The universal phase i^D+2 of the Euclidean de Sitter path integral obstructs a straightforward state-counting interpretation of the Gibbons–Hawking entropy. Building on Maldacena's proposal that specific black-hole observers can reorganize this phase, we derive a general constraint on when such 'real observers' can succeed. By distinguishing gravitational observers from topological spectators, we show that any sector whose infrared effective action is metric independent at the de Sitter saddle factorizes in the path integral, ${Z}_{\rm{tot}\,}={Z}_{\,\rm{grav}}^{(\,\rm{obs}\,)}{Z}_{\,\rm{top}\,}$, so the imaginary phase persists regardless of the sector's information-processing capabilities. Using confining SU(3) gauge theory and topological orders as examples, we demonstrate that an information-bearing clock is necessary but insufficient: only observers whose fluctuations share the negative modes of the conformal factor belong to the special class that can remove the de Sitter phase.

In this paper, we investigate the Sharma-Mittal Holographic Dark Energy (SMHDE) model within the framework of the recently proposed f(Q) gravity theory. Using the reconstruction technique, we derive the corresponding f(Q) functions for the SMHDE model under three different cosmological scale factors: power-law, intermediate, and unified scenarios. Furthermore, we conduct a comprehensive graphical analysis of these reconstructed models based on key cosmological parameters, including the equation-of-state (EoS) parameter, deceleration parameter, the $\omega -\omega ^{\prime} $ phase plane and statefinder parameters. The results indicate that all reconstructed models are dynamically stable and consistent with the current accelerated expansion of the Universe. Hence, the SMHDE model in f(Q) gravity provides a viable and consistent framework for describing the present dark energy phenomenon.

We investigate a class of spatially covariant vector field theories on a flat background, where the Lagrangians are constructed as polynomials of first-order derivatives of the vector field. Because Lorentz and U(1) invariances are broken, such theories generally propagate three degrees of freedom (DOFs): two transverse modes and one longitudinal mode. We examine the conditions under which the additional longitudinal mode is eliminated so that only two DOFs remain. To this end, we perform a Hamiltonian constraint analysis and identify two necessary and sufficient degeneracy conditions that reduce the number of DOFs from three to two. We find three classes of solutions satisfying these degeneracy conditions, corresponding to distinct types of theories. Type-I theories possess one first-class and two second-class constraints, type-II theories have four second-class constraints, and type-III theories contain two first-class constraints. The Maxwell theory is recovered as a special case of the type-III theories, where Lorentz symmetry is restored.

We model the inspiral and merger dynamics of two co-planar rings in Newtonian mechanics with GR motivated corrections and illustrate their similarity with those of black hole (BH) binary systems on the orbital plane. Our simulation reveals a banana-shape deformation of the ‘BHs' involved, and a typhoon-like spiral structure in the merger product. Using an eXact One-Body approach, we compute the full gravitational waveform of this process and qualitatively reproduce results consistent with those of numerical relativity (NR). Our simulation offers a transparent link between the feature of gravitational waveforms and the internal structure of BHs, thus a complementary interpretation of physics behind NR.

This paper investigates the perturbation dynamics of massless scalar and electromagnetic fields on magnetically charged de Sitter black holes within the framework of string-inspired Euler–Heisenberg (EH) gravity. We calculate the quasinormal frequencies (QNFs) and discuss the influences of black hole magnetic charge Q_m, the cosmological constant Λ, coupling parameter ε and multipole number l on QNFs, emphasizing the relationships between these parameters and quasinormal modes behavior. We find that the results obtained through the asymptotic iteration method are in good agreement with those obtained by the WKB method. Importantly, the Bernstein spectral method is employed as a rigorous cross-check for QNFs in the l = 0 scalar perturbation sector, where the WKB approximation is often unreliable. The greybody factors (GFs) are calculated using WKB method. The effects of the parameters Q_m and ε on the GF are also studied.

