353 research outputs found
Recent Progress in Cosmology and Particle Astrophysics
Recent years have seen dramatic progress in cosmology and particle
astrophysics. So much so that anyone who dares to offer an overview would
certainly risk him- or herself for being incomplete and biased at best, and
even incorrect due to the author's limited expertise. It is with this
understanding and excuse that I risk myself in offering this review. After a
brief summary of Planck mission's first results, I highlight some selected
theoretical and experimental advancement in dark energy, dark matter, and
cosmic neutrinos research. It is hoped that with a glance through these
exciting development, one would be convinced that we are now a step closer to
the ultimate understanding of our universe, while major breakthroughs are still
required.Comment: 10 pages, 2 figure
Generalized Uncertainty Principle and Dark Matter
There have been proposals that primordial black hole remnants (BHRs) are the
dark matter, but the idea is somewhat vague. Recently we argued that the
generalized uncertainty principle (GUP) may prevent black holes from
evaporating completely, in a similar way that the standard uncertainty
principle prevents the hydrogen atom from collapsing. We further noted that the
hybrid inflation model provides a plausible mechanism for production of large
numbers of small black holes. Combining these we suggested that the dark matter
might be composed of Planck-size BHRs. In this paper we briefly review these
arguments, and discuss the reheating temperature as a result of black hole
evaporation.Comment: Contributed to the Proceedings of International Symposium on
Frontiers of Science in Celebration of the 80th Birthday of Chen Ning Yang,
June 17-19, 2002, Beijing, Chin
Constraining GRB as Source for UHE Cosmic Rays through Neutrino Observations
The origin of ultra-high energy cosmic rays (UHECR) has been widely regarded
as one of the major questions in the frontiers of particle astrophysics. Gamma
ray bursts (GRB), the most violent explosions in the universe second only to
the Big Bang, have been a popular candidate site for UHECR productions. The
recent IceCube report on the non-observation of GRB induced neutrinos therefore
attracts wide attention. This dilemma requires a resolution: either the
assumption of GRB as UHECR accelerator is to be abandoned or the expected GRB
induced neutrino yield was wrong. It has been pointed out that IceCube has
overestimated the neutrino flux at GRB site by a factor of . In this
paper we point out that, in addition to the issue of neutrino production at
source, the neutrino oscillation and the possible neutrino decay during their
flight from GRB to Earth should further reduce the detectability of IceCube,
which is most sensitive to the muon-neutrino flavor as far as point-source
identification is concerned. Specifically, neutrino oscillation will reduce the
muon-neutrino flavor ratio from 2/3 per neutrino at GRB source to 1/3 on Earth,
while neutrino decay, if exists and under the assumption of normal hierarchy of
mass eigenstates, would result in a further reduction of muon-neutrino ratio to
1/8. With these in mind, we note that there have been efforts in recent years
in pursuing other type of neutrino telescopes based on Askaryan effect, which
can in principle observe and distinguish all three flavors with comparable
sensitivities. Such new approach may therefore be complementary to IceCube in
shedding more lights on this cosmic accelerator question.Comment: 9 pages, 1 figur
Gauge Theory of Gravity with de Sitter Symmetry as a Solution to the Cosmological Constant Problem and the Dark Energy Puzzle
We propose a solution to the longstanding cosmological constant (CC) problem
which is based on the fusion of two existing concepts. The first is the
suggestion that the proper description of classical gravitational effects is
the gauge theory of gravity in which the connection instead of the metric acts
as the dynamical variable. The resulting field equation does not then contain
the CC term. This removes the connection between the CC and the quantum vacuum
energy, and therefore addresses the {\it old} CC problem of why quantum vacuum
energy does not gravitate. The CC-equivalent in this approach arises from the
constant of integration when reducing the field equation to the Einstein
equation. The second is the assumption that the universe obeys de Sitter
symmetry, with the observed accelerating expansion as its manifestation. We
combine these ideas and identify the constant of integration with the
inverse-square of the radius of curvature of the de Sitter space. The origin of
dark energy (DE) is therefore associated with the inherent spacetime geometry,
with the smallness of DE protected by symmetry. This addresses the {\it new} CC
problem, or the DE puzzle. This approach, however, faces major challenges from
quantum considerations. These are the ghost problem associated with higher
order gravity theories and the quantum instability of the de Sitter spacetime.
We discuss their possible remedies.Comment: 8 page
Laser Cosmology
Recent years have seen tremendous progress in our understanding of the
cosmos, which in turn points to even deeper questions to be further addressed.
Concurrently the laser technology has undergone dramatic revolutions, providing
exciting opportunity for science applications. History has shown that the
symbiosis between direct observations and laboratory investigation is
instrumental in the progress of astrophysics. We believe that this remains true
in cosmology. Current frontier phenomena related to particle astrophysics and
cosmology typically involve one or more of the following conditions: (1)
extremely high energy events; (2) very high density, high temperature
processes; (3) super strong field environments. Laboratory experiments using
high intensity lasers can calibrate astrophysical observations, investigate
underlying dynamics of astrophysical phenomena, and probe fundamental physics
in extreme limits. In this article we give an overview of the exciting prospect
of laser cosmology. In particular, we showcase its unique capability of
investigating frontier cosmology issues such as cosmic accelerator and quantum
gravity.Comment: 6 page
Laser Shaping and Optimization of the Laser-Plasma Interaction
The physics of energy transfer between the laser and the plasma in laser
wakefield accelerators is studied. We find that wake excitation by arbitrary
laser shapes can be parameterized using the total pulse energy and pulse
depletion length. A technique for determining laser profiles that produce the
required plasma excitation is developed. We show that by properly shaping the
longitudinal profile of the driving laser pulse, it is possible to maximize
both the transformer ratio and the wake amplitude, achieving optimal
laser-plasma coupling. The corresponding family of laser pulse shapes is
derived in the nonlinear regime of laser-plasma interaction. Such shapes
provide theoretical upper limit on the magnitude of the wakefield and
efficiency of the accelerating stage by allowing for uniform photon
deceleration inside the laser pulse. We also construct realistic optimal pulse
shapes that can be produced in finite-bandwidth laser systems and propose a
two-pulse wake amplification scheme using the optimal solution.Comment: 12 pages, 5 figures, contributed to the Advanced Accelerator Concepts
2000 worksho
- …