Gravity in the infrared and effective nonlocal models

Abstract

We provide a systematic and updated discussion of a research line carried out by our group over the last few years, in which gravity is modified at cosmological distances by the introduction of nonlocal terms, assumed to emerge at an effective level from the infrared behavior of the quantum theory. The requirement of producing a viable cosmology turns out to be very stringent and basically selects a unique model, in which the nonlocal term describes an effective mass for the conformal mode. We discuss how such a specific structure could emerge from a fundamental local theory of gravity, and we perform a detailed comparison of this model with the most recent cosmological datasets, confirming that it fits current data at the same level as ΛCDM.Most notably, the model has striking predictions in the sector of tensor perturbations, leading to a very large effect in the propagation of gravitational wave (GWs) over cosmological distances. At the redshifts relevant for the next generation of GW detectors such as Einstein Telescope, Cosmic Explorer and LISA, this leads to deviations from GR that could be as large as 80%, and could be verified with the detection of just a single coalescing binary with electromagnetic counterpart. This would also have potentially important consequences for the search of the counterpart since, for a given luminosity distance to the source, as inferred through the GW signal, the actual source redshift could be significantly different from that predicted by ΛCDM. At the redshifts relevant for advanced LIGO/Virgo/Kagra the effect is smaller, but still potentially observable over a few years of runs at target sensitivity. arXiv:2001.07619v1 [astro-ph.CO] 21 Jan 2020 4 Conclusions 66 A Difficulties of alternative nonlocal models 68 -1 -2 The Lichnerowicz operator is defined by E µν,ρσ ≡ 1 2, where 2 = η µν ∂µ∂ν is the flat-space d'Alembertian. We use the signature ηµν = (−, +, +, +) and MTW [14] sign conventions. 3 We assume here 3 + 1 spacetime dimensions. See [13] for the corresponding equations in d + 1 spacetime dimensions, with d generic.

Science case for the Einstein telescope

“…For instance, one can have a cosmological model that is observationally indistinguishable from ΛCDM in terms of current electromagnetic observations, but still, as shown in the left panel of Fig. 11, predicts a value of Ξ 0 that can be as large as Ξ 0 1.6, representing a 60% deviation from ΛCDM [194] (and in fact even up to 80% [204]). Such a large effect could be detectable even with just a single standard siren at ET.…”

Quantum corrected gravitational potential beyond monopole-monopole interactions

“…In this case, the exponential factors strongly suppress the second and third term in Eq. (46) and the large distance behavior is dominated by the 1=r-contribution. As it is noted in Ref.…”

“…For an up-to-date overview on the various aspects of cosmological evolution driven by this class of nonlocalities see Ref. [46]. Furthermore, contributions like R□ −2 R and C μναβ □ −2 C μναβ were earlier obtained as a consequence of a decoupling mechanism in a renormalization group analysis [47].…”

Searching for modified gravity in the astrophysical gravitational wave background: Application to ground-based interferometers

“…Current detectors are not sensitive enough to probe the Universe at cosmological distances, not more than z ≲ 1. But, certainly, some promising projects like ET, CE and LISA will be able to detect GW events with great accuracy at cosmological distances and provide information for powerful cosmological tests [53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70].…”