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## ‣ Observational Constraints on Dark Energy and Cosmic Curvature

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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Current observational bounds on dark energy depend on assumptions about the
curvature of the universe. We present a simple and efficient method for
incorporating constraints from CMB anisotropy data, and use it to derive
constraints on cosmic curvature and dark energy density as a free function of
cosmic time using current data. We show that there are two CMB shift
parameters, R=sqrt{\Omega_m H_0^2} r(z_{CMB}) (scaled distance to
recombination) and l_a=\pi r(z_{CMB})/r_s(z_{CMB})(angular scale of the sound
horizon at recombination), with measured values that are nearly uncorrelated
with each other. Allowing nonzero cosmic curvature, the three-year WMAP data
give R =1.71 +/- 0.03, l_a =302.5 +/- 1.2, and \Omega_b h^2 = 0.02173 +/-
0.00082, independent of the dark energy model. The corresponding bounds for a
flat universe are R =1.70 +/- 0.03, l_a =302.2 +/- 1.2, and \Omega_b h^2 =
0.022 +/- 0.00082. We give the covariance matrix of (R, l_a, \Omega_b h^2) from
the three-year WMAP data. We find that (R, l_a, \Omega_b h^2) provide an
efficient and intuitive summary of CMB data as far as dark energy constraints
are concerned. Using current CMB, SN Ia, and BAO data, we find that dark energy
density is consistent with a constant in cosmic time...

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## ‣ The High Energy Budget Allocations in Shocks and GRB

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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The statistical distribution of energies among particles responsible for long
Gamma Ray Burst (GRB) emission is analyzed in light of recent results of the
Fermi Observatory. The allsky flux, $F_{\gamma}$, recorded by the Gamma Ray
Burst Monitor (GBM) is shown, despite its larger energy range, to be not
significantly larger than that reported by the Burst and Transient Explorer
(BATSE), suggesting a relatively small flux in the 3 - 30 MeV energy range. The
present-day energy input rate in $\gamma$-rays recorded by the GBM from long
GRB is found, assuming star-formation rates in the literature, to be $\dot
W(0)=0.5 F_{\gamma} H/c = 5 \times 10^{42}\ \rm{erg/Mpc^3 yr}$. The Large Area
Telescope (LAT) fluence, when observed, is about 5-10\% per decade of the
total, in good agreement with the predictions of saturated, non-linear shock
acceleration.
The high-energy component of long GRBs, as measured by Fermi, is found to
contain only $\sim 10^{-2.5}$ of the energy needed to produce ultrahigh-energy
cosmic rays (UHECR) above 4 Eev, assuming the latter to be extragalactic, when
various numerical factors are carefully included, if the cosmic ray source
spectrum has a spectral index of -2. The observed $\gamma$-ray fraction of the
required UHECR energy is even smaller if the source spectrum is softer than
$E^{-2}$.
The AMANDA II limits rule out such a GRB origin for UHECR if much more than
$10^{-2}$ of the cosmic ray energy goes into neutrinos that are within...

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## ‣ High-energy fluxes of atmospheric neutrinos

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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High-energy neutrinos from decays of mesons, produced in collisions of cosmic
ray particles with air nuclei, form unavoidable background for detection of
astrophysical neutrinos. More precise calculations of the high-energy neutrino
spectrum are required since measurements in the IceCube experiment reach the
intriguing energy region where a contribution of the prompt neutrinos and/or
astrophysical ones should be discovered. Basing on the referent hadronic models
QGSJET II-03, SIBYLL 2.1, we calculate high-energy spectra, both of the muon
and electron atmospheric neutrinos, averaged over zenith-angles. The
computation is made using three parameterizations of cosmic ray spectra which
include the knee region. All calculations are compared with the atmospheric
neutrino measurements by Frejus and IceCube. The prompt neutrino flux
predictions obtained with thequark-gluon string model (QGSM) for the charm
production by Kaidalov & Piskunova do not contradict to the IceCube
measurements and upper limit on the astrophysical muon neutrino flux. Neutrino
flavor ratio, $\phi_{\nu_ mu}/\phi_{\nu_e}$, extracted from IceCube data
decreases in the energy range $0.1 - 5$ TeV energy contrary to that one might
expect from the conventional neutrino flux. Presumable reasons of such behavior
are: i) early arising contribution from decays of charmed particle...

