Physics of the Dark Universe最新文献

In this paper, we introduce a notion of null Raychaudhuri matrix motivated by the matrix representation of tensor fields. The evolution of this matrix gives the matrix form of null Raychaudhuri equation. Using this distinct geometric approach, we have reformulated the original Penrose’s singularity theorem on Black-Hole and have commented on the characteristic of the Raychaudhuri matrix at the singularity. The paper also suggests two possible mitigations for the physical singularity of Schwarzschild black hole via the Wheeler–DeWitt formalism and Bohmian formalism.

The search for compact astrophysical objects, such as black holes and wormholes, along with the testing of gravity theories, are crucial topics in relativistic astrophysics. In this context, theoretical and observational studies of quasi periodic oscillations (QPOs) detected in (micro)quasars prove useful for investigating their central object. We explore the massive particle motion around black hole in Schwarzschild-like spacetime and using the equation of motion, the dependence of innermost stable circular orbits (ISCO) on the parameter $l$ was obtained. The frequencies of radial and vertical oscillations of particles around stable circular orbits have been studied and used to explain the mechanism of quasi periodic oscillations in the relativistic precession (RP) model. Using observational data from GRO J1655-40 and XTE J1550-564, we have determined a constraint on the parameter $l$ in Schwarzschild-like spacetime through Monte-Carlo-Markovian-Chain (MCMC) analyses.

We present a novel perspective on the Schwarzschild, Reissner–Nordstrom, and Massless Black Holes solutions, a cornerstone of general relativity, by employing a distributional approach. The conventional solutions are shown to be incomplete, failing to capture the true nature of the gravitational field near the black hole’s singularity. Drawing from advanced concepts in functional analysis and distribution theory, we derive generalized solutions that resolves the apparent singularities and provides new insights into the behavior of matter and energy in the vicinity of a black hole. Our findings suggest the possibility of quantum tunneling, allowing particles to traverse the event horizon under specific conditions. This work not only offers a more rigorous treatment of the Schwarzschild, Reissner–Nordstrom and Massless Black Hole solutions but also paves the way for a deeper understanding of black hole physics and the interplay between general relativity and quantum mechanics.

Deploying multiple sharp transitions (MSTs) under a unified framework, we investigate the formation of Primordial Black Holes (PBHs) and the production of Scalar Induced Gravitational Waves (SIGWs) by incorporating one-loop corrected renormalized-resummed scalar power spectrum. With effective sound speed parameter, $1⩽c_s⩽1.17$, the direct consequence is the generation of PBH masses spanning $M_{\mathrm{PBH}}\sim O(10^{-31}{M}_{\odot }-10^4{M}_{\odot })$, thus evading well known No-go theorem on PBH mass. Our results align coherently with the extensive NANOGrav 15-year data and the sensitivities outlined by other terrestrial and space-based experiments (e.g.: LISA, HLVK, BBO, HLV(O3), etc.).

This study investigates the impact of the quantum-gravity correction at the third-order curvature ($c_6$) on the black hole’s shadow and deflection angle on the weak field regime, both involving finite distances of observers. While the calculation of the photonsphere and shadow radius $R_{\mathrm{sh}}$ can easily be achieved by the standard Lagrangian for photons, the deflection angle $\alpha$ employs the finite-distance version of the Gauss–Bonnet theorem (GBT). We find that the photonsphere reduces to the classical expression $r_{\mathrm{ph}}=3M$ for both the Planck mass and the theoretical mass limit for BH, thus concealing the information about the applicability of the metric on the quantum and astrophysical grounds. Our calculation of the shadow, however, revealed that $c_6$ is strictly negative and constrains the applicability of the metric to quantum black holes. For instance, the bounds for the mass is $M/l_{\mathrm{Pl}}\in [0.192,4.315]$. We also derived the analytic formula for the observer-dependent shadow, which confirms $c_6$’s influence on quantum black holes even for observers in the asymptotic regions. The influence of such a parameter also strengthens near the quantum black hole. Our analytic calculation of $\alpha$ is shown to be independent of $c_6$ if the finite distance $u\to 0$, and $c_6$ is not coupled to any time-like geodesic. Finally, the effect of $c_6$ manifests in two ways: if $M^2$ is large enough to offset the small value of $l_{\mathrm{Pl}}$ (which is beyond the theoretical mass limit), or if $b$ is comparable to $l_{\mathrm{Pl}}$ for a quantum black hole.

We present a non-abelian cousin of the model presented in Lee et al. (2024) which induces cosmological anisotropies on top of standard FLRW geometry. This is in some sense doing a cosmological mean field approximation, where the mean field cosmological model under consideration would be the standard FLRW, and the induced anisotropies are small perturbative corrections on top of it. Here we mostly focus on the non-abelian $SU\left(2\right)$ gauge fields coupled to the gravity to generate the anisotropies, which can be a viable model for the axion-like particle (ALP) dark sector. The induced anisotropies are consequences of the non-trivial back-reaction of the gauge fields on the gravity sector, and by a clever choice of the parametrization, one can generate the Bianchi model we have studied in this note. We also show that the anisotropies influence the Sachs–Wolfe effect and we discuss the implications.

