617 research outputs found
Inflationary spacetimes are not past-complete
Many inflating spacetimes are likely to violate the weak energy condition, a
key assumption of singularity theorems. Here we offer a simple kinematical
argument, requiring no energy condition, that a cosmological model which is
inflating -- or just expanding sufficiently fast -- must be incomplete in null
and timelike past directions. Specifically, we obtain a bound on the integral
of the Hubble parameter over a past-directed timelike or null geodesic. Thus
inflationary models require physics other than inflation to describe the past
boundary of the inflating region of spacetime.Comment: We improve the basic argument to apply to a wider class of
spacetimes, use a better title and add a discussion of cyclic models. 4
pages, 1 figure, RevTe
Updated results on prototype chalcogenide fibers for 10-um wavefront spatial filtering
The detection of terrestrial planets by Darwin/TPF missions will require
extremely high quality wavefronts. Single-mode fibers have proven to be
powerful beam cleaning components in the near-infrared, but are currently not
available in the mid-infrared where they would be critically needed for
Darwin/TPF. In this paper, we present updated measurements on the prototype
chalcogenide fibers we are developing for the purpose of mid-infrared spatial
filtering. We demonstrate the guiding property of our 3rd generation component
and we characterize its filtering performances on a 4 mm length: the far-field
radiation pattern matches a Gaussian profile at the level of 3% rms and 13%
pk-pk.Comment: 4 pages, 5 figures, to appear in the proceedings of the conference
"Toward Other Earths, Darwin/TPF and the search for extrasolar terrestrial
planets", held in Heidelberg, Germany, 22-25 April 2003, ESA SP-53
Neutrino oscillations in a stochastic model for space-time foam
We study decoherence models for flavour oscillations in four-dimensional
stochastically fluctuating space times and discuss briefly the sensitivity of
current neutrino experiments to such models. We pay emphasis on demonstrating
the model dependence of the associated decoherence-induced damping coefficients
in front of the oscillatory terms in the respective transition probabilities
between flavours. Within the context of specific models of foam, involving
point-like D-branes and leading to decoherence-induced damping which is
inversely proportional to the neutrino energies, we also argue that future
limits on the relevant decoherence parameters coming from TeV astrophysical
neutrinos, to be observed in ICE-CUBE, are not far from theoretically expected
values with Planck mass suppression. Ultra high energy neutrinos from Gamma Ray
Bursts at cosmological distances can also exhibit in principle sensitivity to
such effects.Comment: 12 pages RevTex4, no figure
VINCI / VLTI observations of Main Sequence stars
Main Sequence (MS) stars are by far the most numerous class in the Universe.
They are often somewhat neglected as they are relatively quiet objects (but
exceptions exist), though they bear testimony of the past and future of our
Sun. An important characteristic of the MS stars, particularly the solar-type
ones, is that they host the large majority of the known extrasolar planets.
Moreover, at the bottom of the MS, the red M dwarfs pave the way to
understanding the physics of brown dwarfs and giant planets. We have measured
very precise angular diameters from recent VINCI/VLTI interferometric
observations of a number of MS stars in the K band, with spectral types between
A1V and M5.5V. They already cover a wide range of effective temperatures and
radii. Combined with precise Hipparcos parallaxes, photometry, spectroscopy as
well as the asteroseismic information available for some of these stars, the
angular diameters put strong constraints on the detailed models of these stars,
and therefore on the physical processes at play.Comment: 5 pages, 3 figures. To appear in the Proceedings of IAU Symposium
219, "Stars as Suns", Editors A. Benz & A. Dupree, Astronomical Society of
the Pacifi
Non-Singular Charged Black Hole Solution for Non-Linear Source
A non-singular exact black hole solution in General Relativity is presented.
The source is a non-linear electromagnetic field, which reduces to the Maxwell
theory for weak field. The solution corresponds to a charged black hole with
|q| \leq 2s_c m \approx 0.6 m, having metric, curvature invariants, and
electric field bounded everywhere.Comment: 3 pages, RevTe
Regular Black Hole in General Relativity Coupled to Nonlinear Electrodynamics
The first regular exact black hole solution in General Relativity is
presented. The source is a nonlinear electrodynamic field satisfying the weak
energy condition, which in the limit of weak field becomes the Maxwell field.