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## ‣ Unbiased Estimate of Dark Energy Density from Type Ia Supernova Data

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 17/09/2001
Português

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Type Ia supernovae (SNe Ia) are currently the best probes of the dark energy
in the universe. To constrain the nature of dark energy in a model-independent
manner, we allow the density of dark energy, $\rho_X(z)$, to be an arbitrary
function of redshift. Using simulated data from a space-based supernova pencil
beam survey, we find that by optimizing the number of parameters used to
parametrize the dimensionless dark energy density,
$f(z)=\rho_X(z)/\rho_X(z=0)$, we can obtain an unbiased estimate of both f(z)
and $\Omega_m$ (assuming a flat universe and that the weak energy condition is
satisfied). A plausible supernova pencil beam survey (with a square degree
field of view and for an observational duration of one year) can yield about
2000 SNe Ia with $0\le z \le 2$. Such a survey in space would yield SN peak
luminosities with a combined intrinsic and observational dispersion of $\sigma
(m_{int})=0.16$ mag. We find that for such an idealized survey, $\Omega_m$ can
be measured to 10% accuracy, and f(z) can be estimated to $\sim$ 20% to $z \sim
1.5$, and $\sim$ 20-40% to $z \sim 2$, depending on the time dependence of the
true dark energy density. Dark energy densities which vary more slowly can be
more accurately measured. For the anticipated SNAP mission...

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## ‣ Energy Theorem for 2+1 dimensional gravity

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 08/06/1994
Português

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We prove a positive energy theorem in 2+1 dimensional gravity for open
universes and any matter energy-momentum tensor satisfying the dominant energy
condition. We consider on the space-like initial value surface a family of
widening Wilson loops and show that the energy-momentum of the enclosed
subsystem is a future directed time-like vector whose mass is an increasing
function of the loop, until it reaches the value $1/4G$ corresponding to a
deficit angle of $2\pi$. At this point the energy-momentum of the system
evolves, depending on the nature of a zero norm vector appearing in the
evolution equations, either into a time-like vector of a universe which closes
kinematically or into a Gott-like universe whose energy momentum vector, as
first recognized by Deser, Jackiw and 't Hooft is space-like.
This treatment generalizes results obtained by Carroll, Fahri, Guth and Olum
for a system of point-like spinless particle, to the most general form of
matter whose energy-momentum tensor satisfies the dominant energy condition.
The treatment is also given for the anti de Sitter 2+1 dimensional gravity.; Comment: (Revtex), 24 pages,MIT-CTP#2324 snd IFUP-TH-33/94

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## ‣ Can high energy neutrino annihilation on relic neutrinos generate the observed highest energy cosmic-rays?

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 02/04/1998
Português

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Annihilation of high energy, $\sim 10^{21}$eV, neutrinos on big bang relic
neutrinos of $\sim 1$eV mass, clustered in the Galactic halo or in a nearby
galaxy cluster halo, has been suggested to generate, through hadronic Z decay,
high energy nucleons and photons which may account for the detected flux of
>10^{20}eV cosmic-rays. We show that the flux of high energy nucleons and
photons produced by this process is dominated by annihilation on the uniform,
non-clustered, neutrino background, and that the energy generation rate of
10^{21}eV neutrinos required to account for the detected flux of >10^{20}eV
particles is >10^{48} erg/Mpc^3 yr. This energy generation rate, comparable to
the total luminosity of the universe, is 4 orders of magnitude larger than the
rate of production of high energy nucleons required to account for the flux of
>10^{19}eV cosmic-rays. Thus, in order for neutrino annihilation to contribute
significantly to the detected flux of >10^{20}eV cosmic-rays, the existence of
a new class of high-energy neutrino sources, likely unrelated to the sources of
>10^{19}eV cosmic-rays, must be invoked.; Comment: Submitted to Astropar. Phys. (11 pages, LaTeX)

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## ‣ Clustering, GUT scale and neutrino masses from ultrahigh energy cosmic rays

