15,033 research outputs found
A Thousand Invisible Cords Binding Astronomy and High-Energy Physics
The traditional realm of astronomy is the observation and study of the
largest objects in the Universe, while the traditional domain of high-energy
physics is the study of the smallest things in nature. But these two sciences
concerned with opposite ends of the size spectrum are, in Muir's words, bound
fast by a thousand invisible cords that cannot be broken. In this essay I
propose that collaborations of astronomers and high-energy physicists on common
problems are beneficial for both fields, and that both astronomy and
high-energy physics can advance by this close and still growing relationship.
Dark matter and dark energy are two of the binding cords I will use to
illustrate how collaborations of astronomers and high-energy physicists on
large astronomical projects can be good for astronomy, and how discoveries in
astronomy can guide high-energy physicists in their quest for understanding
nature on the smallest scales. Of course, the fields have some different
intellectual and collaborative traditions, neither of which is ideal. The
cultures of the different fields cannot be judged to be right or wrong; they
either work or they don't. When astronomers and high-energy physicists work
together, the binding cords can either encourage or choke creativity. The
challenge facing the astronomy and high-energy physics communities is to adopt
the best traditions of both fields. It is up to us to choose wisely.Comment: Why "Fundamentalist" Physics Is Good for Astronomy (in response to
the paper of Simon White, arXiv:0704.2291
Elliptic Flow from a Transversally Thermalized Fireball
The agreement of elliptic flow data at RHIC at central rapidity with the
hydrodynamic model has led to the conclusion of very rapid thermalization. This
conclusion is based on the intuitive argument that hydrodynamics, which assumes
instantaneous local thermalization, produces the largest possible elliptic flow
values and that the data seem to saturate this limit. We here investigate the
question whether incompletely thermalized viscous systems may actually produce
more elliptic flow than ideal hydrodynamics. Motivated by the extremely fast
primordial longitudinal expansion of the reaction zone, we investigate a toy
model which exhibits thermalization only in the transverse directions but
undergoes collisionless free-streaming expansion in the longitudinal direction.
For collisions at RHIC energies, elliptic flow results from the model are
compared with those from hydrodynamics. With the final particle yield and
\kt-distribution fixed, the transversally thermalized model is shown not to
be able to produce the measured amount of elliptic flow. This investigation
provides further support for very rapid local kinetic equilibration at RHIC. It
also yields interesting novel results for the elliptic flow of massless
particles such as direct photons.Comment: revtex4, 15 pages + 10 embedded EPS figure
A fully relativistic lattice Boltzmann algorithm
Starting from the Maxwell-Juettner equilibrium distribution, we develop a
relativistic lattice Boltzmann (LB) algorithm capable of handling
ultrarelativistic systems with flat, but expanding, spacetimes. The algorithm
is validated through simulations of quark-gluon plasma, yielding excellent
agreement with hydrodynamic simulations. The present scheme opens the
possibility of transferring the recognized computational advantages of lattice
kinetic theory to the context of both weakly and ultra-relativistic systems.Comment: 12 pages, 8 figure
Cosmology with moving dark energy and the CMB quadrupole
We study the consequences of a homogeneous dark energy fluid having a
non-vanishing velocity with respect to the matter and radiation large-scale
rest frames. We consider homogeneous anisotropic cosmological models with four
fluids (baryons, radiation, dark matter and dark energy) whose velocities can
differ from each other. Performing a perturbative calculation up to second
order in the velocities, we obtain the contribution of the anisotropies
generated by the fluids motion to the CMB quadrupole and compare with
observations. We also consider the exact problem for arbitrary velocities and
solve the corresponding equations numerically for different dark energy models.
