I first review the three main classes of observational challenges to the standard cosmological model (ΛCDM) the Hubble tension, the growth of perturbations tension and the cosmic dipoles. I then focus on essence of the Hubble tension and describe the assumptions involved in its existence. I also classify the models that have been proposed for its resolution in three broad classes Early time...
The non-linear X-ray to UV luminosity relation in quasars can be used to derive their distances, and to build a Hubble diagram up to z~7 which shows a strong tension with the standard LCDM model. I will present a series of observational results strongly supporting the redshift stability of the relation and its intrinsic precision. In particular, I will show that (1) the slope of the relation...
I will briefly introduce into the problem of nonextensive entropies (Bekenstein, Tsallis, Barrow, Renyi, Sharma-Mittal) and their applications to black holes and, in particular, to cosmology. Then, I will concentrate onto the Barrow entropy (strongly related to Tsallis entropy) and show how the full set of most updated dynamical and geometrical data in cosmology points towards an extensive...
Recently, the cosmological tensions, $H_0$ and $S_8$ in particular, have inspired modification of both the pre and post recombination physics simultaneously. Early dark energy is a promising solution of the $H_0$ tension known to be compatible with CMB. However, the compatibility of early dark energy, as well as general early resolutions, with CMB is no longer obvious if the late Universe is...
"Nonparametric reconstructions of cosmological parameters from observational data sets are usually associated with Gaussian processes (GP). It is known though, that GPs are plagued with overfitting issues and they introduce some statistical bias through the selection of the kernel. The last few years, with the advent of Machine Learning, artificial neural networks are being used in cosmology...
We investigate the cosmological applications of Tsallis entropy and we analyze them in the light of both H0 and σ8 tensions. We obtain the modified Friedmann equations, which contain new extra terms that constitute an effective dark energy sector depending on the new Tsallis exponent δ, that quantifies the departure from standard entropy. For particular δ choices we investigate the effect of...
We propose a modification of no-scale supergravity models which incorporates sgoldstino stabilization and supersymmetry (SUSY) breaking with a tunable cosmological constant by introducing a Kahler potential which yields a kinetic pole of order one. The resulting scalar potential may develop an inflection point close to which an inflationary period can be realized for subplanckian field values...
To understand dark matter and energy, large cosmological surveys are designed to reach a few percent precision. To be fully exploited, this large quantity of data needs to be analyzed in light of cosmological simulations. Preliminary analyses brought out tensions between the standard cosmological model and observations. Reaching a 1% precision, systematics of the same order of magnitude, due...
In this talk I am going to present our recent work where we derive for the first time a three-peaked GW signal associated to no-scale Supergravity within the frequency ranges of nHz, Hz and kHz, with the former being in excellent agreement with NANOGrav/PTA GW data. We concentrate on the primordial gravitational wave (GW) spectrum induced due to second-order gravitational interactions by...
I will discuss cosmic domain walls with the tension which red-shifts with the expansion of the Universe. These melting domain walls emit gravitational waves (GW) with the low-frequency spectral shape corresponding to the spectral index favoured by the recent NANOGrav 15 yrs data. I will discuss a concrete high-energy physics scenario, which leads to such a melting domain wall network in the...
I will argue on the possibility that the smallness of some physical parameters signals a universe at a large distance corner in the string landscape of vacua. Such parameters can be the scales of dark energy and supersymmetry breaking, which should then be tied to a large `dark' dimension at the micron scale. I will discuss the theoretical framework and some of its main physical implications.
In this work, we test the robustness of the constancy of the Supernova absolute magnitude MB using Non-parametric Reconstruction Techniques (NRT). We isolate the luminosity distance parameter dL(z) from the Baryon Acoustic Oscillations (BAO) data set and cancel the expansion part from the observed distance modulus dL(z). Consequently, the degeneracy between the absolute magnitude and the...
In this talk, I will review my research on using the "reconstruction" method to study Dark Energy. I'll discuss different types of reconstructions, how they work, and the results I've obtained. My focus is on model-independent reconstructions of the equation of state and density parameter of Dark Energy. The findings suggest oscillatory behavior and a possible transition from positive to...
One of the main tensions in cosmology is that of the identity of dark matter. Primordial black holes (PBHs) are a longstanding candidate that has garnered a tremendous amount of attention and re-examination in recent years due to possible connections with gravitational wave signatures and regions of parameter space where they can form all of the dark matter, while well-studied WIMP candidates...
We investigate the invariability of the Gauss-Bonnet action term in 4D in the case where gravitational instatons appear in the quantum foam level. The key hypotheses of Euclidean Quantum Gravity, that produce instaton solutions of distinct topology, are encapsulated effectively into the quantum field fluctuations. Therefore, under the Effective Topological Variation Conjecture (ETVC) the...
