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Register a new userThe impact of massive neutrinos on the abundance of massive clusters
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Kiyotomo Ichiki Kavlin IPMU
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We study the spherical, top-hat collapse model for a mixed dark matter model including cold dark matter (CDM) and massive neutrinos of mass scales ranging from m_nu= 0.05 to a few 0.1eV, the range of lower- and upper-bounds implied from the neutrino oscillation experiments and the cosmological constraints. To develop this model, we properly take into account relative differences between the density perturbation amplitudes of different components (radiation, baryon, CDM and neutrinos) around the top-hat CDM overdensity region assuming the adiabatic initial conditions. Furthermore, we solve the linearized Boltzmann hierarchy equations to obtain time evolution of the lineariezed neutrino perturbations, yet including the effect of nonlinear gravitational potential due to the nonlinear CDM and baryon overdensities in the late stage. We find that the presence of massive neutrinos slows down the collapse of CDM (plus baryon) overdensity, however, that the neutrinos cannot fully catch up with the the nonlinear CDM perturbation due to its large free-streaming velocity for the ranges of neutrino masses and halo masses we consider. We find that, just like CDM models, the collapse time of CDM overdensity is well monitored by the linear-theory extrapolated overdensity of CDM plus baryon perturbation, smoothed with a given halo mass scale, if taking into account the suppression effect of the massive neutrinos on the linear growth rate. Using these findings, we argue that the presence of massive neutrinos of mass scales 0.05 or 0.1eV may cause a significant decrease in the abundance of massive halos compared to the model without the massive neutrinos; e.g., by 25% or factor 2, respectively, for halos with 10^15Ms and at z=1.
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The possibility that primordial black holes (PBHs) form a part of dark matter has been considered over a wide mass range from the Planck mass ($10^{-5}~rm g$) to the level of the supermassive black hole in the center of the galaxy. Primordial origin might be one of the most important formation channel of massive black holes. We propose the lensing effect of very long baseline interferometer observations of compact radio sources with extremely high angular resolution as a promising probe for the presence of intergalactic PBHs in the mass range $sim10^2$-$10^9~M_{odot}$. For a sample of well-measured 543 compact radio sources, no millilensing multiple images are found with angular separations between $0.2$ milliarcsecond and $50$ milliarcseconds. From this null search result, we derive that the fraction of dark matter made up of PBHs in the mass range $sim10^4$-$10^8~M_{odot}$ is $lesssim0.56%$ at $68%$ confidence level.
We introduce a new set of large N-body runs, the MICE simulations, that provide a unique combination of very large cosmological volumes with good mass resolution. They follow the gravitational evolution of ~ 8.5 billion particles (2048^3) in volumes covering up to 450 (Gpc/h)^3. Our main goal is to accurately model and calibrate basic cosmological probes that will be used by upcoming astronomical surveys. Here we take advantage of the very large volumes of MICE to make a robust sampling of the high-mass tail of the halo mass function (MF). We discuss and avoid possible systematic effects in our study, and do a detailed analysis of different error estimators. We find that available fits to the local abundance of halos (Warren et al. (2006)) match well the abundance in MICE up to M ~ 10^{14}Msun, but significantly deviate for larger masses, underestimating the mass function by 10% (30%) at M = 3.16 x 10^{14}Msun (10^{15}Msun). Similarly, the widely used Sheth & Tormen (1999) fit, if extrapolated to high redshift assuming universality, leads to an underestimation of the cluster abundance by 30%, 20% and 15% at z=0, 0.5, 1 for M ~ [7 - 2.5 - 0.8] x 10^{14}Msun respectively ($ u = delta_c/sigma ~ 3$). We provide a re-calibration of the halo MF valid over 5 orders of magnitude in mass, 10^{10} < M/(Msun) < 10^{15}, that accurately describes its redshift evolution up to z=1. We explore the impact of this re-calibration on the determination of dark-energy, and conclude that using available fits may systematically bias the estimate of w by as much as 50% for medium-depth (z <= 1) surveys. MICE halo catalogues are publicly available at http://www.ice.cat/mice
This is the third of a series of papers in which we derive simultaneous constraints on cosmological parameters and X-ray scaling relations using observations of the growth of massive, X-ray flux-selected galaxy clusters. Our data set consists of 238 clusters drawn from the ROSAT All-Sky Survey, and incorporates extensive follow-up observations using the Chandra X-ray Observatory. Here we present improved constraints on departures from General Relativity (GR) on cosmological scales, using the growth index, gamma, to parameterize the linear growth rate of cosmic structure. Using the method of Mantz et al. (2009a), we simultaneously and self-consistently model the growth of X-ray luminous clusters and their observable-mass scaling relations, accounting for survey biases, parameter degeneracies and systematic uncertainties. We combine the cluster growth data with gas mass fraction, SNIa, BAO and CMB data. This combination leads to a tight correlation between gamma and sigma_8. Consistency with GR requires gamma~0.55. Under the assumption of self-similar evolution and constant scatter in the scaling relations, and for a flat LCDM model, we measure gamma(sigma_8/0.8)^6.8=0.55+0.13-0.10, with 0.79<sigma_8<0.89. Relaxing the assumptions on the scaling relations by introducing two additional parameters to model possible evolution in the normalization and scatter of the luminosity-mass relation, we obtain consistent constraints on gamma that are only ~20% weaker than those above. Allowing the dark energy equation of state, w, to take any constant value, we simultaneously constrain the growth and expansion histories, and find no evidence for departures from either GR or LCDM. Our results represent the most robust consistency test of GR on cosmological scales to date. (Abridged)
The Hubble tension can be significantly eased if there is an early component of dark energy that becomes active around the time of matter-radiation equality. Early dark energy models suffer from a coincidence problem -- the physics of matter-radiation equality and early dark energy are completely disconnected, so some degree of fine-tuning is needed in order for them to occur nearly simultaneously. In this paper we propose a natural explanation for this coincidence. If the early dark energy scalar couples to neutrinos then it receives a large injection of energy around the time that neutrinos become non-relativistic. This is precisely when their temperature is of order their mass, which, coincidentally, occurs around the time of matter-radiation equality. Neutrino decoupling therefore provides a natural trigger for early dark energy by displacing the field from its minimum just before matter-radiation equality. We discuss various theoretical aspects of this proposal, potential observational signatures, and future directions for its study.
Understanding the biasing between the clustering properties of halos and the underlying dark matter distribution is important for extracting cosmological information from ongoing and upcoming galaxy surveys. While on sufficiently larges scales the halo overdensity is a local function of the mass density fluctuations, on smaller scales the gravitational evolution generates non-local terms in the halo density field. We characterize the magnitude of these contributions at third-order in perturbation theory by identifying the coefficients of the non-local invariant operators, and extend our calculation to include non-local (Lagrangian) terms induced by a peak constraint. We apply our results to describe the scale-dependence of halo bias in cosmologies with massive neutrinos. The inclusion of gravity-induced non-local terms and, especially, a Lagrangian $k^2$-contribution is essential to reproduce the numerical data accurately. We use the peak-background split to derive the numerical values of the various bias coefficients from the excursion set peak mass function. For neutrino masses in the range $0leq sum_i m_{ u_i} leq 0.6$ eV, we are able to fit the data with a precision of a few percents up to $k=0.3, h {rm ,Mpc^{-1}}$ without any free parameter.
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