arXiv subjects
Publications and source records attributed to Matts Roos.
Although the cusp-core controversy for dwarf galaxies is seen as a problem, I argue that the cored central profiles can be explained by flattened cusps because they suffer from conflicting measurements and poor statistics and because there is a large number of conventional processes that could have flattened them since their creation, none of which requires new physics. Other problems, such as "too big to fail", are not discussed.
We propose to explain the dimming of distant supernovae as the combined effect of dark energy and a Lemaître-Tolman-Bondi (LTB) metric. We take dark energy to have a shear-dependent pressure $p_{DE}=(w_0+εw_1)ρ_{DE}$ where $ε$ is the ratio of LTB shear to LTB expansion.
Dark matter has been introduced to explain mass deficits noted at different astronomical scales, in galaxies, groups of galaxies, clusters, superclusters and even across the full horizon. Dark matter makes itself felt only through its gravitational effects. This review summarizes phenomenologically all the astrophysical and cosmological probes that have been used to give evidence for its existence.
We study the entropy-corrected version of the new agegraphic dark energy (NADE) model and dark matter in a spatially non-flat Universe and in the framework of Hořava-Lifshitz cosmology. For the two cases containing noninteracting and interacting entropy-corrected NADE (ECNADE) models, we derive the exact differential equation that determines the evolution of the ECNADE density parameter. Also the deceleration parameter is obtained. Furthermore, using a parametrization of the equation of state parameter of the ECNADE model as $ω_Λ(z)=ω_0+ω_1 z$, we obtain both $ω_0$ and $ω_1$. We find that in the presence of interaction, the equation of state parameter $ω_0$ of this model can cross the phantom divide line which is compatible with the observation.
The study started in a former work about the Dilaton mean field stabilization thanks to the effective potential generated by the existence of massive fermions, is here extended. Three loop corrections are evaluated in addition to the previously calculated two loop terms. The results indicate that the Dilaton vacuum field tend to be fixed at a high value close to the Planck scale, in accordance with the need for predicting Einstein gravity from string theory. The mass of the Dilaton is evaluated to be also a high value close to the Planck mass, which implies the absence of Dilaton scalar signals in modern cosmological observations. These properties arise when the fermion mass is chosen to be either at a lower bound corresponding to the top quark mass, or alternatively, at a very much higher value assumed to be in the grand unification energy range. One of the three 3-loop terms is exactly evaluated in terms of Master integrals. The other two graphs are however evaluated in their leading logarithm correction in the perturbative expansion. The calculation of the non leading logarithmic contribution and the inclusion of higher loops terms could made more precise the numerical estimates of the vacuum field value and masses, but seemingly are expected not to change the qualitative behavior obtained. The validity of the here employed Yukawa model approximation is argued for small value of the fermion masses with respect to the Planck one. A correction to the two loop calculation done in the previous work is here underlined.
Dark matter has been introduced to explain many independent gravitational effects at different astronomical scales, in galaxies, groups of galaxies, clusters, superclusters and even across the full horizon. This review describes the accumulated astronomical, astrophysical, and cosmological evidence for dark matter. It is written at a non-specialist level and intended for an audience with little or only partial knowledge of astrophysics or cosmology.
Accelerating Chaplygin gas combined with the decelerating braneworld Dvali-Gabadadze-Porrati (DGP) model can produce an overall accelerated expansion of the order of magnitude seen. Both models have similar asymptotic properties at early and late cosmic times, and are characterized by a length scale. Taking the length scales to be proportional one obtains a combined model with three free parameters, one more than the LCDM model, which fits supernovae data equally well. We further constrain it by the CMB shift parameter, and by requiring that the model yields a longer age of the Universe than that of the oldest star HE 1523-0901, t * = 13.4\pm 0.8(stat)\pm 1.8(syst). In contrast to generalized DGP and Chaplygin gas models, this is a genuine alternative to the cosmological constant model because it does not reduce to it in any limit of the parameter space.
Explanations to the accelerated expansion of the Universe are usually sought either in modifications of Einstein gravity or in new forms of energy density. An example of modified gravity is the braneworld Dvali-Gabadadze-Porrati (DGP) model which is characterized by a length scale which marks the cross-over between physics occurring in our four-dimensional brane and in a five-dimensional bulk space. An example of dark energy is Chaplygin gas which has similar asymptotic properties at early and late cosmic times. Since Chaplygin gas gives too much acceleration we combine it with the self-decelerating branch of the DGP model, taking the cross-over scales to be proportional. This 3-parameter model fits supernovae data with a goodness-of-fit equalling that of the LambdaCDM model. In contrast to generalized DGP models and Chaplygin gas models, this model is unique in the sense that it does not reduce to LambdaCDM for any choice of parameters.
