The purpose of this talk is two-fold: (1) in the first part, I will discuss why thermodynamics is important in cosmology, and then, (2) I will show that the laws of thermodynamics (including the second law) become naturally satisfied during the entire cosmic evolution of the universe, from inflation to the late dark energy era, without imposing any exotic condition. The first part is based on how the matter fields inside the horizon exhibit a flux during the cosmic expansion of the universe. Regarding the thermodynamic laws in cosmology, they are individually shown for two different class of gravitational theories: (a) having one thermodynamic degree of freedom, and (b) having more than one thermodynamics degrees of freedom. These make the inter-connection between cosmology and thermodynamics more concrete. Consequently, it also shows why the matter fields are not in thermal equilibrium with the apparent horizon during most of the cosmic era of the universe, except for the fluids with ω = −1/3, leading to the transitions of the universe from an accelerating to a decelerating era and vice versa. The effect of viscosity on thermodynamic laws will be discussed, as well. We will explicitly show that viscosity makes universe expansion more irreversible compared to the case in the absence of viscosity.

The following papers may be treated as reference for this talk.

REF-1: Phys.Rev.D 111 (2025) 4, 043544

REF-2: Phys.Rev.D 111 (2025) 8, 083540

NANOGrav 15-year data hint towards an alternative inflationary scenario, as opposed to the usual slow-roll one (as the usual slow-roll inflation predicts a low amplitude of gravitational waves around the NANOgrav frequency range), for describing the early time dynamics of the universe. The modified inflation should be such that it leads to a blue-tilted primordial tensor power spectrum around the NANOGrav frequency range, but maintaining the Planck constraints over the CMB scales. Regarding the constraint over CMB scales, the recent the Atacama Cosmology Telescope (combined with the Planck 2018 and BAO) refines the constraint on inflationary observables, compared to the Planck-only 2018 measurements. Based on these observations, we are interested to simultaneously fit these two through the thermodynamic route of cosmology. The entropy of the apparent horizon is considered to be four-parameter generalized entropy (which is able to generalize the other known entropies proposed so far). In particular, we will show that four-parameter generalized entropy proves to be compatible with these observations for common range of entropic parameters. After the inflation ends, the entropic energy density (coming from the generalized entropy) decays to relativistic particles and leads to standard Big Bang cosmology. Therefore we argue that if the future observatories can detect the signal of primordial GWs, then our theoretical expectation carried in the present work may provide a possible testbed for generalized entropic cosmology.

Despite the well-known problem of the appearance of ghost degrees of freedom in gravitational models containing the Gauss–Bonnet invariant, there are special modifications in which this disadvantage can be eliminated. Such models include ghost-free generalizations of the Einstein–Gauss–Bonnet theory. Despite the mathematical attractiveness and theoretical validity of ghost-free f(R, G) models, their cosmological viability is often investigated using external reconstructive approaches that do not rely on the internal structure of the theory. In this paper, we propose an alternative method based on the use of internal model relationships defined by Noether symmetries. This approach makes it possible to significantly narrow down the class of acceptable theories and obtain explicit functional forms of scalar field potentials consistent with both the symmetric requirements and the ghost-free nature of the model. By applying the Noether symmetry method to a homogeneous and isotropic cosmological background, it is possible to obtain a set of allowable potentials with a nontrivial dynamic structure. These potentials are considered as physically motivated candidates for describing the early Universe. Based on them, a study of the inflationary dynamics of the model is conducted, based on the slow-roll parameters and spectral indices. The values obtained are compared with observations from the Planck mission and the Atacama Cosmology Telescope.

Observational evidence, including LIGO, EHT, and astrophysical results, are yet to confirm the existence of the black hole horizon.
Also, it has been pointed out by several research groups that the LIGO observations of merger and ringdown and shadow observed by EHT do not exclude horizon-less exotic compact objects. We present stability analysis and light propagation in wormhole spacetimes for which
we have considered a couple of wormhole geometries. The motivation behind these choices lies in the fact that the geometries may be
constructed out of ordinary matter where the violation of energy conditions are relaxed compared to the standard 4D GR framework.
We investigated the geometrical structure of the spacetime and concentrated on null geodesics with the aim of exploring the shadow features.
The shadow characteristics in a framework where the space is devoid of any sources between the compact object and the observer has been
observed. In addition to this, we have also studied the effect of the environment in particular plasma on the shadow characteristics. Finally, we
explore the stability under scalar perturbation; studying the time domain profile, we find the quasi-normal frequencies. By comparing the
characteristics of the shadow images and the quasi-normal frequencies, we identify the features that could serve as discriminants for
similar compact objects.

