1,397 research outputs found
Supernovae Ia Constraints on a Time-Variable Cosmological "Constant"
The energy density of a scalar field with potential , , behaves like a time-variable cosmological
constant that could contribute significantly to the present energy density.
Predictions of this spatially-flat model are compared to recent Type Ia
supernovae apparent magnitude versus redshift data. A large region of model
parameter space is consistent with current observations. (These constraints are
based on the exact scalar field model equations of motion, not on the widely
used time-independent equation of state fluid approximation equations of
motion.) We examine the consequences of also incorporating constraints from
recent measurements of the Hubble parameter and the age of the universe in the
constant and time-variable cosmological constant models. We also study the
effect of using a non-informative prior for the density parameter.Comment: Accepted for publication in Ap
Beta, Dipole and Noncommutative Deformations of M-theory Backgrounds with One or More Parameters
We construct new M-theory solutions starting from those that contain 5 U(1)
isometries. We do this by reducing along one of the 5-torus directions, then
T-dualizing via the action of an O(4,4) matrix and lifting back to
11-dimensions. The particular T-duality transformation is a sequence of O(2,2)
transformations embedded in O(4,4), where the action of each O(2,2) gives a
Lunin-Maldacena deformation in 10-dimensions. We find general formulas for the
metric and 4-form field of single and multiparameter deformed solutions, when
the 4-form of the initial 11-dimensional background has at most one leg along
the 5-torus. All the deformation terms in the new solutions are given in terms
of subdeterminants of a 5x5 matrix, which represents the metric on the 5-torus.
We apply these results to several M-theory backgrounds of the type AdS_r x
X^{11-r}. By appropriate choices of the T-duality and reduction directions we
obtain analogues of beta, dipole and noncommutative deformations. We also
provide formulas for backgrounds with only 3 or 4 U(1) isometries and study a
case, for which our assumption for the 4-form field is violated.Comment: v2:minor corrections, v3:small improvements, v4:conclusions expanded,
to appear in Class. Quant. Gra
Scaling Cosmologies from Duality Twisted Compactifications
Oscillating moduli fields can support a cosmological scaling solution in the
presence of a perfect fluid when the scalar field potential satisfies
appropriate conditions. We examine when such conditions arise in
higher-dimensional, non-linear sigma-models that are reduced to four dimensions
under a generalized Scherk-Schwarz compactification. We show explicitly that
scaling behaviour is possible when the higher-dimensional action exhibits a
global SL(n,R) or O(2,2) symmetry. These underlying symmetries can be exploited
to generate non-trivial scaling solutions when the moduli fields have
non-canonical kinetic energy. We also consider the compactification of
eleven-dimensional vacuum Einstein gravity on an elliptic twisted torus.Comment: 21 pages, 3 figure
Thermodynamics of Decaying Vacuum Cosmologies
The thermodynamic behavior of vacuum decaying cosmologies is investigated
within a manifestly covariant formulation. Such a process corresponds to a
continuous irreversible energy flow from the vacuum component to the created
matter constituents. It is shown that if the specific entropy per particle
remains constant during the process, the equilibrium relations are preserved.
In particular, if the vacuum decays into photons, the energy density and
average number density of photons scale with the temperature as and . The temperature law is determined and a generalized
Planckian type form of the spectrum, which is preserved in the course of the
evolution, is also proposed. Some consequences of these results for decaying
vacuum FRW type cosmologies as well as for models with ``adiabatic'' photon
creation are discussed.Comment: 21 pages, uses LATE
Black Hole Formation with an Interacting Vacuum Energy Density
We discuss the gravitational collapse of a spherically symmetric massive core
of a star in which the fluid component is interacting with a growing vacuum
energy density. The influence of the variable vacuum in the collapsing core is
quantified by a phenomenological \beta-parameter as predicted by dimensional
arguments and the renormalization group approach. For all reasonable values of
this free parameter, we find that the vacuum energy density increases the
collapsing time but it cannot prevent the formation of a singular point.
However, the nature of the singularity depends on the values of \beta. In the
radiation case, a trapped surface is formed for \beta<1/2 whereas for
\beta>1/2, a naked singularity is developed. In general, the critical value is
\beta=1-2/3(1+\omega), where the \omega-parameter describes the equation of
state of the fluid component.Comment: 9 pages, 8 figure
PHYSICAL, CHEMICAL AND TECHNOLOGICAL FEATURES OF SOME POTATOES KINDS GROWN IN HATAY PROVINCE OF TURKEY
In this research which aimed to determine of physical, chemical and tecnological features of some kinds of potatoes which are grown in Hatay, the exams of Agria, Alaska, Anais, Buchan, Daline, Eladie, Goliat, Harmony, Justine, Lilla, Marabel, Mortana, Safrane, Sebastian, Surde, Szazbouzcp, VanGogh and White Lady as material worked to fresh potatoes, chips and finger potatoes are used. The direction of the knowledge has been obtained Agria, Anais, Alaska, Daline, Harmony, Justine, Safrane, Mortana, Van Gogh and White Lady potatoes variaties are appropriate for potatoes chips process, because of high dry substance rate and low reduced sugar rate. It is determined that the products produced with high dry substance rate potatoes have much more quality and fried products absorb less oil. Sebastian, Szazbouzcp, Goliat and Buchan potatoes varieties are suitable for finger potatoes process. Eladie, Anais, Mortana and Daline are appropiate for boiling process. Because these kinds of potatoes have low dry substance rate. Consequently their absorption rate is high. Van Gogh, Surde, Szazbouzcp, Alaska, Agria and Eladie are suitable for starch production, because of high starch they have. Because it is fixed that starch rates are higher than the others. At the fresh lump, chips and finger potatoes some physical and chemical analysis had been practiced. Obtained results; it had been seen that the kinds of Anais, Buchan, Daline, Szazbouzcp, Surde and Mortana is not suitable for been worked to chips and finger potatoes. The other kinds had been seen suitable for been worked to chips and finger potatoes
Another exact inflationary solution
A new closed-form inflationary solution is given for a hyperbolic interaction
potential. The method used to arrive at this solution is outlined as it appears
possible to generate additional sets of equations which satisfy the model. In
addition a new form of decaying cosmological constant is presented.Comment: 10 pages, 0 figure
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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