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 05/02/2003
Português

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The origin of highest energy cosmic rays (UHECR) is yet unknown. In order to
understand their propagation we determine the probability that an ultrahigh
energy (above 5\cdot 10^{19} eV) proton created at a distance r with energy E
arrives at earth above a threshold E_c. The clustering of ultrahigh energy
cosmic rays suggests that they might be emitted by compact sources. A
statistical analysis on the source density based on the multiplicities is
presented. The ultrahigh energy cosmic ray spectrum is consistent with the
decay of GUT scale particles. Alternatively, we consider the possibility that a
large fraction of the ultrahigh energy cosmic rays are decay products of Z
bosons which were produced in the scattering of ultrahigh energy cosmic
neutrinos on cosmological relic neutrinos. Based on this scenario we determine
the required mass of the heaviest relic neutrino. The required ultrahigh energy
neutrino flux should be detected in the near future by experiments such as
AMANDA, RICE or the Pierre Auger Observatory.; Comment: 13 pages, 8 figures, invited talk presented at the 26th Johns Hopkins
Workshop on Particle Physics, August 2003, Heidelberg, Germany

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## ‣ From Static Potentials to High-Energy Scattering

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 13/01/2003
Português

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We develop a loop-loop correlation model for a unified description of static
color dipole potentials, confining QCD strings, and hadronic high-energy
reactions with special emphasis on saturation effects manifesting S-matrix
unitarity at ultra-high energies. The model combines perturbative gluon
exchange with the non-perturbative stochastic vacuum model which describes
color confinement via flux-tube formation of color fields. We compute the
chromo-field distributions of static color dipoles in various SU(N_c)
representations and find Casimir scaling in agreement with recent lattice QCD
results. We investigate the energy stored in the confining string and use
low-energy theorems to show consistency with the static quark-antiquark
potential. We generalize Meggiolaro's analytic continuation from parton-parton
to dipole-dipole scattering and obtain a Euclidean approach to high-energy
scattering that allows us in principle to calculate S-matrix elements in
lattice QCD. In this approach we compute high-energy dipole-dipole scattering
with the Euclidean loop-loop correlation model. Together with a universal
energy dependence and reaction-specific wave functions, the result forms the
basis for a unified description of proton-proton, pion-proton...

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## ‣ Quasinormal modes of black holes absorbing dark energy

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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#General Relativity and Quantum Cosmology#Astrophysics - High Energy Astrophysical Phenomena#High Energy Physics - Theory

We study perturbations of black holes absorbing dark energy. Due to the
accretion of dark energy, the black hole mass changes. We observe distinct
perturbation behaviors for absorption of different forms of dark energy into
the black holes. This provides the possibility of extracting information
whether dark energy lies above or below the cosmological constant boundary
$w=-1$. In particular, we find in the late time tail analysis that, differently
from the other dark energy models, the accretion of phantom energy exhibits a
growing mode in the perturbation tail. The instability behavior found in this
work is consistent with the Big Rip scenario, in which all of the bound objects
are torn apart with the presence of the phantom dark energy.; Comment: 11 pages, 5 figures, revised version, accepted for publication in
Phys.Lett.B

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## ‣ Nuclear parton distribution functions and energy loss effect in the Drell-Yan reaction off nuclei

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 23/01/2006
Português

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The energy loss effect in nuclear matter is another nuclear effect apart from
the nuclear effects on the parton distribution as in deep inelastic scattering
process. The quark energy loss can be measured best by the nuclear dependence
of the high energy nuclear Drell-Yan process. By means of two typical kinds of
quark energy loss parametrization and the different sets of nuclear parton
distribution functions, we present a analysis of the E866 experiments on the
nuclear dependence of Drell-Yan lepton pair production resulting from the
bombardment of Be, Fe and W targets by 800GeV protons at Fermilab. It is found
that the quark energy loss in cold nuclei is strongly dependent on the used
nuclear parton distribution functions. The further prospects of using
relatively low energy proton incident on nuclear targets are presented by
combining the quark energy loss rate determined from a fit to the E866
nuclear-dependent ratios versus $x_1$, with the nuclear parton distribution
functions given from lA deep inelastic scattering (DIS) data. The experimental
study of the relatively low energy nuclear Drell-Yan process can give valuable
insight in the enengy loss of fast quark propagating a cold nuclei and help to
pin down nuclear parton distributions functions.; Comment: 18 pages...

