1,114 research outputs found
WIMP Dark Matter and the QCD Equation of State
Weakly Interacting Massive Particles (WIMPs) of mass m freeze out at a
temperature T_f ~ m/25, i.e. in the range 400 MeV -- 40 GeV for a particle in
the typical mass range 10 -- 1000 GeV. The WIMP relic density, which depends on
the effective number of relativistic degrees of freedom at T_f, may be measured
to better than 1% by Planck, warranting comparable theoretical precision.
Recent theoretical and experimental advances in the understanding of high
temperature QCD show that the quark gluon plasma departs significantly from
ideal behaviour up to temperatures of several GeV, necessitating an improvement
of the cosmological equation of state over those currently used. We discuss how
this increases the relic density by approximately 1.5 -- 3.5% in benchmark
mSUGRA models, with an uncertainly in the QCD corrections of 0.5 -- 1 %. We
point out what further work is required to achieve a theoretical accuracy
comparable with the expected observational precision, and speculate that the
effective number of degrees of freedom at T_f may become measurable in the
foreseeable future.Comment: 4pp, 2figs. More info including Matlab scripts used to generate
equation of state curves at
http://www.pact.cpes.sussex.ac.uk/arXiv/hep-ph/0501232
Correlations in Cosmic String Networks
We investigate scaling and correlations of the energy and momentum in an
evolving network of cosmic strings in Minkowski space. These quantities are of
great interest, as they must be understood before accurate predictions for the
power spectra of the perturbations in the matter and radiation in the early
Universe can be made. We argue that Minkowski space provides a reasonable
approximation to a Friedmann background for string dynamics and we use our
results to construct a simple model of the network, in which it is considered
to consist of randomly placed segments moving with random velocities. This
model works well in accounting for features of the two-time correlation
functions, and even better for the power spectra.Comment: 20pp Plain LaTeX, 11 EPS figures, uses epsf.st
Where are the Hedgehogs in Nematics?
In experiments which take a liquid crystal rapidly from the isotropic to the
nematic phase, a dense tangle of defects is formed. In nematics, there are in
principle both line and point defects (``hedgehogs''), but no point defects are
observed until the defect network has coarsened appreciably. In this letter the
expected density of point defects is shown to be extremely low, approximately
per initially correlated domain, as result of the topology
(specifically, the homology) of the order parameter space.Comment: 6 pages, latex, 1 figure (self-unpacking PostScript)
Large Radius Hagedorn Regime in String Gas Cosmology
We calculate the equation of state of a gas of strings at high density in a
large toroidal universe, and use it to determine the cosmological evolution of
background metric and dilaton fields in the entire large radius Hagedorn
regime, (ln S)^{1/d} << R << S^{1/d} (with S the total entropy). The pressure
in this regime is not vanishing but of O(1), while the equation of state is
proportional to volume, which makes our solutions significantly different from
previously published approximate solutions. For example, we are able to
calculate the duration of the high-density "Hagedorn" phase, which increases
exponentially with increasing entropy, S. We go on to discuss the difficulties
of the scenario, quantifying the problems of establishing thermal equilibrium
and producing a large but not too weakly-coupled universe.Comment: 12 pages, 4 figures, more details presented in string thermodynamics
section, to be published in Physical Review
Anti-Proton Evolution in Little Bangs and Big Bang
The abundances of anti-protons and protons are considered within
momentum-integrated Boltzmann equations describing Little Bangs, i.e.,
fireballs created in relativistic heavy-ion collisions. Despite of a large
anti-proton annihilation cross section we find a small drop of the ratio of
anti-protons to protons from 170 MeV (chemical freeze-out temperature) till 100
MeV (kinetic freeze-out temperature) for CERN-SPS and BNL-RHIC energies thus
corroborating the solution of the previously exposed "ani-proton puzzle". In
contrast, the Big Bang evolves so slowly that the anti-baryons are kept for a
long time in equilibrium resulting in an exceedingly small fraction. The
adiabatic path of cosmic matter in the phase diagram of strongly interacting
matter is mapped out
Numerical simulations of string networks in the Abelian-Higgs model
We present the results of a field theory simulation of networks of strings in
the Abelian Higgs model. Starting from a random initial configuration we show
