Journal Club

The Hubble constant H0, the current expansion rate of the universe, is one of the most important parameters in cosmology. The cosmic expansion regulates the mutually approaching motion of a pair of celestial objects due to their gravity. Therefore, the mean pairwise peculiar velocity of celestial objects, which quantifies their relative motion, is sensitive to both H0 and the dimensionless total matter density Ωm. Based on this, using the Cosmicflows-4 data, we measured H0 for the first time via the galaxy pairwise velocity in the nonlinear and quasi-linear range. Our results yield H0=75.5±1.4 km s−1 Mpc−1 and Ωm=0.311+0.029−0.028 . The uncertainties of H0 and Ωm can be improved to around 0.6% and 2%, respectively, if the statistical errors become negligible in the future.

The Dark Energy Spectroscopic Instrument (DESI) is leading a groundbreaking five-year survey to investigate the influence of dark energy on cosmic expansion and the evolution of large-scale structure. Latest baryon acoustic oscillations (BAO) and Full-Shape measurements and cosmological results from DESI will be presented and discussed, based on three years of observations. We considered both solely the data from the survey and in combination with cosmic microwave background (CMB), supernovae (SNe), Big Bang Nucleosynthesis (BBN) priors, and 3x2pt measurements observations. We focused on the measurement of constraints on expansion rate, with particular attention on the comparison with Planck results considering ΛCDM model, on dark energy equation of state and on finding upper limits for the sum of neutrino masses. Y3 BAO results are well described by a flat ΛCDM model, but in ΛCDMthe tension between the DESI+BBN and SH0ES H0 results now stands at 4.5σ independent of the CMB, and now the parameters preferred by BAO are in mild 2.3σ tension with those determined from the cosmic microwave background (CMB). This tension is alleviated by dark energy with a time-evolving equation of state parametrized by w0 and wa, where this solution is preferred over ΛCDM at 3.1σ for the combination of DESI BAO and CMB data and up to 2.8 − 4.2σ when including also SNe (depending on which sample is used). These results provide a critical assessment of the standard cosmological model. Regarding Y1 Full-Shape results on S8, we observe an excellence agreement between our data and CMB, both of which are slightly higher than values inferred the weak lensing survey. This tension is alleviated considering the combination with 3x2pt information from DESY3. Finally, regarding constraints on modified gravity, DESI data alone can constrain only μ0, however, the combination with CMB and information from lensing allows us to constrain Σ0 as well, obtaining GR-compatible results. However, although it is an effect not given by the DESI data, it’s interesting to point out that the use of different versions of the Planck likelihood leads to appreciable variations in the estimate of Σ0 due to differences in lensing potential estimations, even going as far as having a 3σ tension compared to the GR previons in the most extreme case. 

The ΛCDM model has been remarkably successful in describing the evolution of the universe. However, it faces notable challenges, such as
the Hubble tension, a discrepancy between the estimated value of the
Hubble constant H0 inferred from Cosmic Microwave Background
measurements under the assumption of ΛCDM and that obtained from local
measurements. In this talk, i will present how scalar-tensor and
bi-scalar-tensor theories can alleviate this tension. To address this
issue, we investigate scalar-tensor models with shift-symmetric friction
term, showing that they can reduce the effective Newton’s constant at
intermediate times, leading to an increased H₀ value. Additionally, we
examine bi-scalar-tensor theories, where the phantom behavior of the
effective dark-energy equation-of-state parameter plays a crucial role
in resolving the tension. These findings highlight the potential of
modified gravity theories to provide viable alternatives to the ΛCDM
paradigm while maintaining theoretical consistency and observational
viability.

The ∼5σ mismatch between the Hubble parameter measured by SH0ES and the value inferred from the inverse distance ladder (IDL) currently
represents the most significant tension within the Standard Model of Cosmology. In this talk, I will discuss the late-time phenomenology required to solve the Hubble crisis if standard physics before recombination is assumed, particularly emphasizing the crucial role played by baryon acoustic oscillations (BAO) data, employed to build the IDL. I will show that angular (2D) and anisotropic (3D) BAO data, despite being extracted from the same parent catalogues of tracers, leave an imprint on completely different scales and might have a distinct impact on the perturbed observables at play. With the aid of Supernovae of type Ia (SNIa), I will illustrate how this discrepancy can be reframed in terms of a BAO tension through a largely model-independent approach, and how the results change when the angular components of the 3D BAO data from BOSS/eBOSS are substituted by the recent data from DESI Y1. The tension is found to be at the level of ∼2𝜎 and ∼2.5𝜎, respectively, when the SNIa of the Pantheon+ compilation are used, and at ∼4.6𝜎 when the latter are replaced with those of DESY5. In view of these results, I will finally discuss a calibrator-independent method to assess the robustness of the distance duality relation.

