Narrow Results

Direct searches for dark matter lead to serious problems for simple models with stable neutral Weakly Interacting Massive Particles (WIMPs) as candidates for dark matter. A possibility is discussed that new stable quarks and charged leptons exist and are hidden from detection, being bound in neutral dark atoms of composite dark matter. Stable -2 charged particles O^-- are bound with primordial helium in O-helium (OHe) atoms, being specific nuclear interacting form of composite Warmer than Cold dark matter. Slowed down in the terrestrial matter, OHe is elusive for direct methods of underground dark matter detection based on the search for effects of nuclear recoil in WIMP-nucleus collisions. The positive results of DAMA experiments can be explained as annual modulation of radiative capture of O-helium by nuclei. In the framework of this approach, test of DAMA results in detectors with other chemical content becomes a nontrivial task, while the experimental search of stable charged particles at LHC or in cosmic rays acquires a meaning of direct test for composite dark matter scenario.

The nature of cosmological dark matter finds its explanation in physics beyond the Standard Model of elementary particles. The landscape of dark matter candidates contains a wide variety of species, either elusive or hardly detectable in direct experimental searches. Even in case, when such searches are possible the interpretation of their results implies additional sources of information, which provide indirect effects of dark matter. Some nontrivial probes for the nature of the dark matter are presented in the present issue.

The statistical mechanics of collisionless self-gravitating systems is a longstanding puzzle, which has not yet been successfully solved. We performed preliminary investigations and then formulated a framework of the entropy-based equilibrium statistical mechanics for collisionless self-gravitating systems. This theory is based on the Boltzmann–Gibbs entropy and includes the generalized virial equations as additional constraints. With the truncated distribution function to the lowest order, we derived a set of second-order equations for the equilibrium states of the system, and solved the numerical solutions of these equations. It is found that there are three types of solutions for these equations. Both the isothermal and divergent solutions are thermally unstable and have unconfined density profiles with infinite mass, energy and spatial extent. The convergent solutions, however, seem to be reasonable. These solutions are just the lowest-order approximation, but they have already manifested the qualitative success of our theory. The second-order variations of the entropy functional indicate that the stationary solutions are neither maximum nor minimum, but saddle-point solutions. Inspired by the saddle-point solutions, we distinguish between two types of perturbations in self-gravitating systems, namely, the large-scale mass perturbation and the small-scale density perturbation, which correspond to long-range violent relaxation and short-range relaxation/Landau damping, respectively, which operate in different fashions. This result is consistent with Antonov's proof, or Binney's argument, that there are no global maximum entropy states for self-gravitating systems. These investigations indicate the achievements that we have made towards this long-standing unsolved problem on the statistical mechanics of self-gravitating systems.

Galaxy Redshift surveys provide a three-dimensional map of the Universe. Three distinct processes that encode cosmological information in these maps, are commonly used to constrain models: (i) the comoving power spectrum shape depends on the physical properties of the early Universe, including the physical matter, baryon and neutrino densities, the inflation power spectrum and the degree of Gaussianity of density fluctuations; (ii) we can use the statistical clustering of galaxies as a standard ruler by matching it, or parts of it at different redshifts, and to the Cosmic Microwave Background (CMB); (iii) redshift-space distortions, anisotropic patterns caused by peculiar galaxy velocities, reveal structure growth. Following the design of my talk at the 1st Galileo–Xu Guangqi Meeting, I will use these proceedings to briefly review these experiments.

We review the causal backreaction paradigm, in which the need for Dark Energy is eliminated via the generation of an apparent cosmic acceleration from the causal flow of inhomogeneity information coming in from distant structure-forming regions. The formalism detailed here incorporates the effects of "recursive nonlinearities": the process by which already-established metric perturbations will subsequently act to slow-down all future flows of inhomogeneity information. Despite such effects, we find viable cosmological models in which causal backreaction successfully serves as a replacement for Dark Energy, via the adoption of relatively large values for the dimensionless "strength" of the clustering evolution functions being modeled. These large values are justified by the hierarchical nature of clustering and virialization in the universe, which occurs on multiple cosmic length scales simultaneously; moreover, the clustering model amplitudes needed to match the apparent acceleration can be moderated via the incorporation of a model parameter representing the late-time slow-down of clustering due to astrophysical feedback processes. In summary, an alternative cosmic concordance can be achieved for a matter-only universe in which the apparent acceleration observed is generated entirely by causal backreaction effects. Lastly, considering the long-term fate of the universe, while the possibility of an "eternal" acceleration due to causal backreaction seems unlikely, this conclusion does not take into account the large-scale breakdown of cosmological isotropy in the far future, or the eventual ubiquity of gravitationally-nonlinear perturbations.

