Narrow Results

The $\mathrm{\Lambda }$CDM model is considered as a standard model because of its self-consistency in theory and nearly perfect fitting to most cosmological observation. However, in recent years, the so-called “Hubble tension”, which means the discrepancy between the early- and late-time observations of Hubble constant $H_0$, has been more and more prominent. Based on the previous research, we update the data combination with Planck 2018 and BAO, detect the relationship of Hubble constant and effective number of relativistic species $N_{\mathrm{\text{eff}}}$, then revisit the possibility of solving Hubble tension starting from $N_{\mathrm{\text{eff}}}$. Considering a class of $\mathrm{\Lambda }\mathrm{\text{CDM}}+\overline{N_{\mathrm{\text{eff}}}}$ models with fixed values of $N_{\mathrm{\text{eff}}}$, a linear relationship of $\mathrm{\Delta }{H}_0$ and $\mathrm{\Delta }{N}_{\mathrm{\text{eff}}}$ is presented as $\mathrm{\Delta }{H}_0$$\approx 6.06\mathrm{\Delta }{N}_{\mathrm{\text{eff}}}$, which indicates that Hubble tension would be solved thoroughly if $N_{\mathrm{\text{eff}}}\approx 3.899$. The numerical results of model with fixed $N_{\mathrm{\text{eff}}}=3.899$ show that the Hubble tension is reduced to $0.28\sigma$. However, the $\mathrm{\Lambda }$CDM model is still preferred over other models, especially over the model with $N_{\mathrm{\text{eff}}}=3.899$ by $\mathrm{\Delta }{\chi }_{\mathrm{\text{min}}}^2=26.4$. In brief, it is unlikely to solve the Hubble tension by varying $N_{\mathrm{\text{eff}}}$ alone.

We review the origins, motivations, and implications for cosmology and black holes, of our proposal that “dark energy” is not a quantum vacuum energy, but rather arises from a Weyl scaling invariant nonderivative component of the gravitational action.

We show that the cosmic bulk viscosity estimated in our previous works is sufficient to bridge the $H_0$ value inferred from observations of the early universe with the value inferred from the local (late) universe.

In this paper, we show how a modified Friedmann equation, originating from a model of the universe built from a certain $W_3$ algebra, has the potential to explain the difference between the Hubble constants extracted from CMB data and from supernova data.

The estimation of the Hubble constant in the past few decades has increasingly become more accurate with the advancement of new techniques. However, its value seems to depend on the epoch at which the measurements are made. The Planck estimate of the Hubble constant from the observations of the cosmic microwave background radiation in the early universe is about $67\mathrm{\text{km}}\backslash \mathrm{\text{s}}\backslash \mathrm{\text{Mpc}}$, whereas that obtained from the distance indicators at the current epoch is $\sim 73-74\mathrm{\text{km}}\backslash \mathrm{\text{s}}\backslash \mathrm{\text{Mpc}}$. This discrepancy between the two groups of measurement is termed as the Hubble tension which has gained much attention in the past few decades with growing significance as measurements from both, the early and the late universe, studies continue to produce results with increasing precision. In this work, we propose a modification to gravity by considering a finite range gravitational field as an alternate explanation for this discrepancy in the value of the Hubble constant.

The recent measurements of the Hubble constant based on the standard $\mathrm{\Lambda }$CDM cosmology reveal an underlying disagreement between the early-Universe estimates and the late-time measurements. Moreover, as these measurements improve, the discrepancy not only persists but becomes even more significant and harder to ignore. The present situation places the standard cosmology in jeopardy and provides a tantalizing hint that the problem results from some new physics beyond the $\mathrm{\Lambda }$CDM model.

It is shown that a nonconventional theory — the Milne model — which introduces a different evolution dynamics for the Universe, alleviates the Hubble tension significantly. Moreover, the model also averts some long-standing problems of the standard cosmology, for instance, the problems related with the cosmological constant, the horizon, the flatness, the Big Bang singularity, the age of the Universe and the nonconservation of energy.

In this paper, we give a physical explanation to the accelerated expansion of the universe, alleviating the tension between the discrepancy of Hubble constant measurements. By the EulerCauchy stress principle, we identify a controversy on the lack of consideration of the surface forces contemplated in the study of the expansion of the universe. We distinguish a new general relativistic effect that modifies the spacetime fabric by means of the energy conservation equation. The resulting dynamical equations from the proposed hypothesis are contrasted to several testable astrophysical predictions. This paper also explains why we have not found any particle or fluid responsible for dark energy and clarifies the Cosmological Coincidence Problem. These explanations are achieved without assuming the existence of exotic matter of unphysical meaning or having to modify Einstein’s Field Equations.

We extend our previous analysis of a model for “dark energy” based on a Weyl scaling invariant dark energy action. We reexpress all prior results in terms of proper time, using the fluctuation amplitude $\mathrm{\Phi }$ without approximation, and derive a compact formula for the squared effective Hubble parameter. This formula involves effective dark energy and matter densities that differ from their expressions in the standard $\mathrm{\Lambda }\mathrm{\text{CDM}}$ cosmology. We also give new analytic results for the function $\mathrm{\Phi }$ and discuss their implications.