Given that General Relativity is intrinsically nonlinear, it is important to look beyond first-order contributions in cosmological perturbations. In this talk, I will present a non-perturbative approach to the computation of CGWB anisotropies at large scales, providing the extension of the initial conditions and the Sachs-Wolfe effect for the CGWB, which encodes the full non-linearity of the scalar metric perturbations. I’ll also present the non-perturbative expression for three-point correlation of the gravitational wave energy density perturbation in the case of an inflationary CGWB with a scale-invariant power spectrum and negligible primordial non-Gaussianity. Under such conditions, the gravitational wave energy density perturbations are lognormally distributed, leading to the interesting effect of intermittency.

arXiv:2412.15654

The measurements of the Cosmic Microwave Background (CMB) have played a significant role in understanding the nature of dark energy. In this talk, we investigate the dynamics of the dark energy equation of state, utilizing high-precision CMB, BAO, and SNe-Ia data from multiple experiments. We examine the confrontation of several Dark Energy scenarios with contemporary data given that the 2024 Baryon Acoustic Oscillation (BAO) measurements released by DESI, when combined with the CMB data from Planck and different samples of type-Ia supernovae reveal a preference for Dynamical Dark Energy (DDE)

https://arxiv.org/abs/2407.14939
https://arxiv.org/abs/2407.16689
https://arxiv.org/abs/2403.15202

The cosmological upper bound on the total neutrino mass is the dominant limit on this fundamental parameter. Recent observations-soon to be improved-have strongly tightened it, approaching the lower limit set by oscillation data. Understanding its physical origin, robustness, and model-independence becomes pressing. Here, we explicitly separate for the first time the two distinct cosmological neutrino-mass effects: the impact on background evolution, related to the energy in neutrino masses; and the “kinematic” impact on perturbations, related to neutrino free-streaming. We scrutinize how they affect CMB anisotropies, introducing two effective masses enclosing background (∑m_{\nu}^{Backg.}) and perturbations (∑m_{\nu}^{Pert.}) effects. We analyze CMB data, finding that the neutrino-mass bound is mostly a background measurement, i.e., how the neutrino energy density evolves with time. The bound on the “kinematic” variable ∑m_{\nu}^{Pert.} is largely relaxed, ∑m_{\nu}^{Pert.}<0.8eV. This work thus adds clarity to the physical origin of the cosmological neutrino-mass bound, which is mostly a measurement of the neutrino equation of state, providing also hints to evade such a bound.

The ΛCDM model has encountered challenges, including the H_0 and S_8 tensions, potentially hinting at the need for new physics. Resolving both tensions within one framework would boost confidence in any alternative particular model, potentially necessitating exploration beyond General Relativity. Modifications, such as Teleparallel Gravity, which is a torsion-based approach, offers alternatives. This study examines prominent f(T) gravity models using late-time cosmological datasets, including Hubble measurements, Pantheon+ supernovae, and DESI BAO data. Growth rate measurements via f\sigma_8 data are also analysed to address the S_8 tension. Additionally, we explore primordial gravitational waves and their effects on CMB anisotropies and B-mode polarization, by leveraging BICEP/Keck data. By integrating these diverse datasets, we achieve a comprehensive understanding of both the late-time Universe and the tensor perturbations within the f(T) framework, contributing to a deeper comprehension of its dynamics and its potential to resolve long-standing cosmological tensions.

A plethora of modified theories of gravity have been proposed over the last few decades. Testing them all observationally is a considerable challenge, so it is advantageous to develop theory-independent approaches that constrain deviations from General Relativity in a systematic way. Many of the most precise such constraints to date are obtained from astrophysical measurements, for which deviations from GR are described by the parameterised post-Newtonian (PPN) parameters. Some attempts have been made to perform similarly general tests on cosmological scales, but it is not clear that they refer to the same couplings as the PPN formalism does, and so interpreting results from these disparate regimes physically is difficult and potentially misleading. With that problem in mind, I will introduce a framework, called parameterised post-Newtonian cosmology, that allows information from cosmological and astrophysical regimes to be combined consistently. I will present novel constraints on the evolution of the PPN parameters over cosmic history, using data from CMB anisotropies and Solar System experiments concurrently, and I will explain how these ideas can be applied further to test cosmological gravity to high precision.

General relativity (GR) is the gravitational framework that underpins the standard model of cosmology, the ΛCDM model; its predictions have been widely tested at astrophysical and cosmological scales, often with remarkable precision. However, our inability to directly observe the constituents of this model’s so-called ‘dark sector’ (i.e., dark matter and dark energy), along with the tensions characterising some of its parameters, prompts us to question the validity of GR at cosmological scales. That implies substituting GR with a modified gravity theory (MG) that can, for instance, explain the observed accelerated expansion of the universe without the need to introduce a cosmological constant (Λ). How can we test if a MG cosmology is sustained by current observations?

Because MG theories are numerous, we can turn to general, model-independent parameterizations of gravity, able to capture the phenomenology of several classes of deviations from GR at once. In this talk, I will present two such parameterizations: the μ/Σ framework and the growth index γ. I will show how they can be used to solve a discrepancy within different cosmic microwave background measurements (CMB) known as the ‘lensing anomaly’, while also explaining the apparent MG detection reported by the Planck CMB telescope as a collateral effect of the same anomaly.