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## ‣ Probing the dynamics of dark energy with divergence-free parametrizations: A global fit study

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 - Phenomenology#High Energy Physics - Theory

The CPL parametrization is very important for investigating the property of
dark energy with observational data. However, the CPL parametrization only
respects the past evolution of dark energy but does not care about the future
evolution of dark energy, since $w(z)$ diverges in the distant future. In a
recent paper [J.Z. Ma and X. Zhang, Phys.\ Lett.\ B {\bf 699}, 233 (2011)], a
robust, novel parametrization for dark energy, $w(z)=w_0+w_1({\ln (2+z)\over
1+z}-\ln2)$, has been proposed, successfully avoiding the future divergence
problem in the CPL parametrization. On the other hand, an oscillating
parametrization (motivated by an oscillating quintom model) can also avoid the
future divergence problem. In this Letter, we use the two divergence-free
parametrizations to probe the dynamics of dark energy in the whole evolutionary
history. In light of the data from 7-year WMAP temperature and polarization
power spectra, matter power spectrum of SDSS DR7, and SN Ia Union2 sample, we
perform a full Markov Chain Monte Carlo exploration for the two dynamical dark
energy models. We find that the best-fit dark energy model is a quintom model
with the EOS across -1 during the evolution. However, though the quintom model
is more favored, we find that the cosmological constant still cannot be
excluded.; Comment: 6 pages...

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## ‣ Non-variable cosmologically distant gamma-ray emitters as an imprint of propagation of ultra-high-energy protons

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 10/05/2011
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The acceleration cites of ultra-high-energy (UHE) protons can be traced by
the footprint left by these particles propagating through cosmic microwave
background (CMB) radiation. Secondary electrons produced in extended region of
several tens of Mpc emit their energy via synchrotron radiation predominantly
in the initial direction of the parent protons. It forms a non-variable and
compact (almost point-like) source of high energy gamma rays. The importance of
this effect is increased for cosmologically distant objects; because of severe
energy losses, UHE protons cannot achieve us even in the case of extremely weak
intergalactic magnetic fields. Moreover, at high redshifts the energy
conversion from protons to secondary particles becomes significantly more
effective due to the denser and more energetic CMB in the past. This increases
the chances of UHE cosmic rays to be traced by the secondary synchrotron gamma
radiation. We discuss the energy budget and the redshift dependence of the
efficiency of energy transfer from UHE protons to synchrotron radiation. The
angular and spectral distributions of radiation in the gamma- and X-ray energy
bands are calculated and discussed in the context of their detectability by
Fermi LAT and Chandra observatories.

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## ‣ Cosmological Constraint and Analysis on Holographic Dark Energy Model Characterized by the Conformal-age-like Length

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 - Phenomenology#High Energy Physics - Theory

We present a best-fit analysis on the single-parameter holographic dark
energy model characterized by the conformal-age-like length,
$L=\frac{1}{a^4(t)}\int_0^tdt' a^3(t') $. Based on the Union2 compilation of
557 supernova Ia data, the baryon acoustic oscillation results from the SDSS
DR7 and the cosmic microwave background radiation data from the WMAP7, we show
that the model gives the minimal $\chi^2_{min}=546.273$, which is comparable to
$\chi^2_{\Lambda{\rm CDM}}=544.616$ for the $\Lambda$CDM model. The single
parameter $d$ concerned in the model is found to be $d=0.232\pm 0.006\pm
0.009$. Since the fractional density of dark energy $\Omega_{de}\sim d^2a^2$ at
$a \ll 1$, the fraction of dark energy is naturally negligible in the early
universe, $\Omega_{de} \ll 1$ at $a \ll 1$. The resulting constraints on the
present fractional energy density of matter and the equation of state are
$\Omega_{m0}=0.286^{+0.019}_{-0.018}^{+0.032}_{-0.028}$ and
$w_{de0}=-1.240^{+0.027}_{-0.027}^{+0.045}_{-0.044}$ respectively. The model
leads to a slightly larger fraction of matter comparing to the $\Lambda$CDM
model. We also provide a systematic analysis on the cosmic evolutions of the
fractional energy density of dark energy, the equation of state of dark energy...

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## ‣ Indirect Dark Matter Signatures in the Cosmic Dark Ages II. Ionization, Heating and Photon Production from Arbitrary Energy Injections

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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#Astrophysics - Cosmology and Nongalactic Astrophysics#Astrophysics - High Energy Astrophysical Phenomena#High Energy Physics - Phenomenology

Any injection of electromagnetically interacting particles during the cosmic
dark ages will lead to increased ionization, heating, production of Lyman-alpha
photons and distortions to the energy spectrum of the cosmic microwave
background, with potentially observable consequences. In this note we describe
numerical results for the low-energy electrons and photons produced by the
cooling of particles injected at energies from keV to multi-TeV scales, at
arbitrary injection redshifts (but focusing on the post-recombination epoch).
We use these data, combined with existing calculations modeling the cooling of
these low-energy particles, to estimate the resulting contributions to
ionization, excitation and heating of the gas, and production of low-energy
photons below the threshold for excitation and ionization. We compute corrected
deposition-efficiency curves for annihilating dark matter, and demonstrate how
to compute equivalent curves for arbitrary energy-injection histories. These
calculations provide the necessary inputs for the limits on dark matter
annihilation presented in the accompanying Paper I, but also have potential
applications in the context of dark matter decay or de-excitation, decay of
other metastable species, or similar energy injections from new physics. We
make our full results publicly available at
http://nebel.rc.fas.harvard.edu/epsilon...

