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Let us recall the reasons for taking dark matter, and in particular non-baryonic dark matter, very seriously in the context of galaxy formation.

This paper investigates a Bianchi type VIII cosmological model with wet dark fluid in Barber self-creation theory of gravitation. To get the determinate model of the Universe, we have assumed the relationship between metric coefficients R and S. Also, the behavior of the model in the radiation Universe and the physical implications of the model are discussed in detail.

In the framework of the Einstein-Dirac-axion-aether theory we consider the quartet of self-interacting cosmic fields, which includes the dynamic aether, presented by the unit timelike vector field, the axionic dark mater, described by the pseudoscalar field, the spinor field associated with fermion particles, and the gravity field. The key, associated with the mechanism of self-interaction, is installed into the modified periodic potential of the pseudoscalar (axion) field constructed on the base of a guiding function, which depends on one invariant, one pseudo-invariant and two cross-invariants containing the spinor and vector fields. The total system of the field equations related to the isotropic homogeneous cosmological model is solved; we have found the exact solutions for the guiding function for three cases: nonzero, vanishing and critical values of the cosmological constant. Based on these solutions, we obtained the expressions for the effective mass of spinor particles, interacting with the axionic dark matter and dynamic aether. This effective mass is shown to bear imprints of the cosmological epoch and of the state of the cosmic dark fluid in that epoch., 12 pages, 0 figures; revised version published in The European Physical Journal C

In this paper, a fluid model is presented which contains the general linear equation of state including the gravitation term. The obtained spherical symmetric Euler equation and the continuity equations were investigated with the Sedov-type time-dependent self-similar ansatz which is capable of describing physically relevant diffusive and disperse solutions. The result of the space and time-dependent fluid density and radial velocity fields are presented and analyzed. Additionally, the role of the initial velocity on the kinetic and total energy densities of the fluid is discussed. This leads to a model, which can be considered as a simple model for a dark-fluid.

The present research paper consists of the study of an η1-Einstein soliton in general relativistic space-time with a torse-forming potential vector field. Besides this, we try to evaluate the characterization of the metrics when the space-time with a semi-symmetric energy-momentum tensor admits an η1-Einstein soliton, whose potential vector field is torse-forming. In adition, certain curvature conditions on the space-time that admit an η1-Einstein soliton are explored and build up the importance of the Laplace equation on the space-time in terms of η1-Einstein soliton. Lastly, we have given some physical accomplishment with the connection of dust fluid, dark fluid and radiation era in general relativistic space-time admitting an η1-Einstein soliton.

This study presents mesonic tilted cosmological models with wet dark fluid in the F(T) theory of gravity, which is an extension of the Teleparallel theory of gravity, where T is the torsion scalar. In this paper, a new idea is introduced about the F(T) theory of gravity with the help of tilt angle, heat conduction, and mesonic wet dark fluid. The cosmological evolution in F(T) models is investigated. The physical and geometrical aspects of the models are also discussed.

In this paper we study two different cases of inhomogeneous EOS of the form p_d = \omega(t)\rho_d+w_1 f(H, t) . We derive the energy density of dark fluid and dark matter component for both the cases. Further we calculate the evolution of energy density, gravitational constant and cosmological constant. We also explore the finite time singularity and thermodynamic stability conditions for the two cases of EOS. Finally, we discuss the thermodynamics of inhomogeneous EOS with the derivation of internal energy, Temperature and entropy and also show that all the stability conditions are satisfied for the two cases of EOS, Comment: 28 pages, 26 figures

As an approximation to the near horizon regime of black holes, the Rindler fluid was proposed on an accelerating cutoff surface in the flat spacetime. The concept of the Rindler fluid was then generalized into a flat bulk with the cutoff surface of the induced de Sitter and FRW universe, such that an effective description of dark fluid in the accelerating universe can be investigated.

International audience; The observed acceleration of the universe arises a puzzling question. What is the nature of a dark energy that would cause this phenomenon? We recall the arguments against the existence of negative matter on General Relativity grounds. They vanish if the universe is considered as a manifold M4 associated to two coupled metrics, solutions of a coupled field equation system. We build a non-steady solution where the positive species accelerates while the negative one decelerates. So that dark energy is replaced by (dominant) negative matter action.