We find that models whose equation of state is initially stiffer than
radiation, as for instance some tracking models, are unstable against velocity
perturbations, thus spoiling the late-time predictions for the energy
densities. In the case of scaling models, the contributions to the quadrupole
can be non-negligible for a wide range of initial conditions. We also consider
fluids moving at the speed of light (null fluids) with positive energy and show
that, without assuming any particular equation of state, they generically act
as a cosmological constant at late times. We find the parameter region for
which the models considered could be compatible with the measured (low)
quadrupole.Comment: 23 pages, 6 figures. Confidence intervals calculated from WMAP data,
new references and comments included. Final version to appear in PR
Leptogenesis implications in models with Abelian family symmetry and one extra real Higgs singlet
We show that the neutrino models, as suggested by Low, which have an
additional Abelian family symmetry and a real Higgs singlet to the default
see-saw do not hinder the possibility of successful thermal leptogenesis. For
these models (neglecting radiative effects), we have investigated the situation
of strong washout in both the one-flavor approximation and when flavor effects
are included. The result is that while such models predict that theta_{13}=0
and that one light neutrino to be massless, they do not modify or provide
significant constraints on the typical leptogenesis scenario where the final
asymmetry is dominated by the decays of the lightest right-handed neutrinos.Comment: 18 pages, RevTeX4, accepted by Phys. Rev. D. v2: minor corrections,
note and 1 ref. added, same content as published versio
Resonant Leptogenesis and Verifiable Seesaw from Large Extra Dimensions
In the presence of large extra dimensions, the fundamental scale could be as
low as a few TeV. This yields leptogenesis and seesaw at a TeV scale.
Phenomenologically two TeV-scale Majorana fermions with a small mass split can
realize a resonant leptogenesis whereas a TeV-scale Higgs triplet with a small
trilinear coupling to the standard model Higgs doublet can give a verifiable
seesaw. We propose an interesting scenario where the small parameters for the
resonant leptogenesis and the type-II seesaw can be simultaneously generated by
the propagation of lepton number violation from distant branes to our world.Comment: 5 pages. More discussions and references. Published in PR
Dynamics of inflationary cosmology in TVSD model
Within the framework of a model Universe with time variable space dimensions
(TVSD), known as decrumpling or TVSD model, we study TVSD chaotic inflation and
obtain dynamics of the inflaton, scale factor and spatial dimension. We also
study the quantum fluctuations of the inflaton field and obtain the spectral
index and its running in this model. Two classes of examples have been studied
and comparisons made with the standard slow-roll formulae. We compare our
results with the recent Wilkinson Microwave Anisotropy Probe (WMAP) data.Comment: 18 pages, 3 figures, accepted in Mod. Phys. Lett.
Gamma-rays from ultracompact minihalos: potential constraints on the primordial curvature perturbation
Ultracompact minihalos (UCMHs) are dense dark matter structures which can
form from large density perturbations shortly after matter-radiation equality.
If dark matter is in the form of Weakly Interacting Massive Particles (WIMPs),
then UCMHs may be detected via their gamma-ray emission. We investigate how the
{\em{Fermi}} satellite could constrain the abundance of UCMHs and place limits
on the power spectrum of the primordial curvature perturbation. Detection by
{\em Fermi} would put a lower limit on the UCMH halo fraction. The smallest
detectable halo fraction, , is for . If gamma-ray emission from UCMHs is not detected, an
upper limit can be placed on the halo fraction. The bound is tightest, , for . The
resulting upper limit on the power spectrum of the primordial curvature
perturbation in the event of non-detection is in the range on scales . This is substantially tighter than the existing constraints from
primordial black hole formation on these scales, however it assumes that dark
matter is in the form of WIMPs and UCMHs are not disrupted during the formation
of the Milky Way halo.Comment: 5 pages, 2 figures, version to appear in Phys. Rev. D, minor change
Turning off the Lights: How Dark is Dark Matter?
We consider current observational constraints on the electromagnetic charge
of dark matter. The velocity dependence of the scattering cross-section through
the photon gives rise to qualitatively different constraints than standard dark
matter scattering through massive force carriers. In particular, recombination
epoch observations of dark matter density perturbations require that
, the ratio of the dark matter to electronic charge, is less than
for , rising to for .
Though naively one would expect that dark matter carrying a charge well below
this constraint could still give rise to large scattering in current direct
detection experiments, we show that charged dark matter particles that could be
detected with upcoming experiments are expected to be evacuated from the
Galactic disk by the Galactic magnetic fields and supernova shock waves, and
hence will not give rise to a signal. Thus dark matter with a small charge is
likely not a source of a signal in current or upcoming dark matter direct
detection experiments.Comment: 19 pages, 2 figures; v2 - figures fixed, references adde
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