TBA
In light of the Hubble tension, it is important to test the individual components of the distance ladder. For this purpose, we report a measurement of the Hubble constant from 35 extragalactic Cepheid hosts measured by the SH0ES team, using their distances and redshifts at cz ⤠3300 km s-1, instead of any more distant SNe Ia, to measure the Hubble flow. The Cepheid distances are calibrated...
In this talk, we will first give a brief introduction to the $\Lambda_{\rm s}$CDM model, which explores the recent conjecture suggesting a rapid transition of the universe from anti-de Sitter vacua to de Sitter vacua, viz., the cosmological constant switches sign from negative to positive at redshift ${z_\dagger\sim 1.7}$, inspired by the graduated dark energy (gDE). And then, we will present...
Asymptotic Safety is a promising framework towards the understanding, in a non-perturbative way, of Quantum Gravity. It treats the Newtons constant $G_N$ and the cosmological constant $\Lambda$ as running coupling of an effective action. At the phenomenological point of view the values of $G_n$ and $\Lambda$ depend on the energy density of the system under consideration. This fact has...
Various studies, such as references 2207.00878 [gr-qc], 2304.14465 [gr-qc], and 2304.14465 [gr-qc], have explored the potential of Fractional Cosmology to address the H0 tension. They have analyzed the Equation of State's value attained from the Supernova H0 and Planck's value for z<1.5 and have reported a trend of H0 that aligns with these values. However, there is still a discrepancy between...
We have performed the dynamical system analysis of f(T) gravity cosmological models at both background and perturbation levels. We have presented three models pertaining to three distinct functional forms of f(T). The first form is that of the logarithmic form of the torsion scalar T, the second one is in the power law form, and the third one is the combination of the first two forms. For all...
In this talk, I will present a possible way to alleviate tensions in cosmology through relaxing some of the widely-held assumptions in cosmology: the constancy of fundamental parameters, large-scale isotropy, and perfect-fluid cosmic media.
Early Dark Energy (EDE), an additional component of dark energy active in the decade of redshift before recombination, has emerged as one of the most effective models at resolving the Hubble tension. By reducing the size of the `sound horizon’ that calibrates CMB and BAO observations, it is able to fit a variety of datasets including the variety of high-H0 measurements, and may shed light upon...
It appears by now established that solving the Hubble tension requires new physics operating at early times, i.e. prior to recombination. But is that really the end of the story? Based on Miller's law (which states that the number of objects the average person can hold in working memory is about seven), I will present seven independent hints pointing towards the fact that the Hubble tension...
In this talk, we introduce a generalization of the usual vacuum energy via preserving zero inertial mass density. In return for zero inertial mass density, vacuum energy yields a particular form of anisotropic pressure. This âansiotropicallly deformed vacuum energy' couples to the shear scalar in a unique way, such that they together emulate the canonical scalar field with an arbitrary...
The expansion isotropy of the Universe and the dissipation of bulk flows at >200 Mpc scales is a crucial assumption of LCDM. Any significant, observational deviation from this consensus can strongly challenge the standard cosmological model. Multiwavelength scaling relations of galaxy clusters are an excellent and powerful tool to scrutinize both the Hubble constant (H0) isotropy and the...
The three-dimensional expansion rate fluctuation field is an unbiased Gaussian observable that measures deviations from the linearity and isotropy of the redshift-distance relationship in an optimal, robust, and model-independent way (Kalbouneh, Marinoni, Bel 2023). We show how to perform a spherical harmonic analysis of this observable and determine the multipole structure of the Hubble...
Higher-derivative theories of gravity are advocated to be power-counting renormalizable. As such, they might play a role in the possible UV completion of gravity. At the same time, some of these theories suffer from the presence of ghosts -- unphysical degrees of freedom that at the classical level lead to instability, while upon quantization, they cause violation of unitarity. In this talk, I...
An exact scale-invariant Harrison-Zeldovich spectrum, i.e. $n_s=1$, has been ruled out at $8.4\sigma$ level according to Planck results. However, the situation changes when we consider the Hubble tension. Extra energy injection in early cosmology (e.g. Early Dark Energy), which is a class of promising solutions of the Hubble tension, seems to call for the return of $n_s=1$. Using recent...
To explore a possible deviation from standard cosmologies that can explain the current discrepancy on the current Hubble value, in this talk we consider adding to the current local observables new calibrated Quasars datasets using ultraviolet, x-ray and optical plane techniques. While these can be identified as part of the high-redshift standard candels objects, the main characteristics of...
One of the ways to resolve the Hubble tension is to modify the recombination history of the universe. An intriguing proposal to realize this invokes primordial magnetic fields (PMFs) to stir up the plasma on small scales. A clumpy baryon density field recombines faster than a homogenous one, which would push the surface of last scattering to earlier times and increase the inferred distance to...
The search for a solution to the tensions in the Standard Model in the shape
of a single, all-describing model is both extremely needed and extremely hard to
carry out. This is partly because we lack the knowledge of some of the fundamental
aspects that a new theory should include in order to match our observations of the
universe. Even in the case of the ΛCDM model itself, which we take...
