232 research outputs found
Probing the time dependence of dark energy
A new method to investigate a possible time-dependence of the dark energy
equation of state is proposed. We apply this methodology to two of the most
recent data sets of type Ia supernova (Union2 and SDSS) and the baryon acoustic
oscillation peak at . For some combinations of these data, we show
that there is a clear departure from the standard CDM model at
intermediary redshifts, although a non-evolving dark energy component () cannot be ruled out by these data. The approach developed here may be
useful to probe a possible evolving dark energy component when applied to
upcoming observational data.Comment: 6 pages, 3 figures, LaTe
Cosmic Chronometers: Constraining the Equation of State of Dark Energy. I: H(z) Measurements
We present new determinations of the cosmic expansion history from
red-envelope galaxies. We have obtained for this purpose high-quality spectra
with the Keck-LRIS spectrograph of red-envelope galaxies in 24 galaxy clusters
in the redshift range 0.2 < z < 1.0. We complement these Keck spectra with
high-quality, publicly available archival spectra from the SPICES and VVDS
surveys. We improve over our previous expansion history measurements in Simon
et al. (2005) by providing two new determinations of the expansion history:
H(z) = 97 +- 62 km/sec/Mpc at z = 0.5 and H(z) = 90 +- 40 km/sec/Mpc at z =
0.8. We discuss the uncertainty in the expansion history determination that
arises from uncertainties in the synthetic stellar-population models. We then
use these new measurements in concert with cosmic-microwave-background (CMB)
measurements to constrain cosmological parameters, with a special emphasis on
dark-energy parameters and constraints to the curvature. In particular, we
demonstrate the usefulness of direct H(z) measurements by constraining the
dark- energy equation of state parameterized by w0 and wa and allowing for
arbitrary curvature. Further, we also constrain, using only CMB and H(z) data,
the number of relativistic degrees of freedom to be 4 +- 0.5 and their total
mass to be < 0.2 eV, both at 1-sigma.Comment: Submitted to JCA
The Two-Loop Pinch Technique in the Electroweak Sector
The generalization of the two-loop Pinch Technique to the Electroweak Sector
of the Standard Model is presented. We restrict ourselves to the case of
conserved external currents, and provide a detailed analysis of both the
charged and neutral sectors. The crucial ingredient for this construction is
the identification of the parts discarded during the pinching procedure with
well-defined contributions to the Slavnov-Taylor identity satisfied by the
off-shell one-loop gauge-boson vertices; the latter are nested inside the
conventional two-loop self-energies. It is shown by resorting to a set of
powerful identities that the two-loop effective Pinch Technique self-energies
coincide with the corresponding ones computed in the Background Feynman gauge.
The aforementioned identities are derived in the context of the
Batalin-Vilkovisky formalism, a fact which enables the individual treatment of
the self-energies of the photon and the -boson. Some possible
phenomenological applications are briefly discussed.Comment: 50 pages, uses axodra
The staggered domain wall fermion method
A different lattice fermion method is introduced. Staggered domain wall
fermions are defined in 2n+1 dimensions and describe 2^n flavors of light
lattice fermions with exact U(1) x U(1) chiral symmetry in 2n dimensions. As
the size of the extra dimension becomes large, 2^n chiral flavors with the same
chiral charge are expected to be localized on each boundary and the full
SU(2^n) x SU(2^n) flavor chiral symmetry is expected to be recovered. SDWF give
a different perspective into the inherent flavor mixing of lattice fermions and
by design present an advantage for numerical simulations of lattice QCD
thermodynamics. The chiral and topological index properties of the SDWF Dirac
operator are investigated. And, there is a surprise ending...Comment: revtex4, 7 figures, minor revisions, 2 references adde
Observational Constraints to Ricci Dark Energy Model by Using: SN, BAO, OHD, fgas Data Sets
In this paper, we perform a global constraint on the Ricci dark energy model
with both the flat case and the non-flat case, using the Markov Chain Monte
Carlo (MCMC) method and the combined observational data from the cluster X-ray
gas mass fraction, Supernovae of type Ia (397), baryon acoustic oscillations,
current Cosmic Microwave Background, and the observational Hubble function. In
the flat model, we obtain the best fit values of the parameters in regions: ,
, , . In the non-flat
model, the best fit parameters are found in
regions:,
, , ,
. Compared to the constraint results in
the model by using the same datasets, it is shown that
the current combined datasets prefer the model to the
Ricci dark energy model.Comment: 12 pages, 3 figure
Running coupling: Does the coupling between dark energy and dark matter change sign during the cosmological evolution?
