Cosmology of "Visible" Sterile Neutrinos

We point out that in scenarios with a low reheating temperature $T_R << 100$ MeV at the end of (the last episode of) inflation or entropy production, the abundance of sterile neutrinos becomes largely independent of their coupling to active neutrinos. Thus, cosmological bounds become less stringent than usually assumed, allowing sterile neutrinos to be ``visible'' in future experiments. For example, the sterile neutrino required by the LSND result does not have any cosmological problem within these scenarios.Comment: Talk given at the ``8th Workshop on Non-Perturbative Quantum Chromodynamics", June 7-11, 2004, Paris, France; 7 pages, 2 figure

Theory of Dark Matter

The search for dark matter is a very wide and active field of research. Many potential hints of dark matter have appeared recently which led to a burst of theoretical activity and model building. I necessarily concentrate here only in some aspects of it. I review here some recent hints and some of the ways in which they could be explained.Comment: Plenary review talk at "Physics at the LHC 2010", 7-12 June 2010, DESY, Hamburg, Germany; 6 pages, no figures, desyproc.cls file needed to typese

Experimental relevance of low reheating temperature cosmologies

Standard simple assumptions are usually made about the pre-Big Bang Nucleosynthesis epoch, from which we do not have observations. Modifying these assumptions, the predicted density of relic particles such as neutralinos and sterile neutrinos can be very different from that in the standard case. For example, neutralinos could have the dark matter density in (almost) any supersymmetric model, and sterile neutrinos with mixings large enough to be soon detected in neutrino experiments would become cosmologically acceptable. These possibilities are important in view of what the LHC, and neutrino experiments could soon find.Comment: Talk given at the 43rd Rencontres de Moriond- Cosmology, La Thuile, Italy, March 15 - 22, 2008. 4 pages, 3 figures, uses moriond.st

Neutralino with the Right Cold Dark Matter Abundance in (Almost) Any Supersymmetric Model

We consider non-standard cosmological models in which the late decay of a scalar field $\phi$ reheats the Universe to a low reheating temperature, between 5 MeV and the standard freeze-out temperature of neutralinos of mass $m_{\chi}$. We point out that in these models all neutralinos with standard density $\Omega_{\rm std} \gtrsim 10^{-5} (100 {\rm GeV}/m_\chi)$ can have the density of cold dark matter, provided the right combination of the following two parameters can be achieved in the high energy theory: the reheating temperature, and the ratio of the number of neutralinos produced per $\phi$ decay over the $\phi$ field mass. We present the ranges of these parameters where a combination of thermal and non-thermal neutralino production leads to the desired density, as functions of $\Omega_{\rm std}$ and $m_{\chi}$.Comment: 4 pages, 1 figur

Low reheating temperatures in monomial and binomial inflationary potentials

We investigate the allowed range of reheating temperature values in light of the Planck 2015 results and the recent joint analysis of Cosmic Microwave Background (CMB) data from the BICEP2/Keck Array and Planck experiments, using monomial and binomial inflationary potentials. While the well studied $\phi^2$ inflationary potential is no longer favored by current CMB data, as well as $\phi^p$ with $p>2$, a $\phi^1$ potential and canonical reheating ($w_{re}=0$) provide a good fit to the CMB measurements. In this last case, we find that the Planck 2015 $68\%$ confidence limit upper bound on the spectral index, $n_s$, implies an upper bound on the reheating temperature of $T_{re}\lesssim 6\times 10^{10}\,{\rm GeV}$, and excludes instantaneous reheating. The low reheating temperatures allowed by this model open the possiblity that dark matter could be produced during the reheating period instead of when the Universe is radiation dominated, which could lead to very different predictions for the relic density and momentum distribution of WIMPs, sterile neutrinos, and axions. We also study binomial inflationary potentials and show the effects of a small departure from a $\phi^1$ potential. We find that as a subdominant $\phi^2$ term in the potential increases, first instantaneous reheating becomes allowed, and then the lowest possible reheating temperature of $T_{re}=4\,{\rm MeV}$ is excluded by the Planck 2015 $68\%$ confidence limit.Comment: 17 pages, 5 figures, v2: some references added and typos correcte