De Sitter solutions play an important role in cosmology because the knowledge of unstable de Sitter solutions can be useful to describe inflation, whereas stable de Sitter solutions are often used in models of late-time acceleration of the Universe. Some models with scalar fields have both stable and unstable de Sitter solutions that correspond to different fixed values of the scalar field. The models the Gauss-Bonnet term are actively used both as inflationary models and as dark energy models. To modify the Einstein equations one can add a nonlinear function of the Gauss-Bonnet term or a function of the scalar field multiplied on the Gauss-Bonnet term.

In order to find out the de Sitter solutions in a model with a minimally coupled scalar field with a potential $V$ it is enough to find zeros of the first derivative of $V$. The sign of the second derivative of $V$ at a de Sitter point determines the stability of the solution. The effective potential plays the same role in the gravity models with the Gauss-Bonnet term because the stable de Sitter solutions correspond to minima of the effective potential.

So, the effective potential method essentially simplifies the search and stability analysis of de Sitter solutions. My talk is based on the paper by E.O. Pozdeeva, M. Sami, A.V. Toporensky, and S.Yu. Vernov, Phys. Rev. D 100 (2019) 083527, and recent investigations.

Mixing transformations in QFT are non-trivial, since they are related with the unitary inequivalence between Fock spaces for definite mass and flavor fields. The question arises as to which of these two representations should be considered as physical. A clue to a solution has been recently provided in the context of particle decay.

The lifetime of a particle is usually considered as one of its intrinsic properties. For some particles such as the pion or muon, it can be estimated by knowing the interactions they experience. Conversely, other particles such as the electron or proton appear to be stable, at least in the Standard Model. Despite such a rooted belief, decay properties should not be regarded as fundamental, since they can be significantly manipulated by external influences. In this sense, one can affect the lifetime of pions or, even more strikingly, spoil the stability of protons, by exposing them to a sufficiently large acceleration. In this context, the inverse β-decay of uniformly accelerated protons was analyzed in both the laboratory frame (where the proton is accelerated) and the comoving frame (where the proton at rest interacts with a thermal bath due to Unruh effect). The equality between the two rates was exhibited as a theoretical proof of Unruh effect for the general covariance of QFT.

Nevertheless, in the above analysis neutrinos were simplistically considered as massless. Recently, this formalism was refined by embedding neutrino flavor mixing. This inevitably raised the aforementioned problem of which asymptotic representation to use (either flavor or mass states). Here, we show that the only scenario which allows us to: i) preserve the general covariance of QFT, ii) naturally describe neutrino oscillations, iii) take into account CP-violation is the one built upon flavor eigenstates. Phenomenological implications are investigated in connection with Katrin and Ptolemy experiments.

In this talk, I am going to show some recent cutting-edge results associated with the Casimir effect, a phenomenon that can be safely regarded as the first-ever manifestation of the zero-point energy, and thus as one of the earliest experimental verifications of Quantum Field Theory. After a preliminary introduction to the subject, I will focus the attention on the remarkable sensitivity of the Casimir effect to new physics phenomenology. Such an awareness can be readily discerned by virtue of the existence of extra contributions that the measurable quantities (such as the emergent pressure and strength within the physical apparatus) acquire for a given physical setting. In particular, by relying on the above framework, I will outline the possibility of detecting the predictions of a novel quantum field theoretical description for particle mixing according to which the flavor and the mass vacuum are unitarily inequivalent. Furthermore, by extending the very same formalism to curved backgrounds, the opportunity to probe extended models of gravity that encompass local Lorentz symmetry breaking and the strong equivalence principle violation is also discussed. Finally, the influence of quantum gravity on the Casimir effect is briefly tackled by means of heuristic considerations. In a similar scenario, the presence of a minimal length at the Planck scale is the source of the discrepancy with the standard outcomes.

In this work understanding of black holes in the context of chemistry will be presented which includes the concepts of Van der Waals fluids, reentrant phase transition and triple point etc. This is done by giving the comparison of thermodynamic laws of ordinary thermodynamic system to the corresponding thermodynamic laws of black holes. In this framework the cosmological constant is considered as thermodynamic pressure, mass is considered as chemical enthalpy. This comparison will lead us to understand the black holes in a new perspective, called the black hole chemistry. In this framework both charged and rotating black holes will be discussed. Due to this change in perspective on the role of black hole mass and the inclusion of the cosmological constant as a pressure term has recently been shown to have a number of remarkable consequences for black
hole thermodynamics, making their behavior analogous to a variety of “everyday” chemical phenomena. One such indicator was the realization that charged black holes behave as Van der Waals fluids. Van der Waals equation modifies the equation of state for an ideal gas to the one that approximates the behavior of real fluids. In this framework thermodynamic of black holes is presented in extended phase space.

Scalar-tensor theories of gravity provide an intriguing and compelling explanation to the dark energy problem. They have received increased attention in recent years thanks to a wealth of developments both in the theoretical and experimental sides. The class of models known as “degenerate” provide a particularly interesting case. These theories extend general relativity by a single degree of freedom, despite their equations of motion being higher than second order, a virtue made possible by the existence of an additional constraint that removes the would-be instability associated to a ghost. This constraint can however be obstructed by matter fields, even when minimally coupled to the metric. In this talk I will present this issue in detail, explaining through some illustrative examples the precise ways in which the extra degree of freedom may reappear. This occurs in the Hamiltonian language through a loss of constraints, which may happen either when the kinetic matrix is not block-diagonal in the presence of matter fields, or when the matter sector itself has constraints. I will next turn to the more physically relevant case of fermionic matter, and show that spin-1/2 fermions evade these issues and can thus be consistently coupled to degenerate theories of scalar-tensor gravity.

