4,871 research outputs found
Quantum Creation of the Randall-Sundrum Bubble
We investigate the semiclassical instability of the Randall-Sundrum brane
world. We carefully analyze the bubble solution with the Randall-Sundrum
background, which expresses the decay of the brane world. We evaluate the decay
probability following the Euclidean path integral approach to quantum gravity.
Since a bubble rapidly expands after the nucleation, the entire spacetime will
be occupied by such bubbles.Comment: 13 pages, 6 figures, To appear in Prog. Theor. Phy
Toward a Deterministic Model of Planetary Formation VII: Eccentricity Distribution of Gas Giants
The ubiquity of planets and diversity of planetary systems reveal planet
formation encompass many complex and competing processes. In this series of
papers, we develop and upgrade a population synthesis model as a tool to
identify the dominant physical effects and to calibrate the range of physical
conditions. Recent planet searches leads to the discovery of many
multiple-planet systems. Any theoretical models of their origins must take into
account dynamical interaction between emerging protoplanets. Here, we introduce
a prescription to approximate the close encounters between multiple planets. We
apply this method to simulate the growth, migration, and dynamical interaction
of planetary systems. Our models show that in relatively massive disks, several
gas giants and rocky/icy planets emerge, migrate, and undergo dynamical
instability. Secular perturbation between planets leads to orbital crossings,
eccentricity excitation, and planetary ejection. In disks with modest masses,
two or less gas giants form with multiple super-Earths. Orbital stability in
these systems is generally maintained and they retain the kinematic structure
after gas in their natal disks is depleted. These results reproduce the
observed planetary mass-eccentricity and semimajor axis-eccentricity
correlations. They also suggest that emerging gas giants can scatter residual
cores to the outer disk regions. Subsequent in situ gas accretion onto these
cores can lead to the formation of distant (> 30AU) gas giants with nearly
circular orbits.Comment: 54 pages, 14 Figures; accepted for publication in Astrophysical
Journa
Lifting of D1-D5-P states
We consider states of the D1-D5 CFT where only the left-moving sector is
excited. As we deform away from the orbifold point, some of these states will
remain BPS while others can `lift'. We compute this lifting for a particular
family of D1-D5-P states, at second order in the deformation off the orbifold
point. We note that the maximally twisted sector of the CFT is special: the
covering surface appearing in the correlator can only be genus one while for
other sectors there is always a genus zero contribution. We use the results to
argue that fuzzball configurations should be studied for the full class
including both extremal and near-extremal states; many extremal configurations
may be best seen as special limits of near extremal configurations.Comment: 51 pages, 6 figure
Psychophysical and physiological evidence for fast binaural processing
The mammalian auditory system is the temporally most precise sensory modality: To localize low-frequency sounds in space, the binaural system can resolve time differences between the ears with microsecond precision. In contrast, the binaural system appears sluggish in tracking changing interaural time differences as they arise from a low-frequency sound source moving along the horizontal plane. For a combined psychophysical and electrophysiological approach, we created a binaural stimulus, called "Phasewarp," that can transmit rapid changes in interaural timing. Using this stimulus, the binaural performance in humans is significantly better than reported previously and comparable with the monaural performance revealed with amplitude-modulated stimuli. Parallel, electrophysiological recordings of binaural brainstem neurons in the gerbil show fast temporal processing of monaural and different types of binaural modulations. In a refined electrophysiological approach that was matched to the psychophysics, the seemingly faster binaural processing of the Phasewarp was confirmed. The current data provide both psychophysical and physiological evidence against a general, hard-wired binaural sluggishness and reconcile previous contradictions of electrophysiological and psychophysical estimates of temporal binaural performance
Toward a Deterministic Model of Planetary Formation VI: Dynamical Interaction and Coagulation of Multiple Rocky Embryos and Super-Earth Systems around Solar Type Stars
Radial velocity and transit surveys indicate that solar-type stars bear
super-Earths, with mass and period up to ~ 20 M_E and a few months, are more
common than those with Jupiter-mass gas giants. In many cases, these
super-Earths are members of multiple-planet systems in which their mutual
dynamical interaction has influenced their formation and evolution. In this
paper, we modify an existing numerical population synthesis scheme to take into
account protoplanetary embryos' interaction with their evolving natal gaseous
disk, as well as their close scatterings and resonant interaction with each
other. We show that it is possible for a group of compact embryos to emerge
interior to the ice line, grow, migrate, and congregate into closely-packed
convoys which stall in the proximity of their host stars. After the disk-gas
depletion, they undergo orbit crossing, close scattering, and giant impacts to
form multiple rocky Earths or super-Earths in non-resonant orbits around ~
0.1AU with moderate eccentricities of ~0.01-0.1. We suggest that most
refractory super-Earths with period in the range of a few days to weeks may
have formed through this process. These super-Earths differ from Neptune-like
ice giants by their compact sizes and lack of a substantial gaseous envelope.Comment: 37 pages, 10 figures, accepted for publication in Ap
Reinventing spacetime on a dynamical hypersurface
In braneworld models, Space-Time-Matter and other Kaluza-Klein theories, our
spacetime is devised as a four-dimensional hypersurface {\it orthogonal} to the
extra dimension in a five-dimensional bulk. We show that the FRW line element
can be "reinvented" on a dynamical four-dimensional hypersurface, which is {\it
not} orthogonal to the extra dimension, without any internal contradiction.
This hypersurface is selected by the requirement of continuity of the metric
and depends explicitly on the evolution of the extra dimension. The main
difference between the "conventional" FRW, on an orthogonal hypersurface, and
the new one is that the later contains higher-dimensional modifications to the
regular matter density and pressure in 4D. We compare the evolution of the
spacetime in these two interpretations. We find that a wealth of "new" physics
can be derived from a five-dimensional metric if it is interpreted on a
dynamical (non-orthogonal) 4D hypersurface. In particular, in the context of a
well-known cosmological metric in , we construct a FRW model which is
consistent with the late accelerated expansion of the universe, while fitting
simultaneously the observational data for the deceleration parameter. The model
predicts an effective equation of state for the universe, which is consistent
with observations.Comment: References added to the Introduction, and Abstract modified. Accepted
for publication in Mod. Phys. Lett.
The Importance of Disk Structure in Stalling Type I Migration
As planets form they tidally interact with their natal disks. Though the
tidal perturbation induced by Earth and super-Earth mass planets is generally
too weak to significantly modify the structure of the disk, the interaction is
potentially strong enough to cause the planets to undergo rapid type I
migration. This physical process may provide a source of short-period
super-Earths, though it may also pose a challenge to the emergence and
retention of cores on long-period orbits with sufficient mass to evolve into
gas giants. Previous numerical simulations have shown that the type I migration
rate sensitively depends upon the circumstellar disk's properties, particularly
the temperature and surface density gradients. Here, we derive these structure
parameters for 1) a self-consistent viscous-disk model based on a constant
\alpha-prescription, 2) an irradiated disk model that takes into account
heating due to the absorption of stellar photons, and 3) a layered-accretion
disk model with variable \alpha-parameter. We show that in the inner
viscously-heated regions of typical protostellar disks, the horseshoe and
corotation torques of super-Earths can exceed their differential Lindblad
torque and cause them to undergo outward migration. However, the temperature
profile due to passive stellar irradiation causes type I migration to be
inwards throughout much of the disk. For disks in which there is outwards
migration, we show that location and the mass range of the "planet traps"
depends on some uncertain assumptions adopted for these disk models. Competing
physical effects may lead to dispersion in super-Earths' mass-period
distribution.Comment: 12 pages, Submitted to Ap
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