that the resulting vortex tangle approaches a self-similar regime in which the
length density of lines of zeros of reduces as . We demonstrate
that the network loses energy directly into scalar and gauge radiation. These
results support a recent claim that particle production, and not gravitational
radiation, is the dominant energy loss mechanism for cosmic strings. This means
that cosmic strings in Grand Unified Theories are severely constrained by high
energy cosmic ray fluxes: either they are ruled out, or an implausibly small
fraction of their energy ends up in quarks and leptons.Comment: 4pp RevTeX, 3 eps figures, clarifications and new results included,
to be published in Phys. Rev. Let
Defect formation and local gauge invariance
We propose a new mechanism for formation of topological defects in a U(1)
model with a local gauge symmetry. This mechanism leads to definite
predictions, which are qualitatively different from those of the Kibble-Zurek
mechanism of global theories. We confirm these predictions in numerical
simulations, and they can also be tested in superconductor experiments. We
believe that the mechanism generalizes to more complicated theories.Comment: REVTeX, 4 pages, 2 figures. The explicit form of the Hamiltonian and
the equations of motion added. To appear in PRL (http://prl.aps.org/
Cosmic Necklaces and Ultrahigh Energy Cosmic Rays
Cosmic necklaces are hybrid topological defects consisting of monopoles and
strings, with two strings attached to each monopole. We argue that the
cosmological evolution of necklaces may significantly differ from that of
cosmic strings. The typical velocity of necklaces can be much smaller than the
speed of light, and the characteristic scale of the network much smaller than
the horizon. We estimate the flux of high-energy protons produced by monopole
annihilation in the decaying closed loops. For some reasonable values of the
parameters it is comparable to the observed flux of ultrahigh-energy cosmic
rays.Comment: 10 pages, Revtex, 1 figur
Cosmic String Formation from Correlated Fields
We simulate the formation of cosmic strings at the zeros of a complex
Gaussian field with a power spectrum , specifically
addressing the issue of the fraction of length in infinite strings. We make two
improvements over previous simulations: we include a non-zero random background
field in our box to simulate the effect of long-wavelength modes, and we
examine the effects of smoothing the field on small scales. The inclusion of
the background field significantly reduces the fraction of length in infinite
strings for . Our results are consistent with the possibility that
infinite strings disappear at some in the range ,
although we cannot rule out , in which case infinite strings would
disappear only at the point where the mean string density goes to zero. We
present an analytic argument which suggests the latter case. Smoothing on small
scales eliminates closed loops on the order of the lattice cell size and leads
to a ``lattice-free" estimate of the infinite string fraction. As expected,
this fraction depends on the type of window function used for smoothing.Comment: 24 pages, latex, 10 figures, submitted to Phys Rev
The evolution of a network of cosmic string loops
We set up and analyse a model for the non-equilibrium evolution of a network
of cosmic strings initially containing only loops and no infinite strings. Due
to this particular initial condition, our analytical approach differs
significantly from existing ones. We describe the average properties of the
network in terms of the distribution function n(l,t) dl, the average number of
loops per unit volume with physical length between l and l + dl at time t. The
dynamical processes which change the length of loops are then estimated and an
equation, which we call the `rate equation', is derived for (dn/dt). In a
non-expanding universe, the loops should reach the equilibrium distribution
predicted by string statistical mechanics. Analysis of the rate equation gives
results consistent with this. We then study the rate equation in an expanding
universe and suggest that three different final states are possible for the
evolving loop network, each of which may well be realised for some initial
conditions. If the initial energy density in loops in the radiation era is low,
then the loops rapidly disappear. For large initial energy densities, we expect
that either infinite strings are formed or that the loops tend towards a
scaling solution in the radiation era and then rapidly disappear in the matter
era. Such a scenario may be relevant given recent work highlighting the
problems with structure formation from the standard cosmic string scenario.Comment: LaTeX, 27 pages, 10 figures included as .eps file
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