2,290 research outputs found
Lee-Yang zero analysis for the study of QCD phase structure
We comment on the Lee-Yang zero analysis for the study of the phase structure
of QCD at high temperature and baryon number density by Monte-Carlo
simulations. We find that the sign problem for non-zero density QCD induces a
serious problem in the finite volume scaling analysis of the Lee-Yang zeros for
the investigation of the order of the phase transition. If the sign problem
occurs at large volume, the Lee-Yang zeros will always approach the real axis
of the complex parameter plane in the thermodynamic limit. This implies that a
scaling behavior which would suggest a crossover transition will not be
obtained. To clarify this problem, we discuss the Lee-Yang zero analysis for
SU(3) pure gauge theory as a simple example without the sign problem, and then
consider the case of non-zero density QCD. It is suggested that the
distribution of the Lee-Yang zeros in the complex parameter space obtained by
each simulation could be more important information for the investigation of
the critical endpoint in the plane than the finite volume scaling
behavior.Comment: 16 pages, 3 figures, 2 tables, minor change
Multiple time scales hidden in heterogeneous dynamics of glass-forming liquids
A multi-time probing of density fluctuations is introduced to investigate
hidden time scales of heterogeneous dynamics in glass-forming liquids.
Molecular dynamics simulations for simple glass-forming liquids are performed,
and a three-time correlation function is numerically calculated for general
time intervals. It is demonstrated that the three-time correlation function is
sensitive to the heterogeneous dynamics and that it reveals couplings of
correlated motions over a wide range of time scales. Furthermore, the time
scale of the heterogeneous dynamics is determined by the
change in the second time interval in the three-time correlation function. The
present results show that the time scale of the heterogeneous dynamics
becomes larger than the -relaxation time at low
temperatures and large wavelengths. We also find a dynamical scaling relation
between the time scale and the length scale of
dynamical heterogeneity as with .Comment: 4 pages, 5 figures, to appear in Phys. Rev. E (Rapid Communications
Solar system and equivalence principle constraints on f(R) gravity by chameleon approach
We study constraints on f(R) dark energy models from solar system experiments
combined with experiments on the violation of equivalence principle. When the
mass of an equivalent scalar field degree of freedom is heavy in a region with
high density, a spherically symmetric body has a thin-shell so that an
effective coupling of the fifth force is suppressed through a chameleon
mechanism. We place experimental bounds on the cosmologically viable models
recently proposed in literature which have an asymptotic form f(R)=R-lambda R_c
[1-(R_c/R)^{2n}] in the regime R >> R_c. From the solar-system constraints on
the post-Newtonian parameter gamma, we derive the bound n>0.5, whereas the
constraints from the violations of weak and strong equivalence principles give
the bound n>0.9. This allows a possibility to find the deviation from the
LambdaCDM cosmological model. For the model f(R)=R-lambda R_c(R/R_c)^p with
0<p<1 the severest constraint is found to be p<10^{-10}, which shows that this
model is hardly distinguishable from the LambdaCDM cosmology.Comment: 5 pages, no figures, version to appear in Physical Review
On the scalar graviton in n-DBI gravity
n-DBI gravity is a gravitational theory which yields near de Sitter inflation
spontaneously at the cost of breaking Lorentz invariance by a preferred choice
of foliation. We show that this breakdown endows n-DBI gravity with one extra
physical gravitational degree of freedom: a scalar graviton. Its existence is
established by Dirac's theory of constrained systems. Firstly, studying scalar
perturbations around Minkowski space-time, we show that there exists one scalar
degree of freedom and identify it in terms of the metric perturbations. Then, a
general analysis is made in the canonical formalism, using ADM variables. It is
useful to introduce an auxiliary scalar field, which allows recasting n-DBI
gravity in an Einstein-Hilbert form but in a Jordan frame. Identifying the
constraints and their classes we confirm the existence of an extra degree of
freedom in the full theory, besides the two usual tensorial modes of the
graviton. We then argue that, unlike the case of (the original proposal for)
Horava-Lifschitz gravity, there is no evidence that the extra degree of freedom
originates pathologies, such as vanishing lapse, instabilities and strong
self-coupling at low energy scales.Comment: 30 pages, 1 figur
Matter instabilities in general Gauss-Bonnet gravity
We study the evolution of cosmological perturbations in f(G) gravity, where
the Lagrangian is the sum of a Ricci scalar R and an arbitrary function f in
terms of a Gauss-Bonnet term G. We derive the equations for perturbations
assuming matter to be described by a perfect fluid with a constant equation of
state w. We show that density perturbations in perfect fluids exhibit negative
instabilities during both the radiation and the matter domination, irrespective
of the form of f(G). This growth of perturbations gets stronger on smaller
scales, which is difficult to be compatible with the observed galaxy spectrum
unless the deviation from General Relativity is very small. Thus f(G)
cosmological models are effectively ruled out from this Ultra-Violet
instability, even though they can be compatible with the late-time cosmic
acceleration and local gravity constraints.Comment: 9 pages, 2 figures, published PRD versio
Density perturbations in general modified gravitational theories
We derive the equations of linear cosmological perturbations for the general
Lagrangian density , where is a Ricci scalar,
is a scalar field, and is a field kinetic energy. We
take into account a nonlinear self-interaction term recently studied in
the context of "Galileon" cosmology, which keeps the field equations at second
order. Taking into account a scalar-field mass explicitly, the equations of
matter density perturbations and gravitational potentials are obtained under a
quasi-static approximation on sub-horizon scales. We also derive conditions for
the avoidance of ghosts and Laplacian instabilities associated with propagation
speeds. Our analysis includes most of modified gravity models of dark energy
proposed in literature and thus it is convenient to test the viability of such
models from both theoretical and observational points of view.Comment: 17 pages, no figure
Generalized Galileon cosmology
We study the cosmology of a generalized Galileon field with five
covariant Lagrangians in which is replaced by general scalar functions
(i=1,...,5). For these theories, the equations of motion remain
at second-order in time derivatives. We restrict the functional forms of
from the demand to obtain de Sitter solutions responsible for
dark energy. There are two possible choices for power-law functions
, depending on whether the coupling with the Ricci
scalar is independent of or depends on . The former
corresponds to the covariant Galileon theory that respects the Galilean
symmetry in the Minkowski space-time. For generalized Galileon theories we
derive the conditions for the avoidance of ghosts and Laplacian instabilities
associated with scalar and tensor perturbations as well as the condition for
the stability of de Sitter solutions. We also carry out detailed analytic and
numerical study for the cosmological dynamics in those theories.Comment: 24 pages, 10 figures, version to appear in Physical Review
Large N reduction on coset spaces
As an extension of our previous work concerning the large N reduction on
group manifolds, we study the large N reduction on coset spaces. We show that
large N field theories on coset spaces are described by certain corresponding
matrix models. We also construct Chern-Simons-like theories on group manifolds
and coset spaces, and give their reduced models.Comment: 22 pages, typos correcte
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