The solution corresponds to a charged black hole with |q| \leq 2 s_c m \approx
0.6 m, having the metric, the curvature invariants, and the electric field
regular everywhere.Comment: 5 pages, RevTex, 6 figure
Rationale for the use of color information on Eddington
For the Eddington mission, the intrinsic stellar variability can be a major
source of noise in the detection of extrasolar planets by the transit method.
We derive that most detections of terrestrial planets (1-2 R_Earth) will occur
around G or K stars with 15-16th magnitude. When these stars are 7-12 times
more variable than the Sun on a 10 hour timescale, we demonstrate that the
detection can be performed with a higher S/N provided composite lightcurves
obtained with the combination of two colors are used instead of white ones. The
level of 10 hour variability for K stars is quite uncertain. We make two
"guess-estimates" of it and find that it could be several times larger than the
solar value. If these estimates were relevant, the color information would not
provide a significant advantage. Although we do not demonstrate a need for
colors, we point out the risk of an unpleasant surprise regarding the 10 hour
stellar variability. Indeed, there is presently no qualified proxy for this
variability. Besides, if Eddington were designed to provide this information at
the cost of added complexity but not sensitivity, white photometry by channel
summation would still be as efficient. Considering the risk that 10 hour
variability is higher than estimated, the Precaution Imperative points to a
study of practical implementations of photometry in different colors before
taking irreversible decisions about the Eddington instrument.Comment: Submitted to A&A as a Research Not
Open and Closed Universes, Initial Singularities and Inflation
The existence of initial singularities in expanding universes is proved
without assuming the timelike convergence condition. The assumptions made in
the proof are ones likely to hold both in open universes and in many closed
ones. (It is further argued that at least some of the expanding closed
universes that do not obey a key assumption of the theorem will have initial
singularities on other grounds.) The result is significant for two reasons:
(a)~previous closed-universe singularity theorems have assumed the timelike
convergence condition, and (b)~the timelike convergence condition is known to
be violated in inflationary spacetimes. An immediate consequence of this
theorem is that a recent result on initial singularities in open,
future-eternal, inflating spacetimes may now be extended to include many closed
universes. Also, as a fringe benefit, the time-reverse of the theorem may be
applied to gravitational collapse.Comment: 27 pages, Plain TeX (figures are embedded in the file itself and they
will emerge if it is processed according to the instructions at the top of
the file
Eternal inflation and the initial singularity
It is shown that a physically reasonable spacetime that is eternally
inflating to the future must possess an initial singularity.Comment: 11 pages, Tufts University cosmology preprin
Modeling of Alpha Cen and Procyon using VLTI observations
We present a novel approach to model the nearby stars Alpha Cen A & B and
Procyon A using asteroseismic and interferometric constraints. Using the VINCI
instrument installed at the VLT Interferometer (VLTI), the angular diameters of
the Alpha Centauri system were measured with a relative precision of 0.2% and
0.6%, respectively. From these values, we derive linear radii of R[A] = 1.224
+/- 0.003 R_sun and R[B] = 0.863 +/- 0.005 R_sun. These radii are in excellent
agreement with the models of Thevenin et al. (2002), that use asteroseismic
frequencies as constraints (Bouchy & Carrier 2001; Bouchy & Carrier 2002). With
the same instrument, we also measured the angular diameter of Procyon A. Using
the Hipparcos parallax, we obtain a linear radius of 2.048 +/- 0.025 R_sun. We
use this result together with spectroscopic and photometric constraints to
model this star with the CESAM code. We also computed the adiabatic oscillation
spectrum of our model of Procyon A, giving a mean large frequency separation of
Delta nu_0 = 54.8 uHz, in agreement with the seismic observations by Martic et
al. (2001). Our model favours a mass around 1.4 M_sun for Procyon A.Comment: 8 pages, 5 figures, poster presented at the JD 12 of the IAU General
Assembly, Sydney 200
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