Munch

Authors: Nickolay Y. Gnedin (1,2), Andrey V. Kravtsov (2,3) (1 - Fermilab, 2 - U.Chicago, 3 - KICP)
Comments: 8 pages, 9 figures; submitted to ApJ

We use a combination of high-resolution gasdynamics simulations of high-redshift dwarf galaxies and dissipationless simulations of a Milky Way sized halo to estimate the expected abundance and spatial distribution of the dwarf satellite galaxies that formed most of their stars around z~8 and evolved only little since then. Such galaxies can be considered as fossils of the reionization era, and studying their properties could provide a direct window into the early, pre-reionization stages of galaxy formation. We show that 5-15% of the objects existing at z~8 do indeed survive until the present in the MW like environment without significant evolution. This implies that it is plausible that the fossil dwarf galaxies do exist in the Local Group. Because such galaxies form their stellar systems early during the period of active merging and accretion, they should have spheroidal morphology regardless of their current distance from the host galaxy. We show that both the expected luminosity function and spatial distribution of dark matter halos which are likely to host fossil galaxies agree reasonably well with the observed distributions of the luminous (L_V>10^6 Lsun) Local Group fossil candidates near the host galaxy (d<200 kpc). However, the predicted abundance is substantially larger (by a factor of 2-3) for fainter galaxies (L_V<10^6 Lsun) at larger distances (d>300 kpc). We discuss several possible explanations for this discrepancy.

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(From the recent AAS Meeting)
Evidence for Evolving Dark Energy from gamma ray bursts

Observational constraints on the acceleration of the Universe

Authors: Yungui Gong, Anzhong Wang
Comments: 9 figures, revtex

We first parameterize the deceleration parameter to discuss its behavior. The advantage of this parametrization is that we do not need to assume any underlying theory of gravity. By fitting the model to the 157 gold sample supernova Ia data, we find strong evidence that the Universe is currently accelerating and it accelerated in the past. By fitting the model to the 115 nearby and Supernova Legacy Survey supernova Ia data, the evidence that the Universe is currently accelerating is weak and there is strong evidence that the Universe once accelerated in the past. We then use a dark energy parametrization to discuss the problem again. If we fit the model to the supernova Ia data alone, we find weak evidence that the Universe is accelerating and the current matter density is higher than that measured from other experiments. Therefore we add the Sloan Digital Sky Survey data to constrain the dark energy model. The behavior of the deceleration parameter is almost the same as that obtained from parameterizing the deceleration parameter.

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SZE Signals in Cluster Models

Authors: Beth A. Reid, David N. Spergel
Comments: 34 pages, 8 figures, submitted to ApJ

The upcoming generation of SZE surveys will shed fresh light onto the study of clusters. What will this new observational window reveal about cluster properties? What can we learn from combining X-ray, SZE, and optical observations? How do variations in the gas entropy profile, dark matter concentration, accretion pressure, and bound baryon fraction affect SZE observables? We investigate the signature of these important cluster parameters with an analytic model of the intracluster medium (ICM). Given the current uncertainties in ICM physics, our approach is to span the range of plausible models motivated by observations and a small set of assumptions. We find a tight relation between the central Compton parameter and the X-ray luminosity outside the cluster core, suggesting that these observables carry the same information about the ICM. The total SZE luminosity is proportional to the thermal energy of the gas, and is a surprisingly robust indicator of cluster mass: $L_{SZ} \propto f_{b} M^{5/3}$. We show that a combination of $L_{SZ}$ and the half-luminosity radius $r_{SZ}$ provides a measure of the potential energy of the cluster gas, and thus we can deduce the total energy content of the bound ICM. We caution that any systematic variation of the baryon fraction will distort the expected $L_{SZ} - M$ calibration to be used to study the evolution of cluster number density.

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On the baryon mass function for galaxy clusters

Authors: S. E. Nuza (IAFE), A. Blanchard (LATT)
Comments: 5 pages, A&A accepted

The evolution of the cluster abundance with redshift is known to be a powerful cosmological constrain when applied to X-ray clusters. Recently, the use of the evolution of the baryon mass function has been proposed as a new variant that is free of the uncertainties present in the Temperature-Mass relation. A flat model with Omega_matter ~ 0.3 was shown to be preferred in the case of a standard cold dark matter scenario. We have compared the high redshift predictions of the baryon mass in clusters for a more general class of spectra having a varying shape factor Gamma in models normalized to reproduce the local baryon mass function. We have found that models with Omega_matter ~ 1 and Gamma ~ 0.12 reproduce high redshift cluster data equally well as the concordance model and we conclude that the baryon mass function evolution on its own is not an efficiently discriminant among the more general family of flat cosmological models with non-standard power spectra.

