In this paper, we will study the phenomenon of neutrino oscillations when traveling through the gravitational field of a black hole embedded in a spatially homogeneous and isotropic flat universe. Thus, we will compute the phase shift for neutrinos within the McVittie spacetime building upon the description of their geodesic motion. Standard assumptions of ultrarelativistic neutrinos with degeneracy between their energy eigenstates are maintained, while no screening mechanisms are needed; the correctness of our result can therefore be checked against relevant literature limiting scenarios of static isolated Schwarzschild black hole and of the Friedmann universe backgrounds, separately. We will compare and contrast the effects of local galactic and global cosmic physics, accounted for by the black hole mass and the Hubble rate, in the phase shift; its numerical value will be estimated for some realistic scenarios by re-casting it in terms of measurable astrophysical and cosmological parameters. We will identify the range of applicability of our model as for neutrinos sourced in the accelerated expansion phase of the universe, with the roles of the cosmological constant, dark matter and black hole mass scrutinized in the loss of coherence of the wavepackets. Our analysis will allow us to identify similarities and differences in the interaction between neutrinos and <italic>matter</italic> or <italic>gravitational</italic> fields.

China

Daniele Gregoris

Neutrino oscillations in the background of a black hole embedded in an expanding Universe

In this paper, we will propose a novel interacting vacuum model whose energy flow is modeled according to the free Dirac–Born–Infeld theory and hydrodynamically realized via the (Modified) Berthelot equation of state. By employing dynamical system techniques, we will identify a suitable late-time attractor which can realistically account for the present-day configuration of the universe, addressing the coincidence problem, supporting an accelerated expansion without breaking any energy condition, free from fine-tuning issues on initial conditions, and stable also at the perturbative level. Analytical closed-form results for the redshift evolution of both vacuum energy and dark matter will be presented. We will provide as well two distinctive fingerprints of our model, useful for its sharp identification inside the rich zoo of literature interacting vacuum models: a vacuum equation of state via geometric curvature scalars, and an analytical relationship between the statefinder variables. We will eventually show that our scenario actually consists in a running (or decaying) vacuum, with consequent production of dark matter particles, also comparing and contrasting its cosmological applicability with that of other known interacting vacuum paradigms. Our work therefore belongs to the line of research scrutinizing the relevance of quantum field theory approaches to the taming of current observational tensions.

On a free Dirac–Born–Infeld interacting vacuum model

In this paper, we compare and contrast the classical versus quantum dynamics of a cosmological model based on the literature (Modified) Berthelot equation of state for the description of the dark sector of the universe. At the classical background level we identify a Minkowski-like and a de Sitter-like equilibrium epochs, with the latter occurring only beyond a certain threshold for a parameter in the equation of state; at the classical perturbed level we find that this same parameter realizes a duality in the adiabatic speed of sound between the two equilibrium epochs. The quantum evolution of this model is studied in the context of quantum geometrodynamics by solving analytically the Wheeler–DeWitt equation in the Born–Oppenheimer approximation for the scalar field potentials about the two equilibrium epochs. We identify the phenomenon of quantum decoherence to arise at the same threshold which constitutes the bifurcation between the two equilibrium epochs at the classical level. We comment on the quantum modified power spectrum focusing on some consequences dealing with the formation of astrophysical structures within the Press–Schechter framework. Our paper is intended to scrutinize which classical features of a certain cosmological model are preserved at its quantum level, and under which assumptions.

Classical versus quantum features of the Berthelot cosmological model

In this paper, we present some new static and spherically symmetric solutions of the Einstein equation in which the matter sector is accounted for by a free Dirac–Born–Infeld field. Our novel spacetimes can describe either a black hole, a wormhole, or a naked singularity depending on certain boundary conditions. By tracking the dynamical gravitational collapse, we enlighten the importance of the isotropy of the pressure for having an horizon as a result, as required by the Cosmic Censorship Conjecture. Our new spacetime solutions, the amount of exotic matter, its “complexity”, and the equation of state along the tangential direction are analytical and written in closed forms. We identify a taming of the breaking of the null energy condition, customary for wormhole spacetimes in General Relativity, along both the radial and tangential direction. We assess the astrophysical applicability and perform a comparative analysis between our solutions and other literature ones, by identifying an ISO-like density profile of the matter field, which provides a flattening of the rotation curves, by discussing the motion of test particles, and the shadow properties. In our model, those effects are interpreted as a manifestation of a topological defect, and since they can observationally mimic the signatures of other spacetimes, a study of the perturbations is performed within the quasi-normal modes formalism. Having identified the Reissner–Nordström-like quasi-resonance, our paper is intended also to provide some insights on which combinations of background and perturbation properties should be observed, for claiming the nature of astrophysical compact objects.