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## ‣ Energy spectra of massive two-body decay products and mass measurement

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 07/12/2015
Português

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We have recently established a new method for measuring the mass of unstable
particles produced at hadron colliders based on the analysis of the energy
distribution of a massless product from their two-body decays. The central
ingredient of our proposal is the remarkable result that, for an unpolarized
decaying particle, the location of the peak in the energy distribution of the
observed decay product is identical to the (fixed) value of the energy that
this particle would have in the rest-frame of the decaying particle, which, in
turn, is a simple function of the involved masses. In addition, we utilized the
property that this energy distribution is symmetric around the location of peak
when energy is plotted on a logarithmic scale. The general strategy was
demonstrated in several specific cases, including both beyond the SM particles,
as well as for the top quark. In the present work, we generalize this method to
the case of a massive decay product from a two-body decay; this procedure is
far from trivial because (in general) both the above- mentioned properties are
no longer valid. Nonetheless, we propose a suitably modified parametrization of
the energy distribution that was used successfully for the massless case, which
can deal with the massive case as well. We establish the accuracy of this
parametrization using concrete examples of energy spectra of Z bosons from the
decay of a heavier stop into a Z boson and a lighter stop. We then study a
realistic application for the same process...

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## ‣ The Heat-Kernel in a Schwarzschild Geometry and the Casimir Energy

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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We obtain an hybrid expression for the heat-kernel, and from that the density
of the free energy, for a minimally coupled scalar field in a Schwarzschild
geometry at finite temperature. This gives us the zero-point energy density as
a function of the distance from the massive object generating the gravitational
field. The contribution to the zero-point energy due to the curvature is
extracted too, in this way arriving at a renormalised expression for the energy
density (the Casimir energy density). We use this to find an expression for
other physical quantities: internal energy, pressure and entropy. It turns out
that the disturbance of the surrounding vacuum generates entropy. For $\beta$
small the entropy is positive for $r>2M$. We also find that the internal energy
can be negative outside the horizon pointing to the existence of bound states.
The total energy inside the horizon turns out to be finite but complex, the
imaginary part being interpreted as responsible for particle creation.; Comment: LaTeX2e (minor errors corrected, discussion extended and a few new
results added)

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## ‣ Zero-point quantum fluctuations and dark energy

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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#Astrophysics - Cosmology and Nongalactic Astrophysics#General Relativity and Quantum Cosmology#High Energy Physics - Theory

In the Hamiltonian formulation of General Relativity the energy associated to
an asymptotically flat space-time with metric $g_{\mu\nu}$ is related to the
Hamiltonian $H_{GR}$ by $E=H_{GR}[g_{\mu\nu}]-H_{\rm GR}[\eta_{\mu\nu}]$, where
the subtraction of the flat-space contribution is necessary to get rid of an
otherwise divergent boundary term. This classic result indicates that the
energy associated to flat space does not gravitate. We apply the same principle
to study the effect of zero-point fluctuations of quantum fields in cosmology,
proposing that their contribution to the cosmic expansion is obtained computing
the vacuum energy of quantum fields in a FRW space-time with Hubble parameter
$H(t)$ and subtracting from it the flat-space contribution. [...] After
renormalization, this produces a renormalized vacuum energy density $\sim
M^2H^2(t)$, where $M$ is the scale where quantum gravity sets is, so for $M$ of
order of the Planck mass a vacuum energy density of the order of the critical
density can be obtained without any fine tuning. The counterterms can be chosen
so that the renormalized energy density and pressure satisfy $p=w\rho$, with
$w$ a parameter that can be fixed by comparison to the observed value, so in
particular one can chose $w=-1$. An energy density evolving in time as $H^2(t)$
is however observationally excluded as an explanation for the dominant dark
energy component which is responsible for the observed acceleration of the
universe. We rather propose that zero-point vacuum fluctuations provide a new
subdominant "dark" contribution to the cosmic expansion that...

