Dark Matter

Could dark energy be vector-like?

Authors: C. Armendariz-Picon
Comments: 26 pages, 6 figures. Uses RevTeX4

In this paper I explore whether a vector field can be the origin of the present stage of cosmic acceleration. In order to avoid violations of isotropy, the vector has be part of a ``cosmic triad'', that is, a set of three identical vectors pointing in mutually orthogonal spatial directions. A triad is indeed able to drive a stage of late accelerated expansion in the universe, and there exist tracking attractors that render cosmic evolution insensitive to initial conditions. However, as in most other models, the onset of cosmic acceleration is determined by a parameter that has to be tuned to reproduce current observations. The triad equation of state can be sufficiently close to minus one today, and for tachyonic models it might be even less than that. I briefly analyze linear cosmological perturbation theory in the presence of a triad. It turns out that the existence of non-vanishing spatial vectors invalidates the decomposition theorem, i.e. scalar, vector and tensor perturbations do not decouple from each other. In a simplified case it is possible to analytically study the stability of the triad along the different cosmological attractors. The triad is stable during inflation, radiation and matter domination, but it is unstable during (late-time) cosmic acceleration. I argue that this instability is not likely to have a significant impact at present.

Full-text: PostScript,PDF, or Other formats

Direct detection of dark matter with space-based laser interferometers

Authors: A. W. Adams, J. S. Bloom (Harvard)
Comments: Submitted, 12 pages, 3 color figures, latex
Report-no: HUTP-04/A021

Only a small fraction of the mass in the universe has been directly detected through its electromagnetic signature; the existence of the remaining dark matter is inferred from its gravitational fingerprint. This dark matter is believed to comprise a free-streaming gas of massive objects whose detailed nature is largely unconstrained; viable models posit masses anywhere between a few electronvolts and millions of solar masses. Searches on both the high and low ends have so far led to no convincing account for the missing mass in the universe. Dark matter pervades the Solar System, perturbing gravitationally, and thus very weakly, all nearby inertial masses. Making use of this, we propose a novel approach to dark matter detection at previously inaccessible intermediate masses (1e13 -- 1e20 gm, scales relevant for several classes of dark matter candidates). We show that if such dark matter exists it will be unambiguously detectable through its inelastic gravitational interaction with the proposed Laser Interferometer Space Antenna (LISA) experiment. We demonstrate the efficacy of this approach by studying the dark matter signal in numerical simulations of the LISA data stream. We further demonstrate that asteroids of comparable mass and space density will be readily detectable in reflected Solar light, eliminating the major background.

Full-text: PostScript,PDF, or Other formats

Diurnal and Annual Modulation of Cold Dark Matter Signals

Authors: Fu-Sin Ling, Pierre Sikivie, Stuart Wick
Comments: 36 pages, 10 figures
Report-no: UFIFT-HET-04-6

We calculate the diurnal and annual modulation of the signals in axion and WIMP dark matter detectors on Earth caused by a cold flow of dark matter in the Solar neighborhood. The effects of the Sun's and the Earth's gravity, and of the orbital and rotational motions of the Earth are included. A cold flow on Earth produces a peak in the spectrum of microwave photons in cavity detectors of dark matter axions, and a plateau in the nuclear recoil energy spectrum in WIMP detectors. Formulas are given for the positions and heights of these peaks and plateaux as a function of time in the course of day and year, including all corrections down to the 0.1% level of precision. The results can be applied to an arbitrary dark matter velocity distribution $f(\vec{v})$ by integrating the one-flow results over velocities. We apply them to the set of flows predicted by the caustic ring model of the Galactic halo. The caustic ring model predicts the dark matter flux on Earth to be largest in December $\pm$ one month. Nonetheless, because of the role of energy thresholds, the model is consistent with the annual modulation results published by the DAMA collaboration provided the WIMP mass is larger than approximately 100 GeV.

Full-text: PostScript,PDF, or Other formats

Search for Neutrino-Induced Cascades with AMANDA

Authors: the AMANDA Collaboration: M. Ackermann, et al
Comments: 18 pages, 12 figures

We report on a search for electro-magnetic and/or hadronic showers (cascades) induced by high energy neutrinos in the data collected with the AMANDA II detector during the year 2000. The observed event rates are consistent with the expectations for atmospheric neutrinos and muons. We place upper limits on a diffuse flux of extraterrestrial electron, tau and muon neutrinos. A flux of neutrinos with a spectrum $\Phi \propto E^{-2}$ which consists of an equal mix of all flavors, is limited to $E^2 \Phi(E)=8.6 x 10^{-7} GeV/(cm^{2} s sr)$ at a 90% confidence level for a neutrino energy range 50 TeV to 5 PeV. We present bounds for specific extraterrestrial neutrino flux predictions. Several of these models are ruled out.