Persistent discrepancies among low and high redshift probes, most notably, the Hubble tension, motivate systematic investigations of cosmologies involving a dynamical dark energy component, rather than a cosmological constant (Lambda) candidature of the latter which gives rise to the standard Lambda-CDM model. Interacting Dark Energy-Matter (IDEM) scenarios provide a more compelling framework in this direction, especially when viewed through modified gravity or their Scalar–Tensor equivalents, however, in the Jordan frame, such interactions do not arise naturally, as the scalar field is non-minimally coupled to gravity. A conformal transformation to the Einstein frame decouples the gravitational and scalar degrees of freedom, but induces a non-minimal coupling between the scalar field and cosmological matter (including the CDM), thereby generating an IDEM-type interaction when the scalar field acts as dark energy. Within this Einstein-frame Scalar–Tensor formulation, we therefore adopt a scenario in which dark energy interacts exclusively with dark matter, whereas the visible matter component remains minimally coupled to both gravity and the IDEM. Thus, the large-scale dilution rates of the dark matter and the visible matter with the cosmological scale factor differ, and this is in principle feasible as we demonstrate explicitly. We carry out a full Bayesian parameter-inference analysis using the standard Metropolis-Hastings algorithm for the Markov Chain Monte Carlo (MCMC) method, by jointly incorporating the Pantheon+ Type~Ia Supernova sample, Observational Hubble Parameter (OHD) measurements via Cosmic Chronometry (CC), and Baryon Acoustic Oscillation (BAO) data from SDSS-IV and DESI-VI. We present updated constraints on the dark-sector coupling parameter and the Hubble constant, and assess the comparative statistics of this Scalar–Tensor IDEM relative to Lambda-CDM using information criteria such as AIC and BIC. The resulting posteriors and reconstructed expansion history illustrate the framework’s impact on the Hubble tension and strengthen its observational viability within Scalar–Tensor gravity.

Neutron stars may exhibit pressure anisotropy arising from several microphysical processes, including strong magnetic fields, elastic stresses, and viscous effects. In this work, we investigate how pressure anisotropy influences two key observables: the tidal deformability measured in gravitational-wave signals from binary inspirals, and the fundamental quadrupolar mode (f-mode) oscillation frequency associated with stellar perturbations. We model anisotropic neutron stars using a phenomenological quasi-local prescription governed by a single dimensionless anisotropy parameter, allowing us to systematically explore deviations from isotropy. Our analysis demonstrates that, although the universal relation between tidal deformability and f-mode frequency varies with the strength of anisotropy, it remains remarkably insensitive to changes in the underlying equation of state that relates radial pressure to energy density. This behavior mirrors the well-known approximate universality found in isotropic stars. Taking advantage of this anisotropy-dependent universal relation, we perform Bayesian inference to constrain the anisotropy parameter using data from both the gravitational-wave event GW170817 and a simulated GW170817-like event detected by a future network. We show that current data restrict the anisotropy parameter to values of order unity, and that future measurements yield comparable bounds. Notably, these constraints are only weakly influenced by remaining uncertainties in the neutron-star equation of state.

The Schwarzschild black hole (BH) constitutes a fundamental exact solution of Einstein’s field equations in general relativity, representing the exterior geometry of a spherically symmetric mass in a vacuum in the absence of a cosmological constant Λ. However, the accelerated expansion of the universe is attributed to dark energy (DE) which is intrinsically linked to a non-zero cosmological constant Λ. Hence, incorporating DE naturally requires the Λ-modified Einstein field equations. To investigate late-time cosmic acceleration, the Biswas Roy Biswas (BRB) parameterization of the dark-energy equation of state has been adopted. This model emphasizes low-redshift dynamics and offers significant flexibility in constraining observational data from SNeIa, BAO and CMB. Using a dataset of eighty five differential age measurements, we obtain the corresponding confidence contours and marginalized distributions for the free parameters. Spherical accretion of BRB type DE onto a Schwarzschild BH within the framework is studied which has been established by Bondi and Michel. Bondi’s analysis shows that stationary, spherically symmetric flow becomes critical at the sonic point, where the infall velocity equals the local sound speed. In our formulation, the fluid equations similarly exhibit a singular structure at the sonic point, and only particular transonic solutions remain physically admissible. These solutions determine the Bondi accretion rate for BRB type DE onto the Schwarzschild spacetime. Our results indicate that, over the redshift interval 0 to 3, a Schwarzschild BH could gain approximately 55.331% of its present mass due to the accretion of BRB type DE.

