272 research outputs found
Scaling solutions in general non-minimal coupling theories
A class of generalized non-minimal coupling theories is investigated, in
search of scaling attractors able to provide an accelerated expansion at the
present time. Solutions are found in the strong coupling regime and when the
coupling function and the potential verify a simple relation. In such cases,
which include power law and exponential functions, the dynamics is independent
of the exact form of the coupling and the potential. The constraint from the
time variability of , however, limits the fraction of energy in the scalar
field to less than 4% of the total energy density, and excludes accelerated
solutions at the present.Comment: 10 pages, 3 figures, accepted for publication in Phys. Rev.
Attractor Solution of Phantom Field
In light of recent study on the dark energy models that manifest an equation
of state , we investigate the cosmological evolution of phantom field in
a specific potential, exponential potential in this paper. The phase plane
analysis show that the there is a late time attractor solution in this model,
which address the similar issues as that of fine tuning problems in
conventional quintessence models. The equation of state is determined by
the attractor solution which is dependent on the parameter in the
potential. We also show that this model is stable for our present observable
universe.Comment: 9 pages, 3 ps figures; typos corrected, references updated, this is
the final version to match the published versio
Cosmological Tracking Solutions
A substantial fraction of the energy density of the universe may consist of
quintessence in the form of a slowly-rolling scalar field. Since the energy
density of the scalar field generally decreases more slowly than the matter
energy density, it appears that the ratio of the two densities must be set to a
special, infinitesimal value in the early universe in order to have the two
densities nearly coincide today.
Recently, we introduced the notion of tracker fields to avoid this initial
conditions problem. In the paper, we address the following questions: What is
the general condition to have tracker fields? What is the relation between the
matter energy density and the equation-of-state of the universe imposed by
tracker solutions? And, can tracker solutions explain why quintessence is
becoming important today rather than during the early universe
Cosmological models from quintessence
A generalized quintessence model is presented which corresponds to a richer
vacuum structure that, besides a time-dependent, slowly varying scalar field,
contains a varying cosmological term. From first principles we determine a
number of scalar-field potentials that satisfy the constraints imposed by the
field equations and conservations laws, both in the conventional and
generalized quintessence models. Besides inverse-power law solutions, these
potentials are given in terms of hyperbolic functions or the twelve Jacobian
elliptic functions, and are all related to the luminosity distance by means of
an integral equation. Integration of this equation for the different solutions
leads to a large family of cosmological models characterized by luminosity
distance-redshift relations. Out of such models, only four appear to be able to
predict a required accelerating universe conforming to observations on
supernova Ia, at large or moderate redshifts.Comment: 9 pages, RevTex, to appear in Phys. Rev.
The rationality of the moduli spaces of Coble surfaces and of nodal Enriques surfaces
We prove the rationality of the coarse moduli spaces of Coble surfaces and of
nodal Enriques surfaces over the field of complex numbers.Comment: 15 page
Reconstructing the Equation of State of Tachyon
Recent progress in theoretical physics suggests that the dark energy in the
universe might be resulted from the rolling tachyon field of string theory.
Measurements to SNe Ia can be helpful to reconstruct the equation of state of
the rolling tachyon which is a possible candidate of dark energy. We present a
numerical analysis for the evolution of the equation of state of the rolling
tachyon and derive the reconstruction equations for the equation of state as
well as the potential.Comment: 6 pages, 3 figures, to appear Phys. Rev.
Phenomenology of a realistic accelerating universe using only Planck-scale physics
Modern data is showing increasing evidence that the Universe is accelerating.
So far, all attempts to account for the acceleration have required some
fundamental dimensionless quantities to be extremely small. We show how a class
of scalar field models (which may emerge naturally from superstring theory) can
account for acceleration which starts in the present epoch with all the
potential parameters O(1) in Planck units.Comment: 4 pages including 4 figures. Final version accepted for publication
in PRL with expanded discussion of the relationship to other quintessence
research. No changes to our own wor
Born-Infeld-type phantom on the brane world
We study the evolution of Born-Infeld-type phantom in the second
Randall-Sundrum brane scenario, and find that there exists attractor solution
for the potential with a maximum, which implies a cosmological constant at the
late time. Especially, we discuss the BI model of constant potential without
and with dust matter. In the weak tension limit of the brane, we obtain an
exact solution for the BI phantom and scale factor and show that there is no
big rip during the evolution of the brane.Comment: 5 pages, 2 figures, Reference added, Phys. Rev. D in pres
Phantom Field with O(N) Symmetry in Exponential Potential
In this paper, we study the phase space of phantom model with O(\emph{N})
symmetry in exponential potential. Different from the model without O(\emph{N})
symmetry, the introduction of the symmetry leads to a lower bound on the
equation of state for the existence of stable phantom dominated attractor
phase. The reconstruction relation between the potential of O(\textit{N})
phantom system and red shift has been derived.Comment: 5 pages, 3 figures, replaced with the version to appear on Phys. Rev.
Tunneling in Decaying Cosmologies and the Cosmological Constant Problem
The tunneling rate, with exact prefactor, is calculated to first order in
for an empty closed Friedmann-Robertson-Walker (FRW) universe with
decaying cosmological term ( is the scale factor and
is a parameter ). This model is equivalent to a cosmology
with the equation of state . The calculations are
performed by applying the dilute-instanton approximation on the corresponding
Duru-Kleinert path integral.
It is shown that the highest tunneling rate occurs for corresponding to
the cosmic string matter universe. The obtained most probable cosmological
term, like one obtained by Strominger, accounts for a possible solution to the
cosmological constant problem.Comment: 21 pages, REVTEX, The section 3 is considerably completed including
some physical mechanisms supporting the time variation of the cosmological
constant, added references for the section 3. Accepted to be published in
Phys. Rev.
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