In this paper we put forward a running coupling scenario for describing the
interaction between dark energy and dark matter. The dark sector interaction in
our scenario is free of the assumption that the interaction term is
proportional to the Hubble expansion rate and the energy densities of dark
sectors. We only use a time-variable coupling (with the scale factor
of the universe) to characterize the interaction . We propose a
parametrization form for the running coupling in which the
early-time coupling is given by a constant , while today the coupling is
given by another constant, . For investigating the feature of the running
coupling, we employ three dark energy models, namely, the cosmological constant
model (), the constant model (), and the time-dependent
model (). We constrain the models with the current
observational data, including the type Ia supernova, the baryon acoustic
oscillation, the cosmic microwave background, the Hubble expansion rate, and
the X-ray gas mass fraction data. The fitting results indicate that a
time-varying vacuum scenario is favored, in which the coupling crosses
the noninteracting line () during the cosmological evolution and the sign
changes from negative to positive. The crossing of the noninteracting line
happens at around , and the crossing behavior is favored at about
1 confidence level. Our work implies that we should pay more attention
to the time-varying vacuum model and seriously consider the phenomenological
construction of a sign-changeable or oscillatory interaction between dark
sectors.Comment: 8 pages, 5 figures; refs added; to appear in EPJ
Observational constraints on holographic dark energy with varying gravitational constant
We use observational data from Type Ia Supernovae (SN), Baryon Acoustic
Oscillations (BAO), Cosmic Microwave Background (CMB) and observational Hubble
data (OHD), and the Markov Chain Monte Carlo (MCMC) method, to constrain the
cosmological scenario of holographic dark energy with varying gravitational
constant. We consider both flat and non-flat background geometry, and we
present the corresponding constraints and contour-plots of the model
parameters. We conclude that the scenario is compatible with observations. In
1 we find ,
, and
, while for the present value
of the dark energy equation-of-state parameter we obtain
.Comment: 12 pages, 2 figures, version published in JCA
Zero temperature string breaking in lattice quantum chromodynamics
The separation of a heavy quark and antiquark pair leads to the formation of
a tube of flux, or "string", which should break in the presence of light
quark-antiquark pairs. This expected zero-temperature phenomenon has proven
elusive in simulations of lattice QCD. We study mixing between the string state
and the two-meson decay channel in QCD with two flavors of dynamical sea
quarks. We confirm that mixing is weak and find that it decreases at level
crossing. While our study does not show direct effects of internal quark loops,
our results, combined with unitarity, give clear confirmation of string
breaking.Comment: 20 pages, 7 figures. With small clarifications and two additions to
references. Submitted to Phys. Rev.
Quasiparticle Interactions in Fractional Quantum Hall Systems: Justification of Different Hierarchy Schemes
The pseudopotentials describing the interactions of quasiparticles in
fractional quantum Hall (FQH) states are studied. Rules for the identification
of incompressible quantum fluid ground states are found, based upon the form of
the pseudopotentials. States belonging to the Jain sequence nu=n/(1+2pn), where
n and p are integers, appear to be the only incompressible states in the
thermodynamic limit, although other FQH hierarchy states occur for finite size
systems. This explains the success of the composite Fermion picture.Comment: RevTeX, 10 pages, 7 EPS figures, submitted fo Phys.Rev.
The -essence scalar field in the context of Supernova Ia Observations
A -essence scalar field model having (non canonical) Lagrangian of the
form where
with constant is shown to be consistent with luminosity
distance-redshift data observed for type Ia Supernova. For constant ,
satisfies a scaling relation which is used to set up a differential
equation involving the Hubble parameter , the scale factor and the
-essence field . and are extracted from SNe Ia data and using
the differential equation the time dependence of the field is found to
be: . The constants
have been determined. The time dependence is similar to that of the
quintessence scalar field (having canonical kinetic energy) responsible for
homogeneous inflation. Furthermore, the scaling relation and the obtained time
dependence of the field is used to determine the -dependence of the
function .Comment: 8 pages, 5 figures, Late
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