In this paper, using the ensemble-averaged theory, we define the thermodynamic free energy of Einstein–Gauss–Bonnet (EGB) black holes in anti-de Sitter (AdS) spacetime. This approach derives the gravitational partition function by incorporating non-saddle geometries besides the classical solutions. Unlike the sharp transition points seen in free energy calculated via saddle-point approximation, the ensemble-averaged free energy plotted against temperature shows a smoother behavior, suggesting that black hole phase transitions may be viewed as a small-G_N (Newton's gravitational constant) limit of the ensemble theory. This is similar to the behavior of black hole solutions in Einstein's gravity theory in AdS spacetime. We have obtained an expression for the quantum-corrected free energy for EGB-AdS black holes, and in the six-dimensional case, we observe a well-defined local minimum after the transition temperature which was absent in the earlier analysis of the classical free energy landscape. Furthermore, we expand the ensemble-averaged free energy in powers of G_N to identify non-classical contributions. Our findings indicate that the similarities in the thermodynamic behavior between five-dimensional EGB-AdS and Reissner–Nordström-AdS black holes, as well as between six-dimensional EGB-AdS and Schwarzschild-AdS black holes, extend beyond the classical regime.

Recent studies have demonstrated that AdS black holes possess the basic characteristics of a standard thermodynamic system. Concurrently, the thermodynamic properties of spacetimes featuring multiple horizons with distinct radiation temperatures have also attracted research interest. In this work, considering the high-order quantum electrodynamics correction, we initially establish an equivalent thermodynamic system for the coexistence region of black hole and cosmological horizons. On this basis, we conduct a detailed investigation into the thermodynamic properties of this dual-horizon coexistence region. Our results demonstrate that this equivalent thermodynamic system exhibits van der Waals-like thermodynamic behavior. Furthermore, we introduce a nonlinear parameter γ to analyze its impact on phase transitions within the equivalent thermodynamic system. Under specific conditions, the system undergoes first- or second-order phase transitions for γ = 0, and zeroth- or second-order phase transitions for γ ≠ 0. Finally, by utilizing the generalized off-shell Helmholtz free energy within the thermodynamic topological framework for black holes, we extend this methodology to investigate the topological properties of de Sitter (dS) spacetime. We compute the topological number characterizing the coexistence region of dual horizons in Euler–Heisenberg dS spacetime using equivalent thermodynamic state parameters. Additionally, we investigate the influence of the nonlinear parameter γ on the thermodynamic characteristics of the equivalent system.

This work focuses on particle production described by a nonminimally coupled model during inflation. In this model, three parameters determine the characteristic frequency and strength of the induced gravitational waves (GWs). Considering the impact of particle production on inflation, we identify the parameter values that generate the strongest GWs without violating the slow-roll mechanism at the cosmic microwave background (CMB) scale. However, even with such extreme parameters, the power spectrum of induced GWs is only about 0.3% of that of vacuum GWs. This contribution remains insignificant when identifying the primary source of the detected CMB B-mode polarization. Furthermore, when our results are integrated with the constraints driven by P + ACT + LB + BK18, the contribution of induced GWs at CMB scales becomes negligible. In contrast, their impact on the scalar spectral index n_s proves significant. For a range of parameter values, the Starobinsky inflation model yields predictions for n_s that are consistent with the measurements obtained from P+ACT+LB+BK18.

Recent measurements of the scalar spectral index n_s reported by the Atacama Cosmology Telescope (ACT) appear to be in tension with the predictions of many standard inflationary models. In this work, we show that constant-roll tachyon inflation can be compatible with the latest observational data. We consider the constant-roll condition with the slow-roll parameter η_V = 2V_,TT/V² being a constant. To be consistent with the 1σ constraints from the combined P-ACT-LB-BK18 data, the parameter η_V must lie within the range −0.016 < η_V < −0.0096. For consistency with the 2σ constraints, the allowed range is extended to −0.025 < η_V < 0.00063. These results indicate that constant-roll tachyon inflation provides a viable alternative to conventional models under the current observational constraints.

Primordial black holes (PBHs), a major candidate for dark matter, have been extensively constrained across most mass ranges. However, PBHs in the mass range 10¹⁷–10²¹ g remain a viable explanation for all dark matter. In this study, we use observational data from the Hard X-ray Modulation Telescope (Insight-HXMT) to refine constraints on PBHs within the mass range 2 × 10¹⁶–5 × 10¹⁷ g. Our analysis explores three scenarios: directly using observational data, incorporating the astrophysical background model, and employing the power-law spectrum with an exponential cutoff. Our research results indicate that although Insight-HXMT does not have an advantage in the first two scenarios, when considering the power-law model, its exceptional sensitivity in the hard X-ray regime and sufficiently high upper energy limit significantly strengthen the constraints on PBHs with masses greater than 10¹⁷ g compared to previous limits. Furthermore, the exclusion limit for PBHs as dark matter has reached 4 × 10¹⁷ g, which is comparable to the current threshold.