We consider the holographic Friedman-Robertson-Walker (hFRW) universe on the 4-dimensional membrane embedded in the 5-dimensional bulk spacetime and fit the parameters with the observational data. In order to fully account for the phenomenology of this scenario, we consider the models with the brane cosmological constant and the negative bulk cosmological constant. The contribution from the bulk is represented as the holographic dark fluid on the membrane. We derive the universal modified Friedmann equation by including all of these effects in both braneworld and holographic cutoff approaches. For three specific models, namely, the pure hFRW model, the one with the brane cosmological constant, and the one with the negative bulk cosmological constant, we compare the model predictions with the observations. The parameters in the considered hFRW models are constrained with observational data. In particular, it is shown that the model with the brane cosmological constant can fit data as well as the standard $\Lambda$CDM universe. We also find that the $\sigma_8$ tension observed in different large-structure experiments can be effectively relaxed in this holographic scenario., Comment: 20 pages, 3 figures

Two inhomogeneous single-fluid models for the Universe, which are able to naturally solve the H0 tension problem, are discussed. The analysis is based on a Bayesian Machine Learning approach that uses a generative process. The method here adopted allows for constraint of the free parameters of each model by making use of the model itself only. The observable is taken to be the Hubble parameter, obtained from the generative process. Using the full advantages of our method, the models are constrained for two redshift ranges. Namely, first this is done with mock H(z) data over z∈[0,2.5], thus covering known H(z) observational data, which are most helpful to validate the fit results. Then, aiming to extend to redshift ranges to be covered by the most recent ongoing and future planned missions, the models are constrained for the range z∈[0,5], too. Full validation of the results for this extended redshift range will have to wait for some years, for when higher-redshift H(z) data become available. This makes our models fully falsifiable. In addition, our second model is able to explain the BOSS reported value for H(z) at z=2.34., The work was carried out with the financial support of the Ministry of Education and Science of the Republic of Kazakhstan, Grant No. BR05236277 (S. D. Odintsov and R.Myrzakulov), Grant No. AP05135123 (R.Myrzakulov) and Grant No. 0118RK00935 while S. D. Odintsov was visiting ENU. This work has been partially supported by MINECO (Spain), Projects No. FIS2016-76363-P and No. PID2019–104397 GB-I00, and by AGAUR (Catalan Government), Project No. 2017-SGR-247.

Some dark energy cosmological models are constructed in the framework of a generalised Brans-Dicke theory which contains a self interacting potential and a dynamical coupling parameter. The dark sector of the universe is considered through a unified linear equation of state. The parameters of the unified dark fluid have been constrained from some physical basis. Since the universe is believed to have undergone a transition from an early deceleration to a late time acceleration, the deceleration parameter should have a signature flipping behaviour at the transition redshift. We have used a hybrid scale factor to simulate the dynamical behaviour of the deceleration parameter. Basing upon the observational constraints on the transition redshift, we have constructed four different transitioning dark energy models. The constructed models are confronted with observational data. For all the models, the behaviour of the dynamical scalar field, Brans-Dicke parameter, Self interacting potential are investigated. Also, on the basis of the generalised Brans-Dicke theory, we have estimated the time variation of the Newtonian gravitational constant., 17pages, 12 figures

The nature of dark matter and of dark energy which constitute more than $95\%$ of the energy in the Universe remains a great and unresolved question in cosmology. Cold dark matter can be made of an ultralight scalar field dominated by its mass term which interacts only gravitationally. The cosmological constant introduced to explain the recent acceleration of the Universe expansion can be easily replaced by a scalar field dominated by its potential. More generally, scalar fields are ubiquitous in cosmology: inflaton, dilatons, moduli, quintessence, fuzzy dark matter, dark fluid, etc. are some examples. One can wonder whether all these scalar fields are independent. The dark fluid model aims at unifying quintessence and fuzzy dark matter models with a unique scalar field. One step futher is to unify the dark fluid model with inflation. In the very early Universe such scalar fields are not strongly constrained by direct observations, but Big-Bang nucleosynthesis set constraints on scalar field models which lead to a modification on the abundance of the elements. In this talk we will present a scalar field model unifying dark matter, dark energy and inflation, and study constraints from Big-Bang nucleosynthesis on primordial scalar fields., Comment: Contribution to the proceedings of the 40th International Conference on High Energy physics - ICHEP2020, 6 pages, 2 figures

