Speaker
Description
Recent DESI results, combined with other cosmological probes, have revived interest in dynamical dark energy, hinting at phantom crossing and a phantom regime at $z \gtrsim 1$. These observations have brought renewed attention to a class of modified gravity (MG) models consistent with current data but exhibiting rich phenomenology beyond $\Lambda$CDM. Several of these theories introduce a non-minimal coupling between a scalar field and gravity, which manifests in two distinct observational signatures: an alteration of the growth of large-scale structure, and a modification of the propagation of tensorial perturbations, inducing a friction term in the gravitational wave (GW) equation of motion, which in turn produces a systematic offset between GW and electromagnetic luminosity distances. This modified GW propagation offers a unique observable to discriminate among models otherwise degenerate at the background level.
We assess the capability of next-generation GW detectors to constrain these DESI-motivated MG models, comparing standard parametric approaches against a model-independent Gaussian Process (GP) reconstruction to evaluate how well each can distinguish viable MG theories from General Relativity.
To this end, we build a mock multi-messenger dataset of binary neutron star (BNS) mergers with $\gamma$-ray burst (GRB) counterparts detected by Fermi and Swift, and forecast the sensitivity of future GW networks to $d_L^{\rm GW}$ via a prior-informed Fisher matrix approach. Combined with CMB, SnIa, and BAO data to break parameter degeneracies, we show that this sample of $\sim$ 40 multi-messenger events is sufficient to deliver unprecedented constraints on the MG phenomenology motivated by DESI, providing a powerful complementary test of gravity in the late Universe.