Journal Club

Recent data releases from the Atacama Cosmology Telescope (ACT DR6) and the Dark Energy Spectroscopic Instrument (DESI DR2) provide new insights into the viability of Early Dark Energy (EDE) as a resolution to the Hubble tension. In this talk, I will explore the implications of these recent measurements for EDE models. In particular, I will show that while ACT DR6 does not statistically prefer EDE over the standard ΛCDM model, it allows for a significantly larger maximum EDE contribution compared to previous constraints from Planck NPIPE, despite ACT’s improved precision on small angular scales. I will also discuss the role of prior volume effects in Bayesian analyses and highlight the importance of complementing Bayesian inference with frequentist approaches

The σ8 parameter is commonly used to quantify the amplitude of matter fluctuations at linear cosmological scales. However, its intrinsic dependence on h can introduce biases and couples the growth and Hubble tensions in an intricate way, when comparing the predictions of different models and/or datasets. For example, the bias found in models with large values of H0 is more prominent, artificially complicating the search for a model that can resolve the Hubble tension without exacerbating the growth tension. To address these challenges, an alternative parameter has been proposed: σ12. In this scenario, the worsening of the growth tension in different cases is much less pronounced than previously thought or may even be non-existent

Gravitational waves (GWs) from compact binary coalescences (CBCs) offer a novel method to probe cosmic expansion, particularly the Hubble constant H0. A key technique in GW cosmology is the spectral sirens method, which utilizes GW luminosity distance and source-frame mass distribution to infer redshift. With GW detectors, populations of CBCs can be either observed as resolved individual sources or implicitly as a stochastic gravitational-wave background (SGWB) from the unresolved ones. This study explores how both resolved and unresolved CBCs contribute to constraining cosmic expansion within the spectral siren framework. The SGWB provides additional constraints on CBC population properties, potentially enhancing precision in cosmic expansion measurements. Using a five-detector network at O5-designed sensitivity, we find that incorporating the SGWB helps exclude lower values of H0​ and the dark matter energy fraction Ωm. It also helps in refining the redshift distribution of CBCs, improving the determination of a possible CBC peak in redshift. However, while SGWB improves constraints on low values of H0​ and Ωm​, resolved spectral sirens remain the dominant source of precision for H0. We also performed a spectral siren analysis for 59 resolved binary black hole sources detected during the third observing run with an inverse false alarm rate higher than 1 per year jointly with the SGWB. We obtain that with current sensitivities, the cosmological and population results are not impacted by the inclusion of the SGWB.

Axion-dilaton models provide a well-motivated, minimal class of models for which kinetic interactions between multiple scalar fields and their predictions can be explored, particularly in late-time cosmology. I will present the cosmological implications of these interactions when prescribing an axion and a dilaton field to describe dark matter and dark energy, respectively, including the predicted effects on the CMB, late time structure growth, and particle mass evolution.

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.

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

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