Recent advancements in CMB measurements have offered a detailed window into the cosmos. However, this progress has also led to tensions between the outcomes of various cosmological observations, challenging the coherence of our current cosmological model. This talk delves into the implications of CMB polarization measurements on large angular scales, which appear to significantly amplify the...
Recent tensions and anomalies have motivated the need to explore novel theoretical possibilities beyond the standard cosmological model. In this talk, I will argue that small-scale CMB observations can play a crucial role in the study of several physical mechanisms, potentially revealing unique signatures of new physics that would be difficult to detect on larger angular scales. I will provide...
In the context of modified teleparallel gravity, we study the generation of primordial density fluctuations. It is well known that generic modifications of teleparallel gravity are not invariant under six-parameter local Lorentz transformations. In order to restore the local Lorentz symmetry, we have incorporated six additional degrees of freedom in the form of Goldstone modes of the symmetry...
We present the idea that replacing the cosmological constant Λ in the ΛCDM by a distribution of walls with very low tension compared to what one would expect from the “new physics” could help for the tension in the Hubble constant fit in this Standard Cosmological Model. Using parameters from our, since long, model for dark matter as macroscopic pearls, we can get a promising order of...
Fuzzy dark matter (FDM) with mass around 10^{-22} eV is believed to be a more hopeful DM candidate compared to cold dark matter (CDM). Because FDM can solve the small-scale problems for CDM, thanks to its wave nature due to the very light mass. These problems arise from the incorrect prediction for the structure growth. For example, CDM predicts a cusp at the center of DM halo, while recent...
The current study offers a thorough analysis of a dark energy cosmological model in $\mathcal{F}( \mathcal{R}, \mathcal{G})$ gravity, where $\mathcal{R}$ and $ \mathcal{G} $ denote Ricci scalar and Gauss-Bonnet invariant respectively. In order to solve Einstein field equations, we use the parametrized Hubble parameter in terms of the scale factor. We constrain the model parameters using the...
We intend to study QCD-modified scalar field models of dark energy, in the presence of both interaction and viscosity, with varying gravitational constant $G$. The equation of the state parameter of the interacting viscous QCD-modified ghost dark energy (MGDE) and the deceleration parameter of the universe, is derived. Furthermore, we establish the correspondence between the interacting...
The $H_0$-tension problem challenges our conventional application of general relativity to cosmology, otherwise well-described by FRLW universes in terms a Hubble parameter $H(z)$ and a deceleration parameter $q(z)$. A finite dark energy density is expected from the Sitter temperature associated with the de Sitter background scale of acceleration $a_{dS}=cH$, where $c$ is the velocity of...
Galaxies are known to be good but biased tracers of the underlying dark matter field. This bias is mostly driven by the history of hierarchical clustering and galaxy/halo assembly history but is also affected by factors regulating galaxy evolution, usually environment dependent. Moreover, it is easily blurred by observational biases unavoidably present in the data. Thus, the relations between...
By incorporating torsional gravity into the effective field theory (EFT)perspective, the model offers a unified and By Incorporating torsional gravity into the EFT perspective offers a unified and consistent framework to fit observations solving the two tensions. Both the H0 and Ï8 tensions can simultaneously be alleviated within torsional gravity by the EFT approach. Further, we investigate...
Addressing the discrepancy between the late and early time measurements of the Hubble parameter, $H_0$, and the so-called $S_8$ parameter has been a challenge in precision cosmology. Several models are present to address these tensions, but very few of them can do so simultaneously. In the past, we have suggested Banks-Zaks/Unparticles as an emergent Dark Energy model and claimed that it can...
In this talk, relying on the fact that the comoving angular diameter distance to last scattering, $D_M(z_*)$, is strictly constrained almost model-independently, I will present how a dark energy (DE) density that attains negative values in the past can alleviate the $H_0$ tension along with the $S_8$, and Ly-$\alpha$ discrepancies. Observational studies suggest that matching the mean value of...
Due to their low surface brightness, dwarf galaxies are particularly susceptible to tidal forces. The expected degree of disturbance depends on the assumed gravity law and whether they have a dominant dark halo. This makes dwarf galaxies useful for testing different gravity models. In this project, we use the Fornax Deep Survey (FDS) dwarf galaxy catalogue to compare the properties of dwarf...
A simple setup for discussing both mathematical and physical aspects of spacetimes with extended sources, like cosmic strings or cosmic filaments of galaxies and dark matter extending across hundreds of millions of light years is provided by cylindrical symmetry. Investigating cylindrically symmetric configurations is also a suitable precursor to the study of axial symmetry. Unlike in the...
We show how we can alleviate the H0 tension in the framework of f(G) gravity. In particular, we
obtain the Friedmann equations and we show that we obtain extra terms of geometrical origin that
constitute an effective dark energy sector. Then we show that due to the increased friction terms,
one can obtain an increase in H0 at late times. The reason behind the tension alleviation is the...
