Gravitational Field of Spherical Branes

The warped solution of Einstein's equations corresponding to the spherical brane in five-dimensional AdS is considered. This metric represents interiors of black holes on both sides of the brane and can provide gravitational trapping of physical fields on the shell. It is found the analytic form of the coordinate transformations from the Schwartschild to co-moving frame that exists only in five dimensions. It is shown that in the static coordinates active gravitational mass of the spherical brane, in agreement with Tolman's formula, is negative, i.e. such objects are gravitationally repulsive.Comment: Minor corrections, 8 pages, the version accepted by Mod. Phys. Lett.

Triaxiality and the determination of the cubic shape parameter K3 from five observables

The absolute and the relative quadrupole shape invariants q3 and K3 provide a model independent measure of triaxiality for beta-rigid nuclei. We will show that one can obtain q3 and K3 from a small number of observables. The approximations which are made will be shown to hold within a few percent both in the rigid triaxial rotor model and the interacting boson model. The shape parameter K3 is given for an exemplary set of nuclei and is translated into effective values of the geometrical deformation parameters beta and gamma.Comment: 16 pages, 4 figure

Mimicking transPlanckian effects in the CMB with conventional physics

We investigate the possibility that fields coupled to the inflaton can influence the primordial spectrum of density perturbations through their coherent motion. For example, the second field in hybrid inflation might be oscillating at the beginning of inflation rather than at the minimum of its potential. Although this effect is washed out if inflation lasts long enough, we note that there can be up to 30 e-foldings of inflation prior to horizon crossing of COBE fluctuations while still giving a potentially visible distortion. Such pumping of the inflaton fluctuations by purely conventional physics can resemble transPlanckian effects which have been widely discussed. The distortions which they make to the CMB could leave a distinctive signature which differs from generic effects like tilting of the spectrum.Comment: 3 pages, 4 figures; presented at PASCOS 03, TIFR, Mumbai, Indi

Can codimension-two branes solve the cosmological constant problem?

It has been suggested that codimension-two braneworlds might naturally explain the vanishing of the 4D effective cosmological constant, due to the automatic relation between the deficit angle and the brane tension. To investigate whether this cancellation happens dynamically, and within the context of a realistic cosmology, we study a codimension-two braneworld with spherical extra dimensions compactified by magnetic flux. Assuming Einstein gravity, we show that when the brane contains matter with an arbitrary equation of state, the 4D metric components are not regular at the brane, unless the brane has nonzero thickness. We construct explicit 6D solutions with thick branes, treating the brane matter as a perturbation, and find that the universe expands consistently with standard Friedmann-Robertson-Walker (FRW) cosmology. The relation between the brane tension and the bulk deficit angle becomes $\Delta=2\pi G_6(\rho-3 p)$ for a general equation of state. However, this relation does not imply a self-tuning of the effective 4D cosmological constant to zero; perturbations of the brane tension in a static solution lead to deSitter or anti-deSitter braneworlds. Our results thus confirm other recent work showing that codimension-two braneworlds in nonsupersymmetric Einstein gravity do not lead to a dynamical relaxation of the cosmological constant, but they leave open the possibility that supersymmetric versions can be compatible with self-tuning.Comment: Revtex4, 17 pages, references added, typos corrected, minor points clarified. Matches published versio

Reactivation and Precise IPN Localization of the Soft Gamma Repeater SGR1900+14

In 1998 May, the soft gamma repeater SGR1900+14 emerged from several years of quiescence and emitted a series of intense bursts, one with a time history unlike any previously observed from this source. Triangulation using Ulysses, BATSE, and KONUS data give a 1.6 square arcminute error box near the galactic supernova remnant G42.8+0.6. This error box contains a quiescent soft X-ray source which is probably a neutron star associated with the soft repeater.Comment: Accepted for publication in the Astrophysical Journal Letter