t_e)} - \\frac{c \\delta t_r}{a(t_r)}= 0\n", "\\end{equation}\n", "\n", "Given that $\\lambda_e = c\\delta t_e$ and $\\lambda_r = c\\delta t_r$, are the wavelengths of the emitted and received light, we obtain\n", "\n", "\\begin{equation}\n", "\\frac{\\lambda_r}{\\lambda_e} = 1+z = \\frac{a(t_r)}{a(t_e)}\n", "\\end{equation}\n", "\n", "This implies that the wavelength of light changes as it travels in a Universe with varying scale factor. If $a(t)$ is an increasing function of time then this implies that the wavelength of the received light will be redshifted compared to when the light was emitted.\n", "\n", "## Expansion of the Universe (Hubble-Lemaitre's law)\n", "\n", "The FRW metric implies that the physical distances in the Universe scale with the scale factor. We can define mean motion coordinates which move together in a concerted manner due to the change in scale factor. Such coordinates are called comoving coordinates. The physical distance between any two locations at a given point of time is thus given by $R_{\\rm phys} = a(t) R_{\\rm com}$ . The velocity between any two objects is then given by the rate of change of the physical distance\n", "\n", "\\begin{eqnarray}\n", "v = \\frac{d R_{\\rm phys}}{dt} &=& \\frac{d \\left[ a(t) R_{\\rm com} \\right]}{dt} \\\\\n", "&=& \\frac{da}{dt} R_{\\rm com} + a(t) \\frac{d R_{\\rm com}}{dt} \\\\\n", "&=& \\frac{\\dot{a}}{a} R_{\\rm phys} + a(t) \\frac{d R_{\\rm com}}{dt}\n", "\\end{eqnarray}\n", "\n", "For fundamental observers at fixed locations in the comoving coordinates, the second term would be zero. This gives\n", "\n", "\\begin{equation}\n", "v = \\left[\\frac{\\dot{a}}{a}\\right] R_{\\rm phys}\n", "\\end{equation}\n", "\n", "This shows that the velocities should be proportional to the physical distance of galaxies. The constant of proportionality $\\dot{a}/a$ is called the Hubble-Lemaitre parameter. Note that $\\dot{a}/a$ is a function of time, the value at $t={\\rm today}$ is called the Hubble-Lemaitre constant.\n", "\n", "In reality galaxies will not necessarily sit still in comoving coordinates due to forces from other objects nearby. These velocities are called peculiar velocities and are a result of the small scale inhomogenieties in the Universe.\n", "\n", "The measurement of the Hubble Lemaitre constant is one of the biggest scientific inquiries currently. The value that Hubble derived in the 1920s was about $500$ ${\\rm km/s/Mpc}$. It was marred by observational issues. The current accepted value for the Hubble constant ranges from $67$ to $74$ ${\\rm km/s/Mpc}$.\n", "\n", "The units of the Hubble-Lemaitre constant are the inverse of the timescale. Thus the Hubble-Lemaitre constant sets a time scale in the Universe. The value of $1/H_0$ sets the order of magnitude for the timescale for the Universe. With the value of about $70$ ${\\rm km/s/Mpc}$, you get a value which is close to $1.46\\times10^{10}$ years.\n", "\n", "\n", "## Distances in the Universe\n", "\n", "### Angular size of a distant object of known physical size\n", "\n", "Consider the angle subtended at the observer by a known object of physical size $R_{\\rm phys}$ at a coordinate distance $\\chi$. Let us assume that the two ends of the object are aligned at constant angle $\\phi$. Thus in the metric we have $dt=0$, $d\\chi=0$ and $d\\phi=0$. The metric is then given by\n", "\n", "\\begin{equation}\n", "ds^2 = a^2(t) f^2(\\chi) d\\theta^2\n", "\\end{equation}\n", "\n", "The physical size of the object $R_{\\rm phys}=a(t) f(\\chi) \\Delta \\theta$. We can define a distance based on the angular size subtended by such an object as the angular diameter distance.\n", "\n", "\\begin{equation}\n", "D_{\\rm ang} = \\frac{R_{\\rm phys}}{\\Delta\\theta} = a(t) f(\\chi) = \\frac{f(\\chi)}{(1+z)}\n", "\\end{equation}\n", "\n", "