In this paper, we investigate the thermodynamic properties and geometries of phantom (A)dS F(R) black holes in the presence of recently proposed generalized entropy. For this purpose, we evaluate the heat capacity, Helmholtz free energy, Gibbs free energy, compressibility factor Z and the isothermal compressibility (κ_T) to analyze the local and global stability of this black hole. We also analyze the thermal geometries of this black hole, which provide useful insights. Furthermore, we present isotherms on the P-V diagram, illustrating phase behavior. It is interesting to mention here that generalized entropy provides a significant and useful impact on the results of thermal analysis of the aforementioned black holes.

Stimulated radiation and gravitational waves (GWs) are two of the most important predictions made by Albert Einstein. In this work, we demonstrate that stimulated GW radiation can occur within gravitational atoms, which consist of Kerr black holes and the surrounding boson clouds formed through superradiance. The presence of GWs induces mixing between different states of the gravitational atoms, leading to resonant transitions between two states when the GW wavenumber closely matches the energy difference. Consequently, the energy and angular momentum released from these transitions lead to the amplification of GWs, resulting in an exponential increase in the transition rate. Remarkably, the transitions complete within a period much shorter than the lifetime of the cloud. These stimulated transitions give rise to a novel GW signal that is strong and directed, distinguished from the previously predicted continuous GWs originating from clouds of ultralight bosons.

Using a model-independent Gaussian process (GP) method to reconstruct the dimensionless luminosity distance D and its derivatives, we derive the evolution of the dimensionless Hubble parameter E, the deceleration parameter q, and the state parameter w of dark energy. We utilize the PantheonPlus, SH0ES, and Gamma Ray Burst (GRB) data to derive the dimensionless luminosity distance D. Additionally, we employ observational H(z) data (OHD) and baryon acoustic oscillations (BAO) from the Dark Energy Spectroscopic Instrument (DESI) Data Release 2 (DR2) to obtain the first derivative of the dimensionless luminosity distance ${D}^{{\prime} }$. To obtain the reconstructed D and ${D}^{{\prime} }$, we utilize the fiducial value from each dataset, with particular emphasis on the varying H₀. According to the reconstruction results obtained from PantheonPlus+SH0ES+GRB+OHD and PantheonPlus+SH0ES+GRB+OHD+DESI data, we find that E is consistent with the predictions of the $\Lambda$CDM model at a 2σ confidence level within the redshift range of z < 2. However, the reconstruction results for q exhibit deviations from the $\Lambda$CDM model in the range of z < 0.3. Furthermore, we observe that the mean value of w exhibits evolving behavior, transiting from w < -1 to w > -1 around ${z}_{{\rm{wt}}}=0.46{4}_{-0.120}^{+0.235}$. Combining data from DESI DR2 can slightly enhance the accuracy of our constraints.

This study presents an analysis of cosmological parameters, focusing on resolving the Hubble tension and constraining neutrino masses within a coupled quintom model. By utilizing datasets from the cosmic microwave background (CMB), Pantheon+Analysis, cosmic chronometers, baryon acoustic oscillations, and CMB Lensing, we explore the interplay between cosmological parameters and observational constraints. The model effectively reduces the Hubble tension, achieving a consistency in H₀ measurements of 1.37σ and 1.24σ for the CMB+ALL dataset for Planck 2018 and R22, respectively. Additionally, the study refines constraints on the total mass of neutrinos (${{\rm{\Sigma }}m}_{\nu }$), with a finding of 0.115 eV for the CMB+ALL dataset. The analysis examines the effective equation of state parameter (w_eff), indicating a transition towards a Universe dominated by exotic energy forms. The combined datasets refine w_eff to -1.02 ± 0.018, underscoring the importance of multi-dataset integration in understanding dark energy dynamics. Furthermore, the interaction constant β between the quintom scalar field and neutrinos is constrained to 0.65 ± 0.12 for the CMB+ALL dataset. The potential parameters λ_σ = -2.09 ± 0.082 and λ_Φ = 2.43 ± 0.12 are also determined, providing insights into the quintom model's implications for cosmological dynamics. This study offers compelling evidence for the coupled quintom model's capability to resolve the Hubble tension and refine constraints on neutrino properties, enhancing our understanding of the Universe's evolution.

