Physics of the Dark Universe最新文献

Under the assumption of a linear $f\left(T\right)$ function, we present a new exact solution for an anisotropic and charged stars in the background of $f\left(T\right)$-gravity. By considering ansatz for the metric potential, charge function, and anisotropy function, we have arrived at an exact and nonsingular solution to the field problem. The anisotropy and charge function are discovered to be contingent upon the existence of the torsion scalar, and the evolution of the charge and anisotropy of the stellar matter is significantly influenced by variations in the torsion parameter ${\zeta }_1$. The features of anisotropy and charged compact star at the boundary are examined by making the interior metric solution correspond to the exterior Reissner-Nordström-de Sitter solution. The physical validity of the derived quantities is shown graphically, and the energy conditions are found to be satisfied. The causality condition, hydrostatic stable equilibrium, and adiabatic stability are also verified for the chosen values of the torsion parameter. The $M-R$ relations are analyzed for the charged and anisotropic stellar configurations in $f\left(T\right)$-gravity. It is found that increasing values of the torsion parameter ${\zeta }_1$ enhance the maximum mass of the star while increasing values of the charge parameter $q_0$ decrease the maximum mass. The influence of anisotropy via increasing the torsion parameter could be a probable interpretation of the maximum masses exceeding 2.5 $M_{\odot }$, as observed in the case of the secondary companion of GW190814. Interestingly, our study sheds light on the characteristic of non-singular solutions to the field equations in $f\left(T\right)$-gravity with linear $f\left(T\right)$.

Symmetric teleparallel $f\left(Q\right)$-gravity allows for the presence of a perfect fluid with a tilted velocity in the Kantowski–Sachs geometry. In this dipole model, we consider an ideal gas and we investigate the evolution of the physical parameters. The tilt parameter is constrained by the nonlinear function $f\left(Q\right)$ through the non-diagonal equations of the field equations. We find that the dynamics always reduce to the vacuum solutions of STEGR. This includes the Kasner universe, when no cosmological term is introduced by the $f\left(Q\right)$ function, and the isotropic de Sitter universe, where $f\left(Q\right)$ plays the role of the cosmological constant. In the extreme tilt limit, the universe is consistently anisotropic and accelerated. However, the final solution matches that of STEGR.

In Yukawa cosmology, a recent discovery revealed a relationship between baryonic matter and the dark sector. The relation is described by the parameter $\alpha$ and the long-range interaction parameter $\lambda$ - an intrinsic property of the graviton. Applying the uncertainty relation to the graviton raises a compelling question: Is there a quantum mechanical limit to the measurement precision of the Hubble constant ($H_0$)? We argue that the uncertainty relation for the graviton wavelength $\lambda$ can be used to explain a running of $H_0$ with redshift. We show that the uncertainty in time has an inverse correlation with the value of the Hubble constant. That means that the measurement of the Hubble constant is intrinsically linked to length scales (redshift) and is connected to the uncertainty in time. On cosmological scales, we found that the uncertainty in time is related to the look-back time quantity. For measurements with a high redshift value, there is more uncertainty in time, which leads to a smaller value for the Hubble constant. Conversely, there is less uncertainty in time for local measurements with a smaller redshift value, resulting in a higher value for the Hubble constant. Therefore, due to the uncertainty relation, the Hubble tension is believed to arise from fundamental limitations inherent in cosmological measurements. Finally, our findings indicate that the mass of the graviton fluctuates with specific scales, suggesting a possible mass-varying mechanism for the graviton.

We investigate the influence of boundary terms in the warm inflationary scenario, by considering that in the Einstein–Hilbert action the boundary can be described in terms of a Weyl-type geometry. The gravitational action, as well as the field equations, are thus extended to include new geometrical terms, coming from the non-metric nature of the boundary, and depending on the Weyl vector, and its covariant derivatives. We investigate the effects of these new boundary terms by considering the warm inflationary scenario of the early evolution of the Universe, in the presence of a scalar field. We obtain the generalized Friedmann equations in the Universe with a Weylian boundary by considering the Friedmann–Lemaitre–Robertson–Walker metric. We consider the simultaneous decay of the scalar field, and of the creation of radiation, by appropriately splitting the general conservation equation through the introduction of the dissipation coefficient, which can depend on both the scalar field, and the Weyl vector. We consider three distinct warm inflationary models, in which the dissipation coefficients are chosen as different functions of the scalar field and of the Weyl vector. The numerical solutions of the cosmological evolution equations show that the radiation is created during the very early phases of expansion, and, after the radiation reaches its maximum value, the transition from an accelerating inflationary phase to a decelerating one takes place. Moreover, it turns out that the Weyl vector, describing the boundary effects on the cosmological evolution, plays a significant role during the process of radiation creation.

We present a follow-up study of a subsolar black hole candidate identified in the second part of the third observing run of the LIGO-Virgo-KAGRA collaboration. The candidate was identified by the GstLAL search pipeline in the Hanford and Livingston LIGO detectors with a network signal-to-noise ratio of 8.90 and a false-alarm-rate of 1 per 5 years. It is the most significant of the three candidates found below the O3b subsolar mass false-alarm rate threshold of 2 per year, but still not significant enough above the background to claim a clear gravitational wave origin. A Bayesian parameter estimation of this candidate, denoted SSM200308, reveals that if the signal originates from a compact binary coalescence, the component masses are $m_1=0.62_{-0.20}^{+0.46}{M}_{\odot }$ and $m_2=0.27_{-0.10}^{+0.12}{M}_{\odot }$ (90% credible intervals) with at least one component being firmly subsolar, below the minimum mass of a neutron star. This discards the hypothesis that the signal comes from a standard binary neutron star. The signal coherence test between the two LIGO detectors is consistent with, but does not necessarily imply, a compact object coalescence origin.

