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Register a new userCould dark matter be a natural consequence of a dynamical universe?
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We construct the gravitating mass of an isolated composite system on asymptotically-flat spacetimes within conventional general relativity and investigate when this quantity is well defined. For stationary spacetimes, this quantity is known to exactly equal the physical (ADM) mass. However, it remains an open question whether these two masses are equal in the absence of a timelike Killing vector. This is especially apropos since our universe has an `origin and hence no such Killing vector. Further, if these masses failed to agree then composite systems could behave as if they had a `dark component, whose gravitating mass would not equal the physical mass-energy present. The existence of such an apparent discrepancy is indeed ubiquitous in galaxies and galaxy clusters, though currently it is attributed to the presence of dark matter. We conclude that the theoretical question of the relation between these masses for dynamical spacetimes is ripe for attention.
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A cosmological model of an holographic dark energy interacting with dark matter throughout a decaying term of the form $Q=3(lambda_1rho_{DE} + lambda_2rho_m) H$ is investigated. General constraint on the parameters of the model are found when accelerated expansion is imposed and we found a phantom scenarios, without any reference to a specific equation of state for the dark energy. The behavior of equation of stated for dark energy is also discussed.
We present an in-depth exploration of the phenomenon of dynamical friction in a universe where the dark matter is composed entirely of so-called Fuzzy Dark Matter (FDM), ultralight bosons of mass $msimmathcal{O}(10^{-22}),$eV. We review the classical treatment of dynamical friction before presenting analytic results in the case of FDM for point masses, extended mass distributions, and FDM backgrounds with finite velocity dispersion. We then test these results against a large suite of fully non-linear simulations that allow us to assess the regime of applicability of the analytic results. We apply these results to a variety of astrophysical problems of interest, including infalling satellites in a galactic dark matter background, and determine that emph{(1)}~for FDM masses $mgtrsim 10^{-21}, {rm eV}, c^{-2}$, the timing problem of the Fornax dwarf spheroidals globular clusters is no longer solved and emph{(2)}~the effects of FDM on the process of dynamical friction for satellites of total mass $M$ and relative velocity $v_{rm rel}$ should require detailed numerical simulations for $left(M/10^9~M_{odot}right) left(m/10^{-22}~{rm eV}right)left(100~{rm km}~{rm s}^{-1}/v_{rm rel}right) sim 1$, parameters which would lie outside the validated range of applicability of any currently developed analytic theory, due to transient wave structures in the time-dependent regime.
A recent article uncovered a surprising dynamical mechanism at work within the (vacuum) Einstein `flow that strongly suggests that many closed 3-manifolds that do not admit a locally homogeneous and isotropic metric textit{at all} will nevertheless evolve, under Einsteinian evolution, in such a way as to be textit{asymptotically} compatible with the observed, approximate, spatial homogeneity and isotropy of the universe cite{Moncrief:2015}. Since this previous article, however, ignored the potential influence of textit{dark-energy} and its correspondent accelerated expansion upon the conclusions drawn, we analyze herein the modifications to the foregoing argument necessitated by the inclusion of a textit{positive} cosmological constant --- the simplest viable model for dark energy.
We investigate a cosmological model in which dark energy identified with the vacuum energy which is running and decaying. In this model vacuum is metastable and decays into a bare (true) vacuum. This decaying process has a quantum nature and is described by tools of the quantum decay theory of unstable systems. We have found formulas for an asymptotic behavior of the energy density of dark energy in the form of a series of inverse powers of the cosmological time. We investigate the dynamics of FRW models using dynamical system methods as well as searching for exact solutions. From dynamical analysis we obtain different evolutional scenarios admissible for all initial conditions. For the interpretation of the dynamical evolution caused by the decay of the quantum vacuum we study the thermodynamics of the apparent horizon of the model as well as the evolution of the temperature. For the early Universe, we found that the quantum effects modified the evolution of the temperature of the Universe. In our model the adiabatic approximation is valid and the quantum vacuum decay occurs with an adequate unknown particle which constitutes quantum vacuum. We argue that the late-time evolution of metastable energy is the holographic dark energy.
We analyze the causal structure of McVittie spacetime for a classical bouncing cosmological model. In particular, we compute the trapping horizons of the metric and integrate the trajectories of radial null geodesics before, during, and after the bounce takes place. In the contracting phase up to the occurrence of the bounce, a dynamical black hole is present. When the universe reaches a certain minimum scale, the trapping horizons disappear and the black hole ceases to exist. After the bounce, the central weak singularity becomes naked. In the expanding phase, for large positive values of the cosmic time, the behaviour of null geodesics indicates that the solution contains a black hole. These results suggest that neither a contracting nor an expanding universe can accommodate a black hole at all times.
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