In this study, we explore the recently proposed f(R, A) gravity, a novel extension of modified gravity theories, where R represents the Ricci scalar and A denotes the anticurvature scalar. The addition of anticurvature term might play a role in softening or avoiding singularities, offering smoother transitions in regions of high curvature, such as near black holes or during cosmic inflation (Amendol et al 2020 Phys. Lett. B 811 135923). We also incorporate well-established cosmological bouncing scenarios, including the symmetric bounce, oscillatory bounce, matter bounce, little rip, and super bounce, to examine their implications within the framework of underlying gravity theory. In this context, we investigate some cosmic and thermodynamic aspects of flat FRW universe. We analyze the effective equation of state parameter ω_eff, which exhibits a transition from quintessence to phantom regimes across different bouncing cosmologies. The stability of the models is examined through the squared speed of sound parameter ${v}_{s}^{2}$, confirming stability in certain bouncing scenarios. The validity of the generalized second law of thermodynamics is verified by ensuring the positivity of total entropy production ${\dot{S}}_{\mathrm{tot}}$. Additionally, the ${\omega }_{\mathrm{eff}}-{\omega }_{\mathrm{eff}}^{{\prime} }$ plane exhibits the thawing as well as the freezing regions. Our findings demonstrate that the chosen bouncing models provide viable cosmic and thermodynamic behavior, supporting their relevance in modified gravity theories.

Our analysis is particularly motivated by its relevance to understanding compact object instabilities, gravitational collapse thresholds, and the formation of dense structures under the influence of modified gravity theories. The interplay of anisotropic pressures, perturbative dynamics, and modified gravity contributions offers insight into both the stable configuration of dense fluids and the mechanisms leading to dynamical instability. Such considerations directly contribute to the aims of high energy density profiles, particularly in modeling physical systems where extreme pressure, curvature, and matter interactions co-exist. We consider an axially symmetric, dense structure with anisotropic matter content and employ a specific equation of state (EoS) to examine the interplay between static and dynamic quantities via the adiabatic index. To address the complex dynamics of the collapse process, a perturbative scheme is utilized under Newtonian and post-Newtonian approximations, enabling a detailed examination of the stability and structural evolution of the system under the influence of the considered minimally coupled gravity. Our results demonstrate that hydrostatic equilibrium is maintained when effective pressure, gravitational, and anti-gravitational forces are balanced, while deviations from this balance initiate dynamical instability. Graphical representations of stable and unstable regimes are presented, revealing how the choice of gravity functions significantly affects the outcome. This work provides insight into the behavior of dense, self-gravitating configurations under modified gravity, offering broader implications for the modeling of compact astrophysical objects and contributing to the understanding of gravitational collapse in energy density regimes.

We search for the stochastic gravitational-wave background (SGWB) predicted by pre-Big-Bang (PBB) cosmology using data from the first three observing runs of Advanced LIGO and Advanced Virgo. PBB cosmology proposes an alternative to cosmic inflation where the Universe evolves from a weak-coupling, low-curvature state to the hot Big Bang through a high-curvature bounce phase, predicting a distinctive SGWB spectrum. We perform a Bayesian analysis of the cross-correlation data to constrain the model parameters characterizing the PBB spectrum. We find no evidence for a PBB-induced SGWB, with a Bayes factor of 0.03 between the PBB and noise-only model, strongly favoring the noise-only hypothesis. Our analysis establishes a lower bound β ≳ −0.19 at 95% confidence level, which is compatible with the theoretical requirement β ≥ 0 for a smooth bounce transition. While we do not detect a signal, our constraints remain consistent with the basic theoretical framework of PBB cosmology, demonstrating the potential of gravitational-wave observations to test early Universe theories.

In this study, we explore a spherically symmetric charged black hole (BH) with a negative cosmological constant under the influence of a Kalb–Ramond field background. We compute the photon sphere and shadow radii, validating our findings using observational data from the Event Horizon Telescope, with a particular emphasis on the shadow images of Sagittarius A^*. Furthermore, we investigate the greybody factors, emission rate, and partial absorption cross section. It is shown that the Lorentz-violating parameter $\bar{l}$ has an important effect on the absorption cross section. Our analysis also includes an examination of the topological charge, temperature-dependent topology, and generalized free energy. In particular, we regard the AdS charged BH with an antisymmetric tensor background as a topological defect in the thermodynamic space, then the system has the same topological classification to the charged Reissner–Nordström–AdS BH.

We perform the manifestly covariant quantization of f(R) gravity in the de Donder gauge condition (or harmonic gauge condition) for general coordinate invariance. We explicitly calculate various equal-time commutation relations (ETCRs), in particular the ETCR between the metric and its time derivative, and show that it has a nonvanishing and nontrivial expression, whose situation should be contrasted to the previous result in higher-derivative or quadratic gravity where the ETCR was found to be identically vanishing. We also clarify global symmetries, the physical content of f(R) gravity, and clearly show that this theory is manifestly unitary and has a massive scalar and massless graviton as physical modes.

