96 research outputs found
New Isotropic and Anisotropic Sudden Singularities
We show the existence of an infinite family of finite-time singularities in
isotropically expanding universes which obey the weak, strong, and dominant
energy conditions. We show what new type of energy condition is needed to
exclude them ab initio. We also determine the conditions under which
finite-time future singularities can arise in a wide class of anisotropic
cosmological models. New types of finite-time singularity are possible which
are characterised by divergences in the time-rate of change of the
anisotropic-pressure tensor. We investigate the conditions for the formation of
finite-time singularities in a Bianchi type universe with anisotropic
pressures and construct specific examples of anisotropic sudden singularities
in these universes.Comment: Typos corrected. Published versio
Einstein billiards and spatially homogeneous cosmological models
In this paper, we analyse the Einstein and Einstein-Maxwell billiards for all
spatially homogeneous cosmological models corresponding to 3 and 4 dimensional
real unimodular Lie algebras and provide the list of those models which are
chaotic in the Belinskii, Khalatnikov and Lifschitz (BKL) limit. Through the
billiard picture, we confirm that, in D=5 spacetime dimensions, chaos is
present if off-diagonal metric elements are kept: the finite volume billiards
can be identified with the fundamental Weyl chambers of hyperbolic Kac-Moody
algebras. The most generic cases bring in the same algebras as in the
inhomogeneous case, but other algebras appear through special initial
conditions.Comment: 27 pages, 10 figures, additional possibility analysed in section 4.3,
references added, typos correcte
Brans-Dicke Boson Stars: Configurations and Stability through Cosmic History
We make a detailed study of boson star configurations in Jordan--Brans--Dicke
theory, studying both equilibrium properties and stability, and considering
boson stars existing at different cosmic epochs. We show that boson stars can
be stable at any time of cosmic history and that equilibrium stars are denser
in the past. We analyze three different proposed mass functions for boson star
systems, and obtain results independently of the definition adopted. We study
how the configurations depend on the value of the Jordan--Brans--Dicke coupling
constant, and the properties of the stars under extreme values of the
gravitational asymptotic constant. This last point allows us to extract
conclusions about the stability behaviour concerning the scalar field. Finally,
other dynamical variables of interest, like the radius, are also calculated. In
this regard, it is shown that the radius corresponding to the maximal boson
star mass remains roughly the same during cosmological evolution.Comment: 9 pages RevTeX file with nine figures incorporated (uses RevTeX and
epsf
Anisotropy and inflation in Bianchi I brane worlds
After a more general assumption on the influence of the bulk on the brane, we
extend some conclusions by Maartens et al. and Santos et al. on the asymptotic
behavior of Bianchi I brane worlds. As a consequence of the nonlocal
anisotropic stresses induced by the bulk, in most of our models, the brane does
not isotropize and the nonlocal energy does not vanish in the limit in which
the mean radius goes to infinity. We have also found the intriguing possibility
that the inflation due to the cosmological constant might be prevented by the
interaction with the bulk. We show that the problem for the mean radius can be
completely solved in our models, which include as particular cases those in the
references above.Comment: 10 pages, improved discussion on the likeliness of
non-isotropization, completed list of references, matches version to appear
in Class. Quantum Gra
Reconstruction of the equation of state for the cyclic universes in homogeneous and isotropic cosmology
We study the cosmological evolutions of the equation of state (EoS) for the
universe in the homogeneous and isotropic
Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) space-time. In particular, we
reconstruct the cyclic universes by using the Weierstrass and Jacobian elliptic
functions. It is explicitly illustrated that in several models the universe
always stays in the non-phantom (quintessence) phase, whereas there also exist
models in which the crossing of the phantom divide can be realized in the
reconstructed cyclic universes.Comment: 29 pages, 8 figures, version accepted for publication in Central
European Journal of Physic
Screening of cosmological constant for De Sitter Universe in non-local gravity, phantom-divide crossing and finite-time future singularities
We investigate de Sitter solutions in non-local gravity as well as in
non-local gravity with Lagrange constraint multiplier. We examine a condition
