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Register a new userClustering dark energy and halo abundances
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Added by
Ronaldo Carlotto Batista
Publication date
2017
fields
Physics
and research's language is
English
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Within the standard paradigm, dark energy is taken as a homogeneous fluid that drives the accelerated expansion of the universe and does not contribute to the mass of collapsed objects such as galaxies and galaxy clusters. The abundance of galaxy clusters -- measured through a variety of channels -- has been extensively used to constrain the normalization of the power spectrum: it is an important probe as it allows us to test if the standard $Lambda$CDM model can indeed accurately describe the evolution of structures across billions of years. It is then quite significant that the Planck satellite has detected, via the Sunyaev-Zeldovich effect, less clusters than expected according to the primary CMB anisotropies. One of the simplest generalizations that could reconcile these observations is to consider models in which dark energy is allowed to cluster, i.e., allowing its sound speed to vary. In this case, however, the standard methods to compute the abundance of galaxy clusters need to be adapted to account for the contributions of dark energy. In particular, we examine the case of clustering dark energy -- a dark energy fluid with negligible sound speed -- with a redshift-dependent equation of state. We carefully study how the halo mass function is modified in this scenario, highlighting corrections that have not been considered before in the literature. We address modifications in the growth function, collapse threshold, virialization densities and also changes in the comoving scale of collapse and mass function normalization. Our results show that clustering dark energy can impact halo abundances at the level of 10%--30%, depending on the halo mass, and that cluster counts are modified by about 30% at a redshift of unity.
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LCDM cosmological models with Early Dark Energy (EDE) have been proposed to resolve tensions between the Hubble constant H0 = 100h km/s/Mpc measured locally, giving h ~ 0.73, and H0 deduced from Planck cosmic microwave background (CMB) and other early universe measurements plus LCDM, giving h ~ 0.67. EDE models do this by adding a scalar field that temporarily adds dark energy equal to about 10% of the cosmological energy density at the end of the radiation-dominated era at redshift z ~ 3500. Here we compare linear and nonlinear predictions of a Planck-normalized LCDM model including EDE giving h = 0.728 with those of standard Planck-normalized LCDM with h = 0.678. We find that nonlinear evolution reduces the differences between power spectra of fluctuations at low redshifts. As a result, at z = 0 the halo mass functions on galactic scales are nearly the same, with differences only 1-2%. However, the differences dramatically increase at high redshifts. The EDE model predicts 50% more massive clusters at z = 1 and twice more galaxy-mass halos at z = 4. Even greater increases in abundances of galaxy-mass halos at higher redshifts may make it easier to reionize the universe with EDE. Predicted galaxy abundances and clustering will soon be tested by JWST observations. Positions of baryonic acoustic oscillations (BAOs) and correlation functions differ by about 2% between the models -- an effect that is not washed out by nonlinearities. Both standard LCDM and the EDE model studied here agree well with presently available acoustic-scale observations, but DESI and Euclid measurements will provide stringent new tests.
We study the effect of dark matter (DM) being encapsulated in primordial black holes (PBHs) on the power spectrum of density fluctuations $P(k)$; we also look at its effect on the abundance of haloes and their clustering. We allow the growth of Poisson fluctuations since matter and radiation equality and study both monochromatic and extended PBH mass distributions. We present updated monochromatic black hole mass constraints by demanding $<10%$ deviations from the $Lambda$ cold dark matter power spectrum at a scale of $k=1$hMpc$^{-1}$. Our results show that PBHs with masses $>10^4$h$^{-1}M_odot$ are excluded from conforming all of the DM in the Universe. We also apply this condition to our extended Press-Schechter (PS) mass functions, and find that the Poisson power is scale dependent even before applying evolution. We find that characteristic masses $M^*leq10^2 $h$^{-1}M_odot$ are allowed, {leaving only two characteristic PBH mass windows of PS mass functions when combining with previous constraints, at $M^*sim10^2$h$^{-1}M_odot$ and $sim10^{-8}$h$^{-1}M_odot$ where all of the DM can be in PBHs. The resulting DM halo mass functions within these windows are similar} to those resulting from cold dark matter made of fundamental particles. However, as soon as the parameters produce unrealistic $P(k)$, the resulting halo mass functions and their bias as a function of halo mass deviate strongly from the behaviour measured in the real Universe.
We investigate generalized interacting dark matter-dark energy scenarios with a time-dependent coupling parameter, allowing also for freedom in the neutrino sector. The models are tested in the phantom and quintessence regimes, characterized by an equation of state $w_x<-1$ and $w_x>-1$, respectively. Our analyses show that for some of the scenarios the existing tensions on the Hubble constant $H_0$ and on the clustering parameter $S_8$ can be significantly alleviated. The relief is either due to textit{(a)} a dark energy component which lies within the phantom region; or textit{(b)} the presence of a dynamical coupling in quintessence scenarios. The inclusion of massive neutrinos into the interaction schemes does not affect neither the constraints on the cosmological parameters nor the bounds on the total number or relativistic degrees of freedom $N_{rm eff}$, which are found to be extremely robust and, in general, strongly consistent with the canonical prediction $N_{rm eff}=3.045$. The most stringent bound on the total neutrino mass $M_{ u}$ is $M_{ u}<0.116$ eV and it is obtained within a quintessence scenario in which the matter mass-energy density is only mildly affected by the presence of a dynamical dark sector coupling.
The influence of dark matter inhomogeneities on the angular size-redshift test is investigated for a large class of flat cosmological models driven by dark energy plus a cold dark matter component (XCDM model). The results are presented in two steps. First, the mass inhomogeneities are modeled by a generalized Zeldovich-Kantowski-Dyer-Roeder (ZKDR) distance which is characterized by a smoothness parameter $alpha(z)$ and a power index $gamma$, and, second, we provide a statistical analysis to angular size data for a large sample of milliarcsecond compact radio sources. As a general result, we have found that the $alpha$ parameter is totally unconstrained by this sample of angular diameter data.
We use a large dark matter simulation of a LambdaCDM model to investigate the clustering and environmental dependence of the number of substructures in a halo. Focusing on redshift z=1, we find that the halo occupation distribution is sensitive at the tens of percent level to the surrounding density and to a lesser extent to asymmetry of the surrounding density distribution. We compute the autocorrelation function of halos as a function of occupation, building on the finding of Wechsler et al. (2006) and Gao and White (2007) that halos (at fixed mass) with more substructure are more clustered. We compute the relative bias as a function of occupation number at fixed mass, finding a strong relationship. At fixed mass, halos in the top 5% of occupation can have an autocorrelation function ~ 1.5-2 times higher than the mean. We also compute the bias as a function of halo mass, for fixed halo occupation. We find that for group and cluster sized halos, when the number of subhalos is held fixed, there is a strong anticorrelation between bias and halo mass. Such a relationship represents an additional challenge to the halo model.
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