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http://scholarworks.umass.edu/physics_faculty_pubs
Recent documents in Physics Department Faculty Publication Seriesen-usSun, 06 Sep 2015 06:22:03 PDT3600Very effective string model?
http://scholarworks.umass.edu/physics_faculty_pubs/1243
http://scholarworks.umass.edu/physics_faculty_pubs/1243Tue, 22 Feb 2011 12:17:57 PST
Additional evidence is presented for a recently proposed effective string model, conjectured to hold throughout the parameter space of the basic 5 dimensional, triply charged black holes, which includes the effects of brane excitations, as well as momentum modes. We compute the low energy spacetime absorption coefficient σ for the scattering of a triply charged scalar field in the near extremal case, and conjecture an exact form for σ. It is shown that this form of σ arises simply from the effective string model. This agreement encompasses both statistical factors coming from the Bose distributions of string excitations and a prefactor which depends on the effective string radius. An interesting feature of the effective string model is that the change in mass of the effective string system in an emission process is not equal to the change in the energies of the effective string excitations. If the model is valid, this may hold clues towards understanding back reaction due to Hawking radiation. A number of weak spots and open questions regarding the model are also noted.
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Kastor, David et al.An extended Kerr–Schild ansatz
http://scholarworks.umass.edu/physics_faculty_pubs/1242
http://scholarworks.umass.edu/physics_faculty_pubs/1242Tue, 22 Feb 2011 12:17:54 PST
We present an analysis of the vacuum Einstein equations for a recently proposed extension of the Kerr–Schild ansatz that includes a spacelike vector field as well as the usual Kerr–Schild null vector. We show that many, although not all, of the simplifications that occur in the Kerr–Schild case continue to hold for the extended ansatz. In particular, we find a simple set of sufficient conditions on the vectors such that the vacuum field equations truncate beyond quadratic order in an expansion around a general vacuum background solution. We extend our analysis to the electrovac case with a related ansatz for the gauge field.
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Ett, Benjamin et al.BPS force balances via spin-spin interactions
http://scholarworks.umass.edu/physics_faculty_pubs/1241
http://scholarworks.umass.edu/physics_faculty_pubs/1241Tue, 22 Feb 2011 12:17:50 PST
We study two systems of BPS solitons in which spin-spin interactions are important in establishing the force balances which allow static, multi-soliton solutions to exist. Solitons in the Israel-Wilson-Perjes (IWP) spacetimes each carry arbitrary, classical angular momenta. Solitons in the Aichelburg-Embacher `superpartner' spacetimes carry quantum mechanical spin, which originates in the zero modes of the gravitino field of N = 2 supergravity in an extreme Reissner-Nordström background. In each case we find a cancellation between gravitational spin-spin and magnetic dipole-dipole forces, in addition to the usual one between Newtonian gravitational attraction and Coulombic electrostatic repulsion. In both cases, we analyse the forces between two solitons by treating one of the solitons as a probe or test particle, with the appropriate properties, moving in the background of the other. In the IWP case, the equation of motion for a spinning test particle, originally due to Papapetrou, includes a coupling between the background curvature and the spin of the test particle. In the superpartner case, the relevant equation of motion follows from a -symmetric superparticle action.
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Kastor, David et al.Breaking Cosmic Strings without Monopoles
http://scholarworks.umass.edu/physics_faculty_pubs/1240
http://scholarworks.umass.edu/physics_faculty_pubs/1240Tue, 22 Feb 2011 12:17:47 PST
It is shown that topologically stable cosmic strings can, in fact, appear to end or to break, even in theories without monopoles. This can occur whenever the spatial topology of the universe is nontrivial. For the case of Abelian-Higgs strings, we describe the gauge and scalar field configurations necessary for a string to end on a black hole. We give a lower bound for the rate at which a cosmic string will break via black hole pair production, using an instanton calculation based on the Euclidean C-metric.
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Eardley, D et al.Calibrations and Fayyazuddin-Smith spacetimes
http://scholarworks.umass.edu/physics_faculty_pubs/1239
http://scholarworks.umass.edu/physics_faculty_pubs/1239Tue, 22 Feb 2011 12:17:44 PST
We show that a class of spacetimes introduced by Fayyazuddin and Smith to describe intersecting M5-branes admits a generalized Kähler calibration. Equipped with this understanding, we are able to construct spacetimes corresponding to further classes of calibrated p-brane world-volume solitons. We note that these classes of spacetimes also describe the fields of p-branes wrapping certain supersymmetric cycles of Calabi-Yau manifolds.