to avoid a ghost and discuss a screening scenario for a cosmological constant
in de Sitter solutions. Furthermore, we explicitly demonstrate that three types
of the finite-time future singularities can occur in non-local gravity and
explore their properties. In addition, we evaluate the effective equation of
state for the universe and show that the late-time accelerating universe may be
effectively the quintessence, cosmological constant or phantom-like phases. In
particular, it is found that there is a case in which a crossing of the phantom
divide from the non-phantom (quintessence) phase to the phantom one can be
realized when a finite-time future singularity occurs. Moreover, it is
demonstrated that the addition of an term can cure the finite-time future
singularities in non-local gravity. It is also suggested that in the framework
of non-local gravity, adding an term leads to possible unification of the
early-time inflation with the late-time cosmic acceleration.Comment: 42 pages, no figure, version accepted for publication in General
Relativity and Gravitatio
Finite-time future singularities in modified Gauss-Bonnet and gravity and singularity avoidance
We study all four types of finite-time future singularities emerging in
late-time accelerating (effective quintessence/phantom) era from
-gravity, where and are the Ricci scalar and the
Gauss-Bonnet invariant, respectively. As an explicit example of
-gravity, we also investigate modified Gauss-Bonnet gravity,
so-called -gravity. In particular, we reconstruct the -gravity and
-gravity models where accelerating cosmologies realizing the
finite-time future singularities emerge. Furthermore, we discuss a possible way
to cure the finite-time future singularities in -gravity and
-gravity by taking into account higher-order curvature
corrections. The example of non-singular realistic modified Gauss-Bonnet
gravity is presented. It turns out that adding such non-singular modified
gravity to singular Dark Energy makes the combined theory to be non-singular
one as well.Comment: 35 pages, no figure, published version, references adde
Modified gravity and its reconstruction from the universe expansion history
We develop the reconstruction program for the number of modified gravities:
scalar-tensor theory, , and string-inspired, scalar-Gauss-Bonnet
gravity. The known (classical) universe expansion history is used for the
explicit and successful reconstruction of some versions (of special form or
with specific potentials) from all above modified gravities. It is demonstrated
that cosmological sequence of matter dominance, decceleration-acceleration
transition and acceleration era may always emerge as cosmological solutions of
such theory. Moreover, the late-time dark energy FRW universe may have the
approximate or exact CDM form consistent with three years WMAP data.
The principal possibility to extend this reconstruction scheme to include the
radiation dominated era and inflation is briefly mentioned. Finally, it is
indicated how even modified gravity which does not describe the
matter-dominated epoch may have such a solution before acceleration era at the
price of the introduction of compensating dark energy.Comment: LaTeX file, 24 pages, no figure, prepared for the proceedings of ERE
2006, minor correction
First results on ProtoDUNE-SP liquid argon time projection chamber performance from a beam test at the CERN Neutrino Platform
The ProtoDUNE-SP detector is a single-phase liquid argon time projection chamber with an active volume of 7.2× 6.1× 7.0 m3. It is installed at the CERN Neutrino Platform in a specially-constructed beam that delivers charged pions, kaons, protons, muons and electrons with momenta in the range 0.3 GeV/c to 7 GeV/c. Beam line instrumentation provides accurate momentum measurements and particle identification. The ProtoDUNE-SP detector is a prototype for the first far detector module of the Deep Underground Neutrino Experiment, and it incorporates full-size components as designed for that module. This paper describes the beam line, the time projection chamber, the photon detectors, the cosmic-ray tagger, the signal processing and particle reconstruction. It presents the first results on ProtoDUNE-SP\u27s performance, including noise and gain measurements, dE/dx calibration for muons, protons, pions and electrons, drift electron lifetime measurements, and photon detector noise, signal sensitivity and time resolution measurements. The measured values meet or exceed the specifications for the DUNE far detector, in several cases by large margins. ProtoDUNE-SP\u27s successful operation starting in 2018 and its production of large samples of high-quality data demonstrate the effectiveness of the single-phase far detector design
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