Recent cosmological data have provided evidence for a "dark" relativistic background at high statistical significance. Parameterized in terms of the number of relativistic degrees of freedom Neff, however, the current data seems to indicate a higher value than the one expected in the standard scenario based on three active neutrinos. This dark radiation component can be characterized not only by its abundance but also by its clustering properties, as its effective sound speed and its viscosity parameter. It is therefore crucial to study the correlations among the dark radiation properties and key cosmological parameters, as the dark energy equation of state or the running of the scalar spectral index, with current and future CMB data. We find that dark radiation with viscosity parameters different from their standard values may be misinterpreted as an evolving dark energy component or as a running spectral index in the power spectrum of primordial fluctuations.
We study cosmological models with interaction between dark energy (DE) and dark matter (DM). For the interaction term $Q$ in cosmic evolution equations, there is a model-independent degeneracy-breaking (D-B) point when $Q_{1}$ (a part of $Q$) equals to zero, where the interaction can be probed without degeneracy between the constant DE equation of state (EoS).
In this work, we study tracker phantom dark energy models with a general parameterization of the scalar potentials. Our analysis also considers the scenario of having both phantom field and the cosmological constant as the dark energy components. A detailed statistical analysis with current cosmological observations shows an increase in the value of the Hubble parameter due to the presence of phantom dark energy but it can not alleviate the Hubble tension completely. Our results using Bayesian methods suggests a decisive evidence in favor of a phantom field over a positive cosmological constan…
We consider a scenario where dark energy and baryons are dynamically coupled without any energy transfer. In this scenario, the background cosmology is unaffected and, at the perturbations level, the coupling only appears through the corresponding Euler equations of dark energy and baryons. We then explore some phenomenological consequences of this scenario and their signatures in several cosmological observables. In particular, we show its ability to suppress the growth of cosmic structures. We also constrain the parameters of the model with cosmological data and show that an interaction of d…
Evolution of cosmological perturbations is considered in the model with dynamical dark energy which non-gravitationally interacts with dark matter. The dark energy equation of state parameter (EoS) is varying in time and is parameterized by its adiabatic sound speed. Such model of interacting dark energy has advantages over model with constant EoS, because it avoids non-adiabatic instabilities of dark energy at radiation dominated epoch for certain types of interaction in dark sector. The stability conditions for solutions of equations of dark energy perturbations were derived. The impact of s…
The local measurement of $H_0$ is in tension with the prediction of $Λ$CDM model based on the Planck data. This tension may imply that dark energy is strengthened in the late-time Universe. We employ the latest cosmological observations on CMB, BAO, LSS, SNe, $H(z)$ and $H_0$ to constrain several interacting dark energy models. Our results show no significant indications for the interaction between dark energy and dark matter. The $H_0$ tension can be moderately alleviated, but not totally released.
In this work, we study a new quintessence model associated with non-Abelian gauge fields, minimally coupled to Einstein gravity. This gauge theory has been recently introduced and studied as an inflationary model, called gauge-flation. Here, however, we are interested in the late time cosmology of the model in the presence of matter and radiation to explain the present time accelerating Universe. During the radiation and matter eras, the gauge field tracks radiation and basically acts like a dark radiation sector. As we approach lower redshifts, the dark component takes the form of a dark ener…
Fuzzy Dark Matter (FDM), motivated by string theory, has recently become a hot candidate for dark matter. The rest mass of FDM is believed to be $\sim 10^{-22}$eV and the corresponding de-Broglie wave length is $\sim 1$kpc. Therefore, the quantum effect of FDM plays an important role in structure formation. In order to study the cosmological structure formation in FDM model, several simulation techniques have been introduced. We review the current status and challenges in the cosmological simulation for the FDM model in this paper.
The quasi-isotropic expansion for a simple two-fluid cosmological model, including radiation and string gas is constructed. The first non-trivial order expressions for the metric coefficients, energy densities and velocities are explicitly written down. Their small and large time asymptotics are studied. It is found that the large time asymptotic for the anisotropic component of the metric coefficients grows faster than that of the isotropic (trace-proportional) component.