Full-text: PostScript,PDF, or Other formats

Search for Small-Mass Black Hole Dark Matter with Space-Based Gravitational Wave Detectors

Authors: Naoki Seto, Asantha Cooray (Caltech)
Comments: 5 pages, 1 figure

The high sensitivity of upcoming space-based gravitational wave detectors suggests the possibility that if halo dark matter were composed of primordial black holes (PBHs) with mass between $10^{16}$ g and 10$^{20}$ g, the gravitational interaction with detector test masses will lead to a detectable pulse-like signal during the fly-by. For an improved version of the Laser Interferometer Space Antenna with a reduced acceleration noise at the low-end of its frequency spectrum, we find an event rate, with signal-to-noise ratios greater than 5, of $\sim$ a few per decade involving black holes of mass $\sim$ 10$^{17}$ g. The detection rate improves significantly for second generation space based interferometers that are currently envisioned, though these events must be distinguished from those involving perturbations due to near-Earth asteroids. While the presence of primordial black holes below a mass of $\sim$ 10$^{16}$ g is now constrained based on the radiation released during their evaporation, the gravitational wave detectors will extend the study of PBHs to a several orders of magnitude higher masses.

Full-text: PostScript,PDF, or Other formats

Triality between Inflation, Cyclic and Phantom Cosmologies

Authors: James E. Lidsey
Comments: 6 pages

It is shown that any spatially flat and isotropic universe undergoing accelerated expansion driven by a self-interacting scalar field can be directly related to a contracting, decelerating cosmology. The duality is made manifest by expressing the scale factor and Hubble parameter as functions of the scalar field and simultaneously interchanging these two quantities. The decelerating universe can be twinned with a cosmology sourced by a phantom scalar field by inverting the scale factor and leaving the Hubble parameter invariant. The accelerating model can be related to the same phantom universe by identifying the scale factor with the inverse of the Hubble parameter. The duality between accelerating and decelerating backgrounds can be extended to spatially curved cosmologies and models containing perfect fluids. A similar triality and associated scale factor duality is found in the Randall-Sundrum type II braneworld scenario.

Full-text: PostScript,PDF, or Other formats

Large-Scale Correlations in the Lyman-alpha Forest at z = 3-4

Authors: George D. Becker (1), Wallace L. W. Sargent (1), Michael Rauch (2) ((1) Palomar Observatory, California Institute of Technology, (2) The Observatories of the Carnegie Institution of Washington)
Comments: 33 pages, 13 figures, submitted to ApJ

We present a study of the spatial coherence of the intergalactic medium toward two pairs of high-redshift quasars with moderate angular separations observed with Keck/ESI, Q1422+2309A/Q1424+2255 (z_em = 3.63, theta = 39") and Q1439-0034A/B (z_em = 4.25, theta = 33"). The crosscorrelation of transmitted flux in the Lyman-alpha forest shows a 5-7 sigma peak at zero velocity lag for both pairs. This strongly suggests that at least some of the absorbing structures span the 230-300/h_70 proper kpc transverse separation between sightlines. We also statistically examine the similarity between paired spectra as a function of transmitted flux, a measure which may be useful for comparison with numerical simulations. In investigating the dependence of the correlation functions on spectral characteristics, we find that photon noise has little impact for S/N >~ 10 per resolution element. However, the agreement between the autocorrelation along the line sight and the crosscorrelation between sightlines, a potential test of cosmological geometry, depends significantly on instrumental resolution. Finally, we present an inventory of metal lines. These include a a pair of strong C IV systems at z ~ 3.4 appearing only toward Q1439B, and a Mg II + Fe II system present toward Q1439 A and B at z = 1.68.

Full-text: PostScript,PDF, or Other formats

Upper limit of galactic mass loss from globular cluster

Authors: Florian Dubath, Anna Rissone
Comments: 7 pages, no figures

Using tidal disruption of globular clusters by the galactic center, we put limits on the total mass ever enclosed into orbits of observed globular clusters. Under the assumption that the rate of mass loss from the Galaxy is steady, we then deduce a bound on this rate. In particular this bound can be used to constrain the galactic gravitational wave luminosity.