This abstract is primarily based on my recent paper ApJ 896, 69 (2020) along with MNRAS 490, 2692 (2019).

Over the past decades, various researchers have indirectly predicted at least a dozen of super-Chandrasekhar white dwarfs (white dwarfs which violate the Chandrasekhar mass-limit) from the luminosity observations of type Ia supernovae (SNeIa). Of course, there is no direct observations of super-Chandrasekhar white dwarfs so far. As a result, several research groups around the world proposed different models to explain the massive white dwarfs. Among them, the model for increasing mass accounting the presence of magnetic fields and rotation is the most popular one. In my presentation, I will explain that if such white dwarfs are rotating with a specific angular frequency following certain conditions, they can efficiently emit continuous gravitational waves, and these gravitational waves can be detected by various futuristic detectors, such as LISA, BBO, DECIGO, Einstein Telescope, etc., with a significant signal-to-noise ratio. I will also discuss various timescales over which these white dwarfs can emit dipole and quadrupole radiation, and the corresponding time required to obtain the optimum signal-to-noise ratio. In this way, in the future, we can detect the super-Chandrasekhar white dwarfs directly, and thereby can enforce better constraints in the theory of gravity to explain all the white dwarfs simultaneously.

In this paper, we introduced a framework to study the tidal deformation of relativistic anisotropic compact stars. Anisotropic stresses are ubiquitous in nature and widely used in modelling compact stellar object. Tidal deformability of astrophysical compact objects is a natural effect of gravity such as one produced by a companion in a binary system. In general relativity, the existence of this measurable effect of gravity level can be quantified by their tidal Love numbers (TLN) which characterize the deformability of a neutron star (NS) from sphericity. The tidal deformability or polarizability parameter of a NS depends on its complex internal structure and hence the nature of the compact object can study through measuring the love number (TLN). We choose a particular solution which is the anisotropic generalization of Tolman IV Model as the interior of the compact stellar object. The physical acceptability of the model has been shown graphically by considering the pulsar 4U1608−52 with their current estimated mass and radius. By computing quadrupole moment we calculated the tidal love number as a dependent on anisotropy of the compact object. We graphically analyze the variation of tidal love number (TLN) against anisotropy for different compact objects with compactness factor. The numerical value of TLN is given for different compact objects for physically acceptable value of anisotropic parameter.

Numerical simulations have been the primary tool to study compact binary coalescences in the last two decades, supplying a wealth of information. Depending primarily on the total mass of the binary, a possible outcome of a binary neutron star merger is a long-lived (with a lifetime > 10ms) compact remnant supported by differential rotation. In this work, we present equilibrium sequences of rotating relativistic stars, constructed with a new differential rotation law that was proposed by Uryu et al. (2017). We choose rotational parameters motivated by numerical simulations of binary neutron star merger remnants, but otherwise adopt a cold, relativistic N=1 polytropic equation of state, in order to perform a detailed comparison to published equilibrium sequences that used the Komatsu, Eriguchi and Hachisu (1989) differential rotation law. We find a small influence of the choice of rotation law on the mass of the equilibrium models and a somewhat larger influence on their radius. The versatility of the new rotation law allows us to construct models that have a similar rotational profile and axis ratio as observed for merger remnants, while at the same time being quasi-spherical. While our models are highly accurate solutions of the fully general relativistic structure equations, we demonstrate that for models relevant to merger remnants the IWM-CFC approximation still maintains an acceptable accuracy.

Dark matter (DM) came from long-range gravitational observations which actually does not interact with ordinary matter. Though, on much smaller scales, a number of unexpected phenomena contradict this picture for DM. Because, some of the solar activity or the dynamic earth’s atmosphere might arise from DM streams. Gravitational (self-)focusing effects by the Sun or its planets of streaming DM fits as the underlying process, e.g., for the otherwise puzzling 11-year solar cycle, the mysterious heating of the solar corona with its fast temperature inversion, etc. Observationally driven we arrive to an external impact by as yet overlooked “streaming invisible matter”, which reconciles some of the investigated mysterious observations. Unexpected planetary relationships exist for the dynamic Sun and Earth atmosphere and are considered as the signature for streaming DM. Then, focusing of DM streams could also occur in exoplanetary systems, suggesting for the first time investigations by searching for the associated stellar activity as a function of the exoplanetary orbital phases. The entire observationally driven reasoning is suggestive for highly cross-disciplinary approaches including also (puzzling) bio-medical phenomena. Favoured candidates from the dark sector are the highly ionizing anti-quark nuggets, magnetic monopoles, but also particles like dark photons.

There is wide recognition of the immense applications of the concepts of quantum physics in modern technologies such as solar panels, mobile phones, global positioning systems, and medical physics which is promoting the need for the introduction of these concepts at an early age. To make the concepts of quantum physics accessible to students of middle school and above, the Einstein-First project has developed tangible and tactile tools in the form of models and analogies. Until now in this project, we presented the concepts of quantum interference as a modern observational fact. To give a more comprehensive understanding of these concepts, we present a graphical approach for exploring basic quantum mechanical phenomena such as matter-wave interference, diffraction, and reflection based on Feynman’s method of summation of all possible paths (path integrals). We developed tactile tools called phasor wheels which can be used to perform vector summation and obtain the probability amplitude of photons. Supported by videos of single-photon interference and matter-wave interference (such as phthalocyanine molecules), students obtain insights into the quantum world in which observations represent the quantum probability amplitudes. Results from trial programs on high school students (14-15 years old) showed the possibility of teaching these concepts.