A charged compact star is modelled within the framework of general relativity and electromagnetism to investigate the intricate complexities arising from charge accumulation through the accretion of surrounding charged baryonic matter, charged dark matter, or both. The Einstein–Maxwell field equations are solved for the compact star PSRJ0740+6620 in anisotropic regime by employing a Gaussian type density profile over a coherent background. A radially modulated exponential function is used as a seed ansatz for coherently connecting the class-one type metric. The structural stability and feasibility are then probed through physical bounds on stellar parameters at equilibrium. The key findings associated with charge accretion emphasised: (i). the existence of a transition zone close to mass $M\in [1.262,1.271]M_{\odot }$ and radius $R\in [12.497,12.505]$  km, indicating the formation of a core–shell type stellar structure (ii). the plane shifting of intrinsic force fields and (iii) the spin retardation, both suggest a non-vanishing spin–charge coupling between the stellar spin and the accreted charge, whether it be from baryonic matter, dark matter, or both.

In the framework of Einstein’s gravity, we study the thermodynamic equation state, $P=P(V,T)$, associated with a flat Friedmann–Lemaitre–Robertson–Walker (FLRW) universe. In this scenario, we consider the components of the dark sector as non-interacting fluids that dominate the universe’s energy content at late times. Under these circumstances, the functional structure of the cosmological coincidence parameter plays a relevant role in admitting first-order $P-V$ phase transitions; specifically, the dark energy density and the coincidence parameter must be given in terms of the radius of the apparent horizon.

The emergence of the Ultra-Diffuse Galaxies in recent years has posed a severe challenge to the galaxy formation models as well as the Extended Theories of Gravity. The existence of both dark matter lacking and dark matter dominated systems within the same family of astrophysical objects indeed requires the gravity models to be versatile enough to describe very different gravitational regimes. In this work, we study a non-local extension of the theory of General Relativity that has drawn increasing attention in recent years due to its capability to account for the late time cosmic acceleration without introducing any dark energy fluid. We leverage the kinematic data of three Ultra-Diffuse Galaxies: NGC 1052-DF2 and NGC 1052-DF4, which are dark matter lacking, and Dragonfly 44, which exhibits a highly dominant dark matter component. Our analysis shows that the non-local corrections to the Newtonian potential do not affect the kinematic predictions, hence no spoiling effects emerge when the Non-local Gravity model serves as a dark energy model. We additionally provide the minimum value that the characteristic non-local radii can reach at these mass scales.

In the present paper, we investigate the quasinormal modes of an Einstein–Maxwell power-Yang–Mills black hole in four dimensions, considering a specific value of the power parameter $p=1/2$. This particular case represents a black hole with both Abelian and Non-Abelian charges and is asymptotically non-flat. We begin by deriving the effective potential for both a neutral massless particle and a neutral Dirac particle using the aforementioned black hole solution. Subsequently, employing the sixth-order WKB approximation method, we calculate the (scalar) quasinormal modes. Our numerical analysis indicates that these modes are stable within the considered parameter range. This result is also confirmed using the eikonal approximation. Furthermore, we calculate the shadow radius for this class of BH and derive constraints on the electric and Yang–Mills charges ($Q,Q_{\mathrm{Y\; M}}$) by using imaging observational data for Sgr A${}^{\star }$, provided by the Event Horizon Telescope Collaboration. We observe that as the electric charge $Q$ increases, the allowed range shifts towards negative values of $Q_{\mathrm{Y\; M}}$. For instance, for the maximum value $Q\approx 1.1$ obtained, the allowed range becomes $-0.171\lesssim Q_{\mathrm{Y\; M}}\lesssim -0.087$ consistent with KECK and VLTI data, while still retaining a non-vanishing horizon.

This paper investigates the cosmological dynamics arising from the interaction between a 2-form field and cold dark matter within a Bianchi I background. Employing a dynamical system analysis, we identify two attractors yielding to exponential expansion of the Universe, i.e., de-Sitter solutions. Notably, these solutions exhibit a pivotal distinction: one is indistinguishable from the cosmological constant scenario, while the other corresponds to an anisotropic de-Sitter expansion sourced by the 2-form field. To validate the asymptotic behavior of our model, we conduct a numerical exploration of its expansion history. Our analysis reveals that the coupling between the dark sectors amplifies the shear during the matter-dominated epoch, offering a potential avenue to address certain observational discrepancies related to the structure formation process. Then, we constrain the parameter space of the model using recent observational datasets. Remarkably, we find that the current shear is precisely constrained to be approximately ${\Sigma }_0\approx -10^{-4}$. We also discuss some key differences in the expansionary dynamics sourced by the 2-form field compared to a 1-form field, i.e., a vector field, offering insights into their respective impacts on the support they provide to late-time anisotropy.