Difference between revisions of "Cosmography"

Revision as of 10:27, 10 August 2012

In next problems we use an approach to the description of the evolution of the Universe's, which is called "cosmography"$^*$. It is based entirely on the cosmological principle and on some consequences of the equivalence principle. The term "cosmography" is a synonym for "cosmo-kinematics". Let us recall that kinematics represents the part of mechanics which describes motion of bodies regardless of the forces responsible for it. In this sense cosmography represents nothing else than the kinematics of cosmological expansion.

In order to construct the key cosmological quantity $a(t)$ one needs the equations of motion (the Einstein's equation) and some assumptions on the material composition of the Universe, which enable one to obtain the energy-momentum tensor. The efficiency of cosmography lies in the ability to test cosmological models of any kind, that are compatible with the cosmological principle. Modifications of General Relativity or introduction of new components (such as dark matter or dark energy) certainly change the dependence $a(t)$, but they absolutely do not affect the kinematics of the expanding Universe.

The rate of Universe's expansion, determined by Hubble parameter $H(t)$, depends on time. The deceleration parameter $q(t)$ is used to quantify this dependence. Let us define it through the expansion of the scale factor $a(t)$ in a Taylor series in the vicinity of current time ${{t}_{0}}$: \[a(t)=a\left( {{t}_{0}} \right) +\dot{a}\left( {{t}_{0}} \right)\left[ t-{{t}_{0}} \right] +\frac{1}{2}\ddot{a}({t}_{0}) {{\left[ t-{{t}_{0}} \right]}^{2}}+\cdots\] Let us present this in the form \[\frac{a(t)}{a\left( {{t}_{0}} \right)} =1+{{H}_{0}}\left[ t-{{t}_{0}} \right] -\frac{{{q}_{0}}}{2}H_{0}^{2} {{\left[ t-{{t}_{0}} \right]}^{2}}+\cdots\] where the deceleration parameter is \[q(t)\equiv -\frac{\ddot{a}(t)a(t)}{{{{\dot{a}}}^{2}}(t)} =-\frac{\ddot{a}(t)}{a(t)}\frac{1}{{{H}^{2}}(t)}.\]

Note that the accelerated growth of scale factor takes place for $q<0$. When the sign of the deceleration parameter was originally defined, it seemed evident that gravity is the only force that governs the dynamics of Universe and it should slow down its expansion. The choice of the sign was determined then by natural wish to deal with positive quantities. This choice turned out to contradict the observable dynamics and became an example of historical curiosity.

In order to describe the kinematics of the cosmological expansion in more detail it is useful to consider the extended set of the parameters: \begin{align} H(t)\equiv &\frac{1}{a}\frac{da}{dt}\\ q(t)\equiv& -\frac{1}{a}\frac{{{d}^{2}}a}{d{{t}^{2}}}{{\left[ \frac{1}{a}\frac{da}{dt} \right]}^{-2}}\\ j(t)\equiv &\frac{1}{a}\frac{{{d}^{3}}a}{d{{t}^{3}}}{{\left[ \frac{1}{a}\frac{da}{dt} \right]}^{-3}}\\ s(t)\equiv &\frac{1}{a}\frac{{{d}^{4}}a}{d{{t}^{4}}}{{\left[ \frac{1}{a}\frac{da}{dt} \right]}^{-4}}\\ l(t)\equiv&\frac{1}{a}\frac{{{d}^{5}}a}{d{{t}^{5}}}{{\left[ \frac{1}{a}\frac{da}{dt} \right]}^{-5}} \end{align}

$^*$See Weinberg, Gravitation and Cosmology, chapter 14.

Problem 1: scale factor series

Using the cosmographic parameters introduced above, expand the scale factor into a Taylor series in time.

Problem 2: redshift series

Using these cosmographic parameters, expand the redshift into a Taylor series in time.

Problem 3: $q$, $z$ and $H$

Obtain the following relations between the deceleration parameter and Hubble's parameter \[q(t)=\frac{d}{dt}\left( \frac{1}{H} \right)-1;\quad q(z)=\frac{1+z}{H}\frac{dH}{dz}-1;\quad q(z)=\frac{d\ln H}{dz}(1+z)-1.\]

Problem 4: $q(a)$

Show that for the deceleration parameter the following relation holds: \[q\left( a \right)=-\left( 1+ \frac{\frac{dH}{dt}}{{{H}^{2}}} \right) -\left( 1+\frac{a\frac{dH}{da}}{H} \right).\]

Problem 5: derivatives

Show that derivatives of lower cosmographic parameters can expressed through the higher ones.

