Journal Club

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.

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.

The standard ΛCDM model is based on the cosmological principle that the universe is homogeneous and isotropic on large scales (greater than 100 Mpc). A crucial test of this principle involves analyzing the Cosmic Microwave Background (CMB), which shows a dipole anisotropy on the order of  ΔT/T∼10^−3. This dipole is commonly interpreted as owing to our motion with respect to the CMB rest frame.

A model-independent approach to validate this kinematic hypothesis is to determine the dipole moment in the angular distribution of large-scale structures. Recent observations of dipole anisotropies in the distribution of radio galaxies and quasars, obtained from the NVSS and WISE catalogs, show differences in both direction and magnitude when compared to the CMB dipole. These discrepancies pose a challenge to the predictions of the ΛCDM model.

In this talk, we investigate how considering the clustering dipole affects the cosmic large-scale structure distribution dipole using the NVSS catalog, examine the non-linear regime to calculate the correlation between the clustering dipole and the kinematic dipole, and determine whether these outcomes help reconcile previous measurements of the NVSS dipole with predictions based on the CMB. Additionally, we explore a model in which the distribution of matter on large scales might be intrinsically anisotropic. Using the remnant discrepancy between the observed and predicted dipole, we derive an upper limit for the amplitude of intrinsic anisotropy and calculate the clustering for this model.

I’ll present the prospect of reconstructing the “cosmic distance ladder” of the Universe using our novel deep learning framework called LADDER – Learning Algorithm for Deep Distance Estimation and Reconstruction. LADDER was trained on the apparent magnitude data from the Pantheon Type Ia supernovae compilation, incorporating the full covariance information among data points, to produce predictions along with corresponding errors. After employing several validation tests with a number of deep learning models, LADDER was picked as the best performing one. I’ll demonstrate some applications of this framework in the cosmological context, that include serving as a model-independent tool for consistency checks for other datasets like baryon acoustic oscillations, calibration of high-redshift datasets such as gamma ray bursts, use as a model-independent mock catalogue generator for future probes, etc. Our analysis advocates for interesting yet cautious consideration of machine learning applications in these contexts. 

This would be based on the work published in Astrophys. J. Suppl. Ser. 273(2), 27 (2024) [arXiv:2401.17029].

The theory of gravity holds significant importance in our understanding of the large-scale structure and dynamics of the Universe. We first demonstrate how one can measure the gravitational slip, η, in a model-independent way by combining observations from galaxy clustering and weak lensing. Additionally, we propose a method to test the cosmological Poisson equation model-independently while maintaining independence from specific models for the background expansion, the power spectrum shape, and the non-linear corrections. We show that one can only measure the combination M≡Ω_{m,0}μ, where μ quantifies the deviation of the Poisson equation from the standard one and Ω_{m,0} is the present matter density fraction. We also obtain constraints on M for a survey that approximates a combination of the Dark Energy Spectroscopic Instrument and Euclid by employing a recent model-independent forecast for the growth rate f(z) and the expansion rate E(z).

Reference papers: 1210.0439; 1902.06978; astro-ph/0307460; 2107.12990; 2302.09777; 2305.02863; 2307.02117; 2312.07436

The dynamics of the Local Group (LG), especially concerning the contributions of the Milky Way (MW) and Andromeda (M31) galaxies, is sensitive to the presence of dark energy. This work compares the evolution of the LG by considering it as a two-body problem in a homogeneous and isotropic expanding spacetime, i.e. the Mc-Vitte spacetime (McV) versus the spherically symmetric metric for LG dynamics with the Cosmological Constant, i.e. the De Sitter-Schwarzschild spacetime (DsS). Using the Timing Argument (which links LG dynamics to LG mass) we find that the McV spacetime predicts a lower mass for the LG for the McV spacetime vs. the DsS spacetime. By the calibration of the IllustrisTNG simulations, the TA mass is shown to be biased high, and modified to be $~3 10^12$. With the Large Magellanic Cloud (LMC) the center of mass of the MW is shifted and reduces the mass the toal mass to $2.4 10^{12}$. The mass is compatible with other estimations in the literature, such as the Hubble flow and the Virial Theorem with the other dwarf galaxies in the LG and the full sum of the MW and M31 estimated mass.

In this epoch of cosmological tensions, it is essential to question the fundamental assumptions underlying our standard picture of cosmology, including the theory of gravity. 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 my talk, I will demonstrate that this method suffers from an important limitation, since models of dark matter with additional interactions can lead to exactly the same signatures as modified gravity in these two observables. Luckily, I will show that the coming generation of large-scale structure surveys, like the Square Kilometer Array, will allow us to break this degeneracy through measurements of the distortion of time.

