The fifth (and final) data release of the Kilo-Degree Survey (KiDS) provides significantly more than just an increase in survey area. Instead, additional observations couple with improvements in data quality, data reduction, and value-added data-products, to make KiDS-DR5 the most robust and reliable release produced by KiDS to date. We leverage the lensing sample from this state-of-the-art...
Recent cosmological surveys have revealed persistent discrepancies within the context of the concordance model regarding the values of the H0 [1907.10625], σ8 [1610.04606] and ΩK [1908.09139, 1911.02087] when estimated with different datasets. Determining the level of disagreement between multidimensional fits is called "tension quantification" [1902.04029].
We approach this problem by...
We perform a stacking analysis of galaxy cluster velocity phase space using the caustic technique. By stacking 128 clusters, we create four robust stacked clusters with excellent agreement between caustic masses and binned medians. We model the gravitational potential using the NFW profile, validating the $\Lambda$CDM mass-concentration relation. Implementing the Chameleon screening model, we...
The path from Cepheid variables to type Ia supernovae gives a value of the Hubble constant which significantly disagrees with the value determined from observations of conditions in the early universe and a cosmological model. A totally independent measurement of H0 from observed redshifts and distances is needed to evaluate the possibilities of systematic errors. A path is being explored that...
I present an up-to-date compilation of published Hubble constant ($H_0$) measurements that are independent of the CMB sound horizon scale. This compilation is split in two distinct groups: A. Distance Ladder Measurements sample comprising of 20 recent measurements, primarily from the past four years, utilizing various rung 2 calibrators and rung 3 cosmic distance indicators. B.One-Step...
We explore the potential of cross-correlation between gravitational waves (GWs) and 21 cm intensity mapping (IM) from neutral hydrogen emission (HI), focusing on its role in view of future high-precision observatories, specifically Einstein Telescope (ET) and the Square Kilometer Array Observatory (SKAO). We model the large scale structure evolution by making use of 3D lightcones of the dark...
One of the fundamental predictions of modern cosmology is that we should see the distant universe run slowly, with time dilated by the expansion of space. Numerous searches for this cosmic time dilation in the light curves of distant quasars, some of the most luminous objects in the universe, have failed to find the expected signal, with claims that this challenges the standard cosmological...
We present a comprehensive statistical framework for cosmological parameter estimation, focusing on Baryon Acoustic Oscillation (BAO) analyses in the light of the DESI DR2. We focus on two complementary directions: the performance evaluation of various sampling algorithms and the investigation of likelihood formulations. We will discuss the role of the choice of a sampler in both its...
I will review what the prospects of quasars in the context of observational cosmology are, and I will present recent measurements of the expansion rate of the Universe based on a Hubble diagram of quasars detected up to the highest redshift ever observed (z~7.5).
The derived distances are in agreement with the standard flat ΛCDM model up to a redshift of ~1.5, but they show significant...
Context. Gamma-ray bursts (GRBs), observed at redshifts as high as 9.4, could serve as valuable probes for investigating the distant Universe. However, this necessitates an increase in the number of GRBs with determined redshifts, as currently, only 12% of GRBs have known redshifts due to observational biases. Aims. We aim to address the shortage of GRBs with measured redshifts, enabling us to...
The well established ΛCDM model of cosmology suffers from several challenges and disparities like the nature of dark matter and dark energy, accelerated expansion of the Universe and tensions in cosmological parameters derived from different observations. Tomographic cross-correlation measurements from the next-generation CMB experiments and galaxy surveys will allow us to robustly quantify...
Supernova (SN) cosmology is based on the key assumption that the luminosity standardization process of type Ia SNe remains invariant with progenitor age. However, our comprehensive age measurements of 360 SN host galaxies reveal a significant (5.5 sigma) correlation between standardized SN luminosity and progenitor age, which is expected to introduce a serious systematic bias with redshift in...
We explore an extension of the $\Lambda$CDM model that introduces a time-dependent evolution of the pressure parameter $p$ in the dark energy fluid. In this process, the corresponding energy density $\rho$ is derived by a continuity equation. Consequently, the equation of state $w\equiv p/\rho$ evolves during the late-time expansion of the Universe. We model the pressure parameter using a...
Dark energy away from a cosmological constant Λ – like early universe inflation that ends – can be understood in terms of well defined physical behaviors. These guide dark energy into thawing or freezing classes, with w0–wa arising as a physical calibration of the phase space. Other regions of phase space – zones of avoidance – require violation of some basic principle. We explore these cases,...
The Dark Energy Survey (DES) recently released the final results of its two main probes of the expansion history: Type Ia Supernovae (SN) and Baryonic Acoustic Oscillations (BAO). In this work, we explore the cosmological implications of these data in combination with external Cosmic Microwave Background (CMB), Big Bang Nucleosynthesis (BBN), and age-of-the-Universe information. The BAO...
Gravitational wave (GW) and multi-messenger (MM) astronomy provides new ways to gain insights into the Dark Energy (DE) phenomenology, through the distance-redshift relation, as well as into potential deviations from General Relativity (GR), via the distance duality relation. Both analyses involve the same astrophysical observables, making MM astronomy a powerful tool to explore simultaneously...
In this talk I will review how modified gravity parametrizations can, in principle, be exploited to not only test general relativity, but also tackle the problematic tensions that riddle cosmology in its current state, considering these issues by the point of view of both background observables and perturbations. More precisely, I will aim to provide insight into questions such as: can...