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## ‣ Vacuum Fluctuations of Energy Density can lead to the observed Cosmological Constant

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 07/06/2004
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The energy density associated with Planck length is $\rho_{uv}\propto
L_P^{-4}$ while the energy density associated with the Hubble length is
$\rho_{ir}\propto L_H^{-4}$ where $L_H=1/H$. The observed value of the dark
energy density is quite different from {\it either} of these and is close to
the geometric mean of the two: $\rho_{vac}\simeq \sqrt{\rho_{uv} \rho_{ir}}$.
It is argued that classical gravity is actually a probe of the vacuum {\it
fluctuations} of energy density, rather than the energy density itself. While
the globally defined ground state, being an eigenstate of Hamiltonian, will not
have any fluctuations, the ground state energy in the finite region of space
bounded by the cosmic horizon will exhibit fluctuations $\Delta\rho_{\rm
vac}(L_P, L_H)$. When used as a source of gravity, this $\Delta \rho$ should
lead to a spacetime with a horizon size $L_H$. This bootstrapping condition
leads naturally to an effective dark energy density $\Delta\rho\propto
(L_{uv}L_H)^{-2}\propto H^2/G$ which is precisely the observed value. The model
requires, either (i) a stochastic fluctuations of vacuum energy which is
correlated over about a Hubble time or (ii) a semi- anthropic interpretation.
The implications are discussed.; Comment: r pages; revtex; comments welcome

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## ‣ Entanglement system, Casimir energy and black hole

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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We investigate the connection between the entanglement system in Minkowski
spacetime and the black hole using the scaling analysis. Here we show that the
entanglement system satisfies the Bekenstein entropy bound. Even though the
entropies of two systems are the same form, the entanglement energy is
different from the black hole energy. Introducing the Casimir energy of the
vacuum energy fluctuations rather than the entanglement energy, it shows a
feature of the black hole energy. Hence the Casimir energy is more close to the
black hole than the entanglement energy. Finally, we find that the entanglement
system behaves like the black hole if the gravitational effects are included
properly.; Comment: 11 pages, no figure, version to appear in PLB

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## ‣ Modified Weak Energy Condition for the Energy Momentum Tensor in Quantum Field Theory

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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The weak energy condition is known to fail in general when applied to
expectation values of the the energy momentum tensor in flat space quantum
field theory. It is shown how the usual counter arguments against its validity
are no longer applicable if the states $|\psi \r$ for which the expectation
value is considered are restricted to a suitably defined subspace. A possible
natural restriction on $|\psi \r$ is suggested and illustrated by two quantum
mechanical examples based on a simple perturbed harmonic oscillator
Hamiltonian. The proposed alternative quantum weak energy condition is applied
to states formed by the action of scalar, vector and the energy momentum tensor
operators on the vacuum. We assume conformal invariance in order to determine
almost uniquely three-point functions involving the energy momentum tensor in
terms of a few parameters. The positivity conditions lead to non trivial
inequalities for these parameters. They are satisfied in free field theories,
except in one case for dimensions close to two. Further restrictions on $|\psi
\r$ are suggested which remove this problem. The inequalities which follow from
considering the state formed by applying the energy momentum tensor to the
vacuum are shown to imply that the coefficient of the topological term in the
expectation value of the trace of the energy momentum tensor in an arbitrary
curved space background is positive...

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## ‣ TeV Burst of Gamma-Ray Bursts and Ultra High Energy Cosmic Rays

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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Some recent experiments detecting very high energy (VHE) gamma-rays above
10-20 TeV independently reported VHE bursts for some of bright gamma-ray bursts
(GRBs). If these signals are truly from GRBs, these GRBs must emit a much
larger amount of energy as VHE gamma-rays than in the ordinary photon energy
range of GRBs (keV-MeV). We show that such extreme phenomena can be reasonably
explained by synchrotron radiation of protons accelerated to \sim 10^{20-21}
eV, which has been predicted by Totani (1998a). Protons seem to carry about
(m_p/m_e) times larger energy than electrons, and hence the total energy
liberated by one GRB becomes as large as \sim 10^{56} (\Delta \Omega / 4 \pi)
ergs. Therefore a strong beaming of GRB emission is highly likely. Extension of
the VHE spectrum beyond 20 TeV gives a nearly model-independent lower limit of
the Lorentz factor of GRBs, as $\gamma \gtilde 500$. Furthermore, our model
gives the correct energy range and time variability of ordinary keV-MeV
gamma-rays of GRBs by synchrotron radiation of electrons. Therefore the VHE
bursts of GRBs strongly support the hypothesis that ultra high energy cosmic
rays observed on the Earth are produced by GRBs.; Comment: Final version to appear in ApJ Lett. Emphasizing that the extremely
large energy required in this model is not theoretically impossible if GRB
emission is strongly beamed. References updated

