2,116 research outputs found
Dynamics of dark energy
In this paper we review in detail a number of approaches that have been
adopted to try and explain the remarkable observation of our accelerating
Universe. In particular we discuss the arguments for and recent progress made
towards understanding the nature of dark energy. We review the observational
evidence for the current accelerated expansion of the universe and present a
number of dark energy models in addition to the conventional cosmological
constant, paying particular attention to scalar field models such as
quintessence, K-essence, tachyon, phantom and dilatonic models. The importance
of cosmological scaling solutions is emphasized when studying the dynamical
system of scalar fields including coupled dark energy. We study the evolution
of cosmological perturbations allowing us to confront them with the observation
of the Cosmic Microwave Background and Large Scale Structure and demonstrate
how it is possible in principle to reconstruct the equation of state of dark
energy by also using Supernovae Ia observational data. We also discuss in
detail the nature of tracking solutions in cosmology, particle physics and
braneworld models of dark energy, the nature of possible future singularities,
the effect of higher order curvature terms to avoid a Big Rip singularity, and
approaches to modifying gravity which leads to a late-time accelerated
expansion without recourse to a new form of dark energy.Comment: 93 pages, 26 figures, Invited Review to be submitted to International
Journal of Modern Physics D; comments are welcome; Additional references
included in response to over 60 comments received. Rewriting of sub-sections
on anthropic principle and gravitational backreaction. New subsections adde
General analytic formulae for attractor solutions of scalar-field dark energy models and their multi-field generalizations
We study general properties of attractors for scalar-field dark energy
scenarios which possess cosmological scaling solutions. In all such models
there exists a scalar-field dominant solution with an energy fraction
\Omega_{\phi}=1 together with a scaling solution. A general analytic formula is
given to derive fixed points relevant to dark energy coupled to dark matter. We
investigate the stability of fixed points without specifying the models of dark
energy in the presence of non-relativistic dark matter and provide a general
proof that a non-phantom scalar-field dominant solution is unstable when a
stable scaling solution exists in the region \Omega_{\phi}<1. A phantom
scalar-field dominant fixed point is found to be classically stable. We also
generalize the analysis to the case of multiple scalar fields and show that for
a non-phantom scalar field assisted acceleration always occurs for all
scalar-field models which have scaling solutions. For a phantom field the
equation of state approaches that of cosmological constant as we add more
scalar fields.Comment: 11 pages, no figures, version to appear in Physical Review
On the existence of the critical point in finite density lattice QCD
We propose a method to probe the nature of phase transitions in lattice QCD
at finite temperature and density, which is based on the investigation of an
effective potential as a function of the average plaquette. We analyze data
obtained in a simulation of two-flavor QCD using p4-improved staggered quarks
with bare quark mass , and find that a first order phase transition
line appears in the high density regime for \mu_q/T \simge 2.5. We also
discuss the difference between the phase structures of QCD with non-zero quark
chemical potential and non-zero isospin chemical potential.Comment: 17 pages, 9 figure
Collective oscillation period of inter-coupled biological negative cyclic feedback oscillators
A number of biological rhythms originate from networks comprised of multiple
cellular oscillators. But analytical results are still lacking on the
collective oscillation period of inter-coupled gene regulatory oscillators,
which, as has been reported, may be different from that of an autonomous
oscillator. Based on cyclic feedback oscillators, we analyze the collective
oscillation pattern of coupled cellular oscillators. First we give a condition
under which the oscillator network exhibits oscillatory and synchronized
behavior. Then we estimate the collective oscillation period based on a novel
multivariable harmonic balance technique. Analytical results are derived in
terms of biochemical parameters, thus giving insight into the basic mechanism
of biological oscillation and providing guidance in synthetic biology design.Comment: arXiv admin note: substantial text overlap with arXiv:1203.125
Group velocity and causality in standard relativistic resistive magnetohydrodynamics
Group velocity of electromagnetic waves in plasmas derived by standard
relativistic resistive MHD (resistive RMHD) equations is superluminal. If we
assume that the group velocity represents the propagation velocity of a signal,
we have to worry about the causality problem. That is, some acausal phenomena
may be induced, such that information transportation to the absolute past and
spontaneous decrease in the entropy. Here, we tried to find the acausal
phenomena using standard resistive RMHD numerical simulations in the suggested
situation of the acausal phenomena. The calculation results showed that even in
such situations no acausal effect happens. The numerical result with respect to
the velocity limit of the information transportation is consistent with a
linear theory of wave train propagation. Our results assure that we can use
these equations without problems of acausal phenomena.Comment: 28 pages, 10 figure
Charge separation instability in an unmagnetized disk plasma around a Kerr black hole
In almost all of plasma theories for astrophysical objects, we have assumed
the charge quasi-neutrality of unmagnetized plasmas in global scales. This
assumption has been justified because if there is a charged plasma, it induces
electric field which attracts the opposite charge, and this opposite charge
reduces the charge separation. Here, we report a newly discovered instability
which causes a charge separation in a rotating plasma inside of an innermost
stable circular orbit (ISCO) around a black hole. The growth rate of the
instability is smaller than that of the disk instability even in the unstable
disk region and is forbidden in the stable disk region outside of the ISCO.
However, this growth rate becomes comparable to that of the disk instability
when the plasma density is much lower than a critical density inside of the
ISCO. In such case, the charge separation instability would become apparent and
cause the charged accretion into the black hole, thus charge the hole up.Comment: 15pages, 1 figur
Ten milliparsec-scale structure of the nucleus region in Centaurus A
We present the results of a VLBI Space Observatory Programme (VSOP)
observation of the subparsec structure in Centaurus A at 4.9 GHz. Owing to its
proximity, our Centaurus A space-VLBI image is one of the highest spatial
resolution images of an AGN ever made -- 0.01 pc per beam. The elongated core
region is resolved into several components over 10 milli-arcseconds long (0.2
pc) including a compact component of brightness temperature 2.2x10^10K. We
analyze the jet geometry in terms of collimation. Assuming the strongest
component to be the core, the jet opening angle at ~ 5,000 r_s (Schwarzchild
radii) from the core is estimated to be ~ 12 degree, with collimation of the
jet to ~ 3 degree continuing out to ~ 20,000 r_s. This result is consistent
with previous studies of the jet in M87, which favor MHD disk outflow models.
Future space VLBI observations at higher frequencies will probably be able to
image the collimation region, within 1,000 r_s of the center of Centaurus A,
together with the accretion disk itself.Comment: 12 pages, 6 figures, accepted for publication in PASJ, Vol.57 No.6,
VSOP special issu
Large N reduction for Chern-Simons theory on S^3
We study a matrix model which is obtained by dimensional reduction of
Chern-Simon theory on S^3 to zero dimension. We find that expanded around a
particular background consisting of multiple fuzzy spheres, it reproduces the
original theory on S^3 in the planar limit. This is viewed as a new type of the
large N reduction generalized to curved space.Comment: 4 pages, 2 figures, references added, typos correcte
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