Bulk Viscous Cosmological Model with Interacting Dark Fluids

We study a cosmological model for a spatially flat Universe whose constituents are a dark energy field and a matter field comprising baryons and dark matter. The constituents are assumed to interact with each other, and a non-equilibrium pressure is introduced to account for irreversible processes. We take the non-equilibrium pressure to be proportional to the Hubble parameter within the framework of a first-order thermodynamic theory. The dark energy and matter fields are coupled by their barotropic indexes, which depend on the ratio between their energy densities. We adjust the free parameters of the model to optimize the fits to the Hubble parameter data. We compare the viscous model with the non-viscous one, and show that the irreversible processes cause the dark-energy and matter-density parameters to become equal and the decelerated–accelerated transition to occur at earlier times. Furthermore, the density and deceleration parameters and the distance modulus have the correct behavior, consistent with a viable scenario of the present status of the Universe.     

Model for a Universe described by a non-minimally coupled scalar field and interacting dark matter

In this work the evolution of a Universe model is investigated where a scalar field, non-minimally coupled to space-time curvature, plays the role of quintessence and drives the Universe to a present accelerated expansion. A non-relativistic dark matter constituent that interacts directly with dark energy is also considered, where the dark matter particle mass is assumed to be proportional to the value of the scalar field. Two models for dark matter pressure are considered: the usual one, pressureless, and another that comes from a thermodynamic theory and relates the pressure with the coupling between the scalar field and the curvature scalar. Although the model has a strong dependence on the initial conditions, it is shown that the mixture consisted of dark components plus baryonic matter and radiation can reproduce the expected red-shift behavior of the deceleration parameter, density parameters and luminosity distance.     

Neutrino condensate as origin of dark energy

We propose a new solution to the origin of dark energy. We suggest that it was created dynamically from the condensate of a singlet neutrino at a late epoch of the early Universe through its effective self-interaction. This singlet neutrino is also the Dirac partner of one of the three observed neutrinos, hence dark energy is related to neutrino mass. The onset of this condensate formation in the early Universe is also related to matter density and offers an explanation of the coincidence problem of why dark energy (70%) and total matter (30%) are comparable at the present time. We demonstrate this idea in a model of neutrino mass with (right-handed) singlet neutrinos and a singlet scalar.     

A modified Chaplygin gas model with interaction

A modified Chaplygin gas (MCG) model of unifying dark energy and dark matter is considered in this paper, in which dark energy interacts with dark matter. Concretely, the evolution of such a unified dark sectors model is studied and the statefinder diagnostic to the MCG model is performed in our model. By analysis, it is shown that the effective equation of state (EoS) parameter of dark energy can cross the so-called phantom divide ω = −1, the behavior of MCG will be like ΛCDM in the future and therefore our Universe will not end up with Big Rip in the future. Furthermore, we plot the evolution trajectories of the MCG model in the statefinder parameter r–s plane and illustrate the discrimination between this scenario and the generalized Chaplygin gas (GCG) model.     

Theoretical direct WIMP detection rates for inelastic scattering to excited states

The recent WMAP and Planck data have confirmed that exotic dark matter together with the vacuum energy (cosmological constant) dominate in the flat Universe. Many extensions of the standard model provide dark matter candidates, in particular Weakly Interacting Massive Particles (WIMPs). Thus the direct dark matter detection is central to particle physics and cosmology. Most of the research on this issue has hitherto focused on the detection of the recoiling nucleus. In this paper we study transitions to the excited states, possible in some nuclei, which have sufficiently low lying excited states. Good examples are the first excited states of ¹²⁷I and ¹²⁹Xe. We find appreciable branching ratios for the inelastic scattering mediated by the spin cross sections. So, in principle, the extra signature of the gamma ray following the de-excitation of these states can, in principle, be exploited experimentally.     

