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Register a new userWhat is half a neutrino? Reviewing cosmological constraints on neutrinos and dark radiation
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Added by
Signe Riemer-Sorensen
Publication date
2013
fields
Physics
and research's language is
English
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Neutrinos are one of the major puzzles in modern physics. Despite measurements of mass differences, the Standard Model of particle physics describes them as exactly massless. Additionally, recent measurements from both particle physics experiments and cosmology indicate the existence of more than the three Standard Model species. Here we review the cosmological evidence and its possible interpretations.
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The existence of dark radiation that is completely decoupled from the standard model in the early Universe leaves open the possibility of an associated dark radiation isocurvature mode. We show that the presence of dark radiation isocurvature leads to spatial variation in the primordial abundances of helium and deuterium due to spatial variation in $N_{rm eff}$ during Big Bang nucleosynthesis. We use the result to constrain the existence of such an isocurvature mode on scales down to $sim 1$ Mpc scales. By measuring the excess variance in the primordial helium to hydrogen and deuterium to hydrogen ratio in different galaxies, we constrain the variance in average isocurvature in a galaxy to be less than $0.13/Delta bar{N}_{rm eff}$ at 95% confidence. Here $Delta bar{N}_{rm eff}$ is the spatially averaged increase in $N_{rm eff}$ due to the additional dark radiation component.
If active neutrinos undergo non-standard (`secret) interactions (NS$ u$I) the cosmological evolution of the neutrino fluid might be altered, leaving an imprint in cosmological observables. We use the latest publicly available CMB data from Planck to constrain NS$ u$I inducing $ u- u$ scattering, under the assumption that the mediator $phi$ of the secret interaction is very light. We find that the effective coupling constant of the interaction, $g_mathrm{eff}^4 equiv langle sigma vrangle T_ u^2$, is constrained at $< 2.35times10^{-27}$ (95% credible interval), which stregthens to $g_mathrm{eff}^4 < 1.64times10^{-27}$ when Planck non-baseline small-scale polarization is considered. Our findings imply that after decoupling at $Tsimeq 1$ MeV, cosmic neutrinos are free streaming at redshifts $z>3800$, or $z>2300$ if small-scale polarization is included. These bounds are only marginally improved when data from geometrical expansion probes are included in the analysis to complement Planck. We also find that the tensions between CMB and low-redshift measurements of the expansion rate $H_0$ and the amplitude of matter fluctuations $sigma_8$ are not significantly reduced. Our results are independent on the underlying particle physics model as long as $phi$ is very light. Considering a model with Majorana neutrinos and a pseudoscalar mediator we find that the coupling constant $g$ of the secret interaction is constrained at $lesssim 7times 10^{-7}$. By further assuming that the pseudoscalar interaction comes from a dynamical realization of the see-saw mechanism, as in Majoron models, we can bound the scale of lepton number breaking $v_sigma$ as $gtrsim (1.4times 10^{6})m_ u$.
We revise the cosmological phenomenology of Macroscopic Dark Matter (MDM) candidates, also commonly dubbed as Macros. A possible signature of MDM is the capture of baryons from the cosmological plasma in the pre-recombination epoch, with the consequent injection of high-energy photons in the baryon-photon plasma. By keeping a phenomenological approach, we consider two broad classes of MDM in which Macros are composed either of ordinary matter or antimatter. In both scenarios, we also analyze the impact of a non-vanishing electric charge carried by Macros. We derive constraints on the Macro parameter space from three cosmological processes: the change in the baryon density between the end of the Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB) decoupling, the production of spectral distortions in the CMB and the kinetic coupling between charged MDM and baryons at the time of recombination. In the case of neutral Macros we find that the tightest constraints are set by the baryon density condition in most of the parameter space. For Macros composed of ordinary matter and with binding energy $I$, this leads to the following bound on the reduced cross-section: $sigma_X/M_X lesssim 6.8 cdot 10^{-7} left(I/mathrm{MeV}right)^{-1.56} , text{cm}^2 , text{g}^{-1}$. Charged Macros with surface potential $V_X$, instead, are mainly constrained by the tight coupling with baryons, resulting in $sigma_X/M_X lesssim 2 cdot 10^{-11} left(|V_X|/mathrm{MeV}right)^{-2} text{cm}^2 , text{g}^{-1}$. Finally, we show that future CMB spectral distortions experiments, like PIXIE and SuperPIXIE, would have the sensitivity to probe larger regions of the parameter space: this would allow either for a possible evidence or for an improvement of the current bounds on Macros as dark matter candidates.
The increasingly significant tensions within $Lambda$CDM, combined with the lack of detection of dark matter (DM) in laboratory experiments, have boosted interest in non-minimal dark sectors, which are theoretically well-motivated and inspire new search strategies for DM. Here we consider, for the first time, the possibility of DM having simultaneous interactions with photons, baryons, and dark radiation (DR). We have developed a new and efficient version of the Boltzmann code CLASS that allows for one DM species to have multiple interaction channels. With this framework we reassess existing cosmological bounds on the various interaction coefficients in multi-interacting DM scenarios. We find no clear degeneracies between these different interactions and show that their cosmological effects are largely additive. We further investigate the possibility of these models to alleviate the cosmological tensions, and find that the combination of DM-photon and DM-DR interactions can at the same time reduce the $S_8$ tension (from $2.3sigma$ to $1.2sigma$) and the $H_0$ tension (from $4.3sigma$ to $3.1sigma$). The public release of our code will pave the way for the study of various rich dark sectors.
Dark matter interactions with electrons or protons during the early Universe leave imprints on the cosmic microwave background and the matter power spectrum, and can be probed through cosmological and astrophysical observations. We explore these interactions using a diverse suite of data: cosmic microwave background anisotropies, baryon acoustic oscillations, the Lyman-$alpha$ forest, and the abundance of Milky-Way subhalos. We derive constraints using model-independent parameterizations of the dark matter--electron and dark matter--proton interaction cross sections and map these constraints onto concrete dark matter models. Our constraints are complementary to other probes of dark matter interactions with ordinary matter, such as direct detection, big bang nucleosynthesis, various astrophysical systems, and accelerator-based experiments.
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