66,888 research outputs found
Energy Conditions and Supernovae Observations
In general relativity, the energy conditions are invoked to restrict general
energy-momentum tensors on physical grounds. We show that in the standard
Friedmann-Lemaitre-Robertson-Walker approach to cosmological modelling where
the equation of state of the cosmological fluid is unknown, the energy
conditions provide model-independent bounds on the behavior of the distance
modulus of cosmic sources as a function of the redshift. We use both the gold
and the legacy samples of current type Ia supenovae to carry out a
model-independent analysis of the energy conditions violation in the context of
standard cosmology.Comment: 4 pages, 2 figures; v2: References added, misprints corrected,
published in Phys.Rev.D in the present for
Lookback time bounds from energy conditions
In general relativity, the energy conditions are invoked to restrict general
energy-momentum tensors on physical grounds. We show that in the standard
Friedmann-Lemaitre-Robertson-Walker (FLRW) approach to cosmological modeling,
where the energy and matter components of the cosmic fluid are unknown, the
energy conditions provide model-independent bounds on the behavior of the
lookback time of cosmic sources as a function of the redshift for any value of
the spatial curvature. We also confront such bounds with a lookback time sample
which is built from the age estimates of 32 galaxies lying in the interval
and by assuming the total expanding age of the
Universe to be Gyr, as obtained from current cosmic microwave
background experiments. In agreement with previous results, we show that all
energy conditions seem to have been violated at some point of the recent past
of cosmic evolution.Comment: 7 pages, 3 figures. v2: Minor changes, published in Phys.Rev.D in the
present for
Energy Conditions and Cosmic Acceleration
In general relativity, the energy conditions are invoked to restrict general
energy-momentum tensors in different frameworks, and to derive
general results that hold in a variety of general contexts on physical grounds.
We show that in the standard Friedmann-Lemaitre-Robertson-Walker (FLRW)
approach, where the equation of state of the cosmological fluid is unknown, the
energy conditions provide model-independent bounds on the behavior of the
distance modulus of cosmic sources as a function of the redshift for any
spatial curvature. We use the most recent type Ia supernovae (SNe Ia)
observations, which include the new Hubble Space Telescope SNe Ia events, to
carry out a model-independent analysis of the energy conditions violation in
the context of the standard cosmology. We show that both the null (NEC), weak
(WEC) and dominant (DEC) conditions, which are associated with the existence of
the so-called phantom fields, seem to have been violated only recently (), whereas the condition for attractive gravity, i.e., the strong
energy condition (SEC) was firstly violated billions of years ago, at .Comment: 6 pages, 3 figures. v2: References added, misprints corrected,
published in Phys.Rev.D in the present for
Two-sided asymmetric subduction; implications for tectonomagmatic and metallogenic evolution of the Lut Block, Eastern Iran
West directed subduction zones show common characteristics, such as low structural elevation, deep trench, steep slab and a
conjugate back-arc basin that are opposite to those of the east directed subduction zones. The tectonomagmatic and
metallogenic setting of the Lut Block is still a matter of debate and several hypotheses have been put forward. Despite some
authors denying the influence of the operation of Benioff planes, the majority propose that it occurred beneath the Afghan
Block, while others consider that oceanic lithosphere was dragged under the Lut Block. Cu-Au porphyry deposits seem to
occur in an island arc geotectonic setting during the middle Eocene while Mo-bearing deposits are coincident with the
crustal thickening during Oligocene. We introduce new trace element and isotope geochemical data for granitoids and
structural evidences testifying the two-sided asymmetric subduction beneath both Afghan and Lut Blocks, with different
rates of consumption of oceanic lithosphere
Is the transition redshift a new cosmological number?