Letter: A Model of Dark Energy for the Accelerating Universe

Recent observations of large scale structure of the Universe, especially that of Type Ia supernovae, indicate that the Universe is flat and is accelerating, and that the dominant energy density in the Universe is the cosmic dark energy. We propose a model in which the cosmic effective Yang-Mills condensate familiar in particle physics plays the role of the dark energy that causes the acceleration of the Universe. Since the quantum effective Yang-Mills field in certain states has the equation of state p _y = – _y , when employed as the cosmic matter source, it naturally results in an accelerating expansion of the Universe. With the matter components ( _m 1/3) being added into the model, the composition of YM condensate and matter components can give rise to the desired equation of state w –2/3 for the Universe.     

Phantom dark energy with varying-mass dark matter particles: Acceleration and cosmic coincidence problem

We investigate several varying-mass dark matter particle models in the framework of phantom cosmology. We examine whether there exist late-time cosmological solutions, corresponding to an accelerating universe and possessing dark energy and dark matter densities of the same order. Imposing exponential or power-law potentials and exponential or power-law mass dependence, we conclude that the coincidence problem cannot be solved or even alleviated. Thus, if dark energy is attributed to the phantom paradigm, varying-mass dark matter models cannot fulfill the basic requirement that led to their construction.     

Basic properties of the Einstein equations with a Ricci-Scalar-Dependent cosmological term

Basic properties of the Einstein equations modified by a cosmological Λ-term dependent on the Ricci scalar R are considered. We show that in addition to a nonzero divergence of the energy-momentum tensor of the matter and the consequent cold matter mass nonconservation as the Universe expands, this model suggests a significant modification of the equations for the gravitational potential and particle acceleration in the Newtonian approximation. These circumstances allow the necessary criteria for possible functional dependences Λ(R) to be formulated. Nevertheless, by introducing a variable Λ-term, we can look at the problems of dark matter and dark energy anew. In particular, we show that the model in which the cosmological term depends linearly on the Ricci scalar (this corresponds to the approximation of a more complex dependence in the case of low matter densities) makes it possible to satisfactorily describe the rotation curves of galaxies without invoking the dark matter hypothesis and to construct a cosmological model with a variable vacuum energy density, in qualitative agreement with the present views of the early Universe.     

Supersymmetron

We consider a supersymmetric model of dark energy coupled to cold dark matter: the supersymmetron. In the absence of cold dark matter, the supersymmetron converges to a supersymmetric minimum with a vanishing cosmological constant. When cold dark matter is present, the supersymmetron evolves to a matter dependent minimum where its energy density does not vanish. In the early Universe until the recent past of the Universe, the energy density of the supersymmetron is negligible compared to the cold dark matter energy density. Away from the supersymmetric minimum, the equation of state of the supersymmetron is constant and negative. When the supersymmetron reaches the neighbourhood of the supersymmetric minimum, its equation of state vanishes rapidly. This leads to an acceleration of the Universe which is transient unless supersymmetry breaking induces a pure cosmological constant and acceleration of the Universe does not end. Moreover, we find that the mass of supersymmetron is always greater than the gravitino mass. As a result, the supersymmetron generates a short ranged fifth force which evades gravitational tests. On the other hand, we find that the supersymmetron may lead to relevant effects on large scale structures.     

Dark energy interacting with neutrinos and dark matter: a phenomenological theory

A model for a flat homogeneous and isotropic Universe composed of dark energy, dark matter, neutrinos, radiation and baryons is analyzed. The fields of dark matter and neutrinos are supposed to interact with the dark energy. The dark energy is considered to obey either the van der Waals or the Chaplygin equations of state. The ratio between the pressure and the energy density of the neutrinos varies with the red-shift simulating massive and non-relativistic neutrinos at small red-shifts and non-massive relativistic neutrinos at high red-shifts. The model can reproduce the expected red-shift behaviors of the deceleration parameter and of the density parameters of each constituent. Dedicated to Professor Ingo Müller on the occasion of his seventieth birthday.     

Letter: Exact Solutions for the Interacting Tachyonic–Dark Matter System

We find exact solutions leading to power law accelerated expansion for a homogeneous, isotropic and spatially flat universe, dominated by an interacting mixture of cold dark matter and a tachyonic field such that the ratio of the energy densities of both components at late times is constant and no coincidence problem arises.     

