Backreaction of super-Hubble cosmological perturbations beyond perturbation theory

Abstract

We discuss the effect of super-Hubble cosmological fluctuations on the locally measured Hubble expansion rate. We consider a large bare cosmological constant in the early universe in the presence of scalar field matter (the dominant matter component), which would lead to a scale-invariant primordial spectrum of cosmological fluctuations. Using the leading order gradient expansion we show that the expansion rate measured by a (secondary) clock field which is not comoving with the dominant matter component obtains a negative contribution from infrared fluctuations, a contribution whose absolute value increases in time. This is the same effect which a decreasing cosmological constant would produce. This supports the conclusion that infrared fluctuations lead to a dynamical relaxation of the cosmological constant. Our analysis does not make use of any perturbative expansion in the amplitude of the inhomogeneities.

Theoretical cosmology

“…It is unlikely that inhomogeneitites and cosmic variance can resolve the tension [77]. However, there are suggestions that the emergence of spatial curvature may alleviate the tension [140,153,146,77,226]. Any definitive measurement of a non-zero spatial curvature would be crucial in cosmology.…”

“…Indeed, from an investigation of the backreaction effect of long wavelength cosmological perturbations it was found that at one LQG order super-Hubble cosmological perturbations do give rise to a negative contribution to the cosmological constant [145]. It has consequently been proposed that this backreaction could then lead to a late time scaling solution for which the contribution of the cosmological constant tracks the contribution of the matter to the total energy density; that is, the cosmological constant obtains a negative contribution from infrared fluctuations whose magnitude increases with time [146].…”

Swampland conjecture in f(R) gravity by the Noether symmetry approach

“…Recently, it was proposed that an effective field theory, to be consistently embedded into quantum gravity, must satisfy two specific criteria [10]. Based on them, it was argued that if string theory should be the ultimate quantum gravity theory, there are evidences that exact de Sitter solutions, with a positive cosmological constant, cannot describe the fate of the late-time universe [7,[11][12][13]. On the other hand, some models with varying equation of state can still be consistent with the * Electronic address: benettim@na.infn.it † Electronic address: capozzie@na.infn.it ‡ Electronic address: leilagraef@if.uff.br conjecture [12].…”

“…The possibility that string theory does not allow for de Sitter vacua is not new [21] , however, it has recently gained impetus by new evidences on the instability of a de Sitter phase (see e.g. [11]), and especially by the recent quantitative proposal for a more detailed constraint on potentials that are not in the swampland region [7]. Swampland criteria can be investigated also for alternative theories of gravity [26,27].…”

The swampland conjecture and the Higgs expectation value

“…Those constraints are independent of the ones derived from the Q-dependence of the Higgs VEV. 15 Here, we use the tree-level relation, M…”