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Cho, Hyunji et al.C-functions in Lovelock gravity
http://scholarworks.umass.edu/physics_faculty_pubs/1238
http://scholarworks.umass.edu/physics_faculty_pubs/1238Tue, 22 Feb 2011 12:17:41 PST
We present C-functions for static and spherically symmetric spacetimes in Lovelock gravity theories. These functions are monotonically increasing functions of the outward radial coordinate and acquire their minima when evaluated on the horizon. Unlike the case of Einstein gravity, where there is a single C-function, we find that this function is non-unique in the case of Lovelock gravity. We define two C-functions, which agree at the horizon giving the black hole entropy, and state the different energy conditions that must hold in order for these functions to satisfy the monotonicity condition.
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Anber, Mohamed et al.Conserved gravitational charges from Yano tensors
http://scholarworks.umass.edu/physics_faculty_pubs/1237
http://scholarworks.umass.edu/physics_faculty_pubs/1237Tue, 22 Feb 2011 12:17:39 PST
The defining properties of Yano tensors naturally generalize those of Killing vectors to anti-symmetric tensor fields of arbitrary rank. We show how the Yano tensors of flat spacetime can be used to construct new, conserved gravitational charges for transverse asymptotically flat spacetimes. The relationship of these new charges to Yano tensors parallels that of ordinary ADM conserved charges to Killing vectors. Hence, we call them Y-ADM charges. A rank n Y-ADM charge is given by an integral over a co-dimension n slice of spatial infinity. In particular, a rank (p+1) Y-ADM charge in a p-brane spacetime is given by an integral over only the sphere SD−(p+2) surrounding the brane and may be regarded as an intensive property of the brane.
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Kastor, David et al.Cosmological multi-black-hole solutions
http://scholarworks.umass.edu/physics_faculty_pubs/1236
http://scholarworks.umass.edu/physics_faculty_pubs/1236Tue, 22 Feb 2011 12:17:36 PST
We present simple, analytic solutions to the Einstein-Maxwell equation, which describe an arbitrary number of charged black holes in a spacetime with a positive cosmological constant Λ. In the limit Λ=0, these solutions reduce to the well-known Majumdar-Papapetrou (MP) solutions. Like the MP solutions, each black hole in a Λ>0 solution has charge Q equal to its mass M, up to a possible overall sign. Unlike the Λ=0 limit, however, solutions with Λ>0 are highly dynamical. The black holes move with respect to one another, following natural trajectories in the background de Sitter spacetime. Black holes moving apart eventually go out of causal contact. Black holes on approaching trajectories ultimately merge. To our knowledge, these solutions give the first analytic description of coalescing black holes. Likewise, the thermodynamics of the Λ>0 solutions is quite interesting. Taken individually, a |Q|=M black hole is in thermal equilibrium with the background de Sitter Hawking radiation. With more than one black hole, because the solutions are not static, no global equilibrium temperature can be defined. In appropriate limits, however, when the black holes are either close together or far apart, approximate equilibrium states are established.
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Kastor, David et al.Do Killing–Yano tensors form a Lie algebra?
http://scholarworks.umass.edu/physics_faculty_pubs/1235
http://scholarworks.umass.edu/physics_faculty_pubs/1235Tue, 22 Feb 2011 12:17:33 PST
Killing–Yano tensors are natural generalizations of Killing vectors. We investigate whether Killing–Yano tensors form a graded Lie algebra with respect to the Schouten–Nijenhuis bracket. We find that this proposition does not hold in general, but that it does hold for constant curvature spacetimes. We also show that Minkowski and (anti)-deSitter spacetimes have the maximal number of Killing–Yano tensors of each rank and that the algebras of these tensors under the SN bracket are relatively simple extensions of the Poincaré and (A)dS symmetry algebras.
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Kastor, David et al.Dynamics of the Dirac-Born-Infeld spike soliton
http://scholarworks.umass.edu/physics_faculty_pubs/1234
http://scholarworks.umass.edu/physics_faculty_pubs/1234Tue, 22 Feb 2011 12:17:30 PST
We compare oscillations of a fundamental string ending on a D3-brane in two different settings: (1) a test string radially threading the horizon of an extremal black D3-brane and (2) the spike soliton of the DBI effective action for a D3-brane. Previous work has shown that overall transverse modes of the test string appear as l=0 modes of the transverse scalar fields of the DBI system. We identify DBI world-volume degrees of freedom that have dynamics matching those of the test-string relative transverse modes. We show that there is a map, resembling T duality, between relative and overall transverse modes for the test string that interchanges Neumann and Dirichlet boundary conditions and implies equality of the absorption coefficients for both modes. We give general solutions to the overall and relative transverse parts of the DBI coupled gauge and scalar system and calculate absorption coefficients for the higher angular momentum modes in the low frequency limit. We find that there is a nonzero amplitude for l>0 modes to travel out to infinity along the spike, demonstrating that the spike remains effectively 3+1 dimensional.
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Kastor, David et al.