Fundamental uncertainty in the BAO scale from isocurvature modes

Small fractions of isocurvature perturbations correlated with the dominant adiabatic mode are shown to be a significant primordial systematic for Baryon Acoustic Oscillation (BAO) surveys which must be accounted for in future surveys. Isocurvature modes distort the standard ruler distance by broadening and shifting the peak in the galaxy correlation function. While a single isocurvature mode does not significantly degrade dark energy constraints, the general case with multiple isocurvature modes leads to biases that exceed 7σ on average in the dark energy parameters even for isocurvature amplitudes undetectable by Planck. Accounting for all isocurvature modes corrects for this bias but degrades the dark energy figure of merit by at least 50% in the case of the Boss experiment. However the BAO data in turn provides significantly stronger constraints on the nature of the primordial perturbations. Future large galaxy surveys will thus be powerful probes of exotic physics in the early Universe in addition to helping pin down the nature of dark energy.     

Dark energy interacting with dark matter and a third fluid: Possible EoS for this component

A cosmological model of dark energy interacting with dark matter and another general component of the universe is considered. The equations for the coincidence parameters r and s, which represent the ratios between dark energy and dark matter and the other cosmic fluid respectively, are analyzed in terms of the stability of stationary solutions. The obtained general results allow to shed some light on the equations of state of the three interacting fluids, due to the constraints imposed by the stability of the solutions. We found that for an interaction proportional to the sum of the dark energy density and the third fluid density, the hypothetical fluid must have positive pressure, which leads naturally to a cosmological scenario with radiation, unparticle or even some form of warm dark matter as the third interacting fluid.     

A CoGeNT confirmation of the DAMA signal

The CoGeNT Collaboration has recently reported a rising low energy spectrum in their ultra low noise Germanium detector. This is particularly interesting as the energy range probed by CoGeNT overlaps with the energy region in which DAMA has observed their annual modulation signal. We show that the mirror dark matter candidate can simultaneously explain both the DAMA annual modulation signal and the rising low energy spectrum observed by CoGeNT. This constitutes a model dependent confirmation of the DAMA signal and adds weight to the mirror dark matter paradigm.     

Semi-holographic universe

By assuming that a dark component (dark energy) in the universe strictly obeys the holographic principle, that is, its entropy is one fourth of the apparent horizon, we find that the existence of the other dark component (dark matter) is compulsory, as a compensation of dark energy, based on the first law of thermodynamics. By using the method of dynamical system analysis, we find that there exists a stable dark energy-dark matter scaling solution at late time, which is helpful to solve the coincidence problem. For reasonable parameters, the deceleration parameter is well consistent with current observations.     

Unified origin for baryonic visible matter and antibaryonic dark matter

We present a novel mechanism for generating both the baryon and dark matter densities of the Universe. A new Dirac fermion X carrying a conserved baryon number charge couples to the standard model quarks as well as a GeV-scale hidden sector. CP-violating decays of X, produced nonthermally in low-temperature reheating, sequester antibaryon number in the hidden sector, thereby leaving a baryon excess in the visible sector. The antibaryonic hidden states are stable dark matter. A spectacular signature of this mechanism is the baryon-destroying inelastic scattering of dark matter that can annihilate baryons at appreciable rates relevant for nucleon decay searches.     

Top ten accelerating cosmological models

Recent astronomical observations indicate that the Universe is presently almost flat and undergoing a period of accelerated expansion. Basing on Einstein's general relativity all these observations can be explained by the hypothesis of a dark energy component in addition to cold dark matter (CDM). Because the nature of this dark energy is unknown, it was proposed some alternative scenario to explain the current accelerating Universe. The key point of this scenario is to modify the standard FRW equation instead of mysterious dark energy component. The standard approach to constrain model parameters, based on the likelihood method, gives a best-fit model and confidence ranges for those parameters. We always arbitrary choose the set of parameters which define a model which we compare with observational data. Because in the generic case, the introducing of new parameters improves a fit to the data set, there appears the problem of elimination of model parameters which can play an insufficient role. The Bayesian information criteria of model selection (BIC) is dedicated to promotion a set of parameters which should be incorporated to the model. We divide class of all accelerating cosmological models into two groups according to the two types of explanation acceleration of the Universe. Then the Bayesian framework of model selection is used to determine the set of parameters which gives preferred fit to the SNIa data. We find a few of flat cosmological models which can be recommend by the Bayes factor. We show that models with dark energy as a new fluid are favoured over models featuring a modified FRW equation.     

