Cosmological Dark Sector Research Articles

Model-independent reconstruction of the interacting dark energy kernel: Binned and Gaussian process

Abstract The cosmological dark sector remains an enigma, offering numerous possibilities for exploration. One particularly intriguing option is the (non-minimal) interaction scenario between dark matter and dark energy. In this paper, to investigate this scenario, we have implemented Binned and Gaussian model-independent reconstructions for the interaction kernel alongside the equation of state; while using data from BAOs, Pantheon+ and Cosmic Chronometers. In addition to the reconstruction process, we conducted a model selection to analyze how our methodology performed against the standard ΛCDM model. The results revealed a slight indication, of at least 1σ confidence level, for some oscillatory dynamics in the interaction kernel and, as a by-product, also in the DE and DM. A consequence of this outcome is the possibility of a sign change in the direction of the energy transfer between DE and DM and a possible transition from a negative DE energy density in early-times to a positive one at late-times. While our reconstructions provided a better fit to the data compared to the standard model, the Bayesian Evidence showed an intrinsic penalization due to the extra degrees of freedom. Nevertheless these reconstructions could be used as a basis for other physical models with lower complexity but similar behavior.

A unified cosmological dark sector from a Bose–Einstein condensate

We examine the viability of cosmological solution(s) describing a unified picture of the dark side of the universe from a Bose–Einstein condensate (BEC) of light bosons. The energy density of the BEC, together with its quantum potential, can indeed account for such a unification, in the sense that the (dust-like) cold dark matter and the dark energy components emerge from the same source. In particular, the bulk of the dark energy can be attributed to the quantum potential, in the quantum corrected Raychaudhuri–Friedmann equation, when the ‘macroscopic’ BEC wave-function is taken to be such that the corresponding probability density is construed as the energy density of the dusty fluid. However, there arises a purely quantum mechanical back-reaction, of even the visible baryons, on the effective dark energy and dark matter contents, which crucially determines the mass of the BEC. We determine the constraint on such a back-reaction, and hence on the BEC mass, from physical considerations, as well as estimate the same using recent observational data.

One-parameter dynamical dark-energy from the generalized Chaplygin gas

The fact that Einstein's equations connect the space-time geometry to the total matter content of the cosmic substratum, but not to individual contributions of the matter species, can be translated into a degeneracy in the cosmological dark sector. Such degeneracy makes it impossible to distinguish cases where dark energy (DE) interacts with dark matter (DM) from a dynamical non-interacting scenario using observational data based only on time or distance measurements. In this paper, based on the non-adiabatic generalized Chaplygin gas (gCg) model, we derive and study some cosmological consequences of a varying one-parameter dynamical DE parameterization, which does not allow phantom crossing.We perform a parameter selection using the most recent publicly available data, such as the data from Planck 2018, eBOSS DR16, Pantheon and KiDS-1000. In light of this analysis, we assess how our model responds to the S 8 tension. We find that CMB data impose strong constraints on the model and conclude that the S 8 tension can be alleviated only for values of the model parameters very close to the ΛCDM cosmology.

Generalized Logotropic Models and Their Cosmological Constraints

We propose a new class of cosmological unified dark sector models called “Generalized Logotropic Models”. They depend on a free parameter n. The original logotropic model is a special case of our generalized model corresponding to n=1. The ΛCDM model is recovered for n=0. In our scenario, the Universe is filled with a single fluid, a generalized logotropic dark fluid (GLDF), whose pressure P includes higher order logarithmic terms of the rest-mass density ρm. The total energy density ϵ is the sum of the rest-mass energy density ρmc2 and the internal energy density u which play the roles of dark matter energy density ϵm and dark energy density ϵde, respectively. We investigate the cosmological behavior of the generalized logotropic models by focusing on the evolution of the energy density, scale factor, equation of state parameter, deceleration parameter and squared speed of sound. Low values of n≤3 are favored. We also study the asymptotic behavior of the generalized logotropic models. In particular, we show that the model presents a phantom behavior and has three distinct ways of evolution depending on the value of n. For 0<n≤2, it leads to a little rip and for n>2 to a big rip. We predict the value of the big rip time as a function of n without any free (undetermined) parameter.

