Physics of the Dark Universe最新文献

We study the dynamics and oscillations of electrically charged particles along stable circular orbits around a nonrotating electrically charged spacetime. In this work, we consider a regularized Reissner–Nordström (RN) spacetime under the transformation $r^2\to r^2+l^2$, known as the black-bounce-RN (or Simpson–Visser RN) spacetime, which can represent both a black hole and a wormhole depending on the values of $l$, in the presence of braneworld effects. In our analyses, we introduce a new parameter that shows a similar gravitational effect, which includes electric ($Q$) and tidal charges ($B$) of the spacetime as $b=Q^2+B$. We investigate the horizon properties of the spacetime and estimate the parametric distributions that describe a black hole and a wormhole. Furthermore, we analyze the effective potential of the particles and the critical angular momentum for the cases dominated by the tidal charge $b<0$ and the electric charge $b>0$, respectively. Moreover, we apply the epicyclic frequencies in the relativistic precession model to fit twin high-frequency (HF) quasiperiodic oscillations (QPOs) observed in microquasars and active galactic nuclei. Finally, we apply a Monte Carlo Markov Chain (MCMC) simulation to constrain the multidimensional parameter space for the microquasars GRO J1655-40 & GRS 1915-105 and the galactic center, for being a black hole and wormhole candidate using observational data from QPOs. Our findings suggest that the central object of the Milky Way and the microquasars GRS 1915-105 may be a black hole and a wormhole with certain parameters. However, there are no constraints for the microquasar GRO J1655-40 obtained regarding the wormhole case. It implies that the central object fails to be a candidate for being a wormhole in the black-bounce charged spacetimes in the braneworlds model.

We consider a double polytropic cosmological fluid and demonstrate that, when one constituent resembles a bare cosmological constant while the other emulates a generalized Chaplygin gas, a good description of the Universe’s large-scale dynamics is obtained. In particular, our double polytropic reduces to the Murnaghan equation of state, whose applications are already well established in solid state physics and classical thermodynamics. Intriguingly, our model approximates the conventional $\Lambda$CDM paradigm while reproducing the collective effects of logotropic and generalized Chaplygin fluids across different regimes. To check the goodness of our fluid description, we analyze first order density perturbations, refining our model through various orders of approximation, utilizing ${\sigma }_8$ data alongside other cosmological data sets. Encouraging results suggest that our model, based on the Murnaghan equation of state, outperforms the standard cosmological background within specific approximate regimes and, on the whole, surpasses the standard phenomenological reconstruction of dark energy.

In the present article, we uncover striking properties of traversable wormhole solutions generated under a newly developed $f(\Re ,L_m)$ gravitational theory. Two distinct dark matter density profiles—the Einasto spike and the Pseudo-isothermal model—are employed to demonstrate the existence of stable wormhole geometries with throat radii of 1.18 km and 1.41 km, respectively. Through a rigorous analytical and plot-based survey, we delve deeper into the exotic matter characteristics of these wormhole structures and analyze their matter content in the context of energy conditions. Impressively, both wormhole models exhibit an adiabatic index greater than 1.34 ($\Gamma >\frac{4}{3}$), indicating their inherent stability. What is more, our study of the anisotropy parameter reveals the attractive nature of these wormhole solutions, a crucial property for their potential traversability. This novel work not only broadens our understanding of modified gravity theories but also highlights the remarkable possibilities of traversable wormholes, opening up new avenues in theoretical physics and in the exploration of exotic spacetime geometries.

In this work we highlight an important perspective for the complete understanding of the stochastic gravitational background structure. The stochastic gravitational wave background is perhaps the most important current and future tool towards pinpointing the early Universe phenomenology related with the inflationary era and the subsequent reheating era. Many mysteries are inherent to the stochastic spectrum so in this work we highlight the fact that the complete understanding of early Universe physics and of astrophysical processes requires data from many distinct frequency band ranges. The combination of these data will provide a deeper and better understanding of the physics that forms the stochastic gravitational wave background, in both cases that it is of cosmological or astrophysical origin. We also discuss how the reheating temperature may be determined by combining multi-band frequency data from gravitational wave experiments and we also discuss how the shape of the gravitational wave energy spectrum can help us better understand the physical processes that formed it.

