399 research outputs found
Natural Quintessence with Gauge Coupling Unification
We show that a positive accelerating universe can be obtained simply by the
dynamics of a non-abelian gauge group. It is the condensates of the chiral
fields that obtain a negative power potential, below the condensation scale,
and allow for a quintessence interpretation of these fields. The only free
parameters in this model are and and the number of dynamically
gauge singlet bilinear fields generated below the condensation scale. We
show that it is possible to have unification of all coupling constants,
including the standard and non standard model couplings, while having an
acceptable phenomenology of as the cosmological constant. This is done
without any fine tuning of the initial conditions. The problem of coincidence
(why the universe has only recently started an accelerating period) is not
solved but it is put at the same level as what the particle content of the
standard model is.Comment: minor changes(discussion on field normalization included), reference
added, accepted in Phy.Rev.Lett., 5 pages,LateX,2 Figure
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.
A New Cosmological Model of Quintessence and Dark Matter
We propose a new class of quintessence models in which late times
oscillations of a scalar field give rise to an effective equation of state
which can be negative and hence drive the observed acceleration of the
universe. Our ansatz provides a unified picture of quintessence and a new form
of dark matter we call "Frustrated Cold Dark Matter" (FCDM). FCDM inhibits
gravitational clustering on small scales and could provide a natural resolution
to the core density problem for disc galaxy halos. Since the quintessence field
rolls towards a small value, constraints on slow-roll quintessence models are
safely circumvented in our model.Comment: Revised. Important new results added in response to referees comment
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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
An analysis of cosmological perturbations in hydrodynamical and field representations
Density fluctuations of fluids with negative pressure exhibit decreasing time
behaviour in the long wavelength limit, but are strongly unstable in the small
wavelength limit when a hydrodynamical approach is used. On the other hand, the
corresponding gravitational waves are well behaved. We verify that the
instabilities present in density fluctuations are due essentially to the
hydrodynamical representation; if we turn to a field representation that lead
to the same background behaviour, the instabilities are no more present. In the
long wavelength limit, both approachs give the same results. We show also that
this inequivalence between background and perturbative level is a feature of
negative pressure fluid. When the fluid has positive pressure, the
hydrodynamical representation leads to the same behaviour as the field
representation both at the background and perturbative levels.Comment: Latex file, 18 page
Dilatonic Interpretation of the Quintessence?
We discuss the possibility that "quintessential effects", recently displayed
by large scale observations, may be consistently described in the context of
the low-energy string effective action, and we suggest a possible approach to
the problem of the cosmic coincidence based on the link between the strength of
the dilaton couplings and the cosmological state of our Universe.Comment: 6 pages, Revtex, four figures included using epsfig. To appear in
Phys. Rev.
Gravitational field of vacuumless defects
It has been recently shown that topological defects can arise in symmetry
breaking models where the scalar field potential has no minima and is
a monotonically decreasing function of . Here we study the
gravitational fields produced by such vacuumless defects in the cases of both
global and gauge symmetry breaking. We find that a global monopole has a
strongly repulsive gravitational field, and its spacetime has an event horizon
similar to that in de Sitter space. A gauge monopole spacetime is essentially
that of a magnetically charged black hole. The gravitational field of a global
string is repulsive and that of a gauge string is attractive at small distances
and repulsive at large distances. Both gauge and global string spacetimes have
singularities at a finite distance from the string core.Comment: 19 pages, REVTeX, 6 Postscript figure
Curvature of the universe and the dark energy potential
The flatness of an accelerating universe model (characterized by a dark
energy scalar field ) is mimicked from a curved model that is filled
with, apart from the cold dark matter component, a quintessencelike scalar
field . In this process, we characterize the original scalar potential
and the mimicked scalar potential associated to the scalar
fields and , respectively. The parameters of the original model are
fixed through the mimicked quantities that we relate to the present
astronomical data, such that the equation state parameter and the
dark energy density parameter .Comment: References 7 and 8 have been corrected: (7) Riess et al. 1998, AJ,
116, 1009 and (8) Perlmutter et al. 1999, ApJ, 517, 56
A Dynamical Solution to the Problem of a Small Cosmological Constant and Late-time Cosmic Acceleration
Increasing evidence suggests that most of the energy density of the universe
consists of a dark energy component with negative pressure, a ``cosmological
constant" that causes the cosmic expansion to accelerate. In this paper, we
address the puzzle of why this component comes to dominate the universe only
recently rather than at some much earlier epoch. We present a class of theories
based on an evolving scalar field where the explanation is based entirely on
internal dynamical properties of the solutions. In the theories we consider,
the dynamics causes the scalar field to lock automatically into a negative
pressure state at the onset of matter-domination such that the present epoch is
the earliest possible time, consistent with nucleosynthesis restrictions, when
it can start to dominate.Comment: 5 pages, 3 figure
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