509 research outputs found
Can modified gravity explain accelerated cosmic expansion?
We show that the recently suggested explanations of cosmic acceleration by
the modification of gravity at small curvature suffer violent instabilities and
strongly disagree with the known properties of gravitational interactions.Comment: 4 pages, no figure, revised version (one footnote added
Accidental Supersymmetric Dark Matter and Baryogenesis
We show that "accidental" supersymmetry is a beyond-the-Standard Model
framework that naturally accommodates a thermal relic dark matter candidate and
successful electroweak baryogenesis, including the needed strongly first-order
character of the electroweak phase transition. We study the phenomenology of
this setup from the standpoint of both dark matter and baryogenesis. For
energies around the electroweak phase transition temperature, the low-energy
effective theory is similar to the MSSM with light super-partners of the
third-generation quarks and of the Higgs and gauge bosons. We calculate the
dark matter relic abundance and the baryon asymmetry across the accidental
supersymmetry parameter space, including resonant and non-resonant CP-violating
sources. We find that there are regions of parameter space producing both the
observed value of the baryon asymmetry and a dark matter candidate with the
correct relic density and conforming to present-day constraints from dark
matter searches. This scenario makes sharp predictions for the particle
spectrum, predicting a lightest neutralino mass between 200 and 500 GeV, with
all charginos and neutralinos within less than a factor 2 of the lightest
neutralino mass and the heavy Higgs sector within 20-25% of that mass, making
it an interesting target for collider searches. In addition, we demonstrate
that successful accidental supersymmetric dark matter and baryogenesis will be
conclusively tested with improvements smaller than one order of magnitude to
the current performance of electron electric dipole moment searches and of
direct dark matter searches, as well as with IceCube plus Deep Core neutrino
telescope data.Comment: 36 pages, 10 figure
Predictions from Quantum Cosmology
The world view suggested by quantum cosmology is that inflating universes
with all possible values of the fundamental constants are spontaneously created
out of nothing. I explore the consequences of the assumption that we are a
`typical' civilization living in this metauniverse. The conclusions include
inflation with an extremely flat potential and low thermalization temperature,
structure formation by topological defects, and an appreciable cosmological
constant.Comment: (revised version), 15 page
Thermal Decay of the Cosmological Constant into Black Holes
We show that the cosmological constant may be reduced by thermal production
of membranes by the cosmological horizon, analogous to a particle ``going over
the top of the potential barrier", rather than tunneling through it. The
membranes are endowed with charge associated with the gauge invariance of an
antisymmetric gauge potential. In this new process, the membrane collapses into
a black hole, thus the net effect is to produce black holes out of the vacuum
energy associated with the cosmological constant. We study here the
corresponding Euclidean configurations ("thermalons"), and calculate the
probability for the process in the leading semiclassical approximation.Comment: 14 pages, 6 figures. Minor correction
Out of equilibrium: understanding cosmological evolution to lower-entropy states
Despite the importance of the Second Law of Thermodynamics, it is not
absolute. Statistical mechanics implies that, given sufficient time, systems
near equilibrium will spontaneously fluctuate into lower-entropy states,
locally reversing the thermodynamic arrow of time. We study the time
development of such fluctuations, especially the very large fluctuations
relevant to cosmology. Under fairly general assumptions, the most likely
history of a fluctuation out of equilibrium is simply the CPT conjugate of the
most likely way a system relaxes back to equilibrium. We use this idea to
elucidate the spacetime structure of various fluctuations in (stable and
metastable) de Sitter space and thermal anti-de Sitter space.Comment: 27 pages, 11 figure
Cosmological Consequences of String-forming Open Inflation Models
We present a study of open inflation cosmological scenarios in which cosmic
strings form betwen the two inflationary epochs. It is shown that in these
models strings are stretched outside the horizon due to the inflationary
expansion but must necessarily re-enter the horizon before the epoch of equal
matter and radiation densities. We determine the power spectrum of cold dark
matter perturbations in these hybrid models, finding good agreement with
observations for values of and comparable
contributions from the active and passive sources to the CMB. Finally, we
briefly discuss other cosmological consequences of these models.Comment: 11 LaTeX pages with 3 eps figure
The nearly Newtonian regime in Non-Linear Theories of Gravity
The present paper reconsiders the Newtonian limit of models of modified
gravity including higher order terms in the scalar curvature in the
gravitational action. This was studied using the Palatini variational principle
in [Meng X. and Wang P.: Gen. Rel. Grav. {\bf 36}, 1947 (2004)] and
[Dom\'inguez A. E. and Barraco D. E.: Phys. Rev. D {\bf 70}, 043505 (2004)]
with contradicting results. Here a different approach is used, and problems in
the previous attempts are pointed out. It is shown that models with negative
powers of the scalar curvature, like the ones used to explain the present
accelerated expansion, as well as their generalization which include positive
powers, can give the correct Newtonian limit, as long as the coefficients of
these powers are reasonably small. Some consequences of the performed analysis
seem to raise doubts for the way the Newtonian limit was derived in the purely
metric approach of fourth order gravity [Dick R.: Gen. Rel. Grav. {\bf 36}, 217
(2004)]. Finally, we comment on a recent paper [Olmo G. J.: Phys. Rev. D {\bf
72}, 083505 (2005)] in which the problem of the Newtonian limit of both the
purely metric and the Palatini formalism is discussed, using the equivalent
Brans--Dicke theory, and with which our results partly disagree.Comment: typos corrected, replaced to match published versio
Measures for a Transdimensional Multiverse
The multiverse/landscape paradigm that has emerged from eternal inflation and
string theory, describes a large-scale multiverse populated by "pocket
universes" which come in a huge variety of different types, including different
dimensionalities. In order to make predictions in the multiverse, we need a
probability measure. In landscapes, the scale factor cutoff measure
has been previously shown to have a number of attractive properties. Here we
consider possible generalizations of this measure to a transdimensional
multiverse. We find that a straightforward extension of scale factor cutoff to
the transdimensional case gives a measure that strongly disfavors large amounts
of slow-roll inflation and predicts low values for the density parameter
, in conflict with observations. A suitable generalization, which
retains all the good properties of the original measure, is the "volume factor"
cutoff, which regularizes the infinite spacetime volume using cutoff surfaces
of constant volume expansion factor.Comment: 30 pages, 1 figure Minor revisions, reference adde
Cosmological Constant of the -Dimensional World, Embedded in the -Dimensional Bulk Space
In this manuscript we study the cosmological constant of a
-dimensional world, which lives in the higher dimensional bulk space. We
assume the extra dimensions are compact on tori. We consider two cases:
positive and negative bulk cosmological constant. It is pointed out that the
tiny cosmological constant of our world can be obtained by the dynamics of a
scalar field and adjusting the parameters of the model. The cosmological
constant of the dual world also will be discussed. We obtain the Dirac
quantization of these cosmological constants.Comment: 11 pages, Latex, No figure. In the revised version, major changes
have been introduced and also references have been adde
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