On some new black hole, wormhole and naked singularity solutions in the free Dirac–Born–Infeld theory

We address the question of the uniqueness of spatially flat ΛCDM-like evolution for FLRW cosmologies in General Relativity, i.e. whether any model other than the spatially flat ΛCDM can give rise to the same type of scale factor evolution. Firstly, we elaborate on what we exactly imply by a ΛCDM-like evolution or kinematic/cosmographic degeneracy with the ΛCDM model, using the lessons from the statefinder diagnostic. Then, we consider two models with interaction in the dark sector: coupled fluid-fluid model and coupled quintessence model. We enforce the kinematic degeneracy with the spatially flat ΛCDM model via the cosmographic condition <math><mi>j</mi><mo>=</mo><mn>1</mn></math> (j being the jerk parameter), which in turn fixes the function of the interaction term that is a priori unspecified. We argue that in General Relativity this cosmographic condition is consistent only with spatial flatness. Employing a dynamical system approach, we show that the spatially flat coupled fluid-fluid interacting models kinematically degenerate with ΛCDM must necessarily be based on a phantom fluid, whereas the set of physically viable spatially flat coupled quintessence models with power law or exponential potential kinematically degenerate to ΛCDM is of measure zero. Our analysis establishes that coupled fluid-fluid models with non-phantom fluids or coupled quintessence models with power law and exponential potential can never reproduce a cosmological evolution similar to that of the ΛCDM. The astrophysical consequences of our findings are qualitatively discussed in light of observational cosmological tensions.

Thailand

South Africa

Saikat Chakraborty

India

B. Mishra

On the uniqueness of ΛCDM-like evolution for homogeneous and isotropic cosmology in General Relativity

Some cosmological models based on the gravitational theory  $$f(R)=R+\zeta R^2$$  <math> <mrow> <mi>f</mi> <mrow> <mo>(</mo> <mi>R</mi> <mo>)</mo> </mrow> <mo>=</mo> <mi>R</mi> <mo>+</mo> <mi>ζ</mi> <msup> <mi>R</mi> <mn>2</mn> </msup> </mrow> </math>  , and on fluids obeying to the equations of state of Redlich–Kwong, Berthelot, and Dieterici are proposed for describing smooth transitions between different cosmic epochs. A dynamical system analysis reveals that these models contain fixed points which correspond to an inflationary, a radiation dominated and a late-time accelerating epoch, and a nonsingular bouncing solution, the latter being an asymptotic fixed point of the compactified phase space. The infinity of the compactified phase space is interpreted as a region in which the non-ideal behaviors of the previously mentioned cosmic fluids are suppressed. Physical constraints on the adopted dimensionless variables are derived by demanding the theory to be free from ghost and tachyonic instabilities, and a novel cosmological interpretation of such variables is proposed through a cosmographic analysis. The different effects of the equation of state parameters on the number of equilibrium solutions and on their stability nature are clarified. Some generic properties of these models, which are not sensitive to the particular fluid considered, are identified, while differences are critically examined by showing that the Redlich–Kwong scenario admits a second radiation-dominated epoch and a Big Rip Singularity.