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## ‣ Charged Particle and Photon Multiplicity, and Transverse Energy Production in High-Energy Heavy-Ion Collisions

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/08/2014
Português

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We review the charged particle and photon multiplicity, and transverse energy
production in heavy-ion collisions starting from few GeV to TeV energies. The
experimental results of pseudorapidity distribution of charged particles and
photons at different collision energies and centralities are discussed. We also
discuss the hypothesis of limiting fragmentation and expansion dynamics using
the Landau hydrodynamics and the underlying physics. Meanwhile, we present the
estimation of initial energy density multiplied with formation time as a
function of different collision energies and centralities. In the end, the
transverse energy per charged particle in connection with the chemical
freeze-out criteria is discussed. We invoke various models and phenomenological
arguments to interpret and characterize the fireball created in heavy-ion
collisions. This review overall provides a scope to understand the heavy-ion
collision data and a possible formation of a deconfined phase of partons via
the global observables like charged particles, photons and the transverse
energy measurement.; Comment: 27 pages, 43 figures, Invited Review for Advances in High Energy
physics for Special Issue on "Global properties in High Energy Collisions"

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## ‣ Empirical determination of charm quark energy loss and its consequences for azimuthal anisotropy

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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We propose an empirical model to determine the form of energy loss of charm
quarks due to multiple scatterings in quark gluon plasma by demanding a good
description of production of D mesons and non-photonic electrons in
relativistic collision of heavy nuclei at RHIC and LHC energies. Best results
are obtained when we approximate the momentum loss per collision $\Delta p_T
\propto \alpha \, p_T$, where $\alpha$ is a constant depending on the
centrality and the centre of mass energy. Comparing our results with those
obtained earlier for drag coefficients estimated using Langevin equation for
heavy quarks we find that up to half of the energy loss of charm quarks at top
RHIC energy could be due to collisions while that at LHC energy at 2760 GeV/A
the collisional energy loss could be about one third of the total. Estimates
are obtained for azimuthal anisotropy in momentum spectra of heavy mesons, due
to this energy loss. We further suggest that energy loss of charm quarks may
lead to an enhanced production of D-mesons and single electrons at low $p_T$ in
AA collisions.; Comment: 11 pages, 3 figures, Typographical errors corrected, Key-words and
PACS indices added, sequence of figures corrected, references added in
section 3, discussions expanded

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## ‣ Are low-energy nuclear observables sensitive to high-energy phase shifts?

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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Conventional nucleon-nucleon potentials with strong short-range repulsion
require contributions from high-momentum wave function components even for
low-energy observables such as the deuteron binding energy. This can lead to
the misconception that reproducing high-energy phase shifts is important for
such observables. Interactions derived via the similarity renormalization group
decouple high-energy and low-energy physics while preserving the phase shifts
from the starting potential. They are used to show that high-momentum
components (and high-energy phase shifts) can be set to zero when using
low-momentum interactions, without losing information relevant for low-energy
observables.; Comment: 13 pages, 5 figures; reference and acknowledgment added

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## ‣ Gravitational vacuum polarization IV: Energy conditions in the Unruh vacuum

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 28/02/1997
Português

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Building on a series of earlier papers [gr-qc/9604007, gr-qc/9604008,
gr-qc/9604009], I investigate the various point-wise and averaged energy
conditions in the Unruh vacuum. I consider the quantum stress-energy tensor
corresponding to a conformally coupled massless scalar field, work in the
test-field limit, restrict attention to the Schwarzschild geometry, and invoke
a mixture of analytical and numerical techniques. I construct a semi-analytic
model for the stress-energy tensor that globally reproduces all known numerical
results to within 0.8%, and satisfies all known analytic features of the
stress-energy tensor. I show that in the Unruh vacuum (1) all standard
point-wise energy conditions are violated throughout the exterior region--all
the way from spatial infinity down to the event horizon, and (2) the averaged
null energy condition is violated on all outgoing radial null geodesics. In a
pair of appendices I indicate general strategy for constructing semi-analytic
models for the stress-energy tensor in the Hartle-Hawking and Boulware states,
and show that the Page approximation is in a certain sense the minimal ansatz
compatible with general properties of the stress-energy in the Hartle-Hawking
state.; Comment: 40 pages; plain LaTeX; uses epsf.sty (ten encapsulated postscript
figures); two tables (table and tabular environments). Should successfully
compile under both LaTeX 209 and the 209 compatibility mode of LaTeX2e