This study examines the effect of charge on physical features of a gravastar model in the framework of Rastall gravity. A gravastar is an alternative model to a black hole consisting of three separate regions: the inner sector, the intermediate shell and the outer sector. Different values of the barotropic equation of state (EoS) parameter provide the mathematical basis for these regions. Field equations (FEs) are initially developed for a spherically symmetric spacetime coupled with charged matter distribution. We then use the temporal component of Tolman IV spacetime to formulate the radial metric potential for both the inner region and intermediate shell. We also apply the matching criteria to ensure smooth matching of exterior and interior spacetimes so that the constants resulting from integrations can be determined. Afterwards, we explore various physical properties of the developed gravastar model such as the proper length, entropy, energy, and others to analyze how shell thickness and charge affect them. It is concluded that, in the background of Rastall theory, a gravastar model exists and serves as a viable alternative to the black hole.

In this study, we present a comprehensive analysis of a modified Frolov black hole (BH) model that incorporates two types of topological defects, a global monopole (GM) and a cloud of strings (CS). This composite BH solution is examined from multiple theoretical perspectives to explore the impact of these modifications on the BH’s geometric, thermodynamic and dynamical properties. We begin by studying the geometrical optics of the spacetime, focusing on the motion of null geodesics. Key features, such as the effective potential, photon sphere, the force acting on photons and the stability of circular photon orbits, are analyzed in detail. Our results show that the presence of GM and CS significantly affects the spacetime geometry and photon dynamics. In addition, the thermodynamic behavior of the modified BH is also investigated. We derive essential quantities such as the Hawking temperature and entropy, demonstrating how the inclusion of GM and CS leads to deviations from the standard thermodynamic relations observed in classical BH solutions. These deviations may offer valuable insights into quantum gravity and the role of topological defects in BH physics. Furthermore, we examine the BH shadow as an observational signature of the underlying geometry. Our analysis shows that the Frolov parameter tends to reduce the apparent size of the shadow, while the presence of topological defects, particularly GM and CS, enlarges it. In addition, we investigate the perturbative dynamics of the BH by studying both scalar (spin-0), fermionic (spin-1/2) and electromagnetic (spin-1) fields through the massless Klein–Gordon and Maxwell equations, respectively. Using the Wentzel–Kramers–Brillouin approximation, we compute the quasinormal modes (QNMs) for scalar and electromagnetic field perturbations. The results confirm the stability of the BH under small perturbations and show that the QNM frequencies and damping rates are strongly influenced by the Frolov parameter, electric charge, GM and CS.

We show that in Schwarzschild equivalent mediums, the massless spin particles obey the same dynamical equation, from which we obtain remarkably simple formulae for the frequencies of the quasibound states. We find that the quasibound frequencies of different bosons can be identical at the same quantum number l, and the same is true of different fermions, but a quasibound frequency for bosons can never equal a quasibound frequency for fermions. These results mean that in Schwarzschild equivalent mediums with the quasibound-state boundary conditions, characteristics of electromagnetic waves are the same as those for all the massless bosonic waves, thereby allowing electromagnetic waves to simulate gravitational waves. Our predictions can be tested in future experiments, building upon the successful preparation of Schwarzschild equivalent mediums.

This study explores asymptotically flat wormhole solutions within the framework of f(R, T) gravity. We analyze f(R, T) expressed as f(R, T) = R + λT + λ₁T². A linear equation of state (EoS) is employed for both radial and lateral pressures, resulting in a power-law shape function. The investigation encompasses solutions characterized by both negative and positive energy densities. It has been determined that solutions with positive energy density comply with all energy conditions, specifically the null, weak, strong, and dominant energy conditions. Additionally, we identify constraints on the parameters λ, λ₁, and the parameters associated with the EoS and shape function.

A semiclassical particle moving near the horizon of a Schwarzschild black hole is chaotic, and its Lyapunov exponent saturates the chaos bound proposed by Maldacena, Shenker, and Stanford, with the temperature being the Hawking temperature. Motivated by this, we consider the Lyapunov exponents of scalar and spinor fields in Schwarzschild spacetime by calculating their out-of-time-ordered commutators along the radial direction. Numerically, we find that the Lyapunov exponent of the scalar field is smaller than that of the spinor field. They are mainly contributed by the bound states near the horizon and lie below the chaos bound.