In this paper, we present asymptotically flat wormhole geometries in the context of $f\left(Q\right)$ gravity theory, where $Q$ is non-metricity scalar, by assuming a particular equation of state (EOS) $\rho =\Psi [P_r+(n-2)P_{\perp }]$, where $\Psi$ denotes a EOS parameter and $n$ is any integer. In this regard, we have established two different wormhole models by solving the EOS assuming the particular values for the parameter $n$, say for $n=1$ and $n=3$, and investigate the effect of EOS parameter on the wormhole geometries. The conducted analysis shows that the energy density is consistently positive for the wormhole models and the null energy condition is violated which indicates the presence of exotic matter. Furthermore, we have also developed a third model by assuming a specific shape function that satisfies all the necessary conditions of wormholes. Moreover, these solutions are found to be stable utilizing an equilibrium condition. Additionally, we go over the phenomenon of the complexity factor for all wormhole models and conclude that, for larger values of the radial coordinate, it approaches zero which shows the isotropic behavior of pressure and homogeneity of energy density.

We propose a type of k-inflation under the Hamilton–Jacobi approach. We calculate various observables such as the scalar power spectrum, the tensor-to-scalar ratio, the scalar spectra index for the case where the Hubble parameter is a power-law function of k-field. The model’s parameters are constrained with Planck data and the concrete form of the potential is presented. The results show that the model can be in good agreement with observations.

Ultradense dark matter halos (UDMHs) are high concentrations of dark matter, assumed to have formed deep in the radiation-dominated era from amplified primordial perturbations. In this work we improve the previous works for the calculation of UDMH abundance by elaborating on the formation process of these halos by including various physical and geometrical modifications in the analysis. In particular, we investigate the impact of angular momentum, dynamical friction and triaxial collapse on the predicted mass functions for UDMHs. We perform the calculations for four primordial power spectra with different amplified features that allow for primordial black hole and UDHM formation in wide and narrow mass ranges. We also apply this analysis in the context of two possible scenarios for dark matter: the single-component and the multi-component. We find that the abundance of UDMHs is prominently enhanced in the presence of these more realistic mass functions.

An exponential modified gravity with additional logarithmic corrections is considered with the presence of an axion-like scalar field in the role of dark matter. Axion fields are thought to become important at late-times when the axion-like scalar field oscillates around its vacuum expectation value, mimicking dark matter behaviour. The model is compared with the usual pressureless fluid description of dark matter. Both models are tested with observational data including some of the latest sources, providing similar fits in comparison with the $\Lambda$CDM model. Despite results are not statistically relevant to rule out any model, the number of free parameters still favours $\Lambda$CDM model, as shown by computing the goodness of the fits.

With its model-independent approach, cosmography has emerged as an invaluable tool for faithfully representing a wealth of astrophysical observations, free from theoretical biases. This study employs cosmography to investigate two innovative cosmological models, $B\left(z\right)$CDM models 1 and 2, which introduce novel corrections to the standard $\Lambda$CDM model, $B\left(z\right)=sin\left(z\right)$ and $B\left(z\right)=log(1+z)$, respectively. By analyzing a variety of observational datasets, including 31 measurements from cosmic chronometers ($CC$), 1048 data points from the $Pantheon$ dataset, 6 measurements from baryon acoustic oscillations ($BAO$), and data from cosmic microwave background ($CMB$) measurements, we derive constraints on model parameters and assess their compatibility with real-world observations. We find that the present values of the deceleration parameter for $B\left(z\right)$CDM models 1 and 2 are $q_0=-0.54_{-0.23}^{+0.24}$ and $q_0=-0.56_{-0.40}^{+0.45}$, respectively, for the combined datasets. The results reveal intriguing transitions in the expansion of the universe, confirming the presence of DE and quintessence-like behavior within the framework of $B\left(z\right)$CDM models.