Among dark atom scenarios, the simplest and most predictive one is that of O-helium (OHe) dark atoms, in which a leptonlike doubly charged particle O^–– is bound to a primordial helium nucleus, and is the main constituent of dark matter. The OHe cosmology has several successes: it leads to a warmer-than-cold-dark matter scenario for large-scale-structure formation, it can provide an explanation for the excess in positron annihilation line in the galactic bulge and it may explain the results of direct dark matter searches. This model liberates the physics of dark atoms from many unknown features of new physics, but it is still not free from astrophysical uncertainties. It also demands a deeper understanding of the details of known nuclear and atomic physics, which are still somewhat unclear in the case of nuclear interacting “atomic” shells. These potential problems of the OHe scenario are also discussed.

The standard paradigm of cosmology assumes General Relativity (GR) is a valid theory for gravity at scales in which it has not been properly tested. Developing novel tests of GR and its alternatives is crucial if we want to give strength to the model or find departures from GR in the data. Since alternatives to GR are usually defined through nonlinear equations, designing new tests for these theories implies a jump in complexity and thus, a need for refining the simulation techniques. We summarize existing techniques for dealing with modified gravity (MG) in the context of cosmological simulations. $N$-body codes for MG are usually based on standard gravity codes. We describe the required extensions, classifying the models not according to their original motivation, but by the numerical challenges that must be faced by numericists. MG models usually give rise to elliptic equations, for which multigrid techniques are well suited. Thus, we devote a large fraction of this review to describing this particular technique. Contrary to other reviews on multigrid methods, we focus on the specific techniques that are required to solve MG equations and describe useful tricks. Finally, we describe extensions for going beyond the static approximation and dealing with baryons.

In this contribution we review and discuss several aspects of Cosmic Voids. Voids are a major component of the large scale distribution of matter and galaxies in the Universe. Their instrumental importance for understanding the emergence of the Cosmic Web is clear. Their relatively simple shape and structure makes them into useful tools for extracting the value of a variety cosmic parameters, possibly including even that of the influence of dark energy. Perhaps most promising and challenging is the issue of the galaxies found within their realm. Not only does the pristine environment of voids provide a promising testing ground for assessing the role of environment on the formation and evolution of galaxies, the dearth of dwarf galaxies may even represent a serious challenge to the standard view of cosmic structure formation.

Based on the cosmological hydrodynamic simulation, we study the properties of shock waves formed during the formation of the large scale structure (LSS) of the universe, and investigate their contribution to the cosmic ray (CR) fraction in the intergalactic medium (IGM). It is found that while strong accretion shocks prevail at high redshift, weak internal shocks become dominant in the intracluster medium (ICM) as galaxy clusters form and virialize at low redshift, z < 1. The accumulated CR proton energy is likely to be less than 10 % of the thermal energy in the ICM, since weak shocks of M ≲ 3 are most abundant. This is consistent with the upper limit constrained by radio and gamma-ray observations of galaxy clusters. In the warm-hot medium (WHIM) inside filaments, CRs and gas could be almost in energy equipartition, since relatively stronger shocks of 5 ≲ M ≲ 10 are dominant there. We suggest that the non-thermal emissions from the CR electrons and protons accelerated by cosmological shock waves could provide a new way to detect the WHIM of the universe.

A wide bandwidth, dual polarized, modified four-square antenna is presented as a feed antenna for radio astronomical measurements. A linear array of these antennas is used as a line-feed for cylindrical reflectors for Tianlai, a radio interferometer designed for 21cm intensity mapping. Simulations of the feed antenna beam patterns and scattering parameters are compared to experimental results at multiple frequencies across the 650–1420MHz range. Simulations of the beam patterns of the combined feed array/reflector are presented as well.

We constructed a new all-sky Compton parameter map (y-map) of the thermal Sunyaev-Zel’dovich (tSZ) effect by applying the MILCA component separation algorithm to the 100 to 857 GHz frequency channel maps from the Planck data release 4. The Planck team performed several improvements for the new channel maps in terms of noises and systematics, and it allowed us to produce a new y-map with reduced noises by ∼7% and minimal survey strips compared to the previous version released in 2015. We computed the tSZ angular power spectrum of the new y-map and performed a cosmological analysis. The results showed $S_8={0.764}_{-0.018}^{+0.015}{(stat)}_{-0.016}^{+0.031}(sys)$, including systematic uncertainties from a hydrostatic mass bias and pressure profile model. The value is fully consistent with recent KiDS and DES weak-lensing observations. It is also consistent with the Planck CMB’s result within 2σ, while our result is slightly lower.

