Abstract

We probe the cosmological consequences of a recently proposed class of solutions to the cosmological constant problem. In these models, the universe undergoes a long period of inflation followed by a contraction and a bounce that sets the stage for the hot big bang era. A requirement of any successful early universe model is that it must reproduce the observed scale-invariant density perturbations at CMB scales. While these class of models involve a long period of inflation, the inflationary Hubble scale during their observationally relevant stages is at or below the current Hubble scale, rendering the de Sitter fluctuations too weak to seed the CMB anisotropies. We show that sufficiently strong perturbations can still be sourced thermally if the relaxion field serving as the inflaton interacts with a thermal bath, which can be generated and maintained by the same interaction. We present a simple model where the relaxion field is derivatively (i.e. technically naturally) coupled to a non-abelian gauge sector, which gets excited tachyonically and subsequently thermalizes due to its nonlinear self-interactions. This model explains both the smallness of the cosmological constant and the amplitude of CMB anisotropies.

Highlights

  • Cosmological observations have shown that the universe is presently undergoing accelerated expansion due to a form of energy density dubbed “dark energy.” The inferred measured value of the dark energy density is ∼10 meV4

  • This measurement constrains the value of the cosmological constant (CC) to be no bigger than ∼10 meV4, an energy density that is at least 60 orders of magnitude smaller than known perturbative contributions to it from the Standard Model

  • Problem and having the dynamics of the CC relaxation be within the regime of validity of our effective field theory (EFT) puts an upper bound of g ≲ 10−18 GeV, corresponding to Vi ≲ GeV4

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Summary

INTRODUCTION

Cosmological observations have shown that the universe is presently undergoing accelerated expansion due to a form of energy density dubbed “dark energy.” The inferred measured value of the dark energy density is ∼10 meV4. The energy density produced in the contraction could conceivably excite UV degrees of freedom that could trigger a bounce, causing the universe to reexpand as a hot universe but with a small, dynamically relaxed CC In this scenario, our current hot big bang epoch exists during this stage of reexpansion. It permits UV physics at very high energies to play a role in solving the CC problem In these models, the tuning necessary to cancel the CC is accomplished in the infrared by a slowly rolling scalar field. The blue-shifting gauge-field radiation soon dominates the universe Once it hits a high enough temperature, we assume that some UV dynamics are triggered causing the universe to bounce and reexpand, entering the standard hot big bang epoch.

Cold inflation
Thermalization of the gauge sector
Warm inflation
Crunching
Bouncing and expanding to the present epoch
EFT validity
PERTURBATIONS
Scalar amplitude
Scalar spectral index
DISCUSSION AND CONCLUSION
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