We devise a fully self-consistent simulation pipeline for the first time to study the interaction between dark matter and dark energy. We perform convergence tests and show that our code is accurate on different scales. Using the parameters constrained by Planck, Type Ia Supernovae, Baryon Acoustic Oscillations (BAO) and Hubble constant observations, we perform cosmological N-body simulations. We calculate the resulting matter power spectra and halo mass functions for four different interacting dark energy models. In addition to the dark matter density distribution, we also show the inhomogene…
A cosmological approach based on considering a cosmic background with non-zero torsion is shown in order to give an option of explaining a possible phantom evolution, not ruled out according to the current observational data. We revise some aspects of the formal schemes on torsion and, according them, we develop a formalism which can be an interesting alternative for exploring Cosmology.
One of the open questions in modern cosmology is the small scale crisis of the cold dark matter paradigm. Increasing attention has recently been devoted to self-interacting dark matter models as a possible answer. However, solving the so-called "missing satellites" problem requires in addition the presence of an extra relativistic particle (dubbed dark radiation) scattering with dark matter in the early universe. Here we investigate the impact of different theoretical models devising dark matter dark radiation interactions on large scale cosmological observables. We use cosmic microwave backgr…
If even a relatively small number of black holes were created in the early universe, they will constitute an increasingly large fraction of the total energy density as space expands. It is thus well-motivated to consider scenarios in which the early universe included an era in which primordial black holes dominated the total energy density. Within this context, we consider Hawking radiation as a mechanism to produce both dark radiation and dark matter. If the early universe included a black hole dominated era, we find that Hawking radiation will produce dark radiation at a level $ΔN_{\rm eff} …
Evidence has continued to accumulate over the last few decades as to the existence and nature of dark matter. Depending on the particle candidate, the dark matter can exhibit one of several cosmologically defined models: hot dark matter, cold dark matter, warm dark matter, self-interacting dark matter, and fuzzy dark matter. In this paper I review the relevance and status of these models, whether it is possible for more than one of these models to each constitute some fraction of the dark matter, and discuss the prospects for determining if any of these models can successfully describe the pro…
The standard paradigm of collisionless cold dark matter is in tension with measurements on large scales. In particular, the best fit values of the Hubble rate $H_0$ and the matter density perturbation $σ_8$ inferred from the cosmic microwave background seem inconsistent with the results from direct measurements. We show that both problems can be solved in a framework in which dark matter consists of two distinct components, a dominant component and a subdominant component. The primary component is cold and collisionless. The secondary component is also cold, but interacts strongly with dark ra…
Primordial black holes in the mass range from $10^{-5}$ to $10^9$ g might have existed in the early universe. Via their evaporation mechanism (completed before Big Bang Nucleosynthesis), they might have released stable particles beyond the Standard Model. We reconsider the possibility that such particles might constitute the main part or a fraction of the dark matter observed today, updating the impact on this scenario from warm dark matter constraints. If sufficiently light, stable particles from primordial black holes evaporation might also provide a significant contribution to dark radiatio…
We consider a cosmological scenario where a relativistic particle and a stable massive particle are simultaneously produced from the decay of a late-decaying particle after Big-Bang Nucleosynthesis but before matter-radiation equality. The relativistic and massive particles behave as dark radiation and warm dark matter, respectively. Due to a common origin, the warmness and abundances are closely related. We refer to the models that lead to such a scenario as Common Origin of Warm and Relativistic Decay Products (COWaRD). We show that COWaRD predicts a correlation between the amount of dark ra…
The standard model of particle physics is extended by adding a purely neutrinophilic dark sector. It is shown that theories which accommodate standard model neutrinos as dark radiation are resurrected. Sterile neutrinos bridge the visible and the dark sector and can keep their mutual interactions effective even after the epoch of big bang nucleosythesis. This prolonged contact between dark matter and standard model neutrinos solves the well-known small-scale structure problems of the cosmological standard model. The dark sector presented in this work satisfies all experimental and observationa…
It is well known that gravitons can convert into photons, and vice versa, in the presence of cosmological magnetic fields. We study this conversion process in the context of atomic dark matter scenario. In this scenario, we can expect cosmological dark magnetic fields, which are free from the stringent constraint from the cosmic microwave observations. We find that gravitons can effectively convert into dark photons in the presence of cosmological dark magnetic fields. The graviton-dark photon conversion effect may open up a new window for ultra high frequency gravitational waves.
