196,172 research outputs found
Velocity distributions and annual-modulation signatures of weakly-interacting massive particles
No full textAn annual modulation in the event rate of the NaI detector of the DAMA collaboration has been used to infer the existence of particle dark matter in the Galactic halo. Bounds on the WIMP mass and WIMP-nucleon cross section have been derived. These analyses have assumed that the local dark-matter velocity distribution is either isotropic or has some bulk rotation. Here we consider the effects of possible structure in the WIMP velocity distribution on the annual-modulation amplitude. We show that if we allow for a locally anisotropic velocity dispersion tensor, the interpretation of direct detection experiments could be altered significantly. We also show that uncertainties in the velocity distribution function that arise from uncertainties in the radial density profile are less important if the velocity dispersion is assumed to be isotropic
Dark-matter spike at the Galactic center?
No full textThe past growth of the central black hole (BH) might have enhanced the density of cold dark matter halo particles at the galactic center. We compute this effect in realistic growth models of the present (2-3) x 10(6) M-., BH from a low-mass seed BH, with special attention to dynamical modeling in a realistic galaxy environment with merger and orbital decay of a seed BH formed generally outside the exact center of the halo. An intriguing "very-dense spike" of dark matter has been claimed in models of Gondolo and Silk with a density high enough to contradict with experimental upper bounds of neutralino annihilation radiation. This "spike" disappears completely or is greatly weakened when we include important dynamical processes neglected in their idealized or restrictive picture with cold particles surrounding an at-the-center zero-seed adiabatically growing BH. For the seed BH to spiral in and settle to the center within a Hubble time by dynamical friction, the seed mass must be at least a significant fraction of the present BH. Any subsequent at-the-center growth of the BH and steepening of the central Keplerian potential well can squeeze the halo density distribution only mildly, whether the squeezing happens adiabatically or instantaneously
Spin-Dependent WIMPs in DAMA?
No full textWe investigate whether the annual modulation observed in the DAMA experiment can be due to a weakly-interacting massive particle (WIMP) with an axial-vector (spin-dependent; SD) coupling to nuclei. We evaluate the SD WIMP-proton cross section under the assumption that such scattering accounts for the DAMA modulation, and we do the same for a SD WIMP-neutron cross section. We show that SD WIMP-proton scattering is ruled out in a model-independent fashion by null searches for energetic neutrinos from WIMP annihilation in the Sun, and that SD WIMP-neutron scattering is ruled out for WIMP masses greater than or similar to 20 GeV by the null result with the DAMA Xe detector. A SD WIMP with mass less than or similar to 20 GeV is still compatible, but only if the SD WIMP-neutron interaction is four orders of magnitude greater than the WIMP-proton interaction
A running spectral index in supersymmetric dark-matter models with quasistable charged particles
No full textWe show that charged particles decaying in the early Universe can induce a scale-dependent or running spectral index in the small-scale linear and nonlinear matter power spectrum and discuss examples of this effect in minimal supersymmetric models in which the lightest neutralino is a viable cold-dark-matter candidate. We find configurations in which the neutralino relic density is set by coannihilations with a long-lived stau and the late decay of staus partially suppresses the linear matter power spectrum. Nonlinear evolution on small scales then causes the modified linear power spectrum to evolve to a nonlinear power spectrum similar to (but different in detail) models parametrized by a constant running alpha(s)=dn(s)/dlnk by redshifts of 2 to 4. Thus, Lyman-alpha forest observations, which probe the matter power spectrum at these redshifts, might not discriminate between the two effects. However, a measurement of the angular power spectrum of primordial 21-cm radiation from redshift zapproximate to30-200 might distinguish between this charged-decay model and a primordial running spectral index. The direct production of a long-lived charged particle at future colliders is a dramatic prediction of this model
Violation of statistical isotropy and homogeneity in the 21-cm power spectrum
No full textMost inflationary models predict primordial perturbations to be statistically isotropic and homogeneous. Cosmic microwave background (CMB) observations, however, indicate a possible departure from statistical isotropy in the form of a dipolar power modulation at large angular scales. Alternative models of inflation, beyond the simplest single-field slow-roll models, can generate a small power asymmetry, consistent with these observations. Observations of clustering of quasars show, however, agreement with statistical isotropy at much smaller angular scales. Here, we propose to use off-diagonal components of the angular power spectrum of the 21-cm fluctuations during the dark ages to test this power asymmetry. We forecast results for the planned SKA radio array, a future radio array, and the cosmic-variance-limited case as a theoretical proof of principle. Our results show that the 21-cm line power spectrum will enable access to information at very small scales and at different redshift slices, thus improving upon the current CMB constraints by ∼2 orders of magnitude for a dipolar asymmetry and by ∼1-3 orders of magnitude for a quadrupolar asymmetry case