After a brief introduction to the sixteenth and seventeenth century views of the Universe and the nineteenth century paradox of Olbers, we start the history of the cosmic expansion with Hubble's epochal discovery of the recession velocities of spiral galaxies. By then Einstein's theories of relativity were well known, but no suitable metric was known. Prior to introducing General Relativity we embark on a non-chronological derivation of the Robertson-Walker metric directly from Special Relativity and the Minkowski metric endowed with a Gaussian curvature. This permits the definition of all relativistic distance measures needed in observational astronomy. Only thereafter do we come to General Relativity, and describe some of its consequences: gravitational lensing, black holes, various tests, and the cornerstone of the standard Big Bang model, the Friedmann-Lemaitre equations. Going backwards in time towards Big Bang we first have to trace the thermal history, and then understand the needs for a cosmic inflation and its predictions. The knowledge of the Big Bang model is based notably on observations of the Cosmic Microwave Background Radiation, large scale structures, and the redshifts of distant supernovae. They tell us that gravitating matter is dominated by a dark and dissipationless component of unknown composition, and that the observable part of the Universe exhibits an accelerated expansion representing a fraction of the energy even larger than gravitating matter.
The expansion of the Universe is accelerating, as testified by observations of supernovae of type Ia as a function of redshift. Explanations are of two types: modifications of Einstein gravity or new forms of energy, coined dark energy.The accelerated expansion is explained here by a combination of Dvali-Gabadadze-Porrati (DGP) model gravity and Chaplygin gas dark energy. Both models are characterized by a length scale L which may be the same. The continuity equation for the combined model is derived in flat geometry, and solved by numerical methods. The solution is shown to have the expected properties: at very small scales (a< >L) as a cosmological constant. The modifications to the DGP model and the Chaplygin gas model occur for values of a L. The results show an increase in the present dark energy density relative to the plain DGP model.
Our procedure to obtain best values for cosmological parameters from five recent multiparameter fits is as follows. We first study the values quoted for $r, α_s, w+1 (w_0+1), w_1$ and $Ω_k$, arriving at the conclusion that they do not differ significantly from zero, and their correlations to other parameters are insignificant. In what follows they can be therefore fixed. The neutrino mass sum $Σm_ν$ also does not differ significantly from zero, but since neutrinos are massive their sum must be included as a free parameter. We then compare the values obtained in five large flat-space determinations of the parameters $Σm_ν, ω_b, ω_m, h, τ, n_s, A_s$ and $σ_8$. For these we compute the medians and the 17-percentile and 83-percentile errors by a described procedure.
Excesses of Galactic gamma rays in the 1-100 GeV region observed by EGRET and by WMAP have been interpreted as signals of dark matter (LSP) annihilation. We argue that the excess of TeV gamma rays from the Galactic center observed by H.E.S.S. signals a heavier dark matter component with a very small relic density.
This conference covered dark matter particle properties and searches, lensing by dark matter, and dark matter distributions in galaxies and clusters; dark energy phenomenology and theoretical models; surveys of structure and galaxy formation, large scale simulations and neutrino cosmology; astronomy and star formation; cosmic rays; cosmic microwave background; inflation in the primordial universe, cosmic strings, brane cosmology, and the fine-structure constant.
We update the curves of the effective degrees of freedom for the energy density $g_*(T)$ and for the entropy density $g_{*S}(T)$ during the era of radiation domination in the Universe. We find that a plain count of effective degrees of freedom sets an upper limit to the temperature of the quark-hadron transition at $T_c< 235$ MeV for the energy density and $T_c< 245$ MeV for the entropy density.
The results of different analyses of the dynamical parameters of the Universe are converging towards agreement. Remaining disagreements reflect systematic errors coming either from the observations or from differences in the methods of analysis. Compiling the most precise parameter values with our estimates of such systematic errors added, we find the following best values: the baryonic density parameter Omega_bh^2 = 0.019 +/- 0.02, the density parameter of the matter component Omega_m = 0.29 +/- 0.06, the density parameter of the cosmological constant Omega_lambda = 0.71 +/- 0.07, the spectral index of scalar fluctuations n_s = 1.02 +/- 0.08, the equation of state of the cosmological constant w_lambda < -0.86, and the deceleration parameter q_0 = -0.56 +/- 0.04. We do not modify the published best values of the Hubble parameter H_0 = 0.73 +/- 0.07 and the total density parameter Omega_0 ^{+0.03}_{-0.02}.
For a flat universe presently dominated by smooth energy, either cosmological constant (LCDM) or quintessence (QCDM), we calculate the asymptotic collapsed mass fraction as function of the present ratio of smooth energy to matter energy $\mathcal R_0$. Identifying the normalized collapsed fraction as a conditional probability for habitable galaxies, we observe that the observed present ratio $\mathcal R_0 \sim 2$ is likely in LCDM, but more likely in QCDM. Inverse application of Bayes' Theorem makes the Anthropic Principle a predictive scientific principle: the data implies that the prior probability for $\mathcal R_0$ must be essentially flat over the anthropically allowed range. Interpreting this prior as a distribution over {\em theories} lets us predict that any future theory of initial conditions must be indifferent to $\mathcal R_0$. This application of the Anthropic Principle does not demand the existence of other universes.
A new approach to the study of the AGN phenomenon is proposed, in which the nucleus activity is related to the metric of the inner massive black hole. The possibility of a Toroidal Black Hole (TBH), in contrast to the usual Spherical Black Hole (SBH), is discussed as a powerful tool in understanding AGN related phenomena, such as the energetics, the production of jets and the acceleration of particles, the shape of the magnetic field and the lifetime of nucleus activity.
We combine the two balloon experiments BOOMERANG and MAXIMA-1 with our previous fit which used 9 constraints and concluded that the Universe is flat. The result is that the flatness is robust, $Ω_0 = 0.97 \pm 0.05$.