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Register a new userTrading kinetic energy: How late kinetic decoupling of dark matter changes $N_{textrm{eff}}$
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Elastic scattering between dark matter particles and a relativistic species such as photons or neutrinos leads to a transfer of energy from the latter due to their intrinsically different temperature scaling relations. In this work, we point out that this siphoning of energy from the radiation bath manifests as a change in the effective number of neutrinos $N_{rm eff}$, and compute the expected shift $Delta N_{rm eff}$ for dark matter-photon and dark matter-neutrino elastic scattering as a function of the dark matter mass $m_psi$ and scattering cross section $sigma_{psi-X}$. For $(m_psi,sigma_{psi-X})$-parameter regions already explored by nonlinear probes such as the Lyman-$alpha$ forest through collisional and/or free-streaming damping, we find shifts of $|Delta N_{rm eff}| simeq O(10^{-2})$, which may be within the reach of the proposed CMB-S4 experiment. For most of the as-yet-unexplored parameter space, however, we expect $|Delta N_{rm eff}| lesssim O(10^{-3})$. An ideal 21 cm tomography survey of the dark ages limited only by cosmic variance is potentially sensitive to $|Delta N_{rm eff}| simeq O(10^{-6})$, in which case dark matter masses up to $m_{psi} sim 10 , textrm{MeV}$ may be probed via their effect on $N_{rm eff}$.
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We argue that the acoustic damping of the matter power spectrum is not a generic feature of the kinetic decoupling of dark matter, but even the enhancement can be realized depending on the nature of the kinetic decoupling when compared to that in the standard cold dark matter model. We consider a model that exhibits a ${it sudden}$ kinetic decoupling and investigate cosmological perturbations in the ${it standard}$ cosmological background numerically in the model. We also give an analytic discussion in a simplified setup. Our results indicate that the nature of the kinetic decoupling could have a great impact on small scale density perturbations.
We investigate whether the $4.4sigma$ tension on $H_0$ between SH$_{0}$ES 2019 and Planck 2018 can be alleviated by a variation of Newtons constant $G_N$ between the early and the late Universe. This changes the Hubble rate before recombination, similarly to adding $Delta N_{rm eff}$ extra relativistic degrees of freedom. We implement a varying $G_N$ in a scalar-tensor theory of gravity, with a non-minimal coupling $(M^2+beta phi^2)R$. If the scalar $phi$ starts in the radiation era at an initial value $phi_I sim 0.5~M_p$ and with $beta<0$, a dynamical transition occurs naturally around the epoch of matter-radiation equality and the field evolves towards zero at late times. As a consequence, the $H_0$ tension between SH$_{0}$ES (2019) and Planck 2018+BAO slightly decreases, as in $Delta N_{rm eff}$ models, to the 3.8$sigma$ level. We then perform a fit to a combined Planck, BAO and supernovae (SH$_0$ES and Pantheon) dataset. When including local constraints on Post-Newtonian (PN) parameters, we find $H_0=69.08_{-0.71}^{+0.6}~text{km/s/Mpc}$ and a marginal improvement of $Deltachi^2simeq-3.2$ compared to $Lambda$CDM, at the cost of 2 extra parameters. In order to take into account scenarios where local constraints could be evaded, we also perform a fit without PN constraints and find $H_0=69.65_{-0.78}^{+0.8}~text{km/s/Mpc}$ and a more significant improvement $Deltachi^2=-5.4$ with 2 extra parameters. For comparison, we find that the $Delta N_{rm eff}$ model gives $H_0=70.08_{-0.95}^{+0.91}~text{km/s/Mpc}$ and $Deltachi^2=-3.4$ at the cost of one extra parameter, which disfavors the $Lambda$CDM limit just above 2$sigma$, since $Delta N_{rm eff}=0.34_{-0.16}^{+0.15}$. Overall, our varying $G_N$ model performs similarly to the $Delta N_{rm eff}$ model in respect to the $H_0$ tension, if a physical mechanism to remove PN constraints can be implemented.
An extension to the Einstein-Cartan (EC) action is discussed in terms of cosmological solutions. The torsion incorporated in the EC Lagrangian is assumed to be totally anti-symmetric, and written by of a vector $S^mu$. Then this torsion model, compliant with the Cosmological Principle, is made dynamical by introducing its quadratic, totally anti-symmetric derivative. The EC Lagrangian then splits up into the Einstein-Hilbert portion and a (mass) term $sim s_0^2$. While for the quintessence model, dark energy arises from the potential, here the kinetic term, $frac{1}{mu^2} dot{s}_0^2$, plays the role of dark energy. The quadratic torsion term, on the other hand, gives rise to a stiff fluid that leads to a bouncing solution. A bound on the bouncing solution is calculated.
We propose a novel technique to separate the late-time, post-reionization component of the kinetic Sunyaev-Zeldovich (kSZ) effect from the contribution to it from a (poorly understood and probably patchy) reionization history. The kSZ effect is one of the most promising probe of the {em missing baryons} in the Universe. We study the possibility of reconstructing it in three dimensions (3D), using future spectroscopic surveys such as the Euclid survey. By reconstructing a 3D template from galaxy density and peculiar velocity fields from spectroscopic surveys we cross-correlate the estimator against CMB maps. The resulting cross-correlation can help us to map out the kSZ contribution to CMB in 3D as a function of redshift thereby extending previous results which use tomographic reconstruction. This allows the separation of the late time effect from the contribution owing to reionization. By construction, it avoids contamination from foregrounds, primary CMB, tSZ effect as well as from star forming galaxies. Due to a high number density of galaxies the signal-to-noise (S/N) for such cross-correlational studies are higher, compared to the studies involving CMB power spectrum analysis. Using a spherical Bessel-Fourier (sFB) transform we introduce a pair of 3D power-spectra: ${cal C}^{parallel}_ell(k)$ and ${cal C}^{perp}_ell(k)$ that can be used for this purpose. We find that in a future spectroscopic survey with near all-sky coverage and a survey depth of $zapprox 1$, reconstruction of ${cal C}^{perp}_ell(k)$ can be achieved in a few radial wave bands $kapprox(0.01-0.5 h^{-1}rm Mpc)$ with a S/N of upto ${cal O}(10)$ for angular harmonics in the range $ell=(200-2000)$ (abrdiged).
The rate of structure formation in the Universe is different in homogeneous and clustered dark energy models. The degree of dark energy clustering depends on the magnitude of its effective sound speed $c^{2}_{rm eff}$ and for $c_{rm eff}=0$ dark energy clusters in a similar fashion to dark matter while for $c_{rm eff}=1$ it stays (approximately) homogeneous. In this paper we consider two distinct equations of state for the dark energy component, $w_{rm d}=const$ and $w_{rm d}=w_0+w_1left(frac{z}{1+z}right)$ with $c_{rm eff}$ as a free parameter and we try to constrain the dark energy effective sound speed using current available data including SnIa, Baryon Acoustic Oscillation, CMB shift parameter ({em Planck} and {em WMAP}), Hubble parameter, Big Bang Nucleosynthesis and the growth rate of structures $fsigma_{8}(z)$. At first we derive the most general form of the equations governing dark matter and dark energy clustering under the assumption that $c_{rm eff}=const$. Finally, performing an overall likelihood analysis we find that the likelihood function peaks at $c_{rm eff}=0$, however the dark energy sound speed is degenerate with respect to the cosmological parameters, namely $Omega_{rm m}$ and $w_{rm d}$.
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