We investigate possible cosmological effects of interacting scalar radiation and dark matter. After its decoupling, scalar radiation can stream freely as neutrinos or self-interact strongly as perfect fluid, highly depending on the magnitude of its self-couplings. We obtain the general and novel structure for self-scattering rate and compare it with the expansion rate of our Universe. If its trilinear/cubic coupling is non-zero, scalar radiation can be eventually treated as perfect fluid. Possible effects on CMB are also discussed. When this scalar also mediates interaction among dark matter p…
During the process of structure formation in the universe matter is converted into radiation through a variety of processes such as light from stars, infrared radiation from cosmic dust and gravitational waves from binary black holes/neutron stars and supernova explosions. The production of this astrophysical radiation background (ARB) could affect the expansion rate of the universe and the growth of perturbations. Here, we aim at understanding to which level one can constraint the ARB using future cosmological observations. We model the energy transfer from matter to radiation through an effe…
We construct high-precision models of the Universe that contain radiation, a cosmological constant, and periodically distributed inhomogeneous matter. The density contrasts in these models are allowed to be highly non-linear, and the cosmological expansion is treated as an emergent phenomenon. This is achieved by employing a generalised version of the post-Newtonian formalism, and by joining together inhomogeneous regions of space-time at reflection symmetric junctions. Using these models, we find general expressions that precisely and unambiguously quantify the effect of small-scale inhomogen…
Non-thermalized dark matter is a cosmologically viable alternative to the widely studied weakly interacting massive particle. We study the evolution of the dark matter phase-space distributions arising from freeze-in and superWIMP production as well as the combination of both. Utilizing our implementation in CLASS, we investigate the cosmological imprints on the matter power spectrum, constrained by Lyman-$α$ forest observations. For the explicit example of a colored $t$-channel mediator model, we explore the cosmologically allowed parameter space highlighting the interplay of Lyman-$α$ constr…
Decaying cold dark matter (CDM) has been considered as a mechanism to tackle the tensions in the Hubble expansion rate and the clustering of matter. However, polarization measurements of the cosmic microwave background (CMB) severely constrain the fraction of dark matter decaying before recombination, and lensing of the CMB anisotropies by large-scale structure set strong constraints on dark matter decaying after recombination. Together, these constraints make an explanation of the Hubble tension in terms of decaying dark matter unlikely. In response to this situation, we investigate whether a…
The dark axion portal is a coupling of an axion-like particle to a dark photon kinetically mixed with the visible photon. We show how this portal, when applied to the relaxion, can lead to cosmological relaxation of the weak scale using dark photon production. The key backreaction mechanism involves the Schwinger effect: As long as electroweak symmetry is unbroken, Schwinger production of massless Standard Model fermions, which carry dark millicharges, suppresses the dark photon production. Once the electroweak symmetry is broken, the fermions acquire mass and the suppression is lifted. An enh…
The existence of dark radiation that is completely decoupled from the standard model in the early Universe leaves open the possibility of an associated dark radiation isocurvature mode. We show that the presence of dark radiation isocurvature leads to spatial variation in the primordial abundances of helium and deuterium due to spatial variation in $N_{\rm eff}$ during Big Bang nucleosynthesis. We use the result to constrain the existence of such an isocurvature mode on scales down to $\sim 1$ Mpc scales. By measuring the excess variance in the primordial helium to hydrogen and deuterium to hy…
This brief contribution is devoted to phenomenological consequences of deviations from Lorentz invariance in gravity and dark matter. We will discuss main effects on cosmological observables and current constraints derived from astrophysical and cosmological data.
Dark energy is investigated from the perspective of quantum cosmology. It is found that, together with an appropriate normal ordering factor $q$, only when there is dark energy then can the cosmological wave function be normalized. This interesting observation may require further attention.
