arXiv subjects

Sveva Castello

Publications and source records attributed to Sveva Castello.

9 recordsLinked to original sources

arXiv2026

Testing the Equivalence Principle in Galaxy Clusters

Clusters of galaxies have been used to measure a subtle effect predicted by Einstein: gravitational redshift. This signal encodes pristine information about our Universe, since it is sensitive to the depth of the clusters' gravitational potential wells. In this work, we show how gravitational redshift can be used to test a fundamental physical principle: the weak equivalence principle. This principle stipulates that all matter falls in the same way in a gravitational potential, regardless of its nature. By comparing the amplitude of the gravitational redshift signal with the velocity dispersion in galaxy clusters, we build a novel test of this principle targeted to the unknown dark matter. Our test is sensitive to any additional interaction that would alter the way dark matter falls in gravitational potentials, hence leading to a violation of the equivalence principle. We show that currently available data can constrain the presence of a fifth force in clusters at the level of 7-14%, while the newest surveys will reach a precision of a few percents. This demonstrates the crucial role played by galaxy clusters in testing fundamental properties of dark matter.

astro-ph.CO↗

arXiv2025

Gravitational Redshift from Galaxy Clusters -- a Relativistic Approach

The light that we receive from clusters of galaxies is redshifted by the presence of the clusters' gravitational potential. This effect, known as gravitational redshift, was first detected from a sample of stacked clusters in 2011, by taking redshift differences between the centre of each cluster and the respective member galaxies. However, the interpretation of this result was later challenged by several studies, which emphasised the possible influence of additional kinematic effects on the observed signal, like the transverse Doppler effect. In this work, we present the first derivation of all such effects within a relativistic framework, accurate to third order in the weak-field approximation. This framework allows us to correctly capture the hierarchy of terms on the scale of clusters and at the same time account for all relativistic effects. We compare our result with previous literature and show that some terms of the same order of the transverse Doppler effect were not properly included, leading to an overestimation of the kinematic contamination. In particular, we do not find any contribution arising from the so-called light-cone effect and obtain a larger correction due the motion of the central galaxy. Our derivation is independent of the Euler equation, providing a straightforward framework to test the weak equivalence principle.

astro-ph.CO↗

arXiv2024

Testing the equivalence principle across the Universe: a model-independent approach with galaxy multi-tracing

We present a test of the equivalence principle on cosmological scales. This cornerstone of general relativity has been tested with high precision for standard matter, but its validity for the unknown dark matter remains a crucial open question. We construct a measurable quantity $E_P$ that acts as a null test, i.e. it deviates from unity whenever the weak equivalence principle is violated. This quantity can be directly measured from the cross-correlation of two different galaxy populations, thanks to the inclusion of large-scale relativistic corrections. A key feature of our approach is that it only involves minimal assumptions, without the need to specify the power spectrum shape, the background evolution, the growth rate of cosmic structure, the galaxy bias function or a model for the potential violation of the equivalence principle. We provide forecasts for the Dark Energy Spectroscopic Instrument and the Square Kilometre Array (SKA). While the relativistic corrections can be detected with high significance by both surveys, $E_P$ can be constrained up to an interesting level only by SKA, with a precision around 7-15$\%$ within the redshift range $z < 0.6$.

astro-ph.CO↗

arXiv2024

Disentangling modified gravity from a dark force with gravitational redshift

The standard approach to test for deviations from general relativity on cosmological scales is to combine measurements of the growth rate of structure with gravitational lensing. In this study, we show that this method suffers from an important limitation with regard to these two probes: models of dark matter with additional interactions can lead to the very same observational signatures found in modified gravity and vice versa. Using synthetic data of redshift-space distortions, weak lensing, and cosmic microwave background, we demonstrate that this degeneracy is inevitable between modifications of gravity and a dark fifth force. We then show that the coming generation of surveys, in particular the Square Kilometre Array, will allow us to break the degeneracy between such models through measurements of gravitational redshift. Performing a Markov chain Monte Carlo analysis of the synthetic dataset, we quantify the extent to which gravitational redshift can distinguish between two representative classes of models, generalized Brans-Dicke (modified gravity) and coupled quintessence (fifth force).

astro-ph.CO↗

arXiv2023

Gravitational Redshift Constraints on the Effective Theory of Interacting Dark Energy

Upcoming galaxy surveys provide the necessary sensitivity to measure gravitational redshift, a general relativistic effect that generates a dipole in galaxy clustering data when correlating two distinct populations of galaxies. Here, we study the constraining power of gravitational redshift within the framework of the effective theory of interacting dark energy. This formalism describes linear cosmological perturbations in scalar-tensor theories of gravity with a limited number of free functions, and allows each particle species to be coupled differently to the gravitational sector. In this work, we focus on Horndeski theories with a non-minimal coupling of dark matter to the scalar degree of freedom, yielding a breaking of the weak equivalence principle for this cosmic component, a scenario that is yet untested. We show that the dipole generated by gravitational redshift significantly breaks degeneracies and tightens the constraints on the parameters of the effective theory compared to the standard redshift-space distortion analysis solely based on the even multipoles in the galaxy correlation function, with an improvement of up to $\sim 50\%$ for populations with a galaxy bias difference equal to 1. We make the Python package EF-TIGRE (Effective Field Theory of Interacting dark energy with Gravitational REdshift) developed for this work publicly available.

