Solitonic structures in a relativistic thermodynamical fluid spacetime and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si3.svg" display="inline" id="d1e22"> <mml:mrow> <mml:mi>f</mml:mi> <mml:mrow> <mml:mo>(</mml:mo> <mml:mi>r</mml:mi> <mml:mo>,</mml:mo> <mml:mi>T</mml:mi> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> -gravity

The biological cosmological constant ΛB: Exploratory propensity, dynamical habitability and the geometric origin of life.

This article is a new completed version of EJAS 11804 (Vol. 10, N°1, Feb 25, 2022). It presents in a synthetic way three articles published in JHEPGC ([1], [2]), EJAS [12] and develops some aspects. Two hypotheses are studied. In the first one, the vacuum is endowed with a quantum structure in which the vacuum particles are Friedmann-Planck micro-universes. For this, the article introduces a quantization of a closed Friedmann universe, then a quantization of the photon spheres filling this universe. This approach gives a numerical value consistent with cosmological measurements for the current dark energy density and the cosmological constant of our Universe. Next, the second hypothesis takes the content of a model published in Physics Essays in 2013 [3], assuming that elementary particles are Schwarzschild photon spheres; these could be derived from the Friedmann photon spheres composing the vacuum particles. It is further recalled that the model presents au nified structure of elementary particles and allows us to calculate the value of the elementary electric charge and the fine structure constant. The masses of some elementary particles are calculated in a complementary model. Finally, this article summarizes a model of closed cyclic universe described in reference [2].This universe begins as one alone Friedmann-Planck micro-universe, then multiplying to constitute our Universe. Further a Big-Rip suddenly transforms it into a Friedmann-Planck macro-universe on a much larger scale. This one is the beginning of a new Big-Bang with the same evolution as ours. This process can be assumed to explain the existence of the initial FP micro-universe: it would be the result of a Big-Rip at the end of the evolution of a much smaller scale universe.

Abstract Dark energy(DE) remains one of the most important topics in modern cosmology, and its physical origin is still under intensive discussion. In this study, we explore a new model of DE induced by black holes, in which cosmic accelerated expansion is caused by de-Sitter like space-time regions around the non-singular black holes. It is difficult to examine such a phenomena by measuring black hole mass because the energy density of the cosmological constant is much smaller than the mass density of a black hole near a black hole. On the other hand, this modification becomes dominant on the cosmological scale. Therefore, we focus on the cosmological probes and perform the MCMC analysis using Planck2018+DESI DR2+Supernovae. Since the total amount of DE density depends on the contributions of all black holes, we use the simulated results for the evolution of the number of black holes. As a result, we obtain the best-fitted total chi-squared value, χ 2 total = 2871.13 compared to ΛCDM case χ 2 total, ΛCDM = 2819.00, and ∆χ 2 ∼ 50. We conclude that this ∆χ 2 is enough large to rule out this model, because the number of parameters is same between this model and ΛCDM.

The recent DESI BAO measurements have revealed a potential deviation from a cosmological constant, suggesting a dynamic nature of dark energy. To rigorously test this result, complementary probes such as weak gravitational lensing are crucial, demanding highly accurate and efficient predictions of the nonlinear matter power spectrum within the w 0 wa CDM framework. However, most existing emulators fail to cover the full parameter posterior from DESI DR2+CMB constraints in the w 0-wa plane. In this work, we extend the spectral equivalence method outlined in Casarini et al. 2016 JCAP 08 (2016) 008 to use auxiliary w 0 wa CDM models for approximating the power spectrum of a target w 0 wa CDM cosmology, moving beyond the previous use of wCDM auxiliaries. Incorporating this enhanced module, the extended CSST Emulator achieves a prediction accuracy of ≤ 1% over the 1σ confidence region from DESI DR2+CMB constraints for z ≤ 3, with a mild degradation in accuracy outside this posterior region. This performance is rigorously validated by additional simulations of dynamic dark energy cosmologies. The emulator's applicable parameter space has been generalized to fully encompass the 2σ region, greatly enhancing its utility for cosmological analysis in the post-DESI era.