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Constraining Lorentz violations with Gamma Ray Bursts

Authors: Maria Rodriguez Martinez, Tsvi Piran
Comments: 16 pages, 4 figures

Gamma ray bursts are excellent candidates to constrain physical models which break Lorentz symmetry. We consider deformed dispersion relations which break the boost invariance and lead to an energy-dependent speed of light. In these models, simultaneously emitted photons from cosmological sources reach Earth with a spectral time delay that depends on the symmetry breaking scale. We estimate the possible bounds which can be obtained by comparing the spectral time delays with the time resolution of available telescopes. We discuss the best strategy to reach the strongest bounds. We compute the probability of detecting bursts that improve the current bounds. The results are encouraging. Depending on the model, it is possible to build a detector that within several years will improve the present limits of 0.015 m_pl.

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Microwave sky and the local Rees-Sciama effect

Authors: Aleksandar Rakic, Syksy Rasanen, Dominik J. Schwarz
Comments: 5 pages, 2 figures, 2 tables

The microwave sky shows unexpected features at the largest angular scales, among them the alignments of the dipole, quadrupole and octopole. Motivated by recent X-ray cluster studies, we investigate the possibility that local structures at the 100 h^-1 Mpc scale could be responsible for such correlations. These structures give rise to a local Rees-Sciama contribution to the microwave sky that may amount to \Delta T/T \sim 10^-5 at the largest angular scales. We model local structures by a spherical overdensity (Lemaitre-Tolman-Bondi model) and assume that the Local Group is falling toward the centre. We superimpose the local Rees-Sciama effect on a statistically isotropic, gaussian sky. As expected we find alignments among low multipoles, but a closer look reveals that they do not agree with the type of correlations revealed by the data.

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Running Spectral Index as a Probe of Physics at High Scales

Authors: G. Ballesteros, J.A. Casas, J.R. Espinosa
Comments: 33 pages, LaTeX, 5 ps figures
Report-no: IFT-UAM/CSIC-05-51

The WMAP results on the scalar spectral index n and its running with scale, though preliminary, open a very interesting window to physics at very high energies. We address the problem of finding inflaton potentials well motivated by particle physics which can accomodate WMAP data. We make a model independent analysis of a large class of models: those with flat tree-level potential lifted by radiative corrections, which cause the slow rolling of the inflaton and the running of n. This includes typical hybrid inflation models. In the small-coupling regime the predictions for the size and running of n are remarkably neat, e.g. -dn/dln k=(n-1)^2 << 1, and n does not cross n=1, contrary to WMAP indications. On the other hand, n can run significantly if the couplings are stronger but at the price of having a small number of e-folds, Ne. We also examine the effect of mass thresholds crossed during inflation. Finally, we show that the presence of non-renormalizable operators for the inflaton, suppressed by a mass scale above the inflationary range, is able to give both dn/dln k ~ O(-0.05) and Ne ~ 50.

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The Lyth Bound and the End of Inflation

Authors: Richard Easther (Yale), William H. Kinney, Brian A. Powell (Univ. at Buffalo, SUNY)
Comments: 10 pages, RevTeX, 3 figures

We derive an extended version of the well-known Lyth Bound on the total variation of the inflaton field, incorporating higher order corrections in slow roll. We connect the field variation $\Delta\phi$ to both the spectral index of scalar perturbations and the amplitude of tensor modes. We then investigate the implications of this bound for ``small field'' potentials, where the field rolls off a local maximum of the potential. The total field variation during inflation is {\em generically} of order $m_{\rm Pl}$, even for potentials with a suppressed tensor/scalar ratio. Much of the total field excursion arises in the last e-fold of inflation and in single field models this problem can only be avoided via fine-tuning or the imposition of a symmetry. Finally, we discuss the implications of this result for inflationary model building in string theory and supergravity.

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Observing Brane Inflation

Authors: Sarah E. Shandera, S.-H. Henry Tye
Comments: 31 pages, 12 figures

Linking the slow-roll scenario and the Dirac-Born-Infeld scenario of ultra-relativistic roll (where, thanks to the warp factor, the inflaton moves slowly even with an ultra-relativistic Lorentz factor), we find that the KKLMMT D3/anti-D3 brane inflation is robust, that is, enough e-folds of inflation is quite generic in the parameter space of the model. We show that the intermediate regime of relativistic roll can be quite interesting observationally. Introducing appropriate inflationary parameters, we explore the parameter space and give the constraints and predictions for the cosmological observables in this scenario. Among other properties, this scenario allows the saturation of the present observational bound of either the tensor/scalar ratio r (in the intermediate regime) or the non-Gaussianity f_NL (in the ultra-relativistic regime), but not both.

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The Paths of Quintessence

Authors: Eric V. Linder
Comments: 12 pages, 7 figures

The structure of the dark energy equation of state phase plane holds important information on the nature of the physics. We explain the bounds of the freezing and thawing models of scalar field dark energy in terms of the tension between the steepness of the potential vs. the Hubble drag. Additionally, we extend the phase plane structure to modified gravity theories, examine trajectories of models with certain properties, and categorize regions in terms of scalar field hierarchical parameters, showing that dark energy is generically not a slow roll phenomenon.

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