Difference between revisions of "The role of curvature in the dynamics of the Universe"

In this case the first Friedman equation takes the form \[\left(\frac{\dot a}{a}\right)^2 =\frac{1}{a^2}\] therefore $a\sim t$ and thus $\rho(t) =\rho_{0}( t_{0}/t )^3$.

The contribution of curvature to $H^{2}$ (the first Friedman equation) is $\sim a^{-2}$, that of non-relativistic matter is $\sim a^{-3}$, of radiation $\sim a^{-4}$. Therefore for sufficiently small $a$, i.e. close enough to the Big Bang, the curvature term can be neglected.

The first Friedman equation is often written as \[H^2 = \frac{8\pi G}{3}\rho - \frac{k}{a^2}.\] Dividing both sides by $H^2$ and rewriting it in terms of \[\Omega = \frac{\rho }{\rho _{cr}}, \:\rho _{cr} = \frac{8\pi G}{3H^2},\] we get \[\frac{k}{a^2 H^2}=\Omega - 1.\] As $a^2H^2>0$ and $k$ can only, by definition, take values $-1, 0, 1$, \[k = \mbox{sign}\left(\Omega-1\right).\] On the other hand, taking the absolute value of the same equation, for the present moment we get \begin{equation}\label{a-H-Om} a_{0}=\frac{H_{0}^{-1}}{\sqrt{|\Omega_0 -1|}}. \end{equation}

The result of the previous problem (\ref{a-H-Om}), on restoring the speed of light $c$ by dimensionality, straightforwardly gives us \begin{align*} a_{0}&>7.5\cdot cH_{0}^{-1} \quad\text{for}\quad k<0;\\ a_{0}&>12.5\cdot cH_{0}^{-1} \quad\text{for}\quad k>0. \end{align*} There is no upper bound, as the observational data does not exclude (or rather, tends to imply) the possibility of a spatially flat Universe, with $a_{0}\to\infty$.

The first Friedman equation can be expressed as \[\Omega - 1 = \frac{k}{a^2 H^2}.\] As for a constant EoS parameter we have $a(t)\sim t^{2/[3(1+w)]}$, then $H\sim 1/t$ and \[\Omega-1 \sim k\;t^{\frac{2}{3}\frac{1+3w}{1+w}}.\] Finally, a) \[\Omega-1\sim k t^{2/3};\] b) \[\Omega-1\sim k t.\]

Dividing the first Friedman equation by the second one, we obtain the condition for the accelerated expansion in the form \[\frac{1}{1 + 3w} \left(1 - \frac{k}{8\pi G a^{2}\rho}\right) < 0.\] Then we immediately see that
1) For a spatially flat or open Universe ($k\leq 0$) accelerated expansion corresponds to $w\leq -1/3$.
2) For a spatially closed Universe the expansion is accelerating if the following conditions hold: \[\left\{\begin{array}{l} 1+3w>0;\\ \rho>(8\pi G a^2)^{-1}; \end{array}\right.\qquad\text{or}\qquad \left\{\begin{array}{l} 1+3w<0\\ \rho<(8\pi G a^2)^{-1}. \end{array}\right.\] In both variants the first inequality is the restriction on pressure, and the second one on the spatial curvature.