Reference paper: arXiv:2404.09379

At high energies, such as during inflation, the quartic coupling of the Standard Model (SM) Higgs potential runs negative, according to current measurements. This can lead the potential into a tachyonic regime, where the square of the mass of the SM Higgs becomes negative. This tachyonic instability can exponentially enhance Higgs particle production via Hubble-induced effects and via the dynamics of the Higgs field itself. Furthermore the enhanced Higgs particle production can draw energy out of the Higgs field and produce stabilizing thermal corrections. The early produced Higgs particles would then modify the curvature perturbations of the early universe which in turn can cause hot or cold spots on the cosmic microwave background (CMB). The aim of our work is to look into this enhanced Higgs particle production and calculate the temperature of the CMB hotspots, as well as looking into CMB hotspots from other sources such as primordial black holes.

A few reference papers: arXiv:2301.08754; 2107.09061

Although General Relativity remains an impressive description of gravitational phenomena, it suffers from specific issues, such as the incapacity of producing an accelerated expansion without introducing a cosmological constant, non-renormalizability, and lack of uniqueness. These problems could hint that new physics is needed, and one possible avenue is to assume that a more general action describes gravity, which is the core idea behind Modified Gravity Theories (MTGs). In this talk, I provide an overview of a particular class of MTGs, the non-minimal geometry-matter coupling theories. More specifically, I offer my perspective on why this class of theories remains a valid option to put our knowledge of gravity to the test. Moreover, I highlight some projects I have been working on in this line of research, such as gravitationally induced particle production, late-time cosmic evolution, and regular black hole solutions. Finally, I plan to briefly mention a work-in-progress Hubble tension research program involving a theory of this type.

arXiv:2205.12545, 2309.15497, 2310.15018

In modern cosmology, cosmological tensions are a leading topic in the field. One of the most controversial subjects in the wake of these discrepancies is the difference between the direct and indirect measurements of the Hubble constant. This has led to alternative scenarios becoming the subject of study in modern cosmology, with dynamical dark energy being one of the most promising. Studies have shown that the key to distinguishing between different dynamical dark energy models lies in their effects on large scales. Therefore, the Integrated Sachs-Wolfe (ISW) effect could be a hint for finding the most convincing scenarios for dark energy. In this work, we have investigated the effect of dynamical dark energy models (CPL, JBP and BA) in comparison with constant dark energy models (LCDM and wCDM) on structure formation in late time by evaluating the matter power spectrum and ISW effect. Our study confirms that the matter power spectrum and the ISW effect are suitable probes for studying dark energy.

arXiv:2403.15202

We found general solutions of matter stress-energy (non-)conservation in scalar-tensor FLRW-type cosmological models by extending the logotropic formalism to the case of non-minimal coupling between the scalar field and new dark fluid candidates. The energy conditions expressed by the generating function are introduced. Next, we investigate the possibility of separating baryonic from dark matter and explain their ratio as a chameleon effect in the presence of non-minimal coupling. To answer the question affirmatively we analyze simple extensions of the Λ-CDM model by adding a non-minimally coupled scalar field in the Einstein frame. Two scenarios involving either a scalaron (quintessence) or a phantom (ghost) are numerically solved and compared. As a result, it is shown that in both cases LCDM model can be reproduced with a high accuracy in the region covered by observations. As expected, in the case of the phantom (ghost) field the Big-Bang scenario is replaced by the (matter) Bounce.

The Hubble tension refers to the discrepancy in the value of the Hubble constant H0 inferred from the cosmic microwave background observations, assuming the concordance ΛCDM model of the Universe, and that from the distance ladder and other direct measurements. In order to alleviate this tension, we construct a plausible dark energy scenario, within the framework of Horndeski gravity which is one of the most general scalar-tensor theories yielding second-order equations. In our set-up, we include the self-interactions and nonminimal coupling of the dynamical dark energy scalar field which enable very interesting dynamics leading to a phantom behaviour at low redshifts along with negative dark energy densities at high redshifts. These two features together make this model a promising scenario to alleviate the Hubble tension for appropriate choices of the model parameters. Towards a consistent model building, we show that this set-up is also free from both the gradient and ghost instabilities. Finally, we confront the predictions of the model with low redshift observations from Pantheon, SH0ES, cosmic chronometers and BAO, to obtain best fit constraints on model parameters.