Cosmological tests of an axiverse-inspired quintessence field
No full textInspired by the string axiverse idea, it has been suggested that the recent transition from decelerated to accelerated cosmic expansion is driven by an axion-like quintessence field with a sub-Planckian decay constant. The scenario requires that the axion field be rather near the maximum of its potential but is less finely tuned than other explanations of cosmic acceleration. The model is parametrized by an axion decay constant f, the axion mass m, and an initial misalignment angle |θi| which is close to π. In order to determine the m and θi values consistent with observations, these parameters are mapped onto observables: the Hubble parameter H(z) at an angular-diameter distance dA(z) to redshift z=0.57, as well as the angular sound horizon of the cosmic microwave background (CMB). Measurements of the baryon acoustic oscillation (BAO) scale at z≃0.57 by the BOSS survey and Planck measurements of CMB temperature anisotropies are then used to probe the {m,f,θi} parameter space. With current data, CMB constraints are the most powerful, allowing a fraction of only ∼0.2 of the parameter-space volume. Measurements of the BAO scale made using the SPHEREx or SKA experiments could go further, observationally distinguishing all but ∼10-2 or ∼10-5 of the parameter-space volume (allowed by simple priors) from the ΛCDM model
Cosmological bounds on dark matter-neutrino interactions
No full textWe investigate the cosmological effects of a neutrino interaction with cold dark-matter. We postulate a neutrino that interacts with a "neutrino-interacting dark-matter" (NIDM) particle with an elastic-scattering cross section that either decreases with temperature as T2 or remains constant with temperature. The neutrino-dark-matter interaction results in a neutrino-dark-matter fluid with pressure, and this pressure results in diffusion-damped oscillations in the matter power spectrum, analogous to the acoustic oscillations in the baryon-photon fluid. We discuss the bounds from the Sloan Digital Sky Survey on the NIDM opacity (ratio of cross section to NIDM-particle mass) and compare with the constraint from observation of neutrinos from supernova 1987A. If only a fraction of the dark matter interacts with neutrinos, then NIDM oscillations may affect current cosmological constraints from measurements of galaxy clustering. We discuss how detection of NIDM oscillations would suggest a particle-antiparticle asymmetry in the dark-matter sector
Tests for Primordial Non-Gaussianity
No full textWe investigate the relative sensitivities of several tests for deviations from Gaussianity in the primordial distribution of density perturbations. We consider models for non-Gaussianity that mimic that which comes from inflation as well as that which comes from topological defects. The tests we consider involve the cosmic microwave background (CMB), large-scale structure, high-redshift galaxies, and the abundances and properties of clusters. We find that the CMB is superior at finding non-Gaussianity in the primordial gravitational potential (as inflation would produce), while observations of high-redshift galaxies are much better suited to find non-Gaussianity that resembles that expected from topological defects. We derive a simple expression that relates the abundance of high-redshift objects in non-Gaussian models to the primordial skewness
Detecting the integrated Sachs-Wolfe effect with high-redshift 21-cm surveys
No full textWe investigate the possibility of detecting the integrated Sachs-Wolfe (ISW) effect by cross-correlating 21-cm surveys at high redshifts with galaxies in a way similar to the usual CMB-galaxy cross-correlation. The high-redshift 21-cm signal is dominated by CMB photons that travel freely without interacting with the intervening matter, and hence its late-time ISW signature should correlate extremely well with that of the CMB at its peak frequencies. Using the 21-cm temperature brightness instead of the CMB would thus be a further check of the detection of the ISW effect, measured by different instruments at different frequencies and suffering from different systematics. We also study the ISW effect on the photons that are scattered by HI clouds. We show that a detection of the unscattered photons is achievable with planned radio arrays, while one using scattered photons will require advanced radio interferometers, either an extended version of the planned Square Kilometre Array or futuristic experiments such as a lunar radio array
Dark Matter Astrophysics
No full textThese lectures are intended to provide a brief pedagogical review of dark
matter for the newcomer to the subject. We begin with a discussion of the
astrophysical evidence for dark matter. The standard weakly-interacting massive
particle (WIMP) scenario--the motivation, particle models, and detection
techniques--is then reviewed. We provide a brief sampling of some recent
variations to the standard WIMP scenario as well as some alternatives (axions
and sterile neutrinos). Exercises are provided for the reader
- …