We study cosmological perturbations in a model of unified dark matter and dark energy with a sharp transition in the late-time universe. The dark sector is described by a dark fluid which evolves from an early stage at redshifts $z > z_C$ when it behaves as cold dark matter (CDM) to a late time dark energy (DE) phase ($z < z_C$) when the equation of state parameter is $w = -1 + \epsilon$, with a constant $\epsilon$ which must be in the range $0 < \epsilon < 2/3$. We show that fluctuations in the dark energy phase suffer from an exponential instability, the mode functions growing both as a function of comoving momentum $k$ and of conformal time $\eta$. In order that this exponential instability does not lead to distortions of the energy density power spectrum on scales for which we have good observational results, the redshift $z_C$ of transition between the two phases is constrained to be so close to zero that the model is unable to explain the supernova data., Comment: 17 pages; 2 figures; v2: minor changes to match the published version

The generalized Chaplygin gas could be considered as the unified dark fluid model because it might describe the past decelerating matter dominated era and at present time it provides an accelerating expansion of the Universe. In this paper, we employed the Planck 2015 cosmic microwave background anisotropy, type-Ia supernovae, observed Hubble parameter data sets to measure the full parameter space of the generalized Chaplygin gas as an unified dark matter and dark energy model. The model parameters B s and α determine the evolutional history of this unified dark fluid model by influencing the energy density ρ G C G = ρ G C G 0 [ B s + ( 1 − B s ) a − 3 ( 1 + α ) ] 1 ∕ ( 1 + α ) . We assume the pure adiabatic perturbation of unified generalized Chaplygin gas. In the light of Markov Chain Monte Carlo method, we found that B s = 0 . 75 9 − 0 . 032 − 0 . 046 + 0 . 020 + 0 . 051 and α = 0 . 080 1 − 0 . 0801 − 0 . 0801 + 0 . 0208 + 0 . 1087 at 2 σ level. The model parameter α is very close to zero, the nature of GCG model is very similar to cosmological standard model Λ CDM.

A 2-year-old castrated male mongrel dog presented with a well-demarcated fluctuant dermal mass, located on the back of the neck. Grossly along with cystic structures filled with a black greasy fluid, when cut open. Microscopically, the mass was multi-lobulated. The lobules consisted of neoplastic basaloid cells and showed central degeneration, forming multiple central cystic structures filled with dark melanin-pigmented materials. Immunohistochemically, the neoplastic cells were strongly positive for CK14 and partially positive for CK19, but negative for CK7, CK8/18, CD34, S-100, Melan-A and α-SMA. Based on the findings, the present case was diagnosed as a feline-type basal cell tumor characterized by cystic structures filled with abundant black fluid.

In present paper a spherically symmetric stellar configuration has been analyzed by assuming the matter distribution of the stellar configuration is anisotropic in nature and compared with the realistic objects, namely, the low mass X-ray binaries (LMXBs) and X-ray pulsars. The analytic solution has been obtained by utilizing the dark energy equation of state for the interior solution corresponding to the Schwarzschild exterior vacuum solution at the junction interface. Several physical properties like energy conditions, stability, mass-radius ratio, and surface redshift are described through mathematical calculations as well as graphical plots. It is found that obtained mass-radius ration of the compact stars candidates like 4U 1820-30, PSR J 1614-2230, Vela X-1 and Cen X-3 are very much consistent with the observed data by Gangopadhyay et al. (Mon. Not. R. Astron. Soc. 431, 3216 (2013)). So our proposed model would be useful in the investigation of the possible clustering of dark energy., Comment: 10 pages & 7 figures, Accepted for publication in Can.J.Phys. It matches with the published version

The algebraic classification of stress-energy tensors includes tensors whose symmetry is partly broken as compared with Einstein’s maximally symmetric cosmological term. This allows us to introduce, in a general setting, a vacuum dark fluid with variable density and pressures approaching the cosmological constant in certain space-time regions. The relevant class of Einstein equations describes cosmological models with time-evolving and spatially inhomogeneous vacuum dark energy and compact objects with de Sitter interiors generically related to vacuum dark energy, which can be responsible for observational effects typical of dark matter. The mass of objects is generically related to breaking of space-time symmetry from the de Sitter group. We outline the basic generic properties of regular cosmological models with vacuum dark energy, and of dark matter candidates with de Sitter interiors, including their observational signatures.