Testing the cosmic coincidence problem and the nature of dark energy

Dark energy models which alter the relative scaling behavior of dark energy and matter could provide a natural solution to the cosmic coincidence problem-why the densities of dark energy and dark matter are comparable today. A generalized class of dark energy models is introduced which allows noncanonical scaling of the ratio of dark matter and dark energy with the Robertson-Walker scale factor a(t). We show that determining whether there is a coincidence problem, and the extent of cosmic coincidence, can be addressed by several forthcoming experiments.     

Extended theories of gravity and their cosmological and astrophysical applications

Astrophysical observations are pointing out huge amounts of “dark matter” and “dark energy” needed to explain the observed large scale structure and cosmic dynamics. The emerging picture is a spatially flat, homogeneous Universe undergoing the today observed accelerated phase. Despite of the good quality of astrophysical surveys, commonly addressed as Precision Cosmology, the nature and the nurture of dark energy and dark matter, which should constitute the bulk of cosmological matter-energy, are still unknown. Furthermore, up to now, no experimental evidence has been found, at fundamental level, to explain such mysterious components. The problem could be completely reversed considering dark matter and dark energy as “shortcomings” of General Relativity in its simplest formulation (a linear theory in the Ricci scalar R, minimally coupled to the standard perfect fluid matter) and claiming for the “correct” theory of gravity as that derived by matching the largest number of observational data, without imposing any theory a priori. As a working hypothesis, accelerating behavior of cosmic fluid, large scale structure, potential of galaxy clusters, rotation curves of spiral galaxies could be reproduced by means of extending the standard theory of General Relativity. In other words, gravity could acts in different ways at different scales and the above “shortcomings” could be due to incorrect extrapolations of the Einstein gravity, actually tested at short scales and low energy regimes. After a survey of what is intended for Extended Theories of Gravity in the so called “metric” and “Palatini” approaches, we discuss some cosmological and astrophysical applications where the issues related to the dark components are addressed by enlarging the Einstein theory to more general f (R) Lagrangians, where f (R) is a generic function of Ricci scalar R, not assumed simply linear. Obviously, this is not the final answer to the problem of “dark-components” but it can be considered as an operative scheme whose aim is to avoid the addition of unknown exotic ingredients to the cosmic pie.     

On the Einstein-Cartan cosmology vs. Planck data

The first comprehensive analyses of Planck data reveal that the cosmological model with dark energy and cold dark matter can satisfactorily explain the essential physical features of the expanding Universe. However, the inability to simultaneously fit the large and small scale TT power spectrum, the scalar power index smaller than unity, and the observations of the violation of the isotropy found by few statistical indicators of the CMB urge theorists to search for explanations. We show that the model of the Einstein-Cartan cosmology with clustered dark matter halos and their corresponding clustered angular momenta coupled to torsion can account for small-scale-large-scale discrepancy and larger peculiar velocities (bulk flows) for galaxy clusters. The nonvanishing total angular momentum (torsion) of the Universe enters as a negative effective density term in the Einstein-Cartan equations causing partial cancellation of the mass density. The integrated Sachs-Wolfe contribution of the Einstein-Cartan model is negative, and it can therefore provide partial cancellation of the large-scale power of the TT CMB spectrum. The observed violation of the isotropy appears as a natural ingredient of the Einstein-Cartan model caused by the spin densities of light Majorana neutrinos in the early stage of the evolution of the Universe and bound to the lepton CP violation and matter-antimatter asymmetry.