Steen Hannestad

Updated bounds on milli-charged particles

We update the bounds on fermions with electric charge e and mass m .F or m . m ewe nd 10 15 . <1 is excluded by laboratory experiments, astrophysics and cosmology. For larger masses, the limits are less restrictive and depend onm. For milli-charged neutrinos, the limits are stronger, especially if the dierentflavors mix as suggested by current experimental evidence.

Probing cosmological parameters with the CMB: Forecasts from Monte Carlo simulations

The Fisher matrix formalism has in recent times become the standard method for predicting the precision with which various cosmological parameters can be extracted from future data. This approach is fast and generally returns accurate estimates for the parameter errors when the individual parameter likelihoods approximate a Gaussian distribution. However, where Gaussianity is not respected (due, for instance, to strong parameter degeneracies), the Fisher matrix formalism loses its reliability. In this paper, we compare the results of the Fisher matrix approach with those from Monte Carlo simulations. The latter method is based on the publicly available CosmoMC code, but uses synthetic realizations of data sets anticipated for future experiments. We focus on prospective cosmic microwave background (CMB) data from the Planck satellite, with or without CMB lensing information, and its implications for a minimal cosmological scenario with eight parameters and an extended model with eleven parameters. We show that in many cases, the projected sensitivities from the Fisher matrix and the Monte Carlo methods differ significantly, particularly for models with many parameters. Sensitivities to the neutrino mass and the dark matter fraction are especially susceptible to change.

What is the lowest possible reheating temperature

We study models in which the universe exits reheating at temperatures in the MeV regime. By combining light element abundance measurements with cosmic microwave background and large scale structure data we find a fairly robust lower limit on the reheating temperature of T_RH > 4 MeV at 95% C.L. However, if the heavy particle whose decay reheats the universe has a direct decay mode to neutrinos, there are some small islands left in parameter space where a reheating temperature as low as 1 MeV is allowed. The derived lower bound on the reheating temperature also leads to very stringent bounds on models with

Searching for a holographic connection between dark energy and the low l CMB multipoles

n

Probing the dark side: Constraints on the dark energy equation of state from CMB, large scale structure, and type Ia supernovae

large extra dimensions. For n=2 the bound on the compactification scale is M > 2000 TeV, and for n=3 it is 100 TeV. These are currently the strongest available bounds on such models.

The neutrino mass bound from WMAP 3 year data, the baryon acoustic peak, the SNLS supernovae and the Lyman-α forest

We consider the angular power spectrum in a finite universe with different boundary conditions and perform a fit to the CMB, LSS and supernova data. A finite universe could be the consequence of a holographic constraint, giving rise to an effective IR cut-off at the future event horizon. In such a model there is a cosmic duality relating the dark energy equation of state and the power spectrum, which shows a suppression and oscillatory behaviour that is found to describe the low l features extremely well. However, much of the discussion here will also apply if we actually live inside an expanding bubble that describes our universe. The best fit to the CMB and LSS data turns out to be better than in the standard ΛCDM model, but when combined with the supernova data, the holographic model becomes disfavoured. We speculate on the possible implications.

Measuring neutrino masses and dark energy with weak lensing tomography

We have reanalyzed constraints on the equation of state parameter,

Stringent Neutron-Star Limits on Large Extra Dimensions

{w}_{Q}\ensuremath{\equiv}P/\ensuremath{\rho},

Neutrino decoupling in the early universe

of the dark energy, using several cosmological data sets and relaxing the usual constraint

Neutrino masses and the dark energy equation of state: relaxing the cosmological neutrino mass bound.

{w}_{Q}g~\ensuremath{-}1.