A complete census of baryons in the late universe is a long-standing challenge due to the intermediate temperate and rarefied character of the majority of cosmic gas. To gain insight into this problem, we extract measurements of the kinematic Sunyaev-Zel’dovich (kSZ) effect from the cross-correlation of angular redshift fluctuations maps, which contain precise information about the cosmic density and velocity fields, and CMB maps high-pass filtered using aperture photometry; we refer to this technique as ARF-kSZ tomography. Remarkably, we detect significant cross-correlation for a wide range of redshifts and filter apertures using 6dF galaxies, BOSS galaxies, and SDSS quasars as tracers, yielding a 11 sigma detection of the kSZ effect. We then leverage these measurements to set constraints on the location, density, and abundance of gas inducing the kSZ effect, finding that this gas resides outside dark matter haloes, presents densities ranging from 10 to 250 times the cosmic average, and comprises half of cosmic baryons. Taken together, these findings indicate that ARF-kSZ tomography provides a nearly complete census of intergalactic gas from z = 0 to 5. This contribution is a summary of the work already published in Ref. 1.

Hydrodynamical simulations predict that the cosmic web contains the majority of the missing baryons in the form of plasma, called the warm-hot intergalactic medium (WHIM). However, its direct measurement through X-ray emissions has been prevented for decades due to the weak signal and the complex morphology of cosmic filaments.

We report the first statistical detection of X-ray emission from cosmic web filaments with the ROSAT data. We identified more than 15,000 large-scale filaments, spanning 30-100 Mpc length, in the SDSS survey and statistically detected X-ray emissions from the WHIM at 4.2 sigma confidence level using the ROSAT maps. Given this detection, we can expect a much more significant detection from SRG/eROSITA and indeed predicted the detectability of the WHIM. The prediction shows that stacking ∼2000 filaments only would lead to a 5σ detection with an average gas temperature of the WHIM as low as ∼0.3 keV.

We present a matching procedure for rapidly calculating the non-linear mass power spectrum in dynamical dark energy cosmologies to percent level accuracy in the range of scales between 0.1 < k < 3. This procedure is verified by large N-body simulations and is valid at any redshift for cosmologies consistent with current observations. Such accuracy in the power spectrum is necessary for next generation cosmological mass probes. Our matching procedure reproduces the CMB distance to last scattering and delivers sub-percent level accuracy in the matter power spectra at z = 0 and z ≈ 3. We discuss the physical implications for probing dark energy with surveys of large scale structure.

We consider the simplest model of modulation of the 3D Gaussian field in k-space by a non-uniformly oscillating function f₁(k) that imitates the baryon acoustic oscillations (BAO), and a model function f₂(k) reproducing the smoothed power spectrum of a galaxy clustering. This model is applied to statistical simulations of radial (shell-like) distributions with calculations of 1D power spectra and auto-correlation functions. It is shown that the radial distributions simulated relatively to different centres in a region of study include quasi-periodical components with a characteristic (BAO) scale 2π/k ∼ 100 h⁻¹ M_pc which can appear with rather high probability depending on a sampling length L_R and an amplitude A_m of the modulation. The averaging of a number of 1D correlation functions built from different centres of radial distributions leads to the mean 1D correlation function, which is qualitatively consistent with the standard 3D correlation function. Both the functions are characterized by a single wide peak at the BAO scale. We apply 1D statistical procedures to the sample of spectroscopic redshifts of the brightest cluster galaxies (BCGs) and show that the results turn out to be consistent with those obtained in our model simulations.

We examine the evolution of the spatial counts-in-cells distribution of galaxies and show that the form of the galaxy distribution function does not change significantly as galaxies merge and evolve. In particular, bound merging pairs follow a similar distribution to that of individual galaxies. From the adiabatic expansion of the universe we show how clustering, expansion and galaxy mergers affect the clustering parameter b. We also predict the evolution of b with respect to redshift.

As a hot dark-matter component, massive neutrinos modify the expansion history of the Universe as well as the evolution of cosmological perturbations, in a distinct way than cold dark matter or dark energy do. We look for such modifications in CFHTLS cosmic-shear data. Adding CMB, baryonic acoustic oscillations, and supernovae data, we are able to constrain the sum of the masses of the neutrinos species.

We showed how the shape of cosmic voids can be used to distinguish between different models of dark energy using galaxy positions.

Galaxies are not distributed randomly in the cosmic web but are instead arranged in filaments and sheets surrounding cosmic voids. Observationally there is still no convincing evidence of a link between the properties of galaxies and their host structures. Using the largest spectroscopic galaxy redshift survey (SDSS) we study the connection between the spin axes of galaxies and the orientation of their host filaments.

We found evidence that the spin axes of bright spiral galaxies have a weak tendency to be aligned parallel to filaments. For elliptical galaxies, we have a statistically significant result that their spin axes are aligned preferentially perpendicular to the host filaments.

A novel method allowing to compute density, velocity and other fields in cosmological N–body simulations with unprecedentedly high spatial resolution is described. It is based on the tessellation of the three-dimensional manifold representing cold dark matter in six-dimensional phase space. The density, velocity and other fields are computed by projecting the tessellation on configuration space. The application of this technique to cosmological N–body simulations in ΛCDM cosmology reveals a far more elaborate cosmic web then dot plots or self–adaptive SPH. In addition, this method allows to uniquely define physical voids and identify and study the caustic surfaces directly.