Are halos of collisionless cold dark matter collisionless?

We study whether gravitational scattering of halo dark matter particles by subhalos can connect two seemingly independent problems: the abundance of subhalos in dark matter halos and the cuspiness of the halos' inner density profiles. Our numerical experiments indicate that subhalos can cause the collisionless dark matter particles in the centers of main halos to diffuse. Combined with tidal mass loss of the subhalos, this process introduces significant scatter in the inner density profiles and offers an explanation for the range of profiles seen in both observations and cosmological simulations.     

Can the morphology of γ-ray emission distinguish annihilating from decaying dark matter?

Recent results from the PAMELA, ATIC, FERMI and HESS experiments have focused attention on the possible existence of high energy cosmic rays e+ e- that may originate from dark matter annihilations or decays in the Milky Way. Here we examine the morphology of the associated γ-ray emission after propagation of the electrons generated by both annihilating and decaying dark matter models. We focus on photon energies of 1, 10, and 50 GeV (relevant for the FERMI satellite) and consider different propagation parameters. Our main conclusion is that distinguishing annihilating from decaying dark matter may only be possible if the propagation parameters correspond to the most optimistic diffusion models. In addition, we point to examples where morphology can lead to an erroneous interpretation of the source injection energy.     

PandaX-Ⅱ set constraints to self-interacting dark matter using the full dataset

正In spite of the success of the cold dark matter(CDM) models working well at large scales, several discrepancies have arisen regarding CDM on galaxy scales. Some of these are described as the cusp-core, diversity, missing satellites, and too-big-to-fail anomalies. Among various suggestions to resolve some of these anomalies, the self-interacting dark matter(SIDM) [1] has attracted considerable attention in recent years.     

What is modified gravity and how to differentiate it from particle dark matter?

An obvious criterion to classify theories of modified gravity is to identify their gravitational degrees of freedom and their coupling to the metric and the matter sector. Using this simple idea, we show that any theory which depends on the curvature invariants is equivalent to general relativity in the presence of new fields that are gravitationally coupled to the energy-momentum tensor. We show that they can be shifted into a new energy-momentum tensor. There is no a priori reason to identify these new fields as gravitational degrees of freedom or matter fields. This leads to an equivalence between dark matter particles gravitationally coupled to the standard model fields and modified gravity theories designed to account for the dark matter phenomenon. Due to this ambiguity, it is impossible to differentiate experimentally between these theories and any attempt of doing so should be classified as a mere interpretation of the same phenomenon.     

Can sterile neutrinos be the dark matter?

We use the Ly-alpha forest power spectrum measured by the Sloan Digital Sky Survey and high-resolution spectroscopy observations in combination with cosmic microwave background and galaxy clustering constraints to place limits on a sterile neutrino as a dark matter candidate in the warm dark matter scenario. Such a neutrino would be created in the early Universe through mixing with an active neutrino and would suppress structure on scales smaller than its free-streaming scale. We ran a series of high-resolution hydrodynamic simulations with varying neutrino masses to describe the effect of a sterile neutrino on the Ly-alpha forest power spectrum. We find that the mass limit is m(s) >13 keV at 95% C.L. (9 keV at 99.9%), which is above the upper limit allowed by x-ray constraints, excluding this candidate from being all of the dark matter in this model.     

Is it possible to estimate the Higgs mass from the CMB power spectrum?

General Relativity and Standard Model are considered as a theory of dynamical scale symmetry with definite initial data compatible with the accepted Higgs mechanism. In this theory the Early Universe behaves like a factory of electroweak bosons and Higgs scalars, and it gives a possibility to identify three peaks in the Cosmic Microwave Background power spectrum with the contributions of photonic decays and annihilation processes of primordial Higgs, W and Z bosons in agreement with the QED coupling constant, Weinberg’s angle, and Higgs’ particle mass of about 118 GeV. The text was submitted by the authors in English.     

Primordial power spectrum from the Dapor–Liegener model of loop quantum cosmology

We compute the predictions for the power spectrum of scalar perturbations from a recent new proposal for the effective Hamiltonian of loop quantum cosmology. The model provides an attractive picture of the early cosmos, in which our classical Friedmann–Lemaître–Robertson–Walker universe emerges from a quantum phase where the spacetime curvature remains constant and of Planckian size. We compare the predictions for the cosmic microwave background with previous results obtained within loop quantum cosmology, and discuss the differences and similarities. The analysis provides an example of the way differences between quantization schemes can be translated to physical observables.     

First dark matter search results from the PandaX-Ⅰ experiment

We report on the first dark-matter(DM)search results from PandaX-I,a low threshold dual-phase xenon experiment operating at the China JinPing Underground Laboratory.In the 37-kg liquid xenon target with 17.4 live-days of exposure,no DM particle candidate event was found.This result sets a stringent limit for low-mass DM particles and disfavors the interpretation of previously-reported positive experimental results.The minimum upper limit,3.7×10-44cm2,for the spin-independent isoscalar DM-particle-nucleon scattering cross section is obtained at a DM-particle mass of 49 GeV/c2at 90%confidence level.     

Do baryons trace dark matter in the early universe?