Motivated by the first image of a black hole captured by the Event Horizon Telescope (EHT), there has been a surge of research using observations of black hole shadows to test theories of gravity. In this paper, we carry out a study related to the shadow of Kerr black holes surrounded by a cloud of strings in Rastall gravity, which deviates from the Kerr black hole due to the presence of the string parameter $a_0$ and the parameter $\beta$. The horizons, ergospheres, and photon region of the black hole are shown. Moreover, we explore the shadow and observations of the black hole, which are closely linked to the parameters $a_0$ and $\beta$. By treating M87* as a Kerr black hole surrounded by a cloud of strings under Rastall gravity, we constrain the black hole parameters using the EHT observations. For a given $\beta$, the circularity deviation of the black hole obeys $\Delta C\lesssim 0.1$ in all regions. The angular diameter ${\theta }_d=42\pm 3\mu as$ provides the upper bound of parameters $a$ and $a_0$ for fixed $\beta$. The shadow axis ratio satisfies the observation data of EHT ($1<D_x\lesssim 4/3$) in the whole space for a given $\beta$. These results are consistent with the public information from EHT. In other words, candidates for real astrophysical black holes can be Kerr black holes surrounded by a cloud of strings in Rastall gravity.

In this paper, we develop a geometric generalization of General Relativity based on the semi-symmetric metric connection introduced by Friedmann and Schouten in 1924. Although the mathematical properties of this connection, which allows for torsion, have been extensively studied, its physical implications remain underexplored. We provide a detailed exposition of the differential geometric aspects of semi-symmetric connections and formulate the corresponding field equations induced by the specific form of torsion we are investigating. We consider the cosmological applications of the theory by deriving the generalized Friedmann equations in a flat, homogeneous and isotropic geometry. The Friedmann equations also include some supplementary terms as compared to their general relativistic counterparts, which can be interpreted as a geometric type dark energy. To evaluate the proposed theory, we consider three cosmological models — the first with constant effective density and pressure, the second with the dark energy satisfying a linear equation of state, and a third one with a polytropic equation of state. We also compare the predictions of the semi-symmetric metric gravitational theory with the observational data for the Hubble function, and with the predictions of the $\Lambda$CDM model. Our findings indicate that the semi-symmetric metric cosmological models give a good description of the observational data, and for certain values of the model parameters, they can reproduce almost exactly the predictions of the $\Lambda$CDM paradigm. Consequently, Friedmann’s initially proposed geometry emerges as a credible alternative to standard general relativity, in which dark energy has a purely geometric origin.

We investigate a generalized Chaplygin-like gas with an anisotropic equation of state, characterizing a dark fluid within which a static spherically symmetric black hole is assumed. By solving the Einstein equations for this black hole spacetime, we explicitly derive the metric function. The spacetime is parametrized by two critical parameters, $B$ and $\alpha$, which measure the deviation from the Schwarzschild black hole and the extent of the dark fluid’s anisotropy, respectively. We explore the behavior of light rays in the vicinity of the black hole by calculating its shadow and comparing our results with the Event Horizon Telescope observations. This comparison constrains the parameters to $0\le B\lesssim 0.03$ and $0<\alpha \lesssim 0.1$. Additionally, we calculate the deflection angles to determine the extent to which light is bent by the black hole. These calculations are further utilized to formulate possible Einstein rings, estimating the angular radius of the rings to be approximately $37.6\mathrm{\mu as}$. Throughout this work, we present analytical solutions wherever feasible, and employ reliable approximations where necessary to provide comprehensive insights into the spacetime characteristics and their observable effects.

James Webb Space Telescope’s (JWST) observations since its launch have shown us that there could be very massive and very large galaxies, as well as massive quasars very early in the history of the Universe, conflicting expectations of the $\Lambda$CDM model. This so-called “impossibly early galaxy problem” requires too rapid star formation in the earliest galaxies than appears to be permitted by the $\Lambda$CDM model. In fact, this might not be a high masses problem, but a “time-compression problem”: time too short for the observed large and massive structures to form from the initial seeds. A cosmological model that could allocate more time for the earliest large structures to form would be more conforming to the data than the $\Lambda$CDM model. In this work we are going to discuss how the recently proposed $\gamma \delta$CDM model might ease and perhaps resolve the time-compression problem. In the $\gamma \delta$CDM model, different energy densities contribute to the Hubble parameter with different weights. Additionally, in the formula for the Hubble parameter, energy densities depend on the redshift differently than what their physical nature dictates. This new way of relating Universe’s energy content to the Hubble parameter leads to a modified relation between cosmic time and redshift. We test the observational relevance of the $\gamma \delta$CDM model to the age problem by constraining its parameters with the ages of the oldest astronomical objects (OAO) together with the cosmic chronometers (CC) Hubble data and the Pantheon+ Type Ia supernovae data of the late Universe at low redshift. We find that, thanks to a modified time-redshift relation, the $\gamma \delta$CDM model has a more plausible time period at high redshift for large and massive galaxies and massive quasars to form, whereas the age of the Universe today is not modified significantly.