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## ‣ Power, energy, and spectrum of a naked singularity explosion

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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Naked singularity occurs in the gravitational collapse of an inhomogeneous
dust ball from an initial density profile which is physically reasonable. We
show that explosive radiation is emitted during the formation process of the
naked singularity. The energy flux is proportional to $(t_{\rm CH}-t)^{-3/2}$
for a minimally coupled massless scalar field, while is proportional to
$(t_{\rm CH}-t)^{-1}$ for a conformally coupled massless scalar field, where
$t_{\rm CH}-t$ is the `remained time' until the distant observer could observe
the singularity if the naked singularity was formed. As a consequence, the
radiated energy grows unboundedly for both scalar fields. The amount of the
power and the energy depends on parameters which characterize the initial
density profile but do not depend on the gravitational mass of the cloud. In
particular, there is characteristic frequency $\nu_{s}$ of singularity above
which the divergent energy is radiated. The energy flux is dominated by
particles of which the wave length is about $t_{\rm CH}-t$ at each moment. The
observed total spectrum is nonthermal, i.e., $\nu dN/d\nu \sim
(\nu/\nu_{s})^{-1}$ for $\nu>\nu_{s}$. If the naked singularity formation could
continue until a considerable fraction of the total energy of the dust cloud is
radiated...

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## ‣ Remarks on Dynamical Dark Energy Measured by the Conformal Age of the Universe

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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#High Energy Physics - Theory#Astrophysics#General Relativity and Quantum Cosmology#High Energy Physics - Phenomenology

We elaborate on a model of conformal dark energy (dynamical dark energy
measured by the conformal age of the universe) recently proposed in [H. Wei and
R.G. Cai, arXiv:0708.0884] where the present-day dark energy density was taken
to be $\rho_q \equiv 3 \alpha^2 m_P^2/\eta^2$, where $\eta$ is the conformal
time and $\alpha$ is a numerical constant. In the absence of an interaction
between the ordinary matter and dark energy field $q$, the model may be
adjusted to the present values of the dark energy density fraction $\Omega\Z{q}
\simeq 0.73$ and the equation of state parameter $w\Z{q} < -0.78$, if the
numerical constant $\alpha$ takes a reasonably large value, $\alpha\gtrsim
2.6$. However, in the presence of a nontrivial gravitational coupling of
$q$-field to matter, say $\widetilde{Q}$, the model may be adjusted to the
values $\Omega\Z{q}\simeq 0.73$ and $w\Z{q}\simeq -1$, even if $\alpha\sim
{\cal O}(1)$, given that the present value of $\widetilde{Q}$ is large. Unlike
for the model in [R.G. Cai, arXiv:0707.4049], the bound $\Omega\Z{q} <0.1$
during big bang nucleosynthesis (BBN) may be satisfied for almost any value of
$\alpha$. Here we discuss some other limitations of this proposal as a viable
dark energy model. The model draws some parallels with the holographic dark
energy; we also briefly comment on the latter model.; Comment: 16 pages...

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## ‣ The Phenomenology of Elastic Energy Loss

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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The unexpectedly strong suppression of high p_T heavy-quarks in heavy-ion
collisions has given rise to the idea that partons propagating through a medium
in addition to energy loss by induced radiation also undergo substantial energy
loss due to elastic collisions. However, the precise magnitude of this elastic
energy loss component is highly controversial. While it is for a parton inside
a medium surprisingly difficult to define the difference between elastic and
radiative processes rigorously, the main phenomenological difference is in the
dependence of energy loss on in-medium pathlength: in a constant medium
radiative energy loss is expected to grow quadratically with pathlength,
elastic energy loss linearly. In this paper, we investigate a class of energy
loss models with such a linear pathlength dependence and demonstrate that they
are incompatible with measured data on hard hadronic back-to-back correlations
where a substantial variation of pathlength is probed. This indicates that any
elastic energy loss component has to be small.; Comment: 7 pages, 3 figures

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