astro-ph.CO↗

arXiv2022

Rescuing constraints on modified gravity using gravitational redshift in large-scale structure

The distribution of galaxies provides an ideal laboratory to test for deviations from General Relativity. In particular, redshift-space distortions are commonly used to constrain modifications to the Poisson equation, which governs the strength of dark matter clustering. Here, we show that these constraints rely on the validity of the weak equivalence principle, which has never been tested for the dark matter component. Relaxing this restrictive assumption leads to modifications in the growth of structure that are fully degenerate with modifications induced by the Poisson equation. This in turns strongly degrades the constraining power of redshift-space distortions. Such degeneracies can however be broken and tight constraints on modified gravity can be recovered by measuring gravitational redshift from the galaxy distribution, an effect that will be detectable by the coming generation of large-scale structure surveys.

astro-ph.CO↗

arXiv2022

A Cautionary Tale: Dark Energy in Single-Field, Slow-Roll Inflationary Models

The current epoch of accelerated cosmic expansion is postulated to be driven by dark energy, which in the standard model takes the form of a cosmological constant with equation of state parameter $w=-1$. We propose an innovative perspective over the nature of dark energy by drawing a parallel with inflation, which we assume to be driven by a single scalar field, the inflaton. The inflaton was not a cosmological constant, as indicated by the fact that inflation ended and by the Planck satellite's constraint of $n_s\neq 1$ at $8\sigma$ confidence. Therefore, it is interesting to verify whether its equation of state parameter was measurably different from $-1$. We analyze this question for a class of single-field slow-roll inflationary models, where the hierarchy of Hubble slow-roll parameters is truncated at different orders. Based on the latest Planck and BICEP2/Keck data, we obtain a $68\%$ upper bound of $1+w < 0.0014$ for the three-parameter model, which gives the best description to the data. This provides a cautionary tale for drawing conclusions about the nature of today's dark energy based upon the non-detection of a deviation from $w=-1$ with current and upcoming cosmological surveys.

astro-ph.CO↗

arXiv2021

A Cosmological Underdensity Does Not Solve the Hubble Tension

A potential solution to the Hubble tension is the hypothesis that the Milky Way is located near the center of a matter underdensity. We model this scenario through the Lema\^itre-Tolman-Bondi formalism with the inclusion of a cosmological constant ($\Lambda$LTB) and consider a generalized Gaussian parametrization for the matter density profile. We constrain the underdensity and the background cosmology with a combination of data sets: the Pantheon Sample of type Ia supernovae (both the full catalogue and a redshift-binned version of it), a collection of baryon acoustic oscillations data points and the distance priors extracted from the latest Planck data release. The analysis with the binned supernovae suggests a preference for a $-13 \%$ density drop with a size of approximately 300 Mpc, interestingly matching the prediction for the so-called KBC void already identified on the basis of independent analyses using galaxy distributions. The constraints obtained with the full Pantheon Sample are instead compatible with a homogeneous cosmology and we interpret this radically different result as a cautionary tale about the potential bias introduced by employing a binned supernova data set. We quantify the level of improvement on the Hubble tension by analyzing the constraints on the B-band absolute magnitude of the supernovae, which provides the calibration for the local measurements of $H_0$. Since no significant difference is observed with respect to an analogous fit performed with a standard $\Lambda$CDM cosmology, we conclude that the potential presence of a local underdensity does not resolve the tension and does not significantly degrade current supernova constraints on $H_0$.

astro-ph.CO↗

arXiv2021

An updated dark energy view of inflation

The present epoch of accelerated cosmic expansion is supposed to be driven by an unknown constituent called dark energy, which in the standard model takes the form of a cosmological constant, characterized by a constant equation of state w=-1. An interesting perspective over the role and nature of dark energy can be achieved by drawing a parallel with a previous epoch of accelerated expansion, inflation, which we assume to be driven by a single scalar field, the inflaton. Since the Planck satellite has constrained the value of $n_s$ away from 1, the inflaton cannot be identified with a pure cosmological constant, as is also suggested by the fact that inflation ended. Thus, it is interesting to verify whether a hypothetical observer would have been able to measure the deviation of the w of the inflaton from -1. To do so, we consider a class of single-field slow-roll inflationary models dubbed HSR{i}, where the hierarchy of Hubble slow-roll parameters is truncated at the i-th order. The models are tested through a MCMC analysis based on combinations of the latest Planck and BICEP2/Keck data sets, and the resulting chains are converted into sets of allowed evolution histories of w. HSR{1} is excluded observationally since it would predict that $n_s=1$, in contrast with the recent Planck constraints, while we find that HSR{2} would prefer w>-1, but is disfavoured by the addition of the BICEP2/Keck data. The overall best description for the data is provided by HSR{3}, which yields a 68% upper bound of 1+w<0.0014. Therefore, if the current era of accelerated expansion happens to have the same equation of state as inflation during the observable epoch, then current and upcoming cosmological observations will not be able to detect that w$\neq$-1. This provides a cautionary tale for drawing conclusions about the nature of dark energy on the basis of the non-observation of a deviation from w=-1.

astro-ph.CO↗