13,778 research outputs found
Equilibrium Temperature Structure in the Mesosphere and Lower Thermosphere
Radiative equilibrium temperature structure in earth mesosphere and lower thermospher
Light-like Signals in General relativity and Cosmology
The modelling of light-like signals in General Relativity taking the form of
impulsive gravitational waves and light-like shells of matter is examined.
Systematic deductions from the Bianchi identities are made. These are based
upon Penrose's hierarchical classification of the geometry induced on the null
hypersurface history of the surface by its imbedding in the space-times to the
future and to the past of it. The signals are not confined to propagate in a
vacuum and thus their interaction with matter (a burst of radiation propagating
through a cosmic fluid, for example) is also studied. Results are accompanied
by illustrative examples using cosmological models, vacuum space-times, the de
sitter univers and Minkowskian space-time.Comment: 21 pages, latex, no figure
Recommended from our members
Estimating drizzle drop size and precipitation rate using two-colour lidar measurements
A method to estimate the size and liquid water content of drizzle drops using lidar measurements at two wavelengths is described. The method exploits the differential absorption of infrared light by liquid water at 905 nm and 1.5 μm, which leads to a different backscatter cross section for water drops larger than ≈50 μm. The ratio of backscatter measured from drizzle samples below cloud base at these two wavelengths (the colour ratio) provides a measure of the median volume drop diameter D0. This is a strong effect: for D0=200 μm, a colour ratio of ≈6 dB is predicted. Once D0 is known, the measured backscatter at 905 nm can be used to calculate the liquid water content (LWC) and other moments of the drizzle drop distribution.
The method is applied to observations of drizzle falling from stratocumulus and stratus clouds. High resolution (32 s, 36 m) profiles of D0, LWC and precipitation rate R are derived. The main sources of error in the technique are the need to assume a value for the dispersion parameter μ in the drop size spectrum (leading to at most a 35% error in R) and the influence of aerosol returns on the retrieval (≈10% error in R for the cases considered here). Radar reflectivities are also computed from the lidar data, and compared to independent measurements from a colocated cloud radar, offering independent validation of the derived drop size distributions
Probing the Reionization History of the Universe using the Cosmic Microwave Background Polarization
The recent discovery of a Gunn--Peterson (GP) trough in the spectrum of the
redshift 6.28 SDSS quasar has raised the tantalizing possibility that we have
detected the reionization of the universe. However, a neutral fraction (of
hydrogen) as small as 0.1% is sufficient to cause the GP trough, hence its
detection alone cannot rule out reionization at a much earlier epoch. The
Cosmic Microwave Background (CMB) polarization anisotropy offers an alternative
way to explore the dark age of the universe. We show that for most models
constrained by the current CMB data and by the discovery of a GP trough
(showing that reionization occurred at z > 6.3), MAP can detect the
reionization signature in the polarization power spectrum. The expected 1-sigma
error on the measurement of the electron optical depth is around 0.03 with a
weak dependence on the value of that optical depth. Such a constraint on the
optical depth will allow MAP to achieve a 1-sigma error on the amplitude of the
primordial power spectrum of 6%. MAP with two years (Planck with one year) of
observation can distinguish a model with 50% (6%) partial ionization between
redshifts of 6.3 and 20 from a model in which hydrogen was completely neutral
at redshifts greater than 6.3. Planck will be able to distinguish between
different reionization histories even when they imply the same optical depth to
electron scattering for the CMB photons.Comment: ApJ version. Added Figure 2 and reference
A Class of Collisions of Plane Impulsive Light--Like Signals in General Relativity
We present a systematic study of collisions of homogeneous, plane--fronted,
impulsive light--like signals which do not interact after head--on collision.
For the head--on collision of two such signals, six real parameters are
involved, three from each of the incoming signals. We find two necessary
conditions to be satisfied by these six parameters for the signals to be
non--interacting after collision. We then solve the collision problem in
general when these necessary conditions hold. After collision the two signals
focus each other at Weyl curvature singularities on each others signal front.
