Model-independent cosmology with gravitational waves, large-scale structure, and high-energy surveys
IFPU
13-17 May 2024
The ΛCDM model of cosmology has been incredibly successful in explaining a number of independent observations, such as the cosmic microwave background (CMB) anisotropy data, 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 dark matter (DM) and the dark energy (DE) / cosmological constant (Λ) – remains unknown. Moreover, when the ΛCDM model is confronted against complementary low-redshift observations, it shows some anomalies, such as the Hubble tension, 4–5 σ discrepancy between the local measurement of the expansion rate of the universe, and its value inferred from early-universe data. The persistence of this (as well as other) issue(s), besides the fact that the nature of DM and DE is still unveiled, has triggered huge theoretical efforts, aiming at developing alternative well-motivated scenarios providing a better combined fit – compared to ΛCDM – to all present data, restoring cosmological concordance.
To unambiguously verify/falsify signals of new physics beyond ΛCDM it is thus crucial to combine CMB and LSS data with complementary astrophysical observations probing different redshifts and scales. However, any alternative theoretical framework must 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 model-independent methods based on Machine Learning (ML), as well as flexible parameterizations reproducing the phenomenology induced by broad classes of theoretical models. The resulting constraints on the phenomenological parameter space can then be easily translated into limits on the fundamental properties of the underlying theoretical scenarios. Regarding the nature of the DE/Λ, most of its observable consequences follow from its impact on the expansion history of the universe, which in turn affects the growth of density perturbations, and the age of the universe. The expansion history of the universe maps to a distance-redshift relation.
Gravitational Wave (GW) signals from binary compact object mergers act as standard sirens, as they provide a self-calibrated luminosity distance to the source, while redshift information can be obtained from their electromagnetic (EM) counterparts and/or inferred through LSS observations. The next generation GW detectors, such as Einstein Telescope (ET) and Cosmic Explorer (CE) will increase the detection rate of binary neutron star (BNS) mergers and binary black hole (BBH) mergers by an enormous factor, reaching more than 100,000 events per year. A fraction of BNS mergers are going to be sources of EM bursts in γ-rays (GRBs), on-axis, as well as off-axis jet emission (optical from kilonovae and X-rays/optical in GRB afterglows). This will allow us to probe the expansion rate of the universe without the distance calibration issues that currently affect analyses with type Ia supernovae. At high-z, where GW signals from BNS are undetectable, we will exploit intrinsic correlations in the properties of short GRBs, previously calibrated on the low-z joint GW-GRB detections.
Concerning GW signals from BBH mergers, at low-z we will focus on GWxLSS cross-correlations, and investigate the synergies of ET (and CE) with photometric and spectroscopic optical surveys such as Euclid or the Dark Energy Spectroscopic Instrument (DESI). At high-z, in the absence of optical emission, as a LSS tracer we will use the intensity mapping (IM) of neutral hydrogen (HI) 21 cm emission.
The goal of this Focus Week is to exploit advanced model-independent data-driven methods to test the late-time expansion history of the universe (z<10) against the incredibly large sets of data that will soon be provided by ongoing and future GW, LSS, and high-energy (HE) surveys. In order to foster new collaborations, we will bring together a group of international experts belonging to 3 scientific communities: theorists and experts on model-independent cosmological inference; experts on GW and multi-messenger (MM) astrophysics with very high-energy (HE) γ-ray observations; experts on LSS probes of cosmology.
Meeting webpage: https://indico.gssi.it/event/606/
Organisers:
- Riccardo Murgia (GSSI, L’Aquila)
- Annalisa Celotti (SISSA, Trieste)
- Matteo Viel (SISSA, Trieste)
Participants:
- TBA