1,068 research outputs found
Dynamics of tachyon field in spatially curved FRW universe
The dynamics of a tachyon field plus a barotropic fluid is investigated in
spatially curved FRW universe. We perform a phase-plane analysis and obtain
scaling solutions accompanying with a discussion on their stability.
Furthermore, we construct the form of scalar potential which may give rise to
stable solutions for spatially open and closed universe separately.Comment: 16 pages, 2 figures, version to be published in PL
Cosmological Scaling Solutions with Tachyon:Modified Gravity Model
Modifying the Einstein's gravity at large distance scales is one of the
interesting proposals to explain the late time acceleration of the universe. In
this paper, we analyse scaling solutions in modified gravity models where the
universe is sourced by a background matter fluid together with a tachyon type
scalar field. We describe a general prescription to calculate the scaling
potential in such models. Later on, we consider specific examples of
modifications and apply our method to calculate the scaling potential and the
scale factor. Our method can be applied to any modified gravity model, in
presence of a tachyon field.Comment: 6 pages, latex style, modified version, two new figures included, new
refernces added, Accepted for publication in Physics Letters
Vortex-Antivortex Pair Production in a First Order Phase Transition
We carry out numerical simulation of a first order phase transition in 2+1
dimensions by randomly nucleating bubbles, and study the formation of global
U(1) vortices. Bubbles grow and coalesce and vortices are formed at junctions
of bubbles via standard Kibble mechanism as well as due to a new mechanism,
recently proposed by us, where defect-antidefect pairs are produced due to
field oscillations. We make a comparative study of the contribution of both of
these mechanisms for vortex production. We find that, for high nucleation rate
of bubbles, vortex-antivortex pairs produced via the new mechanism have
overlapping configurations, and annihilate quickly; so only those vortices
survive till late which are produced via the Kibble mechanism. However, for low
nucleation rates, bubble collisions are energetic enough to lead to many well
separated vortex-antivortex pairs being produced via the new mechanism. For
example, in a simulation involving nucleation of 20 bubbles, a total of 14
non-overlapping vortices and antivortices formed via this new mechanism of pair
creation (6 of them being very well separated), as compared to 6 vortices and
antivortices produced via the Kibble mechanism. Our results show the
possibility that in extremely energetic bubble collisions, such as those in the
inflationary models of the early Universe, this new mechanism may drastically
affect the defect production scenario.Comment: 8 pages, Revtex, 14 figures. Figs.1a,b and 5a,d are included, rest
are availaible on reques
Student Housing: Trends, Preferences And Needs
To attract and retain students, universities are confronted with increased demand to provide housing options that meet the new expectations of the millennial generation. Recent trends and housing preferences are examined. The results of surveys detailing some of these new demands and how universities are attempting to address these demands are discussed. Additionally, universities are under pressure to efficiently use their limited resources and, as government support for higher education declines, public universities are seeking other sources of funding for major projects. Suggestions are made for unique partnering and financing options. 
The Mass, Normalization and Late Time behavior of the Tachyon Field
We study the dynamics of the tachyon field . We derive the mass of the
tachyon as the pole of the propagator which does not coincide with the standard
mass given in the literature in terms of the second derivative of or
. We determine the transformation of the tachyon in order to have a
canonical scalar field . This transformation reduces to the one obtained
for small but it is also valid for large values of . This is
specially interesting for the study of dark energy where . We
also show that the normalized tachyon field is constrained to the
interval where are zeros of the original
potential . This results shows that the field does not know of the
unboundedness of , as suggested for bosonic open string tachyons. Finally
we study the late time behavior of tachyon field using the L'H\^{o}pital rule.Comment: 9 pages, 10 figure
Inflation: From Theory to Observation and Back
Alan Guth introduced cosmologists to inflation at the 1980 Texas Symposium.
