514 research outputs found
An improved model of HII bubbles during the epoch of reionization
The size distribution of ionized regions during the epoch of reionization --
a key ingredient in understanding the HI power spectrum observable by 21cm
experiments -- can be modelled analytically using the excursion set formalism
of random walks in the smoothed initial density field. To date, such
calculations have been based on simplifying assumptions carried forward from
the earliest excursion set models of two decades ago. In particular, these
models assume that the random walks have uncorrelated steps and that haloes can
form at arbitrary locations in the initial density field. We extend these
calculations by incorporating recent technical developments that allow us to
(a) include the effect of correlations in the steps of the walks induced by a
realistic smoothing filter and (b) more importantly, account for the fact that
dark matter haloes preferentially form near peaks in the initial density. A
comparison with previous calculations shows that including these features,
particularly the peaks constraint on halo locations, has large effects on the
size distribution of the HII bubbles surrounding these haloes. For example,
when comparing models at the same value of the globally averaged ionized volume
fraction, the typical bubble sizes predicted by our model are more than a
factor 2 larger than earlier calculations. Our results can potentially have a
significant impact on estimates of the observable HI power spectrum.Comment: 13 pages, 6 figures; v2 - added clarifications and fixed typos.
Accepted in MNRA
Recent mathematical developments in the Skyrme model
In this review we present a pedagogical introduction to recent, more
mathematical developments in the Skyrme model. Our aim is to render these
advances accessible to mainstream nuclear and particle physicists. We start
with the static sector and elaborate on geometrical aspects of the definition
of the model. Then we review the instanton method which yields an analytical
approximation to the minimum energy configuration in any sector of fixed baryon
number, as well as an approximation to the surfaces which join together all the
low energy critical points. We present some explicit results for B=2. We then
describe the work done on the multibaryon minima using rational maps, on the
topology of the configuration space and the possible implications of Morse
theory. Next we turn to recent work on the dynamics of Skyrmions. We focus
exclusively on the low energy interaction, specifically the gradient flow
method put forward by Manton. We illustrate the method with some expository toy
models. We end this review with a presentation of our own work on the
semi-classical quantization of nucleon states and low energy nucleon-nucleon
scattering.Comment: 129 pages, about 30 figures, original manuscript of published Physics
Report
Radiation from collapsing shells, semiclassical backreaction and black hole formation
We provide a detailed analysis of quantum field theory around a collapsing
shell and discuss several conceptual issues related to the emission of
radiation flux and formation of black holes. Explicit calculations are
performed using a model for a collapsing shell which turns out to be
analytically solvable. We use the insights gained in this model to draw
reliable conclusions regarding more realistic models. We first show that any
shell of mass which collapses to a radius close to will emit
approximately thermal radiation for a period of time. In particular, a shell
which collapses from some initial radius to a final radius
(where ) without forming a black hole,
will emit thermal radiation during the period . Later on (), the flux from such a
shell will decay to zero exponentially. We next study the effect of
backreaction computed using the vacuum expectation value of the stress tensor
on the collapse. We find that, in any realistic collapse scenario, the
backreaction effects do \emph{not} prevent the formation of the event horizon.
The time at which the event horizon is formed is, of course, delayed due to the
radiated flux -- which decreases the mass of the shell -- but this effect is
not sufficient to prevent horizon formation. We also clarify several conceptual
issues and provide pedagogical details of the calculations in the Appendices to
the paper.Comment: 26 pages, 6 figures, revtex4; v2 -- minor reformatting, some typos
fixed, one reference added, to appear in PR
Entropy of Null Surfaces and Dynamics of Spacetime
The null surfaces of a spacetime act as one-way membranes and can block information for a corresponding family of observers (time-like curves). Since lack of information can be related to entropy, this suggests the possibility of assigning an entropy to the null surfaces of a spacetime. We motivate and introduce such an entropy functional in terms of the normal to the null surface and a fourth-rank divergence free tensor with the algebraic symmetries of the curvature tensor. Extremising this entropy then leads to field equations for the background metric of the spacetime. When is constructed from the metric alone, these equations are identical to Einstein's equations with an undetermined cosmological constant (which arises as an integration constant). More generally, if is allowed to depend on both metric and curvature in a polynomial form, one recovers the Lanczos-Lovelock gravity. In all these cases: (a) We only need to extremise the entropy associated with the null surfaces; the metric is not a dynamical variable in this approach. (b) The extremal value of the entropy agrees with standard results, when evaluated on-shell for a solution admitting a horizon. The role of full quantum theory of gravity will be to provide the specific form of which should be used in the entropy functional. With such an interpretation, it seems reasonable to interpret the Lanczos-Lovelock type terms as quantum corrections to classical gravity
Soft Bootstrap and Supersymmetry
The soft bootstrap is an on-shell method to constrain the landscape of
effective field theories (EFTs) of massless particles via the consistency of
the low-energy S-matrix. Given assumptions on the on-shell data (particle
spectra, linear symmetries, and low-energy theorems), the soft bootstrap is an
efficient algorithm for determining the possible consistency of an EFT with
those properties. The implementation of the soft bootstrap uses the recently
discovered method of soft subtracted recursion. We derive a precise criterion
for the validity of these recursion relations and show that they fail exactly
when the assumed symmetries can be trivially realized by independent operators
in the effective action. We use this to show that the possible pure (real and
complex) scalar, fermion, and vector exceptional EFTs are highly constrained.
Next, we prove how the soft behavior of states in a supermultiplet must be
related and illustrate the results in extended supergravity. We demonstrate the
power of the soft bootstrap in two applications. First, for the N= 1 and N=2
CP^1 nonlinear sigma models, we show that on-shell constructibility establishes
the emergence of accidental IR symmetries. This includes a new on-shell
perspective on the interplay between N=2 supersymmetry, low-energy theorems,
and electromagnetic duality. We also show that N=2 supersymmetry requires
3-point interactions with the photon that make the soft behavior of the scalar
O(1) instead of vanishing, despite the underlying symmetric coset. Second, we
study Galileon theories, including aspects of supersymmetrization, the
possibility of a vector-scalar Galileon EFT, and the existence of
higher-derivative corrections preserving the enhanced special Galileon
symmetry. This is addressed by soft bootstrap and by application of
double-copy/KLT relations applied to higher-derivative corrections of chiral
perturbation theory.Comment: 71 pages, no figures. v2: significant new material about the N=2 CP^1
NLSM; typos correcte
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