Rollercoaster cosmology

Abstract

Does inflation have to happen all in one go? The answer is a resounding no! All cosmological problemscan be solved by a sequence of short bursts of cosmic acceleration, interrupted by short epochs of decelerated expansion. The spectrum of perturbations will still match the CMB and LSS if the earliest stage of the last O(50)- O(60) efolds is at least O(15) efolds long. Other stages can be considerably shorter. But as long as they add up to O(50)- O(60) efolds and the stages of decelerated expansion in between them are shorter and also overall last less, the ensuing cosmology will pass muster. The presence of the interruptions resets the efold clockof each accelerating stage, and changes its value at the CMB pivot point. This change opens up the theory space, loosening the bounds. In particular some models that seem excluded at N=60 fit very wellas shorter stages with N=30. Interesting predictions are that both the scalar and tensor spectra of perturbations are rapidly modified at shortwavelengths. In the simplest cases the perturbations are suppressedrelative to the perturbations at large scales just because when they freeze out, the background curvature is smaller. The modes which do leave the horizon howeverwill remain frozen as long as the subsequent intervening stages of decelerated expansion remain short. Thesefeatures could be tested with future CMB spectroscopy searches and with short wavelength primordial gravity probes. The spatial curvature in these models can be larger than the largest wavelength scalar perturbations, becauseΩk evolves differently than the scalar perturbations δρ/ρ|S. Finally, with many short stages of accelerated expansion, the abundance of reheating products from previous accelerated stages does not get completely wiped out. This implies thatthe universe may contain additional populations of particles, more rare than the visible ones, or even primordialblack holes, created during a late decelerated epoch before last reheating, whichmay be dark matter.