Nonlocal gravity and gravitational-wave observations

Abstract

We discuss a modified gravity model which fits cosmological observations at a level statistically indistinguishable from ΛCDM and at the same time predicts very large deviations from General Relativity (GR) in the propagation of gravitational waves (GWs) across cosmological distances. The model is a variant of the RT nonlocal model proposed and developed by our group, with initial conditions set during inflation, and predicts a GW luminosity distance that, at the redshifts accessible to LISA or to a third-generation GW detector such as the Einstein Telescope (ET), can differ from that in GR by as much as 60%. An effect of this size could be detected with just a single standard siren with counterpart by LISA or ET. At the redshifts accessible to a LIGO/Virgo/Kagra network at target sensitivity the effect is smaller but still potentially detectable. Indeed, for the recently announced LIGO/Virgo NS-BH candidate S190814bv, the RT model predicts that, given the measured GW luminosity distance, the actual luminosity distance, and the redshift of an electromagnetic counterpart, would be smaller by as much as 7% with respect to the value inferred from ΛCDM.

Cosmology and dark energy from joint gravitational wave-GRB observations

“…The most interesting result is the one for Ξ 0 , which (with our rather extreme assumption of 10 yr of data taking) can be measured to an accuracy of order ∆Ξ 0 0.1, i.e. (given that our fiducial value has been taken to be the ΛCDM value Ξ 0 = 1), a relative accuracy ∆Ξ 0 /Ξ 0 10%, which is already in the ballpark of the predictions of interesting modified gravity models [38,94]. With a shorter but more realistic time of data taking, say 3-4 yr, we still expect to get ∆Ξ 0 /Ξ 0 at the level of about 20%.…”

Gaussian processes reconstruction of modified gravitational wave propagation

“…Beside testing the accuracy to which one could confirm the validity of ΛCDM, assuming it as fiducial model, we have also tested the accuracy to which one could validate a modified gravity model. We have focused in particular on a nonlocal modification of gravity, the so-called RT model [48], for which recent work [47] has shown that Ξ 0 can reach values as large as 1.6. The model therefore gives a 60% deviation from ΛCDM in the tensor sector, despite the fact that, both in the background evolution and in scalar perturbations it is very close to ΛCDM, and indeed it fits the existing cosmological datasets at a level statistically indistinguishable from ΛCDM.…”

“…∆N = 0 (blue solid line), and for initial conditions set during inflation at ∆N efolds before the end of inflation, for ∆N = 34 (magenta, dashed), ∆N = 50 (green, dot-dashed) and ∆N = 64 (cyan, dot-dashed). From ref [47]…”

Gravity in the infrared and effective nonlocal models

“…The results are shown in Fig. 15 (see also [115]). These results are quite spectacular, in particular at large ∆N .…”

“…We next compare the predictions for modified GW propagation of the RT model with the sensitivities of current and future GW detectors, elaborating on the analysis in [115,165,203,204] for ground-based detectors and in [191] Table 6. The events in a given realization of the mock catalog of joint GW-GRB detections for the HLVKI network, over 10 yr of simulated data.…”