Two-Higgs-Doublet Model of Type-X in the large $\tan\beta$ limit becomes leptophilic to allow a light pseudo-scalar $A$ and thus provides an explanation of the muon $g-2$ anomaly. Introducing a singlet scalar dark matter $S$ in this context, one finds that two important dark matter properties, nucleonic scattering and self-annihilation, are featured separately by individual couplings of dark matter to the two Higgs doublets. While one of the two couplings is strongly constrained by direct detection experiments, the other remains free to be adjusted for the relic density mainly through the process $SS\to AA$. This leads to the $4\tau$ final states which can be probed by galactic gamma ray detections., Comment: Accepted version in Physics Letters B

In a recent paper, Erik Verlinde has developed the interesting possibility that spacetime and gravity may emerge from the entangled structure of an underlying microscopic theory. In this picture, dark matter arises as a response to the standard model of particle physics from the delocalized degrees of freedom that build up the dark energy component of the Universe. Dark matter physics is then regulated by a characteristic acceleration scale $a_0$, identified with the radius of the (quasi)-de Sitter universe we inhabit. For a point particle matter source, or outside an extended spherically symmetric object, MOND's empirical fitting formula is recovered. However, Verlinde's theory critically departs from MOND when considering the inner structure of galaxies, differing by a factor of 2 at the centre of a regular massive body. For illustration, we use the eight classical dwarf spheroidal satellites of the Milky Way. These objects are perfect testbeds for the model given their approximate spherical symmetry, measured kinematics, and identified missing mass. We show that, without the assumption of a maximal deformation, Verlinde's theory can fit the velocity dispersion profile in dwarf spheroidals with no further need of an extra dark particle component. If a maximal deformation is considered, the theory leads to mass-to-light ratios that are marginally larger than expected from stellar population and formation history studies. We also compare our results with the recent phenomenological interpolating MOND function of McGaugh {\it et al}, and find a departure that, for these galaxies, is consistent with the scatter in current observations., Comment: 12 pages, 5 figures, 1 table. To appear in MNRAS

In this paper, we will study some properties of oscillatons, spherically symmetric object made of a real time-dependent scalar field, using a self-interaction quartic scalar field potential as a class of what is called Self-Interacting Dark Matter (SIDM) models. Since the oscillatons can be regarded as models for astrophysical objects which play the role of dark matter, therefore investigation of their properties could have an important place in present time of cosmology researches. We investigate the properties of these objects by Numerical Solving the system of differential equations obtained from the Einstein–Klein–Gordon (EKG) equations to show their importance as new candidates for dark matter in the galactic scales.

Tilted Bianchi-I wet dark fluid cosmological model is investigated in Saez and Ballester scalar theory of gravitation. Background cosmologies are obtained for a constant deceleration parameter. We consider a linear relationship between the shear scalar and the expansion scalar. We have discussed some physical and geometrical properties of the models. In our models, equation of state of the dark energy is observed to behave like a cosmological constant at late times.

The generalized Chaplygin gas cosmology provides a prime example for the class of unified dark matter models, which substitute the two dark components of the standard cosmological ΛCDM concordance model by a single dark component. The equation of state of the generalized Chaplygin gas is characterised by a parameter α such that the standard ΛCDM model is recovered in the case α = 0 with respect to the background dynamics and the cosmic microwave background (CMB) statistics. This allows to investigate the concordance of different cosmological data sets with respect to α. We compare the supernova data of the Supernova Cosmology Project, the data of the baryon oscillation spectroscopic survey (BOSS) of the third Sloan digital sky survey (SDSS-III) and the CMB data of the Planck 2015 data release. The importance of the BOSS Lyman α forest BAO measurements is investigated. It is found that these data sets possess a common overlap of the confidence domains only for Chaplygin gas cosmologies very close to the ΛCDM model.

We discuss a possible principle for detecting dark matter axions in galactic halos. If axions constitute a condensate in the Milky Way, stimulated emissions of the axions from a type of excitation in condensed matter can be detectable. We provide general mechanism for the dark matter emission, and, as a concrete example, an emission of dark matter axions from magnetic vortex strings in a type II superconductor are investigated along with possible experimental signatures., 20 pages, no figure; corrected typos, added references, and minor changes

We consider a simple extension of the Standard Model that can account for the dark matter and explain the existence of neutrino masses. The model includes a vector-like doublet of SU(2), a singlet fermion, and two scalar singlets, all of them odd under a new Z $$_2$$ symmetry. Neutrino masses are generated radiatively by one-loop processes involving the new fields, while the dark matter candidate is the lightest neutral particle among them. We focus specifically on the case where the dark matter particle is one of the scalars and its relic density is determined by its Yukawa interactions. The phenomenology of this setup, including neutrino masses, dark matter and lepton flavor violation, is analyzed in some detail. We find that the dark matter mass must be below 600 GeV to satisfy the relic density constraint. Lepton flavor violating processes are shown to provide the most promising way to test this scenario. Future $$\mu \rightarrow 3e$$ and $$\mu $$ –e conversion experiments, in particular, have the potential to probe the entire viable parameter space of this model.