Cosmological evolution with quadratic gravity and nonideal fluids

Different theories of gravity can admit the same black hole solution, but the parameters usually have different physical interpretations. In this work we study in depth the linear term  $$\beta r$$  <math> <mrow> <mi>β</mi> <mi>r</mi> </mrow> </math>   in the redshift function of black holes, which arises in conformal gravity, de Rham–Gabadadze–Tolley (dRGT) massive gravity, f(R) gravity (as approximate solution) and general relativity. Geometrically we quantify the parameter  $$\beta $$  <math> <mi>β</mi> </math>   in terms of the curvature invariants. Astrophysically we found that  $$\beta $$  <math> <mi>β</mi> </math>   can be expressed in terms of the cosmological constant, the photon orbit radius and the innermost stable circular orbit (ISCO) radius. The metric degeneracy can be broken once black hole thermodynamics is taken into account. Notably, we show that under Hawking evaporation, different physical theories with the same black hole solution (at the level of the metric) can lead to black hole remnants with different values of their physical masses with direct consequences on their viability as dark matter candidates. In particular, the mass of the graviton in massive gravity can be expressed in terms of the cosmological constant and of the formation epoch of the remnant. Furthermore the upper bound of remnant mass can be estimated to be around  $$0.5 \times 10^{27}$$  <math> <mrow> <mn>0.5</mn> <mo>×</mo> <msup> <mn>10</mn> <mn>27</mn> </msup> </mrow> </math>   kg.

Yen Ong

Bin Wang

A critical assessment of black hole solutions with a linear term in their redshift function

In this paper, after reconstructing the redshift evolution of the Hubble function by adopting Gaussian process techniques, we estimate the best-fit parameters for some flat Friedmann cosmological models based on a modified Chaplygin gas interacting with dark matter. In fact, the expansion history of the Universe will be investigated because passively evolving galaxies constitute cosmic chronometers. An estimate for the present-day values of the deceleration parameter, adiabatic speed of sound within the dark energy fluid, effective dark energy, and dark matter equation of state parameters is provided. By this, we mean that the interaction term between the two dark fluids, which breaks the Bianchi symmetries, will be interpreted as an effective contribution to the dark matter pressure similarly to the framework of the “Generalized Dark Matter”. We investigate whether the estimates of the Hubble constant and of the present-day abundance of dark matter are sensitive to the dark matter–dark energy coupling. We will also show that the cosmic chronometers data favor a cold dark matter, and that our findings are in agreement with the Le Châtelier–Braun principle according to which dark energy should decay into dark matter.

Iraq

Muhsin Aljaf

Russia

Poland

Spain

Martiros Khurshudyan

Constraints on interacting dark energy models through cosmic chronometers and Gaussian process

In this paper, we investigate the existence and time evolution of the cosmological and event horizons in a McVittie universe whose expansion is driven by the Redlich–Kwong, (Modified) Berthelot, Dieterici, and Peng–Robinson fluids, respectively. The equations of state of these fluids are rich enough to account for both exotic and regular, as well as ideal and non-ideal matter contents of the universe. We show that the cosmological horizon is expanding, while the event horizon is shrinking along the cosmic time evolution. The former achieves larger size for regular types of matter, contrary to the latter. The strength of interactions within the cosmic fluid are shown to play a more important role in affecting the evolution of the event horizon, rather than of the cosmological horizon in the case of a singularity-free universe. While the cosmological horizon always exists during the time evolution, the event horizon can exist only when a certain relationship between the Hawking–Hayward quasi-local mass and the Hubble function is fulfilled. In this manner, we can study the role played by the large-scale physics (cosmic evolution) on the local scale physics (evolution of a black hole).

The horizon of the McVittie black hole: on the role of the cosmic fluid modeling

In this paper, applying the Hartman–Grobman theorem we carry out a qualitative late-time analysis of some unified dark energy-matter Friedmann cosmological models, where the two interact through linear energy exchanges, and the dark energy fluid obeys to the dynamical equation of state of Redlich–Kwong, Modified Berthelot, and Dieterici respectively. The identification of appropriate late-time attractors allows to restrict the range of validity of the free parameters of the models under investigation. In particular, we prove that the late-time attractors which support a negative deceleration parameter correspond to a de Sitter universe. We show that the strength of deviation from an ideal fluid for the dark energy does not influence the stability of the late-time attractors, as well as the values of all the cosmological parameters at equilibrium, but for the Hubble function (which represents the age of the universe). Our analysis also shows that a singularity in the effective equation of state parameter for the dark energy fluid is not possible within this class of models.