18 research outputs found
The cosmic ray positron excess and neutralino dark matter
Using a new instrument, the HEAT collaboration has confirmed the excess of
cosmic ray positrons that they first detected in 1994. We explore the
possibility that this excess is due to the annihilation of neutralino dark
matter in the galactic halo. We confirm that neutralino annihilation can
produce enough positrons to make up the measured excess only if there is an
additional enhancement to the signal. We quantify the `boost factor' that is
required in the signal for various models in the Minimal Supersymmetric
Standard Model parameter space, and study the dependence on various parameters.
We find models with a boost factor greater than 30. Such an enhancement in the
signal could arise if we live in a clumpy halo. We discuss what part of
supersymmetric parameter space is favored (in that it gives the largest
positron signal), and the consequences for other direct and indirect searches
of supersymmetric dark matter.Comment: 11 pages, 6 figures, matches published version (PRD
Constraining the dark energy with galaxy clusters X-ray data
The equation of state characterizing the dark energy component is constrained
by combining Chandra observations of the X-ray luminosity of galaxy clusters
with independent measurements of the baryonic matter density and the latest
measurements of the Hubble parameter as given by the HST key project. By
assuming a spatially flat scenario driven by a "quintessence" component with an
equation of state we place the following limits on the
cosmological parameters and : (i) and (1) if the
equation of state of the dark energy is restricted to the interval (\emph{usual} quintessence) and (ii) and
() if violates the null energy condition and assume values (\emph{extended} quintessence or ``phantom'' energy). These results are in
good agreement with independent studies based on supernovae observations,
large-scale structure and the anisotropies of the cosmic background radiation.Comment: 6 pages, 4 figures, LaTe
Solar Wakes of Dark Matter Flows
We analyze the effect of the Sun's gravitational field on a flow of cold dark
matter (CDM) through the solar system in the limit where the velocity
dispersion of the flow vanishes. The exact density and velocity distributions
are derived in the case where the Sun is a point mass. The results are extended
to the more realistic case where the Sun has a finite size spherically
symmetric mass distribution. We find that regions of infinite density, called
caustics, appear. One such region is a line caustic on the axis of symmetry,
downstream from the Sun, where the flow trajectories cross. Another is a
cone-shaped caustic surface near the trajectories of maximum scattering angle.
The trajectories forming the conical caustic pass through the Sun's interior
and probe the solar mass distribution, raising the possibility that the solar
mass distribution may some day be measured by a dark matter detector on Earth.
We generalize our results to the case of flows with continuous velocity
distributions, such as that predicted by the isothermal model of the Milky Way
halo.Comment: 30 pages, 8 figure
Probing Dark Energy with Supernovae: Exploiting Complementarity with the Cosmic Microwave Background
A primary goal for cosmology and particle physics over the coming decade will
be to unravel the nature of the dark energy that drives the accelerated
expansion of the Universe. In particular, determination of the
equation-of-state of dark energy, w equivalent p/rho, and its time variation,
dw/dz, will be critical for developing theoretical understanding of the new
physics behind this phenomenon. Type Ia supernovae (SNe) and cosmic microwave
background (CMB) anisotropy are each sensitive to the dark energy
equation-of-state. SNe alone can determine w(z) with some precision, while CMB
anisotropy alone cannot because of a strong degeneracy between the matter
density Omega_M and w. However, we show that the Planck CMB mission can
significantly improve the power of a deep SNe survey to probe w and especially
dw/dz. Because CMB constraints are nearly orthogonal to SNe constraints in the
Omega_M-w plane, for constraining w(z) Planck is more useful than precise
determination of Omega_M. We discuss how the CMB/SNe complementarity impacts
strategies for the redshift distribution of a supernova survey to determine
w(z) and conclude that a well-designed sample should include a substantial
number of supernovae out to redshifts z ~ 2.Comment: More discussion of CMB systematics and many new references added.
Matches the PRD versio
Higher Dimensional Cosmological Implications Of A Decay Law For Term : Expressions For Some Observable Quantities
Implications of cosmological model with a cosmological term of the form
, where is a constant, are
analyzed in multidimensional space time. The proper distance, the luminosity
distance-redshift, the angular diameter distance-redshift, and look back
time-redshift for the model are presented. It has been shown that such models
are found to be compatible with the recent observations. This work has thus
generalized to higher dimensions the well-know result in four dimensional space
time. It is found that there may be significant difference in principle at
least,from the analogous situation in four dimensional space time.Comment: 11 pages, no figur
Non-minimal Maxwell-Modified Gauss-Bonnet Cosmologies: Inflation and Dark Energy
In this paper we show that power-law inflation can be realized in non-minimal
gravitational coupling of electromagnetic field with a general function of
Gauss-Bonnet invariant. Such a non-minimal coupling may appear due to quantum
corrections. We also consider modified Maxwell- gravity in which
non-minimal coupling between electromagnetic field and occur in the
framework of modified Gauss-Bonnet gravity. It is shown that inflationary
cosmology and late-time accelerated expansion of the universe are possible in
such a theory.Comment: 10 pages, no figur
Cosmological consequences of a Chaplygin gas dark energy
A combination of recent observational results has given rise to what is
currently known as the dark energy problem. Although several possible
candidates have been extensively discussed in the literature to date the nature
of this dark energy component is not well understood at present. In this paper
we investigate some cosmological implications of another dark energy candidate:
an exotic fluid known as the Chaplygin gas, which is characterized by an
equation of state , where is a positive constant. By assuming
a flat scenario driven by non-relativistic matter plus a Chaplygin gas dark
energy we study the influence of such a component on the statistical properties
of gravitational lenses. A comparison between the predicted age of the universe
and the latest age estimates of globular clusters is also included and the
results briefly discussed. In general, we find that the behavior of this class
of models may be interpreted as an intermediary case between the standard and
CDM scenarios.Comment: 7 pages, 5 figures, to appear in Phys. Rev.