Narrow Results

We show that the idea that CDM are decaying superheavy particles which produce UHECR with energies beyond the GZK cutoff may simultaneously solve the problem of subgalactic structure formation in CCDM model. In particular, the Kuzmin-Rubakov's decaying superheavy CDM model may give an explanation to the smallness of the cosmological constant and a new thought to the CDM experimental search.

N-body simulations of dark matter halos show that the density is cusped near the center of the halo. The density profile behaves as r^–γ in the inner parts, where γ ≃ 1 for the NFW model and γ ≃ 1.5 for the Moore model, but in the outer parts the two models agree with each other in the asymptotic behavior of the density profile. The simulations also show information about the anisotropy parameter β(r) of the velocity distribution: β ≈ 0 in the inner part and β ≈ 0.5 (radially anisotropic) in the outer part of the halo. We provide some distribution functions F(E, L) with the constant anisotropy parameter β for the two spherical models of dark matter halos: a new generalized NFW model and a generalized Moore model. There are two parameters α and ∊ for those two generalized models to determine the asymptotic behavior of the density profile. In this paper, we concentrate on the situation of β(r) = 1/2 from the viewpoint of the simulation.

N-body simulations of dark matter halos show that the density profiles of the halos behave as ρ(r) ∝ r^-α(r), where the density logarithmic slope α ≃ 1–1.5 in the center and α ≃ 3–4 in the outer parts of the halos. However, some observations are not in agreement with simulations in the very central region of the halos. The simulations also show that the velocity dispersion anisotropy parameter β ≈ 0 in the inner part of the halo and the so-called pseudo–phase-space density ρ/σ³ behaves as a power law in radius r. With these results in mind, we study the distribution function and the pseudo–phase-space density ρ/σ³ of the center of dark matter halos and find that they are closely related.

In this work, we investigate the dynamical evolution of spherical self-gravitating systems under their own gravity with $N$-body simulations. For this purpose, we study the evolution of the generalized virialization relations, and particularly focus on the time evolution of the coarse-grained entropy of dark matter halos under various perturbations. First, we construct six single perturbation models under four initial conditions to mimic typical disturbances that a realistic gravitating system may encounter. With the simulation results, we show the time evolution of the entropy for the six perturbation models. In all these models, at first the entropy increases rapidly for a short period of time, slowly evolves for a longer period of time and then remains nearly unchanged in the subsequent evolution. The main dynamical mechanisms behind these evolutions should be violent relaxation and phase mixing. However, under repeated perturbations to the system, the evolution of entropy of self-gravitating systems manifests complete differences from that of the usual thermodynamical systems. We see that the entropy of the end states of every single perturbation, according to different repeated perturbation modes, either decreases or increases. We argue that the increasing or decreasing of the end-state entropy should be the reflection of the complexity of the thermodynamical states of self-gravitating systems. These conclusions are independent of the initial conditions. Besides, we demonstrate that the generalized virialization relations can reveal whether or not, or in which radius interval, the collisionless Boltzmann equation is suitable for description of a self-gravitating system, and can be used as good stability criteria of the system.

The theoretical basis of modern cosmology was laid out soon after Einstein’s 1915 discovery of general relativity. Friedmann first worked out models of the expanding Universe, with Lemaitre adding the concepts of redshift and the initial Big Bang. Hubble’s 1929 discovery of cosmic expansion was the first observational milestone. The hot Big Bang theory accumulated more substance as the first theoretical predictions of the cosmic microwave background (CMB) and of Big-Bang nucleosynthesis were made by Alpher, Gamow, and Herman in the 1940’s. A watershed moment in observational cosmology came with the 1965 discovery of the CMB [1], which was the first direct evidence for the hot and dense initial state of the Universe. Meanwhile, astronomical evidence accumulated for the evolution of the Universe over time, as increasingly distant galaxies were discovered. Soon after quasars were identified as high-redshift objects, Gunn & Peterson [2] used their spectra (in 1965) to show that the inter-galactic gas around them was highly ionized; this was the first sign that the gas had undergone cosmic reionization, likely by the stars in early galaxies.

I provide an introduction to experiments designed to detect WIMP dark matter directly, focussing on building intuitive understanding of the characteristics of potential WIMP signals and the experimental techniques. After deriving the characteristics of potential signals in direct-detection experiments for standard WIMP models, I summarize the general experimental methods shared by most direct-detection experiments and review the advantages, challenges, and status of such searches. Experiments are already probing SUSY models, with best limits on the spin-independent coupling below 10^-7 pb. Combined information from direct and indirect detection, along with detection at colliders, promises to teach us much about fundamental particle physics, cosmology, and astrophysics.