In the last years with the increasing precision in cosmological observations we have been able to establish a standard model of cosmology, the so-called ΛCDM, but also find some tensions between cosmological probes that are difficult to explain within the context of this model. We tested several phenomenological extensions of the ΛCDM with the newest datasets from the chain CMB+BAO+SNIa, to see whether they are able to alleviate the aforementioned tensions. We find that when the updated version of the Planck CMB likelihood (PR4 LoLLiPoP and HiLLiPoP ), with respect to the more used likelihoods (PR4 CamSpec and PR3), is considered, the lensing anomaly is reduced, and the preference for A L > 1 and Ω k < 0 is less significant. From the CMB+BAO+SNIa dataset, in the context of the parameterization w 0 w a CDM, we find a preference for a time-evoling dark energy over the rigid cosmological constant which is consistent with the most recent results from DESI collaboration.

In this paper, we have discovered a new avenue of fibre inflation in perturbative large volume scenario (pLVS) due to the redefinition of the base modulus. pLVS offers a novel regime where large volume of the internal space is guaranteed without the need of non-perturbative effects. In this setup, we study the possibility where a base redefinition allows to assess different versions of fibre inflation whose spectral index aligns with Atacama Cosmology Telescope (ACT) data JCAP 11 (2025) 062, JCAP 11 (2025) 063 and produces tensor-to-scalar ratio in the range 0.008 ≲ r ≲ 0.01 in different setups we have considered. The leading order flat direction — which in our case is the fibre modulus — is lifted with the combinations of string loop corrections, leading order α'3 R 4-correction, higher derivative F 4 corrections as well as our new ingredient redefinition of the modulus. Since recent Dark Energy Spectroscopic Instrument (DESI) results appear to favour a dynamical explanation for late-time acceleration over a simple cosmological constant, exploring quintessence offers a more suitable approach. In this lore, we also examine the quintessence sector to complete our model and account for both early- and late-time cosmic acceleration. In this framework, the poly-instanton correction generates a potential along the axionic directions, and we find that the resulting quintessence behaviour and the subsequent cosmological predictions about dark matter closely resemble the predictions of the original fibre inflation scenario studied earlier.

3-forms are natural candidates for describing the late-time accelerated expansion of the Universe, as they can inherently reproduce a positive cosmological constant when lacking an evolving potential. When such a potential is present, a 3-form field may exhibit either quintessence-like or phantom-like behaviour. In this paper, we consider a late-time effective dark energy model described by a 3-form with a Gaussian potential, stable during the dark-energy-dominated era. We constrain this model observationally by performing a Markov Chain Monte Carlo (MCMC) analysis employing a comprehensive cosmological dataset, including Planck PR4 cosmic microwave background (CMB) data, DESI DR1 baryon acoustic oscillation (BAO) measurements, Pantheon+ Type Ia supernovae data, low-z Cepheid calibrators, and DES Y1 large-scale structure observations. We demonstrate that the 3-form model successfully increases the predicted Hubble parameter of CMB and BAO data from 67.89±0.36 km/s/Mpc of ΛCDM model to 68.29+0.56 -0.61 km/s/Mpc by approaching the potential peak at the right time, thus mildly reducing the tension with the late-time observation. Overall, the 3-form field serves as a promising candidate of phantom-like dark energy from both theoretical and observational points of view.

Abstract We develop the covariant phase space formulation of Weyl-transverse gravity (WTG) in the presence of general timelike and spacelike boundaries. WTG is classically equivalent to General Relativity (GR) but possesses a reduced gauge symmetry consisting of Weyl transformations and transverse diffeomorphisms, together with a fixed background volume form. This structure modifies the variational principle and the definition of conserved quantities relative to GR.We derive the symplectic potential, presymplectic current, and Hamiltonian generators associated with transverse diffeomorphisms, and we identify a set of boundary conditions under which the WTG action is differentiable. These include Dirichlet and Neumann conditions for both the auxiliary Weyl-invariant metric and the dynamical metric, as well as a natural implementation of York boundary conditions, for which WTG exhibits a particularly transparent geometric formulation.We obtain the Noether current and surface charge, clarify the role of the Lagrangian ambiguity related to the cosmological constant, and evaluate the Hamiltonian identity on spacetimes containing a bifurcate Killing horizon. The resulting first-law relation shows that variations of the cosmological constant can contribute nontrivially unless additional physical restrictions are imposed.

