Constraints on bimetric gravity from Big Bang nucleosynthesis

Marcus Högås,Edvard Mörtsell

doi:10.1088/1475-7516/2021/11/001

Abstract

Bimetric gravity is a ghost-free and observationally viable extension of general relativity, exhibiting both a massless and a massive graviton. The observed abundances of light elements can be used to constrain the expansion history of the Universe at the period of Big Bang nucleosynthesis. Applied to bimetric gravity, we readily obtain constraints on the theory parameters which are complementary to other observational probes. For example, the mixing angle between the two gravitons must satisfy θ≲ 18^∘ in the graviton mass range ≳ 10-16 eV/c2, representing a factor of two improvement compared with other cosmological probes.

Highlights

Since the Universe is currently expanding, even accelerating [1, 2], tracing the expansion history backwards in time, the Universe must have been in a very hot and dense initial state. These conditions are appropriate for nuclear reactions to take place, leading to the production of the light elements, including D and 4He, referred to as Big Bang nucleosynthesis (BBN)
The resulting abundances of these light elements depend on the conditions at this period, for example on the expansion rate of the Universe and the number of neutrino species
The cosmological concordance model is based on general relativity (GR) and the standard model of particle physics and has been very successful in accounting for cosmological observations, including the abundances of the light elements [3]

Summary

INTRODUCTION

Since the Universe is currently expanding, even accelerating [1, 2], tracing the expansion history backwards in time, the Universe must have been in a very hot and dense initial state. With increasingly precise observations, a tension has appeared recently between the value of the Hubble constant (i.e., the present-day expansion rate) as inferred from small and large distance measurements, see for example [4]. These issues, together with the problem (or impossibility) of quantizing GR, suggest that Einstein’s theory of general relativity is not the ultimate theory for gravity. MFP is the mass of the (massive) graviton measured in units of H0 ∼ 10−33 eV/c2, ΩΛ is the effective cosmological constant that bimetric cosmology approaches in the asymptotic future, and α and β are parameters which (among other things) determine the screening mechanism, see Ref.

BIMETRIC COSMOLOGY

CONSTRAINTS FROM BBN