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Register a new userMass functions in coupled Dark Energy models
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We evaluate the mass function of virialized halos, by using Press & Schechter (PS) and/or Steth & Tormen (ST) expressions, for cosmologies where Dark Energy (DE) is due to a scalar self-interacting field, coupled with Dark Matter (DM). We keep to coupled DE (cDE) models known to fit linear observables. To implement the PS-ST approach, we start from reviewing and extending the results of a previous work on the growth of a spherical top-hat fluctuation in cDE models, confirming their most intriguing astrophysical feature, i.e. a significant baryon-DM segregation, occurring well before the onset of any hydrodynamical effect. Accordingly, the predicted mass function depends on how halo masses are measured. For any option, however, the coupling causes a distortion of the mass function, still at z=0. Furthermore, the z-dependence of cDE mass functions is mostly displaced, in respect to LambdaCDM, in the opposite way of uncoupled dynamical DE. This is an aspect of the basic underlying result, that even a little DM-DE coupling induces relevant modifications in the non-linear evolution. Therefore, without causing great shifts in linear astrophysical observables, the DM-baryon segregation induced by the coupling can have an impact on a number of cosmological problems, it e.g., galaxy satellite abundance, spiral disk formation, apparent baryon shortage, entropy input in clusters, etc..
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We consider theories in which there exists a nontrivial coupling between the dark matter sector and the sector responsible for the acceleration of the universe. Such theories can possess an adiabatic regime in which the quintessence field always sits at the minimum of its effective potential, which is set by the local dark matter density. We show that if the coupling strength is much larger than gravitational, then the adiabatic regime is always subject to an instability. The instability, which can also be thought of as a type of Jeans instability, is characterized by a negative sound speed squared of an effective coupled dark matter/dark energy fluid, and results in the exponential growth of small scale modes. We discuss the role of the instability in specific coupled CDM and Mass Varying Neutrino (MaVaN) models of dark energy, and clarify for these theories the regimes in which the instability can be evaded due to non-adiabaticity or weak coupling.
We present three distinct types of models of dark energy in the form of a scalar field which is explicitly coupled to dark matter. Our construction draws from the pull-back formalism for fluids and generalises the fluid action to involve couplings to the scalar field. We investigate the cosmology of each class of model both at the background and linearly perturbed level. We choose a potential for the scalar field and a specific coupling function for each class of models and we compute the Cosmic Microwave Background and matter power spectra.
We provide a general framework for studying the evolution of background and cosmological perturbations in the presence of a vector field $A_{mu}$ coupled to cold dark matter (CDM). We consider an interacting Lagrangian of the form $Q f(X) T_c$, where $Q$ is a coupling constant, $f$ is an arbitrary function of $X=-A_{mu}A^{mu}/2$, and $T_c$ is a trace of the CDM energy-momentum tensor. The matter coupling affects the no-ghost condition and sound speed of linear scalar perturbations deep inside the sound horizon, while those of tensor and vector perturbations are not subject to modifications. The existence of interactions also modifies the no-ghost condition of CDM density perturbations. We propose a concrete model of coupled vector dark energy with the tensor propagation speed equivalent to that of light. In comparison to the $Q=0$ case, we show that the decay of CDM to the vector field leads to the phantom dark energy equation of state $w_{rm DE}$ closer to $-1$. This alleviates the problem of observational incompatibility of uncoupled models in which $w_{rm DE}$ significantly deviates from $-1$. The maximum values of $w_{rm DE}$ reached during the matter era are bounded from the CDM no-ghost condition of future de Sitter solutions.
Cosmological limits on neutrino masses are softened, by more than a factor 2, if Cold Dark Matter (CDM) and Dark Energy (DE) are coupled. In turn, a neutrino mass yielding $Omega_ u$ up to $sim0.20$ allows coupling levels $beta simeq 0.15, $ or more, already easing the coincidence problem. The coupling, in fact, displaces both $P(k)$ and $C_l$ spectra in a fashion opposite to neutrino mass. Estimates are obtained through a Fisher--matrix technique.
Large primordial Black Hole (PBH) formation is enhanced if strongly coupled scalar and spinor fields ($Phi$ and $psi$) are a stable cosmic component since the primeval radiative expansion (SCDEW models). In particular, we show that PBH formation is easier at a specific time, i.e., when the asymptotic mass $m_H$, acquired by the $psi$ field at the higgs scale, becomes dominant, so that the typical BH mass $M_{BH}$ depends on $m_H$ value. For instance, if $m_H sim 100,$ eV $(1$ keV$)$ and the coupling $beta sim 8.35 (37)$, PBH with $M_{BH} simeq 10^7-10^8 $ $(10^3-10^4), M_odot$ could form. The very mechanism enhancing PBH formation also causes technical difficulties to evaluate the transfer function of SCDEW models at high $k$. A tentative solution of this problem leaves only minor discrepancies from $Lambda$CDM, also at these scales, gradually vanishing for greater $m_H$ values. We conclude that, for suitable parameter choices, SCDEW models could be the real physics underlying $Lambda$ CDM, so overcoming its fine tuning and coincidence problems, with the extra bonus of yielding large BH seeds.
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