Problem 6: $q(z)$

Prove that \[\frac{dq}{d\ln (1+z)}=j-q(2q+1).\]

Problem 7: $d^{n}H/dz^{n}$

Show that the derivatives $\frac{dH}{dz}$ and $\frac{{{d}^{2}}H}{d{{z}^{2}}}$ can be expressed through the parameters $q$ and $j$.

Problem 8: $d^{n}H/dt^{n}$

Show that the time derivatives of the Hubble's parameter can be expressed through the cosmographic parameters as follows: \begin{eqnarray} \dot{H} &=& -{{H}^{2}}(1+q); \\ \ddot{H} &=& {{H}^{3}}\left( j+3q+2 \right); \\ \dddot{H}&=& {{H}^{4}}\left[ s-4j-3q(q+4)-6 \right]; \\ \ddddot{H}&=& {{H}^{5}}\left[ l-5s+10\left( q+2 \right)j+30(q+2)q+24\right]. \end{eqnarray}

Problem 9: Ricci scalar

Consider the case of spatially flat Universe and express the scalar (Ricci) curvature and its time derivatives in terms of the cosmographic parameters $q,j,s,l$.

Problem 10: acceleration and deceleration

Show that the accelerated growth of expansion rate $\dot{H}>0$ takes place on the condition $q<-1$.

Problem 11: $H[z]$

Obtain the relation \[H(z)=-\frac{1}{1+z}\frac{dz}{dt}.\]

Problem 12: inflection point

Let $t_a$ be the moment in the history of the Universe when the decelerated expansion turned to the accelerated one, i.e. $q(t_a)=0$, and let $t_{1}<t_{a}$ and $t_{2}>t_{a}$ be two moments in the vicinity of $t_{a}$. Show that \[\Delta t\equiv t_1-t_2 =\frac{1}{H_1}-\frac{1}{H_2}.\]

Problem 13: $H[q]$

Obtain the following integral relation between the Hubble's parameter and the deceleration parameter \[H=H_0\exp\left[ \int\limits_0^z [q(z^\prime)+1] d\ln(1+z^\prime)\right].\]

Problem 14: Hubble law for close galaxies

Reformulate the Hubble's law in terms of redshift for close galaxies $(z\ll 1)$.

Problem 15: $\tfrac{d}{dt}[\tfrac{d}{dz}]$

Show that \[\frac{d}{dt}=-(1+z)H\frac{d}{dz}.\]

Problem 16: $\tfrac{d^{n}}{dt^{n}}[\tfrac{d}{dz}]$

Obtain the transformation law from the higher time derivatives to the derivatives with respect to redshift: \[\frac{d^{(i)}}{dt}\to \frac{d^{(i)}}{dz}.\]

Problem 17: $\tfrac{d^{n}H^2}{dz^n}$

Calculate the derivatives of Hubble's parameter squared with respect to redshift \[\frac{d^{(i)}H^2}{dz^{(i)}},\quad i=1,2,3,4\] and express them in terms of the cosmographic parameters.

Problem 18: $q[H(1+z)]$

Show that the deceleration parameter $q$ can be presented in the form \[q(x) = \frac{H'(x)}{H(x)}x - 1;\; x = 1 + z.\]

Problem 19: $q[H(z)]$

Show that \[q(z)=\frac{1}{2}\frac{d\ln {H^2}}{d\ln (1+z)}.\]

Problem 20: $dH/dz$

Express the derivatives $dH/dz$ and $d^2H/dz^2$ throgh the parameters $q$ and \[r \equiv \frac{\dddot a}{aH^3}.\]

Problem 21: averaged $q$

Consider the time average of the deceleration parameter \[\bar{q}\left( {{t}_{0}} \right) =\frac{1}{{{t}_{0}}}\int_{0}^{{{t}_{0}}}{q(t)dt}\] and show that it can be evaluated without integration of equation of motion for the scale factor.

Problem 22: age of the Universe via $q$

Show that the current age of the Universe is proportional to $H_{0}^{-1}$ and the proportionality coefficient is determined by the average value of the deceleration parameter.

Problem 23: proper distance through $q_0$

Show that the proper distance to an object with redshift $z$ is related to the current deceleration parameter $q_0$ as \[R=\frac{c}{H_{0}q_{0}^{2}}\,\frac{1}{1+z}\, \Big[q_{0}z +(q_{0}-1)\big(\sqrt{1+2q_0}-1\big)\Big].\]