Understanding the origin of neutrino masses and their mixing patterns remains a central challenge in particle physics, as it provides key insights into the limitations of the Standard Model (SM) and potential avenues for new physics. Flavour symmetries, combined with seesaw mechanisms, offer a promising framework to address these questions while yielding testable predictions in current and future experiments. Motivated by these considerations, we present a novel framework for neutrino masses and mixing based on the interplay of Type-I and Type-II seesaw mechanisms under non-Abelian $A_4$ discrete flavour symmetry. The framework is consistent with the normal hierarchy (NH) of neutrino masses and is governed by three real parameters that span the entire model and its predictions. It determines the neutrino mass eigenvalues and Majorana phases. The model further constrains the effective Majorana mass ($m_{\beta\beta}$) relevant for neutrinoless double beta ($0\nu\beta\beta$) decay​ and the branching ratio of the dominant $\mu \rightarrow e \gamma$ charged lepton flavour violating decay. Both observables are sensitive to the underlying parameters and provide indirect tests of the model. Overall, this minimal $A_4$ symmetric framework demonstrates that a small set of parameters, guided by symmetry and seesaw dynamics, can lead to predictive outcomes. The interplay between theoretical predictions and low-energy observables highlights the model’s relevance and accessibility to future experimental investigations.

Modified gravity based on the non-metricity scalar Q offers an effective alternative to ΛCDM, with several proposed f(Q) models successfully reproducing the late-time cosmic acceleration inferred from Type Ia supernovae. Current observational anomalies—such as hemispherical temperature asymmetry, the Hubble tension, cosmic voids, dipole modulation, and hints of anisotropic inflation—further motivate exploring departures from the cosmological principle. In this work, we study a broad class of f(Q) gravity models in an anisotropic Bianchi-I background to investigate their dynamical behaviour and cosmological viability. Using dynamical systems techniques, we identify stationary points of the cosmic evolution and classify their stability. Perturbative analyses are performed where applicable. Several fixed points arise only when both f(Q) modifications and anisotropies are considered, highlighting genuinely new cosmological features absent in isotropic or GR-based frameworks. In addition to recovering standard inflationary and late-time accelerating phases, we find that some models naturally support ultra-slow-roll inflation. For certain choices of f(Q), the shear evolves toward a non-zero constant, indicating a universe that gradually becomes more homogeneous, while other models allow complete isotropization depending on initial conditions. Special subclasses exhibit multiple stable attractors, each corresponding to distinct cosmological end-states. Our results demonstrate that f(Q) gravity in Bianchi-I spacetime provides a rich dynamical structure capable of addressing key cosmological tensions while offering novel predictions for both early- and late-time anisotropic evolution.

The Snowball Chamber is a novel concept for rare event detection based on the controlled use of supercooled liquids. In this detector, purified liquids such as water are held below their freezing point in a metastable state, where localized energy depositions—such as those from nuclear recoils—can trigger rapid, visible crystallization events. This process results in a unique “snowball” signature, which can be used to detect rare interactions such as those potentially induced by dark matter or other forms of ionizing radiation.
Beyond its immediate particle physics applications, the Snowball Chamber also can serve as a platform for studying the dynamics of phase transitions in extreme environments. Supercooled liquids are of particular interest in planetary science and astrobiology, as they may exist beneath the icy surfaces of moons like Europa or Ganymede. Understanding how radiation, pressure, and impurities affect nucleation and freezing in such metastable systems could shed light on the thermal behavior of extraterrestrial oceans—and their potential for harboring life.
In this presentation, we introduce the operating principles of the Snowball Chamber, report on early experimental tests of supercooling stability and nucleation behavior, and discuss future directions. These include both optimization of the detector for rare event searches and the use of the system as a terrestrial analog for radiation-induced crystallization in planetary environments.
This work thus represents a multidisciplinary opportunity at the intersection of particle detection, thermodynamics, and planetary exploration.