Based on the idea of treating the anti de Sitter (AdS) radius as a fixed parameter, we study the thermodynamics and topology of d-dimensional charged AdS black holes in the restricted phase space utilizing Visser’s holographic approach. For the charged black hole with a cloud of strings and quintessence in the higher-dimensional spacetimes with d = (4, 5, 6), we demonstrate that the topological number remains invariant within the same canonical ensemble; however, a distinct topological number emerges in the grand canonical ensemble for the same black hole system. Notably, these results are independent of the dimension d and other related parameters. The formalism known as restricted phase space thermodynamics is checked in detail and some interesting thermodynamic behavior is revealed in the example case of d-dimensional charged AdS black holes with a cloud of strings and quintessence. This research lays the foundation for establishing a universal framework of restricted phase space thermodynamics and investigating its fundamental thermodynamic properties.

Cosmic inflation is one of the most important paradigms in modern cosmology. In its simplest form, inflation is driven by a single inflaton field. However, multi-field inflation has become increasingly attractive because it can solve many theoretical and observational challenges. In this paper, we propose a particular model involving two axion-like fields with simply monodromy-dominated potentials. We demonstrate that this model is consistent with current cosmological observations.

In this work we study gravitational lensing of the wormhole in the Eddington-inspired Born–Infeld (EiBI) spacetime that incorporates with a cosmic string. It is found that the presence of a cosmic string can enhance the light deflection in the strong-field limit, compared to the Ellis–Bronnikov wormhole. The magnification effects of this composite structure could cause some substantial impacts on the angle separation between the first and the rest of the images, and their relative brightness. Furthermore, based on these observables, we model some observable aspects in the strong- and the weak-field limits. The presence of a cosmic string can affect some distinguishable observables compared to the wormhole without cosmic string. This work could deepen our understanding of the spacetime structure of the wormhole in EiBI spacetime with one-dimensional topological defects.

Verlinde's emergent gravity (VEG) posits that gravity arises as an emergent phenomenon rooted in the entropic properties of spacetime, challenging the traditional view of gravity as a fundamental force. Building on this paradigm, recent developments have introduced a novel class of black holes within the VEG framework, revealing intriguing connections between apparent dark matter effects and the distribution of baryonic matter. In this study, we delve into the observational signatures of a Simpson–Visser (SV) Minkowski core regular black hole in VEG, focusing on its shadow images and intensity profiles. Our analysis highlights the profound influence of model parameters, including A (governing baryonic matter distribution), B (strength of interaction between apparent dark matter and baryonic matter), and n (characterizing diverse spacetime geometries), on the effective potential and observable properties. Notably, we find that the modifications introduced by these parameters lead to distinct changes in the black hole's shadow size and intensity distribution. Comparing our results to the Reissner–Nordström (RN) black hole, we uncover a striking reduction in the apparent shadow size and an enhancement in intensity for the SV solution in VEG.

Using the Melnikov method, the phenomenon of thermal chaos under periodic perturbation in the extended phase space of the modified thermodynamics of Kerr-AdS black holes is investigated. On the (P, v) section in the extended phase space, it is shown that temporal chaos will appear in the unstable spinodal region when the perturbation amplitude is larger than critical value ${\delta }_{c}^{Pv}$. We find ${\delta }_{c}^{Pv}$ is monotonically decreasing with respect to the angular momentum parameter a, which implies a large a leads to chaotic behavior more easily under time-periodic thermal perturbation. Similarly, on the $({\widehat{{\rm{\Omega }}}}_{H},J)$ section, we show there exists a critical value ${\delta }_{c}^{{\rm{\Omega }}J}$ which depends on the cosmological parameter $l=\sqrt{-3/{\rm{\Lambda }}}$. When the perturbation amplitude exceeds ${\delta }_{c}^{{\rm{\Omega }}J}$, temporal chaos occurs. As l increases, chaos becomes easier. For spatial perturbation, chaos always exists irrespective of perturbation amplitude in both the (P, v) section and $({\widehat{{\rm{\Omega }}}}_{H},J)$ section.