In this paper, we review and update constraints on the Early Dark Energy (EDE) model from cosmological data sets, in particular Planck PR3 and PR4 cosmic microwave background (CMB) data and large-scale structure (LSS) data sets including galaxy clustering and weak lensing data from the Dark Energy Survey, Subaru Hyper Suprime-Cam and KiDS+VIKING-450, as well as BOSS/eBOSS galaxy clustering and Lyman-$\alpha$ forest data. We detail the fit to CMB data, and perform the first analyses of EDE using the CAMSPEC and Hillipop likelihoods for Planck CMB data, rather than Plik, both of which yield a tighter upper bound on the allowed EDE fraction than that found with Plik. We then supplement CMB data with LSS data in a series of new analyses. All these analyses are concordant in their Bayesian preference for $\mathrm{\Lambda }$CDM over EDE, as indicated by marginalized posterior distributions. We perform a series of tests of the impact of priors in these results, and compare with frequentist analyses based on the profile likelihood, finding qualitative agreement with the Bayesian results. All these tests suggest prior volume effects are not a determining factor in analyses of EDE. This work provides both a review of existing constraints and several new analyses.

We introduce a theoretical framework to alleviate the Hubble tension. This framework is based on dynamics of a minimally coupled scalar field which either belongs to the Brans–Dicke theory with a self-interacting potential or is the scalar partner of $f(R)$ gravity. These two theories are dynamically equivalent when the Brans–Dicke parameter is zero. We will use the dynamical equivalence to interpret the Hubble tension in the same theoretical framework. For both theories, we write one set of field equations in which the value of a parameter distinguishes between the two theories. We will show that $H_0$ actually evolves with redshift so that its value is consistent with that measured from the local distance ladder and then drops to the value measured from CMB at high redshift. We argue that even though both theories exhibit this behavior, Brans–Dicke theory with an exponential potential is more successful than power-law $f(R)$ gravity to relieve the Hubble tension.

We present a fully relativistic framework to evaluate the impact of stochastic inhomogeneities on the prediction of the Hubble-Lemaître diagram. In this regard, we relate the fluctuations of the luminosity distance-redshift relation in the Cosmic Concordance model to the intrinsic uncertainty associated to the estimation of cosmological parameters from high-redshift surveys (up to z = 4). Within this framework and according to the specific of forthcoming surveys as Euclid Deep Survey and LSST, we show that the cosmic variance associated with the measurement of the Hubble constant will not exceed 0.1 %. Thanks to our results, we infer that deep surveys will provide an estimation of the the Hubble constant H₀ which will be more precise than the one obtained from local sources, at least in regard of the intrinsic uncertainty related to a stochastic distribution of inhomogeneities.

Baryon Acoustic Oscillations (BAO) datasets use very precise measurements of the spatial distribution of large-scale structures as a distance ladder to help constrain cosmological parameters. In a recent article,¹ we combined 17 uncorrelated BAO measurements in the effective redshift range 0.106 ≤ z ≤ 2.36 with the Cosmic Chronometers data, the Pantheon Type Ia supernova and the Hubble Diagram of Gamma Ray Bursts and Quasars to obtain that the ΛCDM model fit infers for the Hubble constant: 69.85 ± 1.27km/sec/Mpc and for the sound horizon distance: 146.1 ± 2.15Mpc. Beyond the ΛCDM model we test Ω_kCDM and wCDM and we get Ω_k = −0.076 ± 0.012, w = − 0.989 ± 0.049 accordingly. In this proceeding we present elaborate on our findings and we compare them to other recent results in the literature.

The actual knowledge of the structure and future evolution of our universe is based on the use of cosmological models, which can be tested through the so-called ‘probes’, namely astrophysical phenomena, objects or structures with peculiar properties that can help to discriminate among different cosmological models. Among all the existing probes, of particular importance are the Supernovae Ia (SNe Ia) and the Gamma Ray Bursts (GRBs): the former are considered among the best standard candles so far discovered but suffer from the fact that can be observed until redshift z = 2.26, while the latter are promising standardizable candles which have been observed up to z = 9.4, surpassing even the farthest quasar known to date, which is at z = 7.64. The standard candles can be used to test the cosmological models and to give the expected values of cosmological parameters, in particular the Hubble constant value. The Hubble constant is affected by the so-called “Hubble constant tension”, a discrepancy in more than 4 σ between its value measured with local probes and its value measured through the cosmological probes. The increase in the number of observed SNe Ia, as well as the future standardization of GRBs through their correlations, will surely be of help in alleviating the Hubble constant tension and in explaining the structure of the universe at higher redshifts. A promising class of GRBs for future standardization is represented by the GRBs associated with Supernovae Ib/c, since these present features similar to the SNe Ia class and obey a tight correlation between their luminosity at the end of the plateau emission in X-rays and the time at the end of the plateau in the rest-frame.