Observations from Supernovae Type Ia (SNe Ia) provided strong evidence for an
expanding accelerating Universe at intermediate redshifts. This means that the
Universe underwent a transition from deceleration to acceleration phases at a
transition redshift of the order unity whose value in principle depends
on the cosmology as well as on the assumed gravitational theory. Since
cosmological accelerating models endowed with a transition redshift are
extremely degenerated, in principle, it is interesting to know whether the
value of itself can be observationally used as a new cosmic
discriminator. After a brief discussion of the potential dynamic role played by
the transition redshift, it is argued that future observations combining SNe
Ia, the line-of-sight (or "radial") baryon acoustic oscillations, the
differential age of galaxies, as well as the redshift drift of the spectral
lines may tightly constrain , thereby helping to narrow the parameter
space for the most realistic models describing the accelerating Universe.Comment: 12 pages, 5 figures. Some discussions about how to estimate the
transition redshift have been added. New data by Planck and H(z) data have
been mentioned. New references have been adde
Evidence for an inflationary phase transition from the LSS and CMB anisotropy data
In the light of the recent Boomerang and Maxima observations of the CMB which
show an anomalously low second acoustic peak, we reexamine the prediction by
Adams et al (1997) that this would be the consequence of a 'step' in the
primordial spectrum induced by a spontaneous symmetry breaking phase transition
during primordial inflation. We demonstrate that a deviation from
scale-invariance around ~Mpc can simultaneously explain both
the feature identified earlier in the APM galaxy power spectrum as well the
recent CMB anisotropy data, with a baryon density consistent with the BBN
value. Such a break also allows a good fit to the data on cluster abundances
even for a critical density matter-dominated universe with zero cosmological
constant.Comment: 4 pages with 3 figures, LaTeX file using espcrc2.sty to appear on the
Proceedings of "Euroconference on Frontiers in Particle Astrophysics and
Cosmology",Sant Feliu de Guixols,Spain,30th September-5th October of 200
Asteroseismology and Magnetic Cycles
Small cyclic variations in the frequencies of acoustic modes are expected to
be a common phenomenon in solar-like pulsators, as a result of stellar magnetic
activity cycles. The frequency variations observed throughout the solar and
stellar cycles contain information about structural changes that take place
inside the stars as well as about variations in magnetic field structure and
intensity. The task of inferring and disentangling that information is,
however, not a trivial one. In the sun and solar-like pulsators, the direct
effect of the magnetic field on the oscillations might be significantly
important in regions of strong magnetic field (such as solar- / stellar-spots),
where the Lorentz force can be comparable to the gas-pressure gradient. Our aim
is to determine the sun- / stellar-spots effect on the oscillation frequencies
and attempt to understand if this effect contributes strongly to the frequency
changes observed along the magnetic cycle. The total contribution of the spots
to the frequency shifts results from a combination of direct and indirect
effects of the magnetic field on the oscillations. In this first work we
considered only the indirect effect associated with changes in the
stratification within the starspot. Based on the solution of the wave equation
and the variational principle we estimated the impact of these stratification
changes on the oscillation frequencies of global modes in the sun and found
that the induced frequency shifts are about two orders of magnitude smaller
than the frequency shifts observed over the solar cycle.Comment: 4 pages, 6 figures, ESF Conference: The Modern Era of Helio- and
Asteroseismology, to be published on 3 December 2012 at Astronomische
Nachrichten 333, No. 10, 1032-103
Gravitational waves in the generalized Chaplygin gas model
The consequences of taking the generalized Chaplygin gas as the dark energy
constituent of the Universe on the gravitational waves are studied and the
spectrum obtained from this model, for the flat case, is analyzed. Besides its
importance for the study of the primordial Universe, the gravitational waves
represent an additional perspective (besides the CMB temperature and
polarization anisotropies) to evaluate the consistence of the different dark
energy models and establish better constraints to their parameters. The
analysis presented here takes this fact into consideration to open one more
perspective of verification of the generalized Chapligin gas model
applicability. Nine particular cases are compared: one where no dark energy is
present; two that simulate the -CDM model; two where the gas acts like
the traditional Chaplygin gas; and four where the dark energy is the
generalized Chaplygin gas. The different spectra permit to distinguish the
-CDM and the Chaplygin gas scenarios.Comment: Latex file, 9 pages, 11 figures eps forma
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