Full-text: PostScript,PDF, or Other formats

The Effect of Gas Cooling on the Shapes of Dark Matter Halos

Authors: Stelios Kazantzidis (1,2), Andrey V. Kravtsov (2), Andrew R. Zentner (2) Brandon Allgood (3), Daisuke Nagai (2), Ben Moore (1) ((1) University of Zurich, (2) KICP, U. Chicago, (3) UC Santa Cruz)
Comments: submitted to ApJL, 5 pages, 3 figures, LaTeX (uses emulateapj5.sty)

We analyze the effect of dissipation on the shapes of dark matter (DM) halos using high-resolution cosmological gasdynamics simulations of clusters and galaxies in the LCDM cosmology. We find that halos formed in simulations with gas cooling are significantly more spherical than corresponding halos formed in adiabatic simulations. Gas cooling results in an average increase of the principle axis ratios of halos by ~0.2-0.4 in the inner regions. The systematic difference decreases slowly with radius, but persists almost to the virial radius. We argue that the differences in simulations with and without cooling arise both during periods of quiescent evolution, when gas cools and condenses toward the center, and during major mergers. We perform a series of high-resolution N-body simulations to study the shapes of remnants in major mergers of DM halos and halos with embedded stellar disks. In the DM halo only mergers, the shape of the remnants depends only on the orbital angular momentum of the encounter and not on the internal structure of the halos. However, significant shape changes in the DM distribution may result if stellar disks are included. In this case the shape of the DM halos is correlated with the morphology of the stellar remnants.

Full-text: PostScript,PDF, or Other formats

Impact of the Gravity of Cosmic Fluctuations on CMB and Matter Clustering

Authors: Sergei Bashinsky (Princeton U.)
Comments: 6 pages, 2 figures

All the particles and dynamical fields that contribute to the energy of the universe leave potentially observable gravitational imprints of their fluctuations. We analyze the gravitational impact of adiabatic perturbations for a photon-baryon plasma, relativistic neutrinos, dark matter, and quintessence. We find that, contrary to the prevailing view, the self-gravity of radiation fluctuations does not boost the cosmic microwave background (CMB) angular power on scales entering the acoustic horizon in the radiation era. Rather, the CMB anisotropy power is suppressed, up to 25 times, by the gravity of collapsing pressureless dark matter on scales exceeding the horizon size at radiation-matter equality, l<200. We verify that the suppression would not occur if matter were unclustered on large scales. Whenever non-standard species contribute to the total energy, the gravity of their perturbations affects the modes that enter the horizon at that time (the impact is generally stronger when pressureless matter is abundant). Neutrinos and early quintessence both shift the phase of the CMB peaks to lower l but change the ratio of the CMB to the matter power spectrum in opposite directions.

Full-text: PostScript,PDF, or Other formats

Compton Echoes from Gamma-Ray Bursts: Unveiling Misaligned Jets in Nearby Type Ib/c Supernovae

Authors: Enrico Ramirez-Ruiz (IAS), Piero Madau (UCSC)
Comments: 9 pages, 2 figures, to appear in the ApJ Letters

There is now compelling evidence of a link between long-duration gamma-ray bursts (GRBs) and Type Ib/c supernovae (SNe). These core-collapse explosions are conjectured to radiate an anisotropic, beamed component associated with a decelerating, relativistic outflow and an unbeamed, isotropic component associated with the slowly expanding stellar debris. The anisotropic emission remains at a very low level until the Doppler cone of the beam intersects the observer's line of sight, making off-axis GRB jets directly detectable only at long wavelengths and late times. Circumstellar material, however, will Compton scatter the prompt gamma-ray and afterglow radiation flux and give rise to a reflection echo. We show that the Compton echo of a misaligned GRB carries an X-ray luminosity that may exceed by many orders of magnitude that produced by the underlying subrelativistic SN during the first few weeks. Bright scattering echoes may therefore provide a means for detecting a population of misaligned GRBs associated with nearby Type Ib/c SNe and yield crucial information on the environment surrounding a massive star at the time of its death. The question of whether the interpretation of GRB980425 as an ordinary GRB observed off-axis is consistent with the lack of an X-ray echo is addressed, along with the constraints derived on the possible existence of misaligned GRB jets in SN1993J, SN1994I, SN1999em, and SN2002ap.

Full-text: PostScript,PDF, or Other formats

Supersymmetric dark matter Q-balls and their interactions in matter

Authors: Alexander Kusenko, Lee Loveridge, Mikhail Shaposhnikov
Comments: 12 pages
Report-no: EPFL-ITP-LPPC-04-2s; UCLA/04/TEP/13

Supersymmetric extensions of the Standard Model contain non-topological solitons, Q-balls, which can be stable and can be a form of cosmological dark matter. We describe interaction of SUSY Q-balls with ordinary matter. A baryon scattering off a baryonic SUSY Q-ball can convert into its antiparticle with a high probability. We explore the resulting astrophysical consequences of dark-matter Q-balls captured by neutron stars and white dwarfs. We find, in particular, that SUSY Q-balls could destroy neutron stars on a time scale of the order of a billion years. While this is not in contradiction with any data, the disruption of neutron stars can produce bursts of energy consistent with short gamma-ray bursts.