Pregeometric first order Yang-Mills theory

The standard description of particles and fundamental interactions is crucially based on a regular metric background. In the language of differential geometry, this dependence is encoded into the action via Hodge star dualization. As a result, the conventional forms of the scalar and Yang-Mills actions break down in a pregeometric regime where the metric is degenerate. This suggests the use of first order formalism, where the metric may emerge from more fundamental constituents and the theory can be consistently extended to the pregeometric phase. We systematically explore different realizations and interpretations of first order formalism, finding that a fundamental vector or spinor substructure brings about continuum magnetization and polarization as integration constants. This effect is analogous to the description of the cosmological dark sector in a recent self-dual formulation of gravity, and the similar form obtained for the first order Yang-Mills theory suggests new paths toward unification.

Mimetic-metric-torsion with induced axial mode and phantom barrier crossing

We extend the basic formalism of mimetic-metric-torsion gravity theory, in a way that the mimetic scalar field can manifest itself geometrically as the source of not just the trace mode of torsion, but its axial (or, pseudo-trace) mode as well. Specifically, we consider the mimetic field to be (i) coupled explicitly to the well-known Holst extension of the Riemann–Cartan action, and (ii) identified with the square of the associated Barbero–Immirzi field, presumably a pseudo-scalar. The conformal symmetry originally prevaling in the theory would still hold, as the associated Cartan transformations do not affect the torsion pseudo-trace, and hence the Holst term. Demanding the theory to preserve the spatial parity symmetry as well, we show a geometric unification of the cosmological dark sector, and the feasibility of a super-accelerating regime in the course of evolution of the universe. From the observational perspective, assuming the cosmological evolution profile to be very close to that for varLambda CDM, we further illustrate a smooth crossing of the so-called phantom barrier at a low red-shift, albeit with a restricted parametric domain. Subsequently, we determine the extent of the super-acceleration by examining the evolution of the relevant torsion parameters.

Evolving ultralight scalars into non-linearity with Lagrangian perturbation theory

ABSTRACT Many models of high energy physics suggest that the cosmological dark sector consists of not just one, but a spectrum of ultralight scalar particles with logarithmically distributed masses. To study the potential signatures of low concentrations of ultralight axion (also known as fuzzy) dark matter, we modify Lagrangian perturbation theory (LPT) by distinguishing between trajectories of different dark matter species. We further adapt LPT to include the effects of a quantum potential, which is necessary to generate correct initial conditions for ultralight axion simulations. Based on LPT, our modified scheme is extremely efficient on large scales and it can be extended to an arbitrary number of particle species at very little computational cost. This allows for computation of self-consistent initial conditions in mixed dark matter models. Additionally, we find that shell-crossing is delayed for ultralight particles and that the deformation tensor extracted from LPT can be used to identify the range of redshifts and scales for which the Madelung formalism of fuzzy dark matter can lead to divergences.

Looking for interactions in the cosmological dark sector

We study observational signatures of non-gravitational interactions between the dark components of the cosmic fluid, which can be either due to creation of dark particles from the expanding vacuum or an effect of the clustering of a dynamical dark energy. In particular, we analyse a class of interacting models (Λ(t)CDM), characterised by the parameter α, that behaves at background level like cold matter at early times and tends to a cosmological constant in the asymptotic future. Our analysis improves previous works, where only the position of the Cosmic Microwave Background (CMB) first peak and supernova and LSS data where used, considering for the first time both background and primordial perturbations evolutions of the model. We use full spectra of the CMB Planck data together with late time observations, such as the Joint Light-curve Analysis (JLA) supernovae data, the Hubble Space Telescope (HST) measurement of the local value of the Hubble-Lemaȋtre parameter, and primordial deuterium abundance from Lyα systems to test the observational viability of the model and some of its extensions, adapting an Einstein-Boltzmann solver code for our purpose. We found that there is no preference for values of α different from zero (characterising interaction), even if there are some indications for positive values when the minimal Λ(t)CDM model is analysed. When extra degrees of freedom in the relativistic component of the cosmic fluid are considered, the data favour negative values of α, which means an energy flux from dark energy to dark matter.

An estimate of the dark matter density from galaxy clusters and supernovae data

In this paper, we discuss a model-independent way to obtain the present dark matter density parameter (Ωc,0) by combining gas mass fraction measurements in galaxy clusters (fgas), type Ia supernovae (SNe Ia) observations and measurements of the cosmic baryon abundance from observations of absorption systems at high redshifts. Our estimate is Ωc,0 = 0.244 ± 0.013 (1σ). By considering the latest local measurement of the Hubble constant, we obtain ΩM,0 = 0.285 ± 0.013 (1σ) for the total matter density parameter. We also investigate departures of the evolution of the dark matter density with respect to the usual a−3 scaling, as usual in interacting models of dark matter and dark energy. As the current data cannot confirm or rule out such an interaction, we perform a forecast analysis to estimate the necessary improvements in number and accuracy of upcoming fgas and SNe Ia observations to detect a possible non-minimal coupling in the cosmological dark sector.

