# Geometry and Destiny

See L.Krauss, M. Turner, arXiv:9904020; B.Ryden, Introduction to cosmology, Addison Wesley; J.E.Felten,R.Isaacman, Rev.Mod.Phys. 58,689 , 1986

Presence of the cosmological constant reevaluates standard notions about the connection between geometry and the fate of the Universe. The traditional philosophy of General Relativity is that "Geometry is Destiny", with "geometry" in this context implying openness or closure of the three-space of constant cosmological time. If energy content is provided by "ordinary" matter (nonrelativistic matter or radiation) this slogan transforms into "Density is Destiny". If the density of matter is less or equal than the critical value (and the Universe is open), then the destiny of Universe is eternal expanding; if the density is greater, and the Universe is closed, then the destiny is recollapse. If the Universe contains cosmological constant (or any energy component with $w<-1/3$) the situation changes radically: an open Universe can recollapse, while a closed Universe can expand forever. Geometry no longer determines the fate of the Universe.

### Problem 1

As $\Omega_{tot}-1=k/(H^2a^2)$ the sign of $k$ is determined by whether $\Omega_{tot}$ is greater or less than $1$. A measurement of $\Omega_{tot}$ at any epoch---including the present---determines the geometry of the Universe. However, as opposed to situation with only non-relativistic matter, we can no longer claim that the magnitude of $\Omega_{tot}$ uniquely determines the fate of the Universe. Explain decoupling between $\Omega_{tot}$ and destiny using deceleration parameter.

### Problem 2

Show that decoupling between $\Omega_{tot}$ and destiny of the Universe is due to violation of strong energy condition.

### Problem 3

Find the maximum value of scale factor for a hypothetical flat Universe with $\Omega_{\Lambda0}<0$.

### Problem 4

For the Universe considered in the previous problem, find the analytical solution $t(a)$ and time of the collapse from $a=a_{\max}$ back down to $a=0$.

### Problem 5

In the flat Universe with $\Omega_{m0}<1$ and positive cosmological constant find the late time asymptotic for the scale factor.

### Problem 6

Show that eternal expansion is inevitable if and only if $\Omega_\Lambda>4\Omega_m\left\{\cos\left[\frac13\arccos(\Omega_m^{-1}-1)+\frac{4\pi}{3}\right]\right\}.$

### Problem 7

Show that if $\Omega_{m0}+\Omega_{\Lambda0}>1$ (positive spatial curvature) and $\Lambda>0$, then it is possible to have a Universe that expands at late times, but without the initial Big Bang ($a=0,\ t=0$).

### Problem 8

Show that collapse of the Universe is possible if the following conditions are satisfied:
1. Closed ($k>0$) Universe: $\rho_m>2\rho_\Lambda$ when $H=0$.
2. Open, flat ($k\le0$) or closed ($k>0$) Universe: $\rho_\Lambda\le0$.

### Problem 9

Show that eternal expansion of the Universe is possible if the following conditions are satisfied
1. Closed ($k>0$) Universe: $\rho_m<2\rho_\Lambda$ before $H=0$.
2. Open or flat ($k\le0$) Universe: $\rho_\Lambda\ge0$.