Year: 2013
Type: article
Abstract: A recent number of analyses of cosmological data have shown indications for the presence of extra radiation beyond the standard model at the equality and nucleosynthesis epochs, which has been usually interpreted as an effective number of neutrinos, ${N}_{\mathrm{eff}}>3.046$. In this work we establish the theoretical basis for a particle physics-motivated model (bound dark matter, BDM) which explains the need for extra radiation. The BDM model describes dark matter particles which are relativistic at a scale below $a<{a}_{c}$; these particles acquire mass with an initial velocity, ${v}_{c}$, at scales $a>{a}_{c}$ due to nonperturbative methods (as protons and neutrons do) and this process is described by a time-dependent equation of state, ${\ensuremath{\omega}}_{\mathrm{BDM}}(a)$. Owing to this behavior the amount of extra radiation changes as a function of the scale factor, and this implies that the extra relativistic degrees of freedom ${N}_{\mathrm{ex}}$ may also vary as a function of the scale factor. This is favored by data on the cosmic microwave background and big bang nucleosynthesis (BBN) epochs. We compute the range of values of the BDM model parameters, ${x}_{c}={a}_{c}{v}_{c}$, that explain the values obtained for the $^{4}\mathrm{He}$ at BBN and ${N}_{\mathrm{eff}}$ at equality. Combining different analyses, we compute the values ${x}_{c}=4.13(\genfrac{}{}{0}{}{+3.65}{\ensuremath{-}4.13})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ and ${v}_{c}={0.37}_{\ensuremath{-}0.17}^{+0.18}$. We conclude that we can account for the apparent extra neutrino degrees of freedom ${N}_{\mathrm{ex}}$ using a phase transition in the dark matter with a time-dependent equation of state without introducing extra relativistic particles.
Authors: Jorge Mastache, Axel de la Macorra
Institution: Universidad Nacional Autónoma de México
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2054225528