18,730 research outputs found
A Concise Introduction to Perturbation Theory in Cosmology
We give a concise, self-contained introduction to perturbation theory in
cosmology at linear and second order, striking a balance between mathematical
rigour and usability. In particular we discuss gauge issues and the active and
passive approach to calculating gauge transformations. We also construct
gauge-invariant variables, including the second order tensor perturbation on
uniform curvature hypersurfaces.Comment: revtex4, 16 pages, 3 figures; v2: minor changes, typos corrected,
reference added, version accepted by CQ
Heat capacity and magnetoresistance in Dy(Co,Si)2 compounds
Magnetocaloric effect and magnetoresistance have been studied in
Dy(Co1-xSix)2 [x=0, 0.075 and 0.15] compounds. Magnetocaloric effect has been
calculated in terms of adiabatic temperatue change (Delta Tad) as well as
isothermal magnetic entropy change (Delta SM) using the heat capacity data. The
maximum values of DeltaSM and DeltaTad for DyCo2 are found to be 11.4 JKg-1K-1
and 5.4 K, respectively. Both DSM and DTad decrease with Si concentration,
reaching a value of 5.4 JKg-1K-1 and 3 K, respectively for x=0.15. The maximum
magnetoresistance is found to about 32% in DyCo2, which decreases with increase
in Si. These variations are explained on the basis of itinerant electron
metamagnetism occurring in these compounds.Comment: Total 8 pages of text and figure
Constraints on the three-fluid model of curvaton decay
A three fluid system describing the decay of the curvaton is studied by
numerical and analytical means. We place constraints on the allowed interaction
strengths between the fluids and initial curvaton density by requiring that the
curvaton decays before nucleosynthesis while nucleosynthesis, radiation-matter
equality and decoupling occur at correct temperatures. We find that with a
continuous, time-independent interaction, a small initial curvaton density is
naturally preferred along with a low reheating temperature. Allowing for a
time-dependent interaction, this constraint can be relaxed. In both cases, a
purely adiabatic final state can be generated, but not without fine-tuning.
Unlike in the two fluid system, the time-dependent interactions are found to
have a small effect on the curvature perturbation itself due to the different
nature of the system. The presence of non-gaussianity in the model is
discussed.Comment: 9 pages, 10 figure
Suppressing Super-Horizon Curvature Perturbations?
We consider the possibility of suppressing superhorizon curvature
perturbations after the end of the ordinary slow-roll inflationary stage. This
is the opposite of the curvaton limit. We assume that large curvature
perturbations are created by the inflaton and investigate if they can be
diluted or suppressed by a second very homogeneous field which starts to
dominate the energy density of the universe shortly after the end of inflation.
We show explicit that the gravitational sourcing of inhomogeneities from the
more inhomogeneous fluid to the more homogeneous fluid makes the suppression
difficult if not impossible to achieve.Comment: 10 pages, 1 figure. Important revision. Conclusions more negativ
Heat capacity and magnetocaloric effect in polycrystalline Gd1-xSmxMn2Si2
We report the magnetocaloric effect in terms of isothermal magnetic entropy
change as well as adiabatic temperature change, calculated using the heat
capacity data. Using the zero field heat capacity data, the magnetic
contribution to the heat capacity has been estimated. The variations in the
magnetocaloric behavior have been explained on the basis of the magnetic
structure of these compounds. The refrigerant capacities have also been
calculated for these compounds
Multiple magnetic transitions and magnetocaloric effect in Gd1-xSmxMn2Ge2 compounds
Magnetic and magnetocaloric properties of polycrystalline samples of
Gd1-xSmxMn2Ge2 have been studied. All the compounds except GdMn2Ge2 show
re-entrant ferromagnetic behavior. Multiple magnetic transitions observed in
these compounds are explained on the basis of the temperature dependences of
the exchange strengths of the rare earth and Mn sublattices. Magnetocaloric
effect is found to be positive at the re-entrant ferromagnetic transition,
whereas it is negative at the antiferro-ferromagnetic transition. In SmMn2Ge2,
the magnetic entropy change associated with the re-entrant transition is found
to decrease with field, which is attributed to the admixture effect of the
crystal field levels. The isothermal magnetic entropy change is found to
decrease with increase in Sm concentration.Comment:
Wigner's little group and BRST cohomology for one-form Abelian gauge theory
We discuss the (dual-)gauge transformations for the gauge-fixed Lagrangian
density and establish their intimate connection with the translation subgroup
T(2) of the Wigner's little group for the free one-form Abelian gauge theory in
four -dimensions (4D) of spacetime. Though the relationship between
the usual gauge transformation for the Abelian massless gauge field and T(2)
subgroup of the little group is quite well-known, such a connection between the
dual-gauge transformation and the little group is a new observation. The above
connections are further elaborated and demonstrated in the framework of
Becchi-Rouet-Stora-Tyutin (BRST) cohomology defined in the quantum Hilbert
space of states where the Hodge decomposition theorem (HDT) plays a very
decisive role.Comment: LaTeX file, 17 pages, Journal-ref. give
Nilpotent Symmetries For Matter Fields In Non-Abelian Gauge Theory: Augmented Superfield Formalism
In the framework of superfield approach to Becchi-Rouet-Stora-Tyutin (BRST)
formalism, the derivation of the (anti-)BRST nilpotent symmetries for the
matter fields, present in any arbitrary interacting gauge theory, has been a
long-standing problem. In our present investigation, the local, covariant,
continuous and off-shell nilpotent (anti-)BRST symmetry transformations for the
Dirac fields are derived in the framework of the augmented
superfield formulation where the four -dimensional (4D) interacting
non-Abelian gauge theory is considered on the six -dimensional
supermanifold parametrized by the four even spacetime coordinates and a
couple of odd elements ( and ) of the Grassmann algebra.
The requirement of the invariance of the matter (super)currents and the
horizontality condition on the (super)manifolds leads to the derivation of the
nilpotent symmetries for the matter fields as well as the gauge- and the
(anti-)ghost fields of the theory in the general scheme of the augmented
superfield formalism.Comment: LaTeX file, 16 pages, printing mistakes in the second paragraph of
`Introduction' corrected, a footnote added, these modifications submitted as
``erratum'' to IJMPA in the final for
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