Abstract We explore how quantum properties of spacetime—specifically the curvature of momentum space—can backreact on classical gravity within a tractable semiclassical $(2+1)$-dimensional framework with negative cosmological constant. Motivated by quantum-gravity scenarios, we investigate how Planck-scale modifications of particle kinematics influence both dynamics and gravitational solutions. Starting from a first-order action, we derive an effective configuration-space description and show that particle trajectories remain geodesic, preserving the weak equivalence principle despite the underlying deformation. Coupling this modified matter sector to Einstein gravity, we obtain a deformed BTZ black hole solution. Remarkably, the local geometric structure and thermodynamic relations retain their standard form, while all quantum-gravity effects are encoded in a nonlinear mapping between the microscopic mass parameter and the ADM mass. This induces a renormalization of the horizon radius and thermodynamic quantities without altering their functional dependence. As a concrete observable consequence, we compute corrections to the return time of massless probes traveling along null geodesics between the horizon and the $AdS_3$ boundary. Our results demonstrate that Planck-scale kinematic effects can leave controlled and potentially measurable imprints on classical geometry, providing a clear and consistent bridge between quantum gravity ideas and semiclassical observables.

A simple form of the gravitational field for a radiating relativistic star with nonzero shear is studied. The junction conditions at the surface of the star reduce to an autonomous nonlinear first order differential equation of the Riccati type: it admits exact solutions and is amenable to a phase plane analysis. We can determine the temporal evolution and asymptotic behaviour of the radiating star including shear, charge and the cosmological constant. The presence of shear leads to behaviour which is distinct from shear-free models and fold bifurcations arise; the model collapses from an initial static configuration or a superdense cold star forms. The areal distance plays a central role in the asymptotic analysis and there is an interplay with the charge and the cosmological constant.

Abstract We investigate the implications of the behavior of geodesics in static, cylindrically symmetric spacetimes with a non-zero cosmological constant. We consider the symmetries of these spacetimes to restrict admissible ranges of the metric parameters and to formulate an intuitively plausible interpretation of the coordinates.

We consider various possible consequences of time-varying dark energy due to a quintessence scalar field whose energy density is partially converted to particles as the field evolves down its potential. This particle production acts as a source of thermal friction on the field that can make it difficult to distinguish whether dark energy is due to a radiating field rolling down a steep potential, a purely self-interacting field moving down a flatter potential, or a cosmological constant. By reducing the acceleration of the scalar field, thermal friction increases the amount of accelerated expansion and can cause a sizable bump in the quintessence equation of state. We take special interest in the case where a steep potential rapidly changes from positive to negative as the field evolves, resulting in the end of cosmic expansion and the beginning of contraction. Even in this case, we find that thermal friction lengthens the period of accelerated expansion and consequently delays the end of cosmic expansion, making it challenging to detect the impending transition to contraction using conventional cosmological tests. However, particle production can also provide alternative avenues for detection by generating a background of thermal dark radiation, partly comprised of neutrinos or other particles, whose energy density exceeds the remnant photon energy density.

In this paper, we construct two spherically symmetric thin-shell gravastar models within a BTZ geometry with minimum length. Therefore, in the inner region of the gravastar, we consider an anti-de Sitter metric with minimum length. Thus, for the first model, we introduce the minimum length effect using the probability density of the ground state of the hydrogen atom in two dimensions. For the second gravastar model, we adopt a Lorentzian-type distribution. Also in the outer region, we consider the BTZ black hole metric. So, by examining the inner spacetime, the thin shell, and the outer spacetime, we find that there are different physical characteristics regarding their energy densities and pressures that make the gravastar stable. This effect persists even when the cosmological constant is zero. In addition, we determined the entropy of the gravastar thin shell. Besides, we explore the thermodynamic properties of the BTZ black hole with minimum length in Schwarzschild-type form and also check its stability.

