9,515 research outputs found
Baryon Asymmetry, Dark Matter and Quantum Chromodynamics
We propose a novel scenario to explain the observed cosmological asymmetry
between matter and antimatter, based on nonperturbative QCD physics. This
scenario relies on a mechanism of separation of quarks and antiquarks in two
coexisting phases at the end of the cosmological QCD phase transition: ordinary
hadrons (and antihadrons), along with massive lumps (and antilumps) of novel
color superconducting phase. The latter would serve as the cosmological cold
dark matter. In certain conditions the separation of charge is C and CP
asymmetric and can leave a net excess of hadrons over antihadrons in the
conventional phase, even if the visible universe is globally baryon symmetric
. In this case an equal, but negative, overall baryon charge must be
hidden in the lumps of novel phase. Due to the small volume occupied by these
dense lumps/antilumps of color superconducting phase and the specific features
of their interaction with "normal" matter in hadronic phase, this scenario does
not contradict the current phenomenological constrains on presence of
antimatter in the visible universe. Moreover, in this scenario the observed
cosmological ratio within an order of magnitude
finds a natural explanation, as both contributions to originated from
the same physics during the QCD phase transition. The baryon to entropy ratio
would also be a natural outcome, fixed by the
temperature T_f \simlt T_{QCD} at which the separation of phases is
completed.Comment: New paragraph added in subsection II.D; version to appear in Physical
Review
Accelerating universes driven by bulk particles
We consider our universe as a 3d domain wall embedded in a 5d dimensional
Minkowski space-time. We address the problem of inflation and late time
acceleration driven by bulk particles colliding with the 3d domain wall. The
expansion of our universe is mainly related to these bulk particles. Since our
universe tends to be permeated by a large number of isolated structures, as
temperature diminishes with the expansion, we model our universe with a 3d
domain wall with increasing internal structures. These structures could be
unstable 2d domain walls evolving to fermi-balls which are candidates to cold
dark matter. The momentum transfer of bulk particles colliding with the 3d
domain wall is related to the reflection coefficient. We show a nontrivial
dependence of the reflection coefficient with the number of internal dark
matter structures inside the 3d domain wall. As the population of such
structures increases the velocity of the domain wall expansion also increases.
The expansion is exponential at early times and polynomial at late times. We
connect this picture with string/M-theory by considering BPS 3d domain walls
with structures which can appear through the bosonic sector of a
five-dimensional supergravity theory.Comment: To appear in Phys. Rev. D, 16 pages, 3 eps figures, minor changes and
references adde
Scale Factor Self-Dual Cosmological Models
We implement a conformal time scale factor duality for
Friedmann-Robertson-Walker cosmological models, which is consistent with the
weak energy condition. The requirement for self-duality determines the
equations of state for a broad class of barotropic fluids. We study the example
of a universe filled with two interacting fluids, presenting an accelerated and
a decelerated period, with manifest UV/IR duality. The associated self-dual
scalar field interaction turns out to coincide with the "radiation-like"
modified Chaplygin gas models. We present an equivalent realization of them as
gauged K\"ahler sigma models (minimally coupled to gravity) with very specific
and interrelated K\"ahler- and super-potentials. Their applications in the
description of hilltop inflation and also as quintessence models for the late
universe are discussed.Comment: v3, improved and extended version to be published in JHEP; new
results added to sect.2; 4 figures; 17pg
Reflection symmetry breaking scenarios with minimal gauge form coupling in brane world cosmology
This article synthesises and extends recent work on the cosmological
consequences of dropping the usual Z_2 reflection symmetry postulate in brane
world scenarios. It is observed that for a cosmological model of homogeneous
isotropic type, the relevant generalised Birkhoff theorem establishing
staticity of the external vacuum in the maximally symmetric ``bulk'' outside a
freely moving world brane will remain valid for the case of motion that is
forced by minimal (generalised Wess Zumino type) coupling to an external
antisymmetric gauge field provided its kinetic action contribution has the
usual homogeneous quadratic form. This means that the geometry on each side of
the brane worldsheet will still be of the generalised Schwarzschild anti de
Sitter type. The usual first integrated Friedmann equation for the Hubble
expansion rate can thereby be straightforwardly generalised by inclusion of new
terms involving 2 extra parameters respectively measuring the strength of the
gauge coupling and the degree of deviation from reflection symmetry. Some
conceivable phenomenological implications are briefly outlined, and
corresponding limitations are derived for possible values of relevant
parameters.Comment: 13 pages latex (revised to rectify a couple of sign transcription
errors
Symmetries and the cosmological constant puzzle
We outline the evaluation of the cosmological constant in the framework of
the standard field-theoretical treatment of vacuum energy and discuss the
relation between the vacuum energy problem and the gauge-group spontaneous
symmetry breaking. We suggest possible extensions of the 't Hooft-Nobbenhuis
symmetry, in particular, its complexification till duality symmetry and discuss
the compatible implementation on gravity. We propose to use the discrete
time-reflection transform to formulate a framework in which one can eliminate
the huge contributions of vacuum energy into the effective cosmological
constant and suggest that the breaking of time--reflection symmetry could be
responsible for a small observable value of this constant.Comment: 11 pages, more relevant refs, refining cutoff definition of
cosmological constant + eq.for regularized pressure adde
Moduli Vacuum Bubbles Produced by Evaporating Black Holes
We consider a model with a toroidally compactified extra dimension giving
rise to a temperature-dependent 4d effective potential with one-loop
contributions due to the Casimir effect, along with a 5d cosmological constant.
The forms of the effective potential at low and high temperatures indicates a
possibility for the formation of a domain wall bubble, formed by the modulus
scalar field, surrounding an evaporating black hole. This is viewed as an
example of a recently proposed black hole vacuum bubble arising from
matter-sourced moduli fields in the vicinity of an evaporating black hole [D.
Green, E. Silverstein, and D. Starr, Phys. Rev. D74, 024004 (2006),
arXiv:hep-th/0605047]. The black hole bubble can be highly opaque to lower
energy particles and photons, and thereby entrap them within. For high
temperature black holes, there may also be a symmetry-breaking black hole
bubble of false vacuum of the type previously conjectured by Moss [I.G. Moss,
Phys. Rev. D32,1333 (1985)], tending to reflect low energy particles from its
wall. A double bubble composed of these two different types of bubble may form
around the black hole, altering the hole's emission spectrum that reaches
outside observers. Smaller mass black holes that have already evaporated away
could have left vacuum bubbles behind that contribute to the dark matter.Comment: 20 pages; to appear in Phys.Rev.
BKL oscillations in 2+1 space-time dimensions
We investigate the question whether there are cosmological models in 2+1
space-time dimensions which exhibit dynamics similar to BKL oscillations, as
the cosmological singularity is approached. Based on intuition, we conceive a
toy model which displays such oscillatory dynamics. We show that in the phase
space of this model, the cosmological singularity is represented by a
separatrix curve and discuss the model's dynamics within the cosmological
billiards picture. Finally, we offer a physical interpretation for a family of
similar cosmological models in terms of the topological degrees of freedom of
gravity in 2+1 dimensions.Comment: 22 pages, 4 figure
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