Can the cosmological dark sector be modeled by a single scalar field?

In a previous paper it was shown that a minimally coupled scalar field of mass $M \sim H_0$ can describe both components of the dark sector in a unified way. In the solution found, the dark energy component decays linearly with the Hubble parameter, with a homogeneous creation of dark matter. In the present note we show that a $\Lambda$CDM dark sector can also be modeled by such a single field. More generally, we show that the system of Klein-Gordon and Einstein equations admits a uniparametric family of solutions that is equivalent to a non-adiabatic (with zero sound speed) generalised Chaplygin gas.

The cosmological dark sector as a scalar \u03c3-meson field

Previous quantum field estimations of the QCD vacuum in the expanding space-time lead to a dark energy component scaling linearly with the Hubble parameter, which gives the correct figure for the observed cosmological term. Here we show that this behaviour also appears at the classical level, as a result of the chiral symmetry breaking in a low energy, effective sigma -model. The dark sector is described in a unified way by the sigma condensate and its fluctuations, giving rise to a decaying dark energy and a homogeneous creation of non-relativistic dark particles. The creation rate and the future asymptotic de Sitter horizon are both determined by the sigma mass scale.

Is the cosmological dark sector better modeled by a generalized Chaplygin gas or by a scalar field?

Both scalar fields and (generalized) Chaplygin gases have been widely used separately to characterize the dark sector of the universe. Here we investigate the cosmological background dynamics for a mixture of both these components and quantify the fractional abundances that are admitted by observational data from supernovae of type Ia and from the evolution of the Hubble rate. Moreover, we study how the growth rate of (baryonic) matter perturbations is affected by the dark-sector perturbations.

Does a generalized Chaplygin gas correctly describe the cosmological dark sector?

Yes, but only for a parameter value that makes it almost coincide with the standard model. We reconsider the cosmological dynamics of a generalized Chaplygin gas (gCg) which is split into a cold dark matter (CDM) part and a dark energy (DE) component with constant equation of state. This model, which implies a specific interaction between CDM and DE, has a $\Lambda$CDM limit and provides the basis for studying deviations from the latter. Including matter and radiation, we use the (modified) CLASS code \cite{class} to construct the CMB and matter power spectra in order to search for a gCg-based concordance model that is in agreement with the SNIa data from the JLA sample and with recent Planck data. The results reveal that the gCg parameter $\alpha$ is restricted to $|\alpha|\lesssim 0.05$, i.e., to values very close to the $\Lambda$CDM limit $\alpha =0$. This excludes, in particular, models in which DE decays linearly with the Hubble rate.

Low-ztest for interacting dark energy

A non-minimal coupling between the dark matter and dark energy components may offer a way of solving the so-called coincidence problem. In this paper we propose a low-$z$ test for such hypothesis using measurements of the gas mass fraction $f_{\rm{gas}}$ in relaxed and massive galaxy clusters. The test applies to any model whose dilution of dark matter is modified with respect to the standard $a^{-3}$ scaling, as usual in interacting models, where $a$ is the cosmological scale factor. We apply the test to current $f_{\rm{gas}}$ data and perform Monte Carlo simulations to forecast the necessary improvements in number and accuracy of upcoming observations to detect a possible interaction in the cosmological dark sector. Our results show that improvements in the present relative error $\sigma_{\rm{gas}}/f_{\rm{gas}}$ are more effective to achieve this goal than an increase in the size of the $f_{\rm{gas}}$ sample.