From negative to positive cosmological constant through decreasing temperature of the universe: Connection with string theory and spacetime foliation results

Cosmology is living through fascinating times, where new observations from ground and space telescopes are questioning the established paradigm, the so-called Λ Cold Dark Matter model. The particle nature of Dark Matter is severely constrained by underground experiments, while recent observations by galaxy surveys indicate that the cosmological constant (Λ) may not be constant after all. Furthermore, observations at high redshift of fully formed galaxies with massive black holes at their centers by the James Webb Space Telescope, as well as black holes with unexpected properties observed by the LIGO-Virgo gravitational wave detectors, are driving an in-depth revision of our assumptions in models of structure formation and the evolution of the Universe. I propose exploring two new paradigms to account for Dark Matter and Dark Energy, based on known physics, without introducing new particles into the Standard Model of Particle Physics. I will extend the primordial spectrum of fluctuations to small scales with new statistical properties to provide a viable Primordial Black Hole scenario for Dark Matter, and will include non-equilibrium thermodynamics in the expanding Universe, in the form of General Relativistic Entropic Acceleration, to explain Dark Energy. My proposal could provide a unified explanation for a plethora of interrelated multi-epoch, multi-scale, and multi-probe observations from present and future Gravitational Wave detectors, Large Scale Structure observatories, and Cosmic Microwave Background experiments. It emphasizes the need to develop new theoretical ideas hand-in-hand with observations to acquire a deeper understanding of our universe. If these ideas are correct, they will open a new window into the early universe and a new fundamental understanding of gravity in the late universe.

Stabilizing charged Dyson shells through the cosmological constant

Abstract We present improved cosmological constraints from a re-analysis of the Dark Energy Survey (DES) 5-year sample of Type Ia supernovae (DES-SN5YR). This re-analysis includes an improved photometric cross-calibration, recent white dwarf observations to cross-calibrate between DES and low redshift surveys, retraining the SALT3 light curve model and fixing a numerical approximation in the host galaxy colour law. Our fully recalibrated sample, which we call DES-Dovekie, comprises ∼1600 likely Type Ia SNe from DES and ∼200 low-redshift SNe from other surveys. With DES-Dovekie, we obtain Ωm = 0.330 ± 0.015 in Flat ΛCDM which changes Ωm by −0.022 compared to DES-SN5YR. Combining DES-Dovekie with CMB data from Planck, ACT and SPT and the DESI DR2 measurements in a Flat w0waCDM cosmology, we find w0 = −0.803 ± 0.054, wa = −0.72 ± 0.21. Our results hold a significance of 3.2σ, reduced from 4.2σ for DES-SN5YR, to reject the null hypothesis that the data are compatible with the cosmological constant. This significance is equivalent to a Bayesian model preference odds of approximately 5:1 in favour of the Flat w0waCDM model. Using generally accepted thresholds for model preference, our updated data exhibits only a weak preference for evolving dark energy.

We obtain the spectra of codimension-2 horizon “edge” degrees of freedom for gravity and higher-spin gauge fields in de Sitter space and in the static Nariai spacetime, advancing previous Lorentzian and Euclidean analyses of one-loop thermodynamics. The edge spectra exhibit universal shift symmetries, revealing a novel symmetry-breaking structure in one-loop partition functions with a positive cosmological constant. For the graviton, these modes admit a geometric interpretation as fluctuations of the cosmic horizon, which also persists in the Nariai case.

In this paper, we investigate the propagation and absorption of Standard Model fields — scalar, electromagnetic and Dirac — on a [Formula: see text]-dimensional brane embedded in a higher-dimensional Schwarzschild–de Sitter (SdS) spacetime. By using the effective four-dimensional projection of the Tangherlini metric, we compute grey-body factors (GBFs) and absorption cross-sections for each spin sector by means of the sixth-order WKB method and, independently, via the recently proposed correspondence between quasinormal modes (QNMs) and transmission coefficients. The results demonstrate that the cosmological constant and field mass crucially affect the transmission probabilities: increasing [Formula: see text] lowers the potential barrier and enhances the transparency of the geometry, while a larger field mass [Formula: see text] suppresses low-frequency emission and shifts the absorption spectrum to higher energies. For all fields, the QNM–GBF correspondence proves reliable to within about one percent for multipoles [Formula: see text], while the correspondence remains less accurate for the lowest multipoles. The total absorption cross-sections exhibit the expected transition from the low-frequency suppression to the geometric-optics regime. Overall, these findings provide quantitative insight into the interplay between dimensionality, cosmological expansion and black-hole radiation on the brane.