796 research outputs found
High-contrast Imaging from Space: Speckle Nulling in a Low Aberration Regime
High-contrast imaging from space must overcome two major noise sources to
successfully detect a terrestrial planet angularly close to its parent star:
photon noise from diffracted star light, and speckle noise from star light
scattered by instrumentally-generated wavefront perturbation. Coronagraphs
tackle only the photon noise contribution by reducing diffracted star light at
the location of a planet. Speckle noise should be addressed with
adaptative-optics systems. Following the tracks of Malbet, Yu and Shao (1995),
we develop in this paper two analytical methods for wavefront sensing and
control that aims at creating dark holes, i.e. areas of the image plane cleared
out of speckles, assuming an ideal coronagraph and small aberrations. The first
method, speckle field nulling, is a fast FFT-based algorithm that requires the
deformable-mirror influence functions to have identical shapes. The second
method, speckle energy minimization, is more general and provides the optimal
deformable mirror shape via matrix inversion. With a NxN deformable mirror, the
size of matrix to be inverted is either N^2xN^2 in the general case, or only
NxN if influence functions can be written as the tensor product of two
one-dimensional functions. Moreover, speckle energy minimization makes it
possible to trade off some of the dark hole area against an improved contrast.
For both methods, complex wavefront aberrations (amplitude and phase) are
measured using just three images taken with the science camera (no dedicated
wavefront sensing channel is used), therefore there are no non-common path
errors. We assess the theoretical performance of both methods with numerical
simulations, and find that these speckle nulling techniques should be able to
improve the contrast by several orders of magnitude.Comment: 31 pages, 8 figures, 1 table. Accepted for publication in ApJ (should
appear in February 2006
What is the Homogeneity of our Universe Telling Us?
The universe we observe is homogeneous on super-horizon scales, leading to
the ``cosmic homogeneity problem''. Inflation alleviates this problem but
cannot solve it within the realm of conservative extrapolations of classical
physics. A probabilistic solution of the problem is possible but is subject to
interpretational difficulties. A genuine deterministic solution of the
homogeneity problem requires radical departures from known physics.Comment: 6 pages. Awarded Honorable Mention in the 1999 Gravity Research
Foundation Essay Competitio
Curing singularities: From the big bang to black holes
Singular spacetimes are a natural prediction of Einstein's theory. Most
memorable are the singular centers of black holes and the big bang. However,
dilatonic extensions of Einstein's theory can support nonsingular spacetimes.
The cosmological singularities can be avoided by dilaton driven inflation.
Furthermore, a nonsingular black hole can be constructed in two dimensions.Comment: To appear as a brief report in Phys. Rev.
Recoil Effects in Microwave Ramsey Spectroscopy
We present a theory of recoil effects in two zone Ramsey spectroscopy,
particularly adapted to microwave frequency standards using laser cooled atoms.
We describe the atoms by a statistical distribution of Gaussian wave packets
which enables us to derive and quantify effects that are related to the
coherence properties of the atomic source and that have not been considered
previously. We show that, depending on the experimental conditions, the
expected recoil frequency shift can be partially cancelled by these effects
which can be significant at microwave wavelengths whilst negligible at optical
ones. We derive analytical expressions for the observed interference signal in
the weak field approximation, and numerical results for realistic caesium
fountain parameters. In the near future Cs and Rb fountain clocks are expected
to reach uncertainties which are of the same order of magnitude (10^{-16}) as
first estimates of the recoil shift at microwave frequencies. We show, however,
that the partial cancellation predicted by the complete theory presented here
leads to frequency shifts which are up to an order of magnitude smaller.
Nonetheless observation of the microwave recoil shift should be possible under
particular experimental conditions (increased microwave power, variation of
atomic temperature and launching height etc.). We hope that the present paper
can provide some guidance for such experiments that would test the underlying
theory and its assumptions, which in turn is essential for the next generation
of microwave frequency standards.Comment: to be submitted to Phys. Rev.
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
Violations of the Weak Energy Condition in Inflating Spacetimes
We argue that many future-eternal inflating spacetimes are likely to violate
the weak energy condition. It is possible that such spacetimes may not enforce
any of the known averaged conditions either. If this is indeed the case, it may
open the door to constructing non-singular, past-eternal inflating cosmologies.
Simple non-singular models are, however, unsatisfactory, and it is not clear if
satisfactory models can be built that solve the problem of the initial
singularity.Comment: 18 pages, 1 figure (which emerges automatically if you use dvips
On T-Duality in Brane Gas Cosmology
In the context of homogeneous and isotropic superstring cosmology, the
T-duality symmetry of string theory has been used to argue that for a
background space-time described by dilaton gravity with strings as matter
sources, the cosmological evolution of the Universe will be nonsingular. In
this Letter we discuss how T-duality extends to brane gas cosmology, an
approximation in which the background space-time is again described by dilaton
gravity with a gas of branes as a matter source. We conclude that the arguments
for nonsingular cosmological evolution remain valid.Comment: 8 pages, Appendix adde
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
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
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