22 research outputs found
General Relativity, the Cosmological Constant and Modular Forms
Strong field (exact) solutions of the gravitational field equations of
General Relativity in the presence of a Cosmological Constant are investigated.
In particular, a full exact solution is derived within the inhomogeneous
Szekeres-Szafron family of space-time line element with a nonzero Cosmological
Constant. The resulting solution connects, in an intrinsic way, General
Relativity with the theory of modular forms and elliptic curves. The
homogeneous FLRW limit of the above space-time elements is recovered and we
solve exactly the resulting Friedmann Robertson field equation with the
appropriate matter density for generic values of the Cosmological Constant
%Lambda and curvature constant K. A formal expression for the Hubble constant
is derived. The cosmological implications of the resulting non-linear solutions
are systematically investigated. Two particularly interesting solutions i) the
case of a flat universe K=0, Lambda not= 0 and ii) a case with all three
cosmological parameters non-zero, are described by elliptic curves with the
property of complex multiplication and absolute modular invariant j=0 and 1728,
respectively. The possibility that all non-linear solutions of General
Relativity are expressed in terms of theta functions associated with
Riemann-surfaces is discussed.Comment: LaTeX file, 34 pages plus 9 EPS figures, Accepted for Publication in
Classical and Quantum Gravit
On the Stability of Thermonuclear Burning Fronts in Type Ia Supernovae
The propagation of cellularly stabilized thermonuclear flames is investigated
by means of numerical simulations. In Type Ia supernova explosions the
corresponding burning regime establishes at scales below the Gibson length. The
cellular flame stabilization - which is a result of an interplay between the
Landau-Darrieus instability and a nonlinear stabilization mechanism - is
studied for the case of propagation into quiescent fuel as well as interaction
with vortical fuel flows. Our simulations indicate that in thermonuclear
supernova explosions stable cellular flames develop around the Gibson scale and
that deflagration-to-detonation transition is unlikely to be triggered from
flame evolution effects here.Comment: 6 pages, 2 figures, to appear in the proceedings of the IAU
Colloquium 192, "Supernovae (10 years of SN1993J)", 22-26 April 2003,
Valencia, Spain, Eds. J.M. Marcaide and K.W. Weiler, Springer Verla
Determining Reheating Temperature at Colliders with Axino or Gravitino Dark Matter
After a period of inflationary expansion, the
Universe reheated and reached full thermal equilibrium at the reheating
temperature T_R. In this work we point out that, in the context of effective
low-energy supersymmetric models, LHC measurements may allow one to determine
T_R as a function of the mass of the dark matter particle assumed to be either
an axino or a gravitino. An upper bound on their mass may also be derived.Comment: 19 pages, some improvements, JHEP versio
Gravitino Dark Matter in the CMSSM and Implications for Leptogenesis and the LHC
In the framework of the CMSSM we study the gravitino as the lightest
supersymmetric particle and the dominant component of cold dark matter in the
Universe. We include both a thermal contribution to its relic abundance from
scatterings in the plasma and a non--thermal one from neutralino or stau decays
after freeze--out. In general both contributions can be important, although in
different regions of the parameter space. We further include constraints from
BBN on electromagnetic and hadronic showers, from the CMB blackbody spectrum
and from collider and non--collider SUSY searches. The region where the
neutralino is the next--to--lightest superpartner is severely constrained by a
conservative bound from excessive electromagnetic showers and probably
basically excluded by the bound from hadronic showers, while the stau case
remains mostly allowed. In both regions the constraint from CMB is often
important or even dominant. In the stau case, for the assumed reasonable ranges
of soft SUSY breaking parameters, we find regions where the gravitino abundance
is in agreement with the range inferred from CMB studies, provided that, in
many cases, a reheating temperature \treh is large, \treh\sim10^{9}\gev. On
the other side, we find an upper bound \treh\lsim 5\times 10^{9}\gev. Less
conservative bounds from BBN or an improvement in measuring the CMB spectrum
would provide a dramatic squeeze on the whole scenario, in particular it would
strongly disfavor the largest values of \treh\sim 10^{9}\gev. The regions
favored by the gravitino dark matter scenario are very different from standard
regions corresponding to the neutralino dark matter, and will be partly probed
at the LHC.Comment: JHEP version, several improvements and update
Ya.B. Zeldovich and Foundation of the Accretion Theory
© 2018, Pleiades Publishing, Ltd. This brief review is dedicated to academician Yakov Borisovich Zeldovich, the science of his epoch and the creation of modern accretion theory
A fussy revisitation of antiprotons as a tool for Dark Matter searches
Antiprotons are regarded as a powerful probe for Dark Matter (DM) indirect
detection and indeed current data from PAMELA have been shown to lead to
stringent constraints. However, in order to exploit their
constraining/discovery power properly and especially in anticipation of the
exquisite accuracy of upcoming data from AMS, great attention must be put into
effects (linked to their propagation in the Galaxy) which may be perceived as
subleasing but actually prove to be quite relevant. We revisit the computation
of the astrophysical background and of the DM antiproton fluxes fully including
the effects of: diffusive reacceleration, energy losses including tertiary
component and solar modulation (in a force field approximation). We show that
their inclusion can somewhat modify the current bounds, even at large DM
masses, and that a wrong interpretation of the data may arise if they are not
taken into account. The numerical results for the astrophysical background are
provided in terms of fit functions; the results for Dark Matter are
incorporated in the new release of the PPPC4DMID.Comment: v3: small comments and references added, matches journal versio