72 research outputs found
Classification of BATSE, Swift, and Fermi Gamma-Ray Bursts from Prompt Emission Alone
Although it is generally assumed that there are two dominant classes of
gamma-ray bursts (GRB) with different typical durations, it has been difficult
to unambiguously classify GRBs as short or long from summary properties such as
duration, spectral hardness, and spectral lag. Recent work used t-distributed
stochastic neighborhood embedding (t-SNE), a machine learning algorithm for
dimensionality reduction, to classify all Swift gamma-ray bursts as short or
long. Here, the method is expanded, using two algorithms, t-SNE and UMAP, to
produce embeddings that are used to provide a classification for the 1911 BATSE
bursts, 1321 Swift bursts, and 2294 Fermi bursts for which both spectra and
metadata are available. Although the embeddings appear to produce a clear
separation of each catalog into short and long bursts, a resampling-based
approach is used to show that a small fraction of bursts cannot be robustly
classified. Further, 3 of the 304 bursts observed by both Swift and Fermi have
robust but conflicting classifications. A likely interpretation is that in
addition to the two predominant classes of GRBs, there are additional, uncommon
types of bursts which may require multi-wavelength observations in order to
separate from more typical short and long GRBs.Comment: ApJ, in pres
Robustness of slow contraction to cosmic initial conditions
We present numerical relativity simulations of cosmological scenarios in
which the universe is smoothed and flattened by undergoing a phase of slow
contraction and test their sensitivity to a wide range of initial conditions.
Our numerical scheme enables the variation of all freely specifiable physical
quantities that characterize the initial spatial hypersurface, such as the
initial shear and spatial curvature contributions as well as the initial field
and velocity distributions of the scalar that drives the cosmological
evolution. In particular, we include initial conditions that are far outside
the perturbative regime of the well-known attractor scaling solution. We
complement our numerical results by analytically performing a complete
dynamical systems analysis and show that the two approaches yield consistent
results.Comment: 41 pages, 18 figures; accepted for publication in JCA
Ultralocality and Slow Contraction
We study the detailed process by which slow contraction smooths and flattens
the universe using an improved numerical relativity code that accepts initial
conditions with non-perturbative deviations from homogeneity and isotropy along
two independent spatial directions. Contrary to common descriptions of the
early universe, we find that the geometry first rapidly converges to an
inhomogeneous, spatially-curved and anisotropic ultralocal state in which all
spatial gradient contributions to the equations of motion decrease as an
exponential in time to negligible values. This is followed by a second stage in
which the geometry converges to a homogeneous, spatially flat and isotropic
spacetime. In particular, the decay appears to follow the same history whether
the entire spacetime or only parts of it are smoothed by the end of slow
contraction.Comment: 27 pages, 10 figure
The Ultimate Fate of Life in an Accelerating Universe
The ultimate fate of life in a universe with accelerated expansion is
considered. Previous work showed that life cannot go on indefinitely in a
universe dominated by a cosmological constant. In this paper we consider
instead other models of acceleration (including quintessence and Cardassian
expansion). We find that it is possible in these cosmologies for life to
persist indefinitely. As an example we study potentials of the form and find the requirement .Comment: 8 pages, RevTex. (V2 has a reference added, additional minor
changes.
Controlling Chaos through Compactification in Cosmological Models with a Collapsing Phase
We consider the effect of compactification of extra dimensions on the onset
of classical chaotic "Mixmaster" behavior during cosmic contraction. Assuming a
universe that is well-approximated as a four-dimensional
Friedmann-Robertson--Walker model (with negligible Kaluza-Klein excitations)
when the contraction phase begins, we identify compactifications that allow a
smooth contraction and delay the onset of chaos until arbitrarily close the big
crunch. These compactifications are defined by the de Rham cohomology (Betti
numbers) and Killing vectors of the compactification manifold. We find
compactifications that control chaos in vacuum Einstein gravity, as well as in
string theories with N = 1 supersymmetry and M-theory. In models where chaos is
controlled in this way, the universe can remain homogeneous and flat until it
enters the quantum gravity regime. At this point, the classical equations
leading to chaotic behavior can no longer be trusted, and quantum effects may
allow a smooth approach to the big crunch and transition into a subsequent
expanding phase. Our results may be useful for constructing cosmological models
with contracting phases, such as the ekpyrotic/cyclic and pre-big bang models.Comment: 1 figure. v2/v3: minor typos correcte
Phase diagram of the Shastry-Sutherland Compound SrCu2(BO3)2 under extreme combined conditions of field and pressure
Motivated by the intriguing properties of the Shastry-Sutherland compound
SrCu2(BO3)2 under pressure, with a still debated intermediate plaquette phase
appearing at around 20 kbar and a possible deconfined critical point at higher
pressure upon entering the antiferromagnetic phase, we have investigated its
high-field properties in this pressure range using tunnel diode oscillator
(TDO) measurements. The two main new phases revealed by these measurements are
fully consistent with those identified by infinite Projected Entangled Pair
states (iPEPS) calculations of the Shastry-Sutherland model, a 1/5 plateau and
a 10 x 2 supersolid. Remarkably, these phases are descendants of the
full-plaquette phase, the prominent candidate for the intermediate phase of
SrCu2(BO3)2. The emerging picture for SrCu2(BO3)2 is shown to be that of a
system dominated by a tendency to an orthorhombic distortion at intermediate
pressure, an important constraint on any realistic description of the
transition into the antiferromagnetic phase
Towards understanding the magnetic properties of the breathing pyrochlore compound Ba3Yb2Zn5O11: A single crystal study
Ba3Yb2Zn5O11 is unique among breathing pyrochlore compounds for being in the
nearly decoupled limit where inter-tetrahedron interactions are weak, hosting
isolated clusters or "molecular magnet" like tetrahedra of magnetic ytterbium
(Yb3+) ions. In this work, we present the first study carried out on
single-crystal samples of the breathing pyrochlore Ba3Yb2Zn5O11, using a
variety of magnetometry and neutron scattering techniques along with
theoretical modeling. We employ inelastic neutron scattering to investigate the
magnetic dynamics as a function of applied field (with respect to both
magnitude and direction) down to a temperature of 70 mK, where inelastic
scattering reveals dispersionless bands of excitations as found in earlier
powder sample studies, in good agreement with a single-tetrahedron model.
However, diffuse neutron scattering at zero field and dc-susceptibility at
finite field exhibit features suggesting the presence of excitations at
low-energy that are not captured by the single tetrahedron model. Analysis of
the local structure down to 2 K via pair distribution function analysis finds
no evidence of structural disorder. We conclude that effects beyond the single
tetrahedron model are important in describing the low-energy, low temperature
physics of Ba3Yb2Zn5O11, but their nature remains undetermined
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