Lighting the Dark: Evolution of the Postinflationary Universe

In simple inflationary cosmological scenarios, the near-exponential growth can be followed by a long period in which the Universe is dominated by the oscillating inflaton condensate. The condensate is initially almost homogeneous, but perturbations grow gravitationally, eventually fragmenting the condensate if it is not disrupted more quickly by resonance or prompt reheating. We show that the gravitational fragmentation of the condensate is well-described by the Schrödinger-Poisson equations and use numerical solutions to show that large overdensities form quickly after the onset of nonlinearity. This is the first exploration of this phase of nonlinear dynamics in the very early Universe, which can affect the detailed form of the inflationary power spectrum and the dark matter fraction when the dark sector is directly coupled to the inflaton.

Figure 1

Power spectrum of the density contrast at the beginning and end of the simulation. Scales are given relative to the horizon scale at the end of inflation.

Figure 2

Root mean square and maximum density contrast, measured at comoving distance scale $\simeq 0.02/H_{\mathrm{end}}$; the scale factor $a$ is unity at the end of inflation. The results diverge as $\delta$ increases and perturbation theory breaks down.

Figure 3

Simulation results for an initial perturbation of amplitude $\sim {10}^{-2}$; comoving simulation box size of 10 times postinflationary Hubble radius; when the Universe has expanded by a factor of $a=200$ since the end of inflation. Top: The density $\rho$ along a slice including the point of highest density. Bottom: Volume rendering of a subset of the box; blue regions $\delta \sim 1$; yellow and white regions $\delta \sim 10–100$.