In the framework of general relativity (GR), gravitational waves (GWs) travel at the speed of light across all frequencies. However, massive gravity and weak equivalence principle (WEP) violation may lead to frequency-dependent variations in the propagation speed of GWs, which can be examined by comparing the theoretical and observed discrepancies in the arrival times of GW signals at various frequencies. This provides us with an opportunity to test these theories. For massive gravity, we consider that gravitons may have a nonzero rest mass. For WEP violations, we hypothesize that different massless particles exposed to the same gravitational source should exhibit varying gravitational time delays. The gravitational time delay induced by massive gravitational sources is proportional to γ + 1, where the parameter γ = 1 in GR. Therefore, we can quantify these two deviations using phenomenological parameters m_g and ∣Δγ∣, respectively. In this study, we use selected GW data from binary black hole coalescences in the LIGO-Virgo catalogs GWTC-2.1 and GWTC-3 to place constraints on the parameters m_g and ∣Δγ∣. We also compute Bayes factors for models that assume the existence of graviton mass and WEP violation compared to the standard GW model, respectively. The absolute value of the natural logarithm of the Bayes factor is generally less than two. Our analysis reveals no significant preference for either model. Additionally, the Bayes factors between these two models do not provide obvious evidence in favor of either one.

Gravitational collapse and bubble evolution in the asymptotic Friedmann–Lemaître–Robertson–Walker (FLRW) Universe is an intriguing and intricate problem. We systematically analyze the dynamics of contact Schwarzschild–FLRW (McVittie) spacetimes, focusing on their general junction conditions and introducing a novel function to simplify the extrinsic curvature and surface stress–energy tensor. Both static and dynamic scenarios are explored, including special cases such as Schwarzschild, FLRW, and Einstein–Straus configurations using our general framework. Numerical calculations further investigate the evolution of concentric McVittie spacetimes under various initial conditions, incorporating Λ-CDM cosmological models to better reflect realistic cosmic backgrounds. These results offer deep insights into the interplay between the McVittie mass parameter, initial peculiar velocity, and the influence of dark energy, providing a unified perspective for understanding gravitational collapse and bubble evolution in cosmology and astrophysics.

This research paper seeks to investigate the characteristics of almost Riemann solitons and almost gradient Riemann solitons within the framework of generalized Robertson–Walker (GRW) spacetimes that incorporate imperfect fluids. Our study begins by defining specific properties of the potential vector field linked to these solitons. We examine the potential vector field of an almost Riemann soliton on GRW imperfect fluid spacetimes, establishing that it aligns collinearly with a unit timelike torse-forming vector field. This leads us to express the scalar curvature in relation to the structures of soliton and spacetime. Furthermore, we explore the characteristics of an almost gradient Riemann soliton with a potential function ψ across a range of GRW imperfect fluid spacetimes, deriving a formula for the Laplacian of ψ. We also categorize almost Riemann solitons on GRW imperfect fluid spacetimes into three types: shrinking, steady, and expanding, when the potential vector field of the soliton is Killing. We prove that a GRW imperfect fluid spacetime with constant scalar curvature and a Killing vector field admits an almost Riemann soliton. Additionally, we demonstrate that if the potential vector field of the almost Riemann soliton is a ν(Ric)-vector, or if the GRW imperfect fluid spacetime is ${{ \mathcal W }}_{2}$-flat or pseudo-projectively flat, the resulting spacetime is classified as a dark fluid.

Light sub-GeV dark matter (DM) particles up-scattered by high-energy cosmic rays (CRs) (referred to as CRDM) can be energetic and become detectable by conventional DM direct detection experiments. Nevertheless, current CRDM theoretical frameworks remain limited by model-dependent parameterizations, whereas the effective operators provides a model-independent computing framework. In this work, we systematically investigate the general relativistic DM-nucleus spin-independent interactions. We first construct effective operators for dark matter with spin up to two, i.e. spin-1/2 fermionic DM (χ), the scalar DM (φ), the vector DM (V_μ), spin-3/2 fermionic DM ($\Psi$) and spin-2 DM (T^μν). We then derive the CRDM flux and the nuclear recoil event rate based on these operators, and employ nuclear recoil data from the LUX-ZEPLIN (LZ) experiment to constrain all effective operators. We set stringent constraints on the CRDM-nucleon scattering cross section for sub-GeV DM. Especially, our results show that the exclusion limits from the spin-2 T^μν operator differ by as much as ten orders of magnitude from those calculated using constant cross section.

