1,813 research outputs found
A conceptual framework for transportation research at Mississippi Test Facility of NASA
Constructing models of transportation system in U
Is cosmic acceleration slowing down?
We investigate the course of cosmic expansion in its `recent past' using the
Constitution SN Ia sample (which includes CfA data at low redshifts), jointly
with signatures of baryon acoustic oscillations (BAO) in the galaxy
distribution and fluctuations in the cosmic microwave background (CMB). Earlier
SN Ia data sets could not address this issue because of a paucity of data at
low redshifts. Allowing the equation of state of dark energy (DE) to vary, we
find that a coasting model of the universe (q_0=0) fits the data about as well
as LCDM. This effect, which is most clearly seen using the recently introduced
`Om' diagnostic, corresponds to an increase of Om(z) and q(z) at redshifts z
\lleq 0.3. In geometrical terms, this suggests that cosmic acceleration may
have already peaked and that we are currently witnessing its slowing down. The
case for evolving DE strengthens if a subsample of the Constitution set
consisting of SNLS+ESSENCE+CfA SN Ia data is analysed in combination with
BAO+CMB using the same statistical methods. The effect we observe could
correspond to DE decaying into dark matter (or something else). A toy model
which mimics this process agrees well with the combined SN Ia+BAO+CMB data.Comment: 6 pages, 5 figures, presentation expanded, results for a new
subsample of the Constitution set are added, new BAO data are accounted for,
main results unchange
Two new diagnostics of dark energy
We introduce two new diagnostics of dark energy (DE). The first, Om, is a
combination of the Hubble parameter and the cosmological redshift and provides
a "null test" of dark energy being a cosmological constant. Namely, if the
value of Om(z) is the same at different redshifts, then DE is exactly
cosmological constant. The slope of Om(z) can differentiate between different
models of dark energy even if the value of the matter density is not accurately
known. For DE with an unevolving equation of state, a positive slope of Om(z)
is suggestive of Phantom (w < -1) while a negative slope indicates Quintessence
(w > -1). The second diagnostic, "acceleration probe"(q-probe), is the mean
value of the deceleration parameter over a small redshift range. It can be used
to determine the cosmological redshift at which the universe began to
accelerate, again without reference to the current value of the matter density.
We apply the "Om" and "q-probe" diagnostics to the Union data set of type Ia
supernovae combined with recent data from the cosmic microwave background
(WMAP5) and baryon acoustic oscillations.Comment: 14 pages, 9 figures. Some new results and an additional reference.
Main conclusions unchanged. Matches published versio
Pseudo-Dirac Neutrino Scenario: Cosmic Neutrinos at Neutrino Telescopes
Within the "pseudo-Dirac" scenario for massive neutrinos the existence of
sterile neutrinos which are almost degenerate in mass with the active ones is
hypothesized. The presence of these sterile neutrinos can affect the flavor
composition of cosmic neutrinos arriving at Earth after traveling large
distances from astrophysical objects. We examine the prospects of neutrino
telescopes such as IceCube to probe the very tiny mass squared differences
10^(-12) eV^2<\Delta m^2<10^(-19) eV^2, by analyzing the ratio of -track
events to shower-like events. Considering various sources of uncertainties
which enter this analysis, we examine the capability of neutrino telescopes to
verify the validity of the pseudo-Dirac neutrino scenario and especially to
discriminate it from the conventional scenario with no sterile neutrino. We
also discuss the robustness of our results with respect to the uncertainties in
the initial flavor ratio of neutrinos at the source.Comment: 24 pages, 5 figure
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