44 research outputs found
Rolling Tachyon in Nonlocal Cosmology
Nonlocal cosmological models derived from String Field Theory are considered.
A new method for constructing rolling tachyon solutions in the FRW metric in
two field configuration is proposed and solutions of the Friedman equations
with nonlocal operator are presented. The cosmological properties of these
solutions are discussed.Comment: 4 pages, 2 figures. Contribution to the proceedings of PASCOS-07, 2-7
July 2007, Imperial College, Londo
Dynamics with Infinitely Many Time Derivatives in Friedmann-Robertson-Walker Background and Rolling Tachyon
Open string field theory in the level truncation approximation is considered.
It is shown that the energy conservation law determines existence of rolling
tachyon solution. The coupling of the open string field theory action to a
Friedmann-Robertson-Walker metric is considered which leads to a new time
dependent rolling tachyon solution is presented and possible cosmological
consequences are discussed.Comment: 18 pages, 8 figure
Time Lumps in Nonlocal Stringy Models and Cosmological Applications
We study lump solutions in nonlocal toy models and their cosmological
applications. These models are motivated by a description of D-brane decay
within string field theory framework. In order to find cosmological solutions
we use the simplest local approximation keeping only second derivative terms in
nonlocal dynamics. We study a validity of this approximation in flat background
where time lump solutions can be written explicitly. We work out the validity
of this approximation. We show that our models at large time exhibit the
phantom behaviour similar to the case of the string kink.Comment: Latex, 24 pages, 13 figures, Typos corrected, references adde
Null Energy Condition Violation and Classical Stability in the Bianchi I Metric
The stability of isotropic cosmological solutions in the Bianchi I model is
considered. We prove that the stability of isotropic solutions in the Bianchi I
metric for a positive Hubble parameter follows from their stability in the
Friedmann-Robertson-Walker metric. This result is applied to models inspired by
string field theory, which violate the null energy condition. Examples of
stable isotropic solutions are presented. We also consider the k-essence model
and analyse the stability of solutions of the form .Comment: 27 pages, references added, accepted for publication in Phys. Rev.
Time Evolution in Superstring Field Theory on non-BPS brane.I. Rolling Tachyon and Energy-Momentum Conservation
We derive equations of motion for the tachyon field living on an unstable
non-BPS D-brane in the level truncated open cubic superstring field theory in
the first non-trivial approximation. We construct a special time dependent
solution to this equation which describes the rolling tachyon. It starts from
the perturbative vacuum and approaches one of stable vacua in infinite time. We
investigate conserved energy functional and show that its different parts
dominate in different stages of the evolution. We show that the pressure for
this solution has its minimum at zero time and goes to minus energy at infinite
time.Comment: 16 pages, 5 figures; minor correction
Bouncing and Accelerating Solutions in Nonlocal Stringy Models
A general class of cosmological models driven by a non-local scalar field
inspired by string field theories is studied. In particular cases the scalar
field is a string dilaton or a string tachyon. A distinguished feature of these
models is a crossing of the phantom divide. We reveal the nature of this
phenomena showing that it is caused by an equivalence of the initial non-local
model to a model with an infinite number of local fields some of which are
ghosts. Deformations of the model that admit exact solutions are constructed.
These deformations contain locking potentials that stabilize solutions.
Bouncing and accelerating solutions are presented.Comment: Minor corrections, references added, published in JHE
Dynamics in nonlocal linear models in the Friedmann-Robertson-Walker metric
A general class of cosmological models driven by a nonlocal scalar field
inspired by the string field theory is studied. Using the fact that the
considering linear nonlocal model is equivalent to an infinite number of local
models we have found an exact special solution of the nonlocal Friedmann
equations. This solution describes a monotonically increasing Universe with the
phantom dark energy.Comment: 18 pages, 3 figures, a few misprints in Section 5 have been correcte
FRW Cosmology with Non-positively Defined Higgs Potentials
We discuss the classical aspects of dynamics of scalar models with
non-positive Higgs potentials in the FRW cosmology. These models appear as
effective local models in non-local models related with string field theories.
After a suitable field redefinition these models have the form of local Higgs
models with a negative extra cosmological term and the total Higgs potential is
non-positively defined and has rather small coupling constant. The
non-positivity of the potential leads to the fact that on some stage of
evolution the expansion mode gives place to the mode of contraction, due to
that the stage of reheating is absent. In these models the hard regime of
inflation gives place to inflation near the hill top and the area of the slow
roll inflation is very small. Meanwhile one can obtain enough e-foldings before
the contraction to make the model under consideration admissible to describe
inflation.Comment: 40 pages, 20 figures, typos correcte