The chiral gravitational wave background (GWB) can be produced by axion-like fields in the early universe. We perform parameter estimation for two types of chiral GWB with the LISA-Taiji network: axion-dark photon coupling and axion-Nieh–Yan coupling. We estimate the spectral parameters of these two mechanisms induced by the axion and determine the normalized model parameters using the Fisher information matrix. For highly chiral GWB signals that we choose to analyze in the mHz band, the normalized model parameters are constrained with a relative error less than 6.7% (dark photon coupling) and 2.2% (Nieh–Yan coupling) at the one-sigma confidence level. The circular polarization parameters are constrained with a relative error around 21% (dark photon coupling) and 6.2% (Nieh–Yan coupling) at the one-sigma confidence level.

We consider the recently developed black hole in massive Einstein-dilaton gravity including the coupling of the dilaton scalar field to massive graviton terms. This model has different horizon structures such as event horizons and inner horizons depending on the values of certain parameters. These variations influence how the black hole interacts with its surroundings. We utilize the well-known Novikov–Thorne model to investigate the thin accretion disks into this interesting model. Our research shows a crucial correlation between the dynamics of the accretion disk and the parameters of dilatonic black holes in dilaton-massive gravity. We observe that dilaton-massive gravity leads to significant contraction and outward expansion. We offer a detailed analysis of accretion by examining both direct and secondary images at various radial distances and observation angles.

This study investigates the key characteristics of compact star configurations within the framework of Rastall’s theory of gravity, employing the Krori–Barua ansatz. By forming the field equations for a spherically symmetric line element with an isotropic matter source through Krori–Barua metric potentials, we derive the modified Tolman–Oppenheimer–Volkov equation. This equation is crucial for studying the mass–radius function, the compactness factor, and the surface redshift. Additionally, we examine various physical aspects, including energy density, pressure evolution, equation of state, adiabatic index, and stability analysis, to assess the model’s viability. Rastall’s theory, which extends general relativity by relaxing the conservation of energy and momentum, plays a central role in our analysis, particularly in understanding the enhanced stability of compact stars. Our results provide strong evidence that within Rastall’s gravitational framework, the proposed stellar structures exhibit significant stability, suggesting that this theory may offer new perspectives on the behavior of such stars.

Understanding the nature of dark matter remains one of the most enigmatic and unresolved issues in astrophysics. Certain theoretical models address this by introducing a novel component to account for dark matter. In this study, we propose a new scalar field derived from string T-duality, where its associated density represents the density of the surrounding matter field, in the spherically symmetric and static medium. Our exploration reveals that this scalar field behaves as the baryonic fluid, characterized by a positive effective state equation, ${\omega }_{{\rm{e}}}\gt 0$. Furthermore, a detailed investigation demonstrates that this model satisfies all energy conditions beyond the event horizon of a central black hole. Considering the light deflection and radar echo delay suggests that in this scalar field, the dark matter grows up in the halo and surrounding regions of galaxy systems. This indicates that dark matter accumulates as an effective field outside the observable regions of galaxies.

In this work, the phase structure of a holographic s+d model with quartic potential terms from 4D Einstein-Gauss-Bonnet gravity is studied in the probe limit. We first show the q_d-μ phase diagram with a very small value of the Gauss-Bonnet coefficient α=1 × 10^-7 and in the absence of the quartic terms to locate the suitable choice of the value of q_d, where the system admits coexistent s+d solutions. Then we consider the various values of the Gauss-Bonnet coefficient α and present the α-μ phase diagram to show the influence of the Gauss-Bonnet term on the phase structure. We also give an example of the re-entrant phase transition which is also realized in the holographic s+s and s+p models. After that we confirm the universality of the influence of the quartic term with coefficient λ_d on the d-wave solutions, which is similar to the case of s-wave and p-wave solutions previously studied in the s+p model. Finally we give the dependence of the special values of the quartic term coefficient λ_d on the Gauss-Bonnet coefficient α, below which the d-wave condensate grows to an opposite direction at the (quasi-)critical point, which is useful in realizing first order phase transitions in further studies of the holographic d-wave superfluids.

In this paper, we present the second post-Newtonian solution for the quasi-Keplerian motion of a test particle in the regular Simpson-Visser black-bounce spacetime which has a bounce parameter a. The obtained solution is formulated in terms of orbital energy, angular momentum, and the bounce parameter of the black hole. We explicitly analyze the leading effects of the bounce parameter which has dimensions of length, on the test particle's orbit, including the periastron advance and orbital period. Then, we apply this model to the precessing motion of OJ 287 and determine the upper limits of the dimensionless bounce parameter as a/m=3.45 ± 0.01, where m is the mass of the regular black hole. Compared with the bound given by the periastron advance of star S2, our bound on a/m is reduced by one order of magnitude, although our upper limit of a still needs further improvement.