Our family of solutions contains some known collision solutions as special
cases.Comment: 14 pages, late
Space missions to detect the cosmic gravitational-wave background
It is thought that a stochastic background of gravitational waves was
produced during the formation of the universe. A great deal could be learned by
measuring this Cosmic Gravitational-wave Background (CGB), but detecting the
CGB presents a significant technological challenge. The signal strength is
expected to be extremely weak, and there will be competition from unresolved
astrophysical foregrounds such as white dwarf binaries. Our goal is to identify
the most promising approach to detect the CGB. We study the sensitivities that
can be reached using both individual, and cross-correlated pairs of space based
interferometers. Our main result is a general, coordinate free formalism for
calculating the detector response that applies to arbitrary detector
configurations. We use this general formalism to identify some promising
designs for a GrAvitational Background Interferometer (GABI) mission. Our
conclusion is that detecting the CGB is not out of reach.Comment: 22 pages, 7 figures, IOP style, References Adde
Mapping the Cosmic Web with Ly-alpha Emission
We use a high-resolution cosmological simulation to predict the distribution
of HI Ly-alpha emission from the low-redshift (z<0.5) intergalactic medium
(IGM). Our simulation can be used to reliably compute the emission from
optically thin regions of the IGM but not that of self-shielded gas. We
therefore consider several models that bracket the expected emission from
self-shielded regions. Most galaxies are surrounded by extended (>10^2 kpc)
``coronae'' of optically thin gas with Ly-alpha surface brightness close to the
expected background. Most of these regions contain smaller cores of dense, cool
gas. Unless self-shielded gas is able to cool to T<10^4.1 K, these cores are
much brighter than the background. The Ly-alpha coronae represent ``cooling
flows'' of IGM gas accreting onto galaxies. We also estimate the number of
Ly-alpha photons produced through the reprocessing of stellar ionizing
radiation in the interstellar medium of galaxies; while this mechanism is
responsible for the brightest Ly-alpha emission, it occurs on small physical
scales and can be separated using high-resolution observations. In all cases,
we find that Ly-alpha emitters are numerous (with a space density ~0.1 h^3
Mpc^-3) and closely trace the filamentary structure of the IGM, providing a new
way to map gas inside the cosmic web.Comment: 4 pages, 3 figures, accepted by ApJ
Gravitational Waves from Mesoscopic Dynamics of the Extra Dimensions
Recent models which describe our world as a brane embedded in a higher
dimensional space introduce new geometrical degrees of freedom: the shape
and/or size of the extra dimensions, and the position of the brane. These modes
can be coherently excited by symmetry breaking in the early universe even on
``mesoscopic'' scales as large as 1 mm, leading to detectable gravitational
radiation. Two sources are described: relativistic turbulence caused by a
first-order transition of a radion potential, and Kibble excitation of
Nambu-Goldstone modes of brane displacement. Characteristic scales and spectral
properties are estimated and the prospects for observation by LISA are
discussed. Extra dimensions with scale between 10 \AA and 1 mm, which enter the
3+1-D era at cosmic temperatures between 1 and 1000 TeV, produce backgrounds
with energy peaked at observed frequencies in the LISA band, between
and Hz. The background is detectable above instrument and
astrophysical foregrounds if initial metric perturbations are excited to a
fractional amplitude of or more, a likely outcome for the
Nambu-Goldstone excitations.Comment: Latex, 5 pages, plus one figure, final version to appear in Phys.
Rev. Let
Gravitational waves from the sound of a first order phase transition
We report on the first three-dimensional numerical simulations of first-order phase transitions in the early Universe to include the cosmic fluid as well as the scalar field order parameter. We calculate the gravitational wave (GW) spectrum resulting from the nucleation, expansion, and collision of bubbles of the low-temperature phase, for phase transition strengths and bubble wall velocities covering many cases of interest. We find that the compression waves in the fluid continue to be a source of GWs long after the bubbles have merged, a new effect not taken properly into account in previous modeling of the GW source. For a wide range of models, the main source of the GWs produced by a phase transition is, therefore, the sound the bubbles make
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