84 research outputs found
Hidden Hot Dark Matter as Cold Dark Matter
We show that hidden hot dark matter, hidden-sector dark matter with
interactions that decouple when it is relativistic, is a viable dark matter
candidate provided it has never been in thermal equilibrium with the particles
of the standard model. This hidden hot dark matter may reheat to a lower
temperature and number density than the visible Universe and thus account,
simply with its thermal abundance, for all the dark matter in the Universe
while evading the typical constraints on hot dark matter arising from structure
formation. We find masses ranging from ~3 keV to ~10 TeV. While never in
equilibrium with the standard model, this class of models may have unique
observational signatures in the matter power spectrum or via extra-weak
interactions with standard model particles.Comment: 5 pages, 1 figur
Morphing the CMB: a technique for interpolating power spectra
The confrontation of the Cosmic Microwave Background (CMB) theoretical
angular power spectrum with available data often requires the calculation of
large numbers of power spectra. The standard practice is to use a fast code to
compute the CMB power spectra over some large parameter space, in order to
estimate likelihoods and constrain these parameters. But as the dimensionality
of the space under study increases, then even with relatively fast anisotropy
codes, the computation can become prohibitive. This paper describes the
employment of a "morphing" strategy to interpolate new power spectra based on
previously calculated ones. We simply present the basic idea here, and
illustrate with a few examples; optimization of interpolation schemes will
depend on the specific application. In addition to facilitating the exploration
of large parameter spaces, this morphing technique may be helpful for Fisher
matrix calculations involving derivatives.Comment: 18 pages, including 6 figures, uses elsart.cls, accepted for
publication in New Astronomy, changes to match published versio
Cosmological Limits on Hidden Sector Dark Matter
We explore the model-independent constraints from cosmology on a dark-matter
particle with no prominent standard model interactions that interacts and
thermalizes with other particles in a hidden sector. Without specifying
detailed hidden-sector particle physics, we characterize the relevant physics
by the annihilation cross section, mass, and temperature ratio of the hidden to
visible sectors. While encompassing the standard cold WIMP scenario, we do not
require the freeze-out process to be nonrelativistic. Rather, freeze-out may
also occur when dark matter particles are semirelativistic or relativistic. We
solve the Boltzmann equation to find the conditions that hidden-sector dark
matter accounts for the observed dark-matter density, satisfies the
Tremaine-Gunn bound on dark-matter phase space density, and has a
free-streaming length consistent with cosmological constraints on the matter
power spectrum. We show that for masses <1.5 keV no region of parameter space
satisfies all these constraints. This is a gravitationally-mediated lower bound
on the dark-matter mass for any model in which the primary component of dark
matter once had efficient interactions -- even if it has never been in
equilibrium with the standard model.Comment: 8 pages, 6 figures, 1 table; References added, Eq. 16 corrected, and
appendix with surface of allowed dark-matter abundance adde
Photon Regeneration from Pseudoscalars at X-ray Laser Facilities
Recently, the PVLAS collaboration has reported an anomalously large rotation
of the polarization of light in the presence of a magnetic field. As a possible
explanation they consider the existence of a light pseudoscalar particle
coupled to two photons. In this note, we propose a method of independently
testing this result by using a high-energy photon regeneration experiment (the
X-ray analogue of "invisible light shining through walls") using the
synchrotron X-rays from a free-electron laser (FEL). With such an experiment
the region of parameter space implied by PVLAS could be probed in a matter of
minutes.Comment: 3 pages, 2 figure
Dark Matter Astrophysics
These 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.Comment: Based on lectures given by MK at the Villa Olmo School on "The Dark
Side of the Universe," 14--18 May 2007 and by KS at the XIX Heidelberg
Physics Graduate Days, 8--12 October 2007. 33 pages, 15 figure
Limits on Neutrino-Neutrino Scattering in the Early Universe
In the standard model neutrinos are assumed to have streamed across the
Universe since they last scattered at the weak decoupling epoch when the
temperature of the standard-model plasma was ~MeV. The shear stress of
free-streaming neutrinos imprints itself gravitationally on the Cosmic
Microwave Background (CMB) and makes the CMB a sensitive probe of neutrino
scattering. Yet, the presence of nonstandard physics in the neutrino sector may
alter this standard chronology and delay neutrino free-streaming until a much
later epoch. We use observations of the CMB to constrain the strength of
neutrino self-interactions G_eff and put limits on new physics in the neutrino
sector from the early Universe. Recent measurements of the CMB at large
multipoles made by the Planck satellite and high-l experiments are critical for
probing this physics. Within the context of conventional LambdaCDM parameters
cosmological data are compatible with G_eff < 1/(56 MeV)^2 and neutrino
free-streaming might be delayed until their temperature has cooled to as low as
~25 eV. Intriguingly, we also find an alternative cosmology compatible with
cosmological data in which neutrinos scatter off each other until z~10^4 with a
preferred interaction strength in a narrow region around , where is
the Fermi constant. This distinct self-interacting neutrino cosmology is
characterized by somewhat lower values of both the scalar spectral index and
the amplitude of primordial fluctuations. While we phrase our discussion here
in terms of a specific scenario in which a late onset of neutrino
free-streaming could occur, our constraints on the neutrino visibility function
are very general.Comment: 9 Pages, 4 figures, 1 table. v2: Version accepted for publication,
enhanced discussion on neutrino interaction beyond the SM, enhanced figures,
references adde
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