2,810 research outputs found
Generalized Chaplygin Gas Model: Dark Energy - Dark Matter Unification and CMBR Constraints
The generalized Chaplygin gas (GCG) model allows for an unified description
of the recent accelerated expansion of the Universe and the evolution of energy
density perturbations. This dark energy - dark matter unification is achieved
through an exotic background fluid whose equation of state is given by , where is a positive constant and .
Stringent constraints on the model parameters can be obtained from recent WMAP
and BOOMERanG bounds on the locations of the first few peaks and troughs of the
Cosmic Microwave Background Radiation (CMBR) power spectrum as well as SNe Ia
data.Comment: 9 pages, 2 figures; essay selected for an honorable mention by the
Gravity Research Foundation, 200
Nonlinear evolution of dark matter and dark energy in the Chaplygin-gas cosmology
The hypothesis that dark matter and dark energy are unified through the
Chaplygin gas is reexamined. Using generalizations of the spherical model which
incorporate effects of the acoustic horizon we show that an initially
perturbative Chaplygin gas evolves into a mixed system containing cold dark
matter-like gravitational condensate.Comment: 11 pages, 3 figures, substantial revision, title changed, content
changed, added references, to appear in JCA
Role of Modified Chaplygin Gas as a Dark Energy Model in Collapsing Spherically Symmetric Cloud
In this work, gravitational collapse of a spherical cloud, consists of both
dark matter and dark energy in the form of modified Chaplygin gas is studied.
It is found that dark energy alone in the form of modified Chaplygin gas forms
black hole. Also when both components of the fluid are present then the
collapse favors the formation of black hole in cases the dark energy dominates
over dark matter. The conclusion is totally opposite to the usually known
results.Comment: 7 Latex Pages, RexTex style, No figure
Constraints on coupling constant between dark energy and dark matter
We have investigated constraints on the coupling between dark matter and the
interacting Chaplygin gas. Our results indicate that the coupling constant
between these two entities can take arbitrary values, which can be either
positive or negative, thus giving arbitrary freedom to the inter-conversion
between Chaplygin gas and dark matter. Thus our results indicate that the
restriction on the coupling constant occurs as a very special case. Our
analysis also supports the existence of phantom energy under certain conditions
on the coupling constant.Comment: 16 Pages, 3 figure
How does Inflation Depend Upon the Nature of Fluids Filling Up the Universe in Brane World Scenario
By constructing different parameters which are able to give us the
information about our universe during inflation,(specially at the start and the
end of the inflationary universe) a brief idea of brane world inflation is
given in this work. What will be the size of the universe at the end of
inflation,i.e.,how many times will it grow than today's size is been speculated
and analysed thereafter. Different kinds of fluids are taken to be the matter
inside the brane. It is observed that in the case of highly positive pressure
grower gas like polytropic,the size of the universe at the end of inflation is
comparitively smaller. Whereas for negative pressure creators (like chaplygin
gas) this size is much bigger. Except thse two cases, inflation has been
studied for barotropic fluid and linear redshift parametrization too. For them the size of the universe after
inflation is much more high. We also have seen that this size does not depend
upon the potential energy at the end of the inflation. On the contrary, there
is a high impact of the initial potential energy upon the size of inflation.Comment: 20 page
Muon conversion to electron in nuclei in type-I seesaw models
We compute the muon to electron conversion in the type-I seesaw model, as a
function of the right-handed neutrino mixings and masses. The results are
compared with previous computations in the literature. We determine the
definite predictions resulting for the ratios between the muon to electron
conversion rate for a given nucleus and the rate of two other processes which
also involve a mu-e flavour transition: mu -> e gamma and mu -> eee. For a
quasi-degenerate mass spectrum of right-handed neutrino masses -which is the
most natural scenario leading to observable rates- those ratios depend only on
the seesaw mass scale, offering a quite interesting testing ground. In the case
of sterile neutrinos heavier than the electroweak scale, these ratios vanish
typically for a mass scale of order a few TeV. Furthermore, the analysis
performed here is also valid down to very light masses. It turns out that
planned mu -> e conversion experiments would be sensitive to masses as low as 2
MeV. Taking into account other experimental constraints, we show that future mu
-> e conversion experiments will be fully relevant to detect or constrain
sterile neutrino scenarios in the 2 GeV-1000 TeV mass range.Comment: 32 pages 14 figures, references added and some minor precisions;
results unchange
Phenomenology of Light Sneutrino Dark Matter in cMSSM/mSUGRA with Inverse Seesaw
We study the possibility of a light Dark Matter (DM) within a constrained
Minimal Supersymmetric Standard Model (cMSSM) framework augmented by a SM
singlet-pair sector to account for the non-zero neutrino masses by inverse
seesaw mechanism. Working within a 'hybrid' scenario with the MSSM sector fixed
at high scale and the singlet neutrino sector at low scale, we find that,
contrary to the case of the usual cMSSM where the neutralino DM cannot be very
light, we can have a light sneutrino DM with mass below 100 GeV satisfying all
the current experimental constraints from cosmology, collider as well as
low-energy experiments. We also note that the supersymmetric inverse seesaw
mechanism with sneutrino as the lightest supersymmetric partner can have
enhanced same-sign dilepton final states with large missing transverse energy
(mET) coming from the gluino- and squark-pair as well as the squark-gluino
associated productions and their cascade decay through charginos. We present a
collider study for the same-sign dilepton+jets+mET signal in this scenario and
propose some distinctions with the usual cMSSM. We also comment on the
implications of such a light DM scenario on the invisible decay width of an 125
GeV Higgs boson.Comment: 24 pages, 4 figures, 7 tables; matches published versio
Gravitational Collapse in Higher Dimensional Husain Space-Time
We investigate exact solution in higher dimensional Husain model for a null
fluid source with pressure and density are related by the following
relations (i) , (ii) (variable
modified Chaplygin) and (iii) (polytropic). We have studied
the nature of singularity in gravitational collapse for the above equations of
state and also for different choices of the of the parameters and
namely, (i) , constant (generalized Chaplygin), (ii) constant
(modified Chaplygin). It is found that the nature of singularity is independent
of these choices of different equation of state except for variable Chaplygin
model. Choices of various parameters are shown in tabular form. Finally,
matching of Szekeres model with exterior Husain space-time is done.Comment: 12 latex pages, No figure, RevTex styl
A realistic pattern of fermion masses from a five-dimensional SO(10) model
We provide a unified description of fermion masses and mixing angles in the
framework of a supersymmetric grand unified SO(10) model with anarchic Yukawa
couplings of order unity. The space-time is five dimensional and the extra flat
spatial dimension is compactified on the orbifold ,
leading to Pati-Salam gauge symmetry on the boundary where Yukawa interactions
are localised. The gauge symmetry breaking is completed by means of a rather
economic scalar sector, avoiding the doublet-triplet splitting problem. The
matter fields live in the bulk and their massless modes get exponential
profiles, which naturally explain the mass hierarchy of the different fermion
generations. Quarks and leptons properties are naturally reproduced by a
mechanism, first proposed by Kitano and Li, that lifts the SO(10) degeneracy of
bulk masses in terms of a single parameter. The model provides a realistic
pattern of fermion masses and mixing angles for large values of . It
favours normally ordered neutrino mass spectrum with the lightest neutrino mass
below 0.01 eV and no preference for leptonic CP violating phases. The right
handed neutrino mass spectrum is very hierarchical and does not allow for
thermal leptogenesis. We analyse several variants of the basic framework and
find that the results concerning the fermion spectrum are remarkably stable.Comment: 30 pages, 7 figures, 4 table
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