HEP Colloquia 2022

by Dr Riccardo Murgia (Gran Sasso Institute (GSSI) - INFN)

Wednesday 14 Dec 2022, 14:30 → 16:30 Europe/Rome

Aula C (Department of Physics)

Probing Cosmology with Gravitational Waves and Structure Formation data

The $\Lambda$CDM model of cosmology has been incredibly successful in explaining most properties of a number of independent observations, such as the cosmic microwave background (CMB) anisotropy data from Planck, and the formation and distribution of the large-scale structure (LSS) of the universe.

Nonetheless, the nature of the main components of the late-time energy budget of the universe – the (cold) dark matter ((C)DM) and the dark energy (DE),  interpreted as a cosmological constant ($\Lambda$) – remains unknown. Moreover, it is currently under debate whether the various sets of CMB data from ground-based experiments available today, e.g. SPT and ACT, are in full agreement with \emph{Planck}.

Additionally, when the $\Lambda$CDM model is confronted against complementary low-redshift observations, it shows some anomalies: the Hubble tension, 4 -- 5 $\sigma$ discrepancy between the local measurement of the expansion rate of the universe, $H_0$,  and its value inferred from early-universe data; the growth tension, 2 -- 3 $\sigma$ discrepancy between the amplitude of matter fluctuations on $\sim$ 8 Mpc scales measured from weak lensing surveys, and its CMB-inferred value.
The persistence of these (as well as other) tensions, besides the fact that the nature of DM and DE is still unveiled, has triggered huge theoretical efforts, aiming to develop alternative well-motivated scenarios capable to restore cosmological concordance, providing a better combined fit -- compared to $\Lambda$CDM -- to all present data.

To unambiguously verify/falsify signals of new physics beyond $\Lambda$CDM it is thus crucial to combine CMB and LSS data with complementary astrophysical observations probing different redshifts and scales. Any alternative theoretical framework must only introduce very limited phenomenological departures, in order to resolve the tensions without spoiling the tight limits from CMB and LSS. An efficient approach to test such departures is to develop flexible parameterizations to cover the phenomenology induced by broad classes of theoretical models, and then translate the constraints on the phenomenological parameter space into limits on the fundamental properties of the underlying theoretical scenarios.
In this talk, I will discuss how to exploit the synergy between CMB data and other complementary probes, such as structure formation data on small scales as traced by the Lyman-$\alpha$ forest, as well as gravitational wave and multi-messenger observations from next generation surveys.
I will present up-to-date bounds and outline future perspectives regarding some of the most promising proposed alternative models, such as early dark energy, sterile neutrinos, interacting DM, and decaying DM.