Baryon-density perturbations of large amplitude may exist if they are compensated by dark-matter perturbations such that the total density is unchanged. Primordial abundances and galaxy clusters allow these compensated isocurvature perturbations (CIPs) to have amplitudes as large as ~10%. CIPs will modulate the power spectrum of cosmic microwave background (CMB) fluctuations--those due to the usual adiabatic perturbations--as a function of position on the sky. This leads to correlations between different spherical-harmonic coefficients of the temperature and/or polarization maps, and induces polarization B modes. Here, the magnitude of these effects is calculated and techniques to measure them are introduced. While a CIP of this amplitude can be probed on large scales with existing data, forthcoming CMB experiments should improve the sensitivity to CIPs by at least an order of magnitude.     

Can dark matter annihilation dominate the extragalactic gamma-ray background?

Annihilating dark matter (DM) has been discussed as a possible source of gamma rays from the galactic center and as a contribution to the extragalactic gamma-ray background. Assuming universality of the density profile of DM halos, we show that it is quite unlikely that DM annihilation is a main constituent of extragalactic gamma-ray background, without exceeding the observed gamma-ray flux from the galactic center. This argument becomes stronger when we include enhancement of the density profiles by supermassive black holes or baryon cooling. The presence of a substructure may loosen the constraint, but only if a very large cross section as well as the rather flat profile are realized.     

How complex is the dynamics of Peptide folding?

Classical molecular dynamics simulations of the folding of alanine peptides in aqueous solution are analyzed by constructing a deterministic model of the dynamics, using methods from nonlinear time series analysis. While the dimension of the free energy landscape increases with system size, a Lyapunov analysis shows that the effective dimension of the dynamic system is rather small and even decreases with chain length. The observed reduction of phase space is a nonlinear cooperative effect that is caused by intramolecular hydrogen bonds that stabilize the secondary structure of the peptides.     

How particular is the physics of the free energy principle?

The free energy principle (FEP) states that any dynamical system can be interpreted as performing Bayesian inference upon its surrounding environment. Although, in theory, the FEP applies to a wide variety of systems, there has been almost no direct exploration or demonstration of the principle in concrete systems. In this work, we examine in depth the assumptions required to derive the FEP in the simplest possible set of systems – weakly-coupled non-equilibrium linear stochastic systems. Specifically, we explore (i) how general the requirements imposed on the statistical structure of a system are and (ii) how informative the FEP is about the behaviour of such systems. We discover that two requirements of the FEP – the Markov blanket condition (i.e. a statistical boundary precluding direct coupling between internal and external states) and stringent restrictions on its solenoidal flows (i.e. tendencies driving a system out of equilibrium) – are only valid for a very narrow space of parameters. Suitable systems require an absence of perception-action asymmetries that is highly unusual for living systems interacting with an environment. More importantly, we observe that a mathematically central step in the argument, connecting the behaviour of a system to variational inference, relies on an implicit equivalence between the dynamics of the average states of a system with the average of the dynamics of those states. This equivalence does not hold in general even for linear stochastic systems, since it requires an effective decoupling from the system's history of interactions. These observations are critical for evaluating the generality and applicability of the FEP and indicate the existence of significant problems of the theory in its current form. These issues make the FEP, as it stands, not straightforwardly applicable to the simple linear systems studied here and suggest that more development is needed before the theory could be applied to the kind of complex systems that describe living and cognitive processes.     

How fundamental is the character of thermal uncertainty relations?

We discuss here two different information measures of the Tsallis type, and their associated probability distributions, in order to repeat the Mandelbrot Cramer–Rao steps that lead to a thermal uncertainty relation for exponential distributions. We deal first with the original Tsallis measure and discuss afterwards a second entropic measure associated with the concept of escort distribution. In neither case it is possible to re-obtain a thermal uncertainty relationship. We conclude therefore that the thermal uncertainty, as derived from the Cramer–Rao inequality, cannot be as fundamental as the quantum one.     

Constraining self-interacting dark matter with the full dataset of PandaX-Ⅱ

Self-interacting dark matter(SIDM) is a leading candidate proposed to solve discrepancies between predictions of the prevailing cold dark matter theory and observations of galaxies. Many SIDM models predict the existence of a light force carrier that mediates strong dark matter self-interactions. If the mediator couples to the standard model particles, it could produce characteristic signals in dark matter direct detection experiments. We report searches for signals of SIDM models with a light mediator using the full dataset of the PandaX-Ⅱ experiment, basing on a total exposure of 132 tonne-days. No significant excess over background is found, and our likelihood analysis leads to a strong upper limit on the dark matter-nucleon coupling strength. We further combine the PandaX-Ⅱ constraints and those from observations of the light element abundances in the early universe, and show that direct detection and cosmological probes can provide complementary constraints on dark matter models with a light mediator.     

Can sub-GeV dark matter coherently scatter on the electrons in the atom?

A novel detection of sub-GeV dark matter is proposed in the paper.The electron cloud is boosted by the dark matter and emits an electron when it is dragged back by the heavy nucleus,namely the coherent scattering of the electron cloud of the atom.The survey in the x-ray diffraction shows that the atomic form factors are much more complex than the naive consideration.The results of the relativistic Hartree-Fock(RHF) method give non-trivial shapes of the atoms.The detailed calculation of the recoi...