784 research outputs found
Brane Inflation and the Overshoot Problem
We investigate recent claims that brane inflation solves the overshoot
problem through a combination of microphysical restrictions on the phase space
of initial conditions and the existence of the Dirac-Born-Infeld (DBI)
attractor in regimes where the slow-roll attractor does not apply. Carrying out
a comprehensive analysis of the parameter space allowed by the latest advances
in brane inflation model-building, we find that these restrictions are
insufficient to solve the overshoot problem. The vast majority of the phase
space of initial conditions is still dominated by overshoot trajectories. We
present an analytic proof that the brane-inflationary attractor must be close
to the slow-roll limit, and update the predictions for observables such as
non-Gaussianity, cosmic string tension and tensor modes.Comment: 14 pages, 3 figures, matches version published in PRD. Minor
clarifications, references adde
Cosmological Landscape From Nothing: Some Like It Hot
We suggest a novel picture of the quantum Universe -- its creation is
described by the {\em density matrix} defined by the Euclidean path integral.
This yields an ensemble of universes -- a cosmological landscape -- in a mixed
state which is shown to be dynamically more preferable than the pure quantum
state of the Hartle-Hawking type. The latter is dynamically suppressed by the
infinitely large positive action of its instanton, generated by the conformal
anomaly of quantum fields within the cosmological bootstrap (the
self-consistent back reaction of hot matter). This bootstrap suggests a
solution to the problem of boundedness of the on-shell cosmological action and
eliminates the infrared catastrophe of small cosmological constant in Euclidean
quantum gravity. The cosmological landscape turns out to be limited to a
bounded range of the cosmological constant . The domain is ruled out by the
back reaction effect which we analyze by solving effective Euclidean equations
of motion. The upper cutoff is enforced by the quantum effects of vacuum energy
and the conformal anomaly mediated by a special ghost-avoidance renormalization
of the effective action. They establish a new quantum scale
which is determined by the coefficient of the topological Gauss-Bonnet term in
the conformal anomaly. This scale is realized as the upper bound -- the
limiting point of an infinite sequence of garland-type instantons which
constitute the full cosmological landscape. The dependence of the cosmological
constant range on particle phenomenology suggests a possible dynamical
selection mechanism for the landscape of string vacua.Comment: Final version, to appear in JCA
On Power Law Inflation in DBI Models
Inflationary models in string theory which identify the inflaton with an open
string modulus lead to effective field theories with non-canonical kinetic
terms: Dirac-Born-Infeld scalar field theories. In the case of a -brane
moving in an AdS throat with a quadratic scalar field potential DBI kinetic
terms allow a novel realization of power law inflation. This note adresses the
question of whether this behaviour is special to this particular choice of
throat geometry and potential. The answer is that for any throat geometry one
can explicitly find a potential which leads to power law inflation. This
generalizes the well known fact that an exponential potential gives power law
inflation in the case of canonical kinetic terms.Comment: References and comments adde
Accelerating universe emergent from the landscape
We propose that the existence of the string landscape suggests the universe
can be in a quantum glass state, where an extremely large viscosity is
generated, and long distance dynamics slows down. At the same time, the short
distance dynamics is not altered due to the separation of time scales. This
scenario can help to understand some controversies in cosmology, for example
the natural existence of slow roll inflation and dark energy in the landscape,
the apparent smallness of the cosmological constant. We see also that moduli
stabilization is no longer necessary. We further identify the glass transition
point, where the viscosity diverges, as the location of the cosmic horizon. We
try to reconstruct the geometry of the accelerating universe from the structure
of the landscape, and find that the metric should have an infinite jump when
crossing the horizon. We predict that the static coordinate metric for dS space
breaks down outside the horizon.Comment: 20 pages, no figures, harvma
Fractional Superstrings with Space-Time Critical Dimensions Four and Six
We propose possible new string theories based on local world-sheet symmetries
corresponding to extensions of the Virasoro algebra by fractional spin
currents. They have critical central charges and Minkowski
space-time dimensions for an integer. We present evidence
for their existence by constructing modular invariant partition functions and
the massless particle spectra. The dimension and strings have
space-time supersymmetry.Comment: 9 page
Enhancement of the Thermal Conductivity in gapped Quantum Spin Chains
We study mechanism of magnetic energy transport, motivated by recent
measurements of the thermal conductivity in low dimensional quantum magnets. We
point out a possible mechanism of enhancement of the thermal conductivity in
gapped magnetic system, where the magnetic energy transport plays a crucial
role. This mechanism gives an interpretation for the recent experiment of
CuGeO_3, where the thermal conductivity depends on the crystal direction.Comment: 4 pages, 2 figure
On the Slow Roll Expansion for Brane Inflation
One possibility for identifying the inflaton in the framework of string
theory is that it is a -brane modulus. This option involves a specific,
non-canonical form of the kinetic energy -- the Dirac-Born-Infeld action. This
note investigates the applicability of the slow roll approximation in
inflationary models of this type. To this end the slow roll expansion of
Liddle, Parsons and Barrow is derived for the case of the DBI action. The
resulting slow roll conditions augment the standard ones valid in the case of
canonical kinetic terms. It is also shown that in DBI models inflation does not
require that the potential dominate the energy density.Comment: References adde
Vacuum energy in conical space with additional boundary conditions
Total vacuum energy of some quantized fields in conical space with additional
boundary conditions is calculated. These conditions are imposed on a
cylindrical surface which is coaxial with the symmetry axis of conical space.
The explicit form of the matching conditions depends on the field under
consideration. In the case of electromagnetic field, the perfectly conducting
boundary conditions or isorefractive matching conditions are imposed on the
cylindrical surface. For a massless scalar field, the semi-transparent
conditions (-potential) on the cylindrical shell are investigated. As a
result, the total Casimir energy of electromagnetic field and scalar field, per
a unit length along the symmetry axis, proves to be finite unlike the case of
an infinitely thin cosmic string. In these studies the spectral zeta functions
are widely used. It is shown briefly how to apply this technique for obtaining
the asymptotics of the relevant thermodynamical functions in the high
temperature limit.Comment: 29 pages, 2 figures, the title was changed for a more adequate one,
the abstract was rewritten, a few typos and minor grammar mistakes were
correcte
A Stringy Mechanism for A Small Cosmological Constant
Based on the probability distributions of products of random variables, we
propose a simple stringy mechanism that prefers the meta-stable vacua with a
small cosmological constant. We state some relevant properties of the
probability distributions of functions of random variables. We then illustrate
the mechanism within the flux compactification models in Type IIB string
theory. As a result of the stringy dynamics, we argue that the generic
probability distribution for the meta-stable vacua typically peaks with a
divergent behavior at the zero value of the cosmological constant. However, its
suppression in the single modulus model studied here is modest.Comment: 36 pages, 8 figure
Critical Behavior of Nuclear-Spin Diffusion in GaAs/AlGaAs Heterostructures near Landau Level Filling \nu=1
Thermal measurements on a GaAs/AlGaAs heterostructure reveal that the state
of the confined two-dimensional electrons dramatically affects the nuclear-spin
diffusion near Landau level filling factor \nu=1. The experiments provide
quantitative evidence that the sharp peak in the temperature dependence of heat
capacity near \nu=1 is due to an enhanced nuclear-spin diffusion from the GaAs
quantum wells into the AlGaAs barriers. We discuss the physical origin of this
enhancement in terms the possible Skyrme solid-liquid phase transition.Comment: 1 LateX file, 3 figures, submitte
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