Since, inflation has had almost as much impact on cosmology as the big-bang
model itself. However, unlike the big-bang model, it has little observational
support. Hopefully, that situation is about to change as a variety and
abundance of data begin to test inflation in a significant way. The
observations that are putting inflation to test involve the formation of
structure in the Universe, especially measurements of the anisotropy of the
cosmic background radiation. The cold dark matter models of structure formation
motivated by inflation are holding up well as the observational tests become
sharper. In the next decade inflation will be tested even more significantly,
with more precise measurements of CBR anisotropy, the mean density of the
Universe, the Hubble constant, and the distribution of matter, as well as
sensitive searches for the nonbaryonic dark matter predicted to exist by
inflation. As an optimist I believe that we may be well on our way to a
standard cosmology that includes inflation and extends back to around 10^{-32}
sec, providing an important window on the earliest moments and fundamental
physics.Comment: 17 pages LaTeX with 2 eps figure
Can Moduli Fields parametrize the Cosmological Constant?
We study the cosmological evolution of string/M moduli fields T. We use
T-duality to fix the potential and show that the superpotential W is a function
of the duality invariant function j(T) only. If W is given as a finite
polynomial of j then the moduli fields {\it do not} give an accelerating
universe, i.e. they {\it cannot} be used as quintessence. Furthermore, at T >>1
the potential is given by a double exponential potential V \simeq e^{-a
e^{\sqrt{2} T}} leading to a fast decaying behaviour at large times. For moduli
potentials with a finite v.e.v. of T the energy density redshift is model
dependent but if T has a finite mass, m < \infty, then the moduli energy
density redshifts faster or equal to matter. Only if the moduli mass is
infinite can the moduli energy density dominate the universe independently of
the initial conditions.Comment: 13 pages, LaTeX, 3 postscript figure
A New Approach to Quintessence and a Solution of Multiple Attractors
We take a new approach to construct Quintessential models. With this
approach, we first easily obtain a tracker solution that is different from
those discovered before and straightforwardly find a solution of multiple
attractors, i.e., a solution with more than one attractor for a given set of
parameters. Then we propose a scenario of Quintessence where the field jumps
out of the scaling attractor to the de-Sitter-like attractor, by introducing a
field whose value changes a certain amount in a short time, leading to the
current acceleration. We also calculate the change the field needs for a
successful jump and suggest a possible mechanism that involves spontaneous
symmetry breaking to realize the sudden change of the field value.Comment: 6 pages, 2 figures, Revtex4; To appear in PL
Quintessence Restrictions on Negative Power and Condensate Potentials
We study the cosmological evolution of scalar fields that arise from a phase
transition at some energy scale \Lm_c. We focus on negative power potentials
given by V=c\Lm_c^{4+n}\phi^{-n} and restrict the cosmological viable values
of \Lm_c and . We make a complete analysis of and impose
conditions on the different cosmological parameters. The cosmological
observations ruled out models where the scalar field has reached its attractor
solution. For models where this is not the case, the analytic approximated
solutions are not good enough to determine whether a specific model is
phenomenologically viable or not and the full differential equations must be
numerically solved. The results are not fine tuned since a change of 45% on the
initial conditions does not spoil the final results. We also determine the
values of that give a condensation scale \Lm_c consistent with
gauge coupling unification, leaving only four models that satisfy unification
and SN1a constraints.Comment: 15 pages, LaTeX, 8 Figures. Minor changes in text, a discussion on
initial conditions added (accepted in Phys.Rev.D
Natural Quintessence?
We formulate conditions for the naturalness of cosmological quintessence
scenarios. The quintessence lagrangian is taken to be the sum of a simple
exponential potential and a non-canonical kinetic term. This parameterization
covers most variants of quintessence and makes the naturalness conditions
particularly transparent. Several ``natural'' scalar models lead, for the
present cosmological era, to a large fraction of homogeneous dark energy
density and an acceleration of the scale factor as suggested by observation.Comment: 11 pages LaTeX, 3 figures included, numerical correctio
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