Baryon Acoustic Oscillations (BAO), together with Type Ia Supernovae (SNeIa), increasingly disfavor late-time modifications as viable resolutions to the Hubble tension. A key argument against post-recombination solutions—the so-called “no-go theorem”—relies critically on BAO data: since BAO constrain angular scales proportional to rdH0, a higher H0 implies a smaller rd, in tension with the...
The recent constraints from DESI (arXiv: 2404.03002, arXiv: 2503.14738), when combined with various SNeIa data, indicate a strong preference ($\sim3-4\sigma$) for a phantom divide line (PDL) crossing in the equation of state parameter of a supposed dynamical dark energy (DE). This is evident from their constraints preferring $w_0>-1$, $w_a<0$ for the CPL parametrization of the DE, i.e.,...
The cosmic dipole measured in surveys of cosmologically distant sources is generally in disagreement with the kinematic expectation of the Cosmic Microwave Background (CMB). This discrepancy represents severe tension with the Cosmological Principle and challenges the standard model of cosmology. We present a Bayesian analysis that quantifies the tension between datasets used to measure the...
I discuss our ongoing efforts to measure the cosmic dipole in new galaxy surveys, referencing the systematic effects which can influence its measurement and forecasting the contribution of forthcoming surveys. This is motivated by the nascent cosmic dipole tension: the disagreement between the CMB dipole and counts of extragalactic sources, which has reached a high level of statistical...
Cosmic voids, the largest observable structures in the Universe and characterized by a scarcity of galaxies, impact the Cosmic Microwave Background (CMB) through gravitational lensing, leaving a negative imprint on the CMB convergence (κ) map. This imprint offers insights into matter distribution within voids and the growth of cosmic structures. Furthermore, voids produce secondary CMB...
We explore an interacting dark sector model in trace-free Einstein gravity where dark energy has a constant equation of state, $w=-1$, and the energy-momentum transfer potential is proportional to the cold dark matter density. Compared to the standard $\Lambda$CDM model, this scenario introduces a single additional dimensionless parameter, $\epsilon$, which determines the amplitude of the...
We formulate topological holographic dark energy and we present how it can alleviate both $H_{0}$ and $\sigma 8$ tensions. The modified cosmological model is obtained by the application of the gravity thermodynamics conjecture when the corresponding Wald Gauss Bonnet entropy term is added to the standard Bekenstein Hawking one. Thus, one obtains modified Friedmann equations that depend on the...
A scale invariant gravity theory containing two scalar fields, dust particles and a measure defined from degrees of freedom independent of the metric. The integration of the degrees of freedom that define the measure spontaneously break the scale symmetry, leaving us in the Einstein frame with an effective potential that is dependent on the density of the particles. The potential contains...
We are currently witnessing a remarkable era in the fields of cosmology and gravitation, as an ever-increasing amount of observational data continues to be collected. One of the most perplexing questions confronting cosmologists today concerns the mysterious source responsible for the Universe’s recent accelerated expansion. The most widely accepted explanations for this phenomenon involve...
The standard Lambda Cold Dark Matter (ΛCDM) cosmological model has proven remarkably successful in describing a broad range of observational data, ranging from the cosmic microwave background (CMB) radiation to the large-scale structure of the Universe. However, recent advances in precision cosmology have revealed persistent statistical discrepancies between independent data sets and...
We propose a novel dark energy mechanism rooted in the microscopic structure of spacetime, where quantum-induced topological transitions give rise to an effective cosmological constant. Within the framework of Euclidean Quantum Gravity, gravitational instantons emerging at the spacetime foam level induce changes in topology, activating the Gauss-Bonnet term in four dimensions. This leads to a...
Hydrodynamic simulations will be instrumental in attempting to resolve cosmological tensions such as the $S_8$ tension as they can assist with modelling key sources of uncertainty such as galaxy bias. However, for these purposes, the simulations need both large box size and high resolution, which is computationally prohibitive. In this work we instead use the connection between galaxies and...
In the era of high precision Cosmic Microwave Background (CMB) anisotropy measurements, detection of the primordial graviational waves (PGW) will be one of the main goals of the upcoming CMB experiments. To make it possible precise cleaning of the CMB maps from diffused contamination coming from polarised galactic emission and CMB gravitational lensing effect is needed. We investigate the...
The effect of gravitational lensing of the Cosmic Microwave Background
(CMB) provides a unique opportunity to obtain a picture of the gravitational potential of the large-scale structure of the Universe at very high redshifts. Tomographic cross-correlation of the gravitational potential with other tracers of the large-scale structure at known redshifts allows tracing the evolution of the...
We investigate a theoretical framework for the Universe dynamics, based on the possible creation of dark energy constituents by the time-varying gravitational field of the expanding Universe. The proposed scenario is compared to specific cases of the reduced Chevallier-Polarski-Linder (CPL) parameterization, with the aim of probing the nature of dark energy.
We construct, from our model and...
In my talk, I will present a model where the Universe is filled with a phantom scalar field with a potential in the form of a hyperbolic tangent, as well as cold dark matter and radiation. This model is free from the shortcomings usually attributed to phantom models and undergoes a rapid Anti-de Sitter to de Sitter transition. At the background level, a detailed analysis of the model is...
Earlier studies investigating the allowed fraction of dark matter as primordial black holes (PBHs) tend to completely rule out PBHs with masses smaller than \~10⁻¹⁵ solar masses. This is due to the lack of evidence for Hawking radiation coming from the final evaporation stages of such small PBHs. These limits, however, make the key assumption that these PBHs can be modelled as uncharged,...
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