Polytropic dark matter flows illuminate dark energy and accelerated expansion

Currently, a large amount of data implies that the matter constituents of the cosmological dark sector might be collisional. An attractive feature of such a possibility is that, it can reconcile dark matter (DM) and dark energy (DE) in terms of a single component, accommodated in the context of a polytropic-DM fluid. Accordingly, we explore the time evolution and the dynamical characteristics of a spatially-flat cosmological model, in which, in principle, there is no DE at all. Instead, in this model, the DM itself possesses some sort of fluid-like properties, i.e., the fundamental units of the Universe matter-energy content are the volume elements of a DM fluid, performing polytropic flows. In this case, the energy of this fluid's internal motions is also taken into account as a source of the universal gravitational field. This form of energy can compensate for the extra energy needed to compromise spatial flatness, namely, to justify that, today, the total-energy density parameter is exactly unity. The polytropic cosmological model, depends on only one free parameter, the corresponding exponent, \Gamma. What makes this model particularly interesting, is that, for \Gamma < 0.541, the (conventional) pressure becomes negative enough, so that the Universe accelerates its expansion at cosmological redshifts below a transition value. Several physical reasons impose further constraints on the value of \Gamma, which, eventually, is settled down to the range -0.089 < \Gamma < 0. Such a cosmological model does not suffer either from the age problem or from the coincidence problem. At the same time, this model reproduces to high accuracy the distance measurements performed with the aid of the supernovae Type Ia standard candles, and most naturally interprets, not only when, but also, why the Universe transits from deceleration to acceleration, thus arising as a mighty contestant for a DE model.

Interactions in the dark sector of the Universe

Interactions inside the cosmological dark sector influence the cosmological dynamics. As a consequence, the future evolution of the Universe may be different from that predicted by the ΛCDM model. We review main features of several recently studied models with nongravitational couplings between dark matter and dark energy.

Cosmology with Ricci dark energy

We assume the cosmological dark sector to consist of pressureless matter and holographic dark energy with a cutoff length proportional to the Ricci scale. The requirement of separate energy-momentum conservation of the components is shown to establish a relation between the matter fraction and the (necessarily time-dependent) equation-of-state parameter of the dark energy. Focusing on intrinsically adiabatic pressure perturbations of the dark-energy component, the matter perturbations are found as linear combinations of the total energy-density perturbations of the cosmic medium and the relative (nonadiabatic) perturbations of the components. The resulting background dynamics is consistent with observations from supernovae of type Ia, baryonic acoustic oscillations and the differential age of old objects. The perturbation dynamics, on the other hand, is plagued by instabilities which excludes any phantom-type equation of state. The only stable configuration is singled out by a fixed relation between the present matter fraction ${\ensuremath{\Omega}}_{m0}$ and the present value ${\ensuremath{\omega}}_{0}$ of the equation-of-state parameter of the dark energy. However, this instability-avoiding configuration is only marginally consistent with the observationally preferred background values of the mentioned parameters.

Influence of small-scale inhomogeneities on the cosmological consistency tests

Abstract The current cosmological dark sector (dark matter plus dark energy) is challenging our comprehension about the physical processes taking place in the Universe. Recently, some authors tried to falsify the basic underlying assumptions of such dark matter–dark energy paradigm. In this Letter, we show that oversimplifications of the measurement process may produce false positives to any consistency test based on the globally homogeneous and isotropic Λ cold dark matter (ΛCDM) model and its expansion history based on distance measurements. In particular, when local inhomogeneity effects due to clumped matter or voids are taken into account, an apparent violation of the basic assumptions (‘Copernican Principle’) seems to be present. Conversely, the amplitude of the deviations also probes the degree of reliability underlying the phenomenological Dyer–Roeder procedure by confronting its predictions with the accuracy of the weak lensing approach. Finally, a new method is devised to reconstruct the effects of the inhomogeneities in a ΛCDM model, and some suggestions of how to distinguish between clumpiness (or void) effects from different cosmologies are discussed.

Interacting model for the cosmological dark sector

We discuss a new interacting model for the cosmological dark sector in which the attenuated dilution of cold dark matter scales as $a^{-3}f(a)$, where f(a) is an arbitrary function of the cosmic scale factor $a$. From thermodynamic arguments, we show that f(a) is proportional to entropy source of the particle creation process. In order to investigate the cosmological consequences of this kind of interacting models, we expand f(a) in a power series and viable cosmological solutions are obtained. Finally, we use current observational data to place constraints on the interacting function f(a).

Unified models of the cosmological dark sector

We model the cosmological substratum by a viscous fluid that is supposed to provide a unified description of the dark sector and pressureless baryonic matter. In the homogeneous and isotropic background the total energy density of this mixture behaves as a generalized Chaplygin gas. The perturbations of this energy density are intrinsically non-adiabatic and source relative entropy perturbations. The resulting baryonic matter power spectrum is shown to be compatible with the 2dFGRS and SDSS (DR7) data. A joint statistical analysis, using also Hubble-function and supernovae Ia data, shows that, different from other studies, there exists a maximum in the probability distribution for a negative present value q0 ≈ −0.53 of the deceleration parameter. Moreover, different from other approaches, the unified model presented here favors a matter content that is of the order of the baryonic matter abundance suggested by big-bang nucleosynthesis.