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## ‣ Comparing the energy spectra of ultra-high energy cosmic rays measured with EAS arrays

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 11/02/2010
Português

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The energy spectra of ultra-high energy cosmic rays (CRs) measured with giant
extensive air shower (EAS) arrays exhibit discrepancies between the flux
intensities and/or estimated CR energies exceeding experimental errors. The
well-known intensity correction factor due to the dispersion of the measured
quantity in the presence of a rapidly falling energy spectrum is insufficient
to explain the divergence. Another source of systematic energy determination
error is proposed concerning the charged particle density measured with the
surface arrays, which arises due to simplifications (namely, the superposition
approximation) in nucleus-nucleus interaction description applied to the shower
modeling. Making use of the essential correction factors results in congruous
CR energy spectra within experimental errors. Residual differences in the
energy scales of giant arrays can be attributed to the actual overall accuracy
of the EAS detection technique used. CR acceleration and propagation model
simulations using the dip and ankle scenarios of the transition from galactic
to extragalactic CR components are in agreement with the combined energy
spectrum observed with EAS arrays.; Comment: Accepted for publication in ApJ

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## ‣ High-energy atmospheric neutrinos

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 12/10/2010
Português

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#Astrophysics - High Energy Astrophysical Phenomena#High Energy Physics - Experiment#High Energy Physics - Phenomenology

High-energy neutrinos, arising from decays of mesons that were produced
through the cosmic rays collisions with air nuclei, form unavoidable background
noise in the astrophysical neutrino detection problem. The atmospheric neutrino
flux above 1 PeV should be supposedly dominated by the contribution of charmed
particle decays. These (prompt) neutrinos originated from decays of massive and
shortlived particles, $D^\pm$, $D^0$, $\bar{D}{}^0$, $D_s^\pm$, $\Lambda^+_c$,
form the most uncertain fraction of the high-energy atmospheric neutrino flux
because of poor explored processes of the charm production. Besides, an
ambiguity in high-energy behavior of pion and especially kaon production cross
sections for nucleon-nucleus collisions may affect essentially the calculated
neutrino flux. There is the energy region where above flux uncertainties
superimpose. A new calculation presented here reveals sizable differences, up
to the factor of 1.8 above 1 TeV, in muon neutrino flux predictions obtained
with usage of known hadronic models, SIBYLL 2.1 and QGSJET-II. The atmospheric
neutrino flux in the energy range $10-10^7$ GeV was computed within the 1D
approach to solve nuclear cascade equations in the atmosphere, which takes into
account non-scaling behavior of the inclusive cross-sections for the particle
production...

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## ‣ Super-Penrose collisions are inefficient - a Comment on: Black hole fireworks: ultra-high-energy debris from super-Penrose collisions

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 08/01/2015
Português

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

In a paper posted on the arXiv a few weeks ago Berti, Brito and Cardoso
\cite{Berti+14} suggest that ultra-high-energy particles can emerge from
collisions in a black hole's ergosphere. This can happen if the process
involves a particle on an outgoing trajectory very close to the black hole.
Clearly such a particle cannot emerge from the black hole. It is argued
\cite{Berti+14} that this particle can arise in another collision. Thus the
process involves two collisions: one in which an outgoing particle is produced
extremely close to the horizon, and a second one in which energy is gained. The
real efficiency of this process should take into account, therefore, the energy
needed to produce the first particle. We show here that while this process is
kinematically possible, it requires a deposition of energy that is divergently
large compared with the energy of the escaping particle. Thus, in contradiction
to claims of infinitely high efficiencies, the efficiency of the combined
process is in fact extremely small, approaching zero for very high output
energies. Even under more general conditions than those considered in
\cite{Berti+14} the total energy gain never diverges, and is larger only by a
factor of a few than the energy gain of the original collisional Penrose
process that takes place between two infalling particles
\cite{Piran+75...

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## ‣ Sharp knee phenomenon of primary cosmic ray energy spectrum

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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Primary energy spectral models are tested in the energy range of 1-200 PeV
using standardized extensive air shower responses from BASJE-MAS, Tibet, GAMMA
and KASCADE scintillation shower arrays. Results point towards the
two-component origin of observed cosmic ray energy spectra in the knee region
(GAPS spectral model) consisting of a pulsar component superimposed upon
rigidity-dependent power law diffuse galactic flux. The two-component energy
spectral model accounts for both the sharp knee shower spectral phenomenon and
observed irregularity of all-particle energy spectrum in the region of 50-100
PeV. Alternatively, tested multi-population primary energy spectra predicted by
non-linear diffusive shock acceleration (DSA) models describe observed shower
spectra in the knee region provided that the cutoff magnetic rigidities of
accelerating particles are 6.0+/-0.3 PV and 45+/-2 PV for the first two
populations respectively. Both tested spectral models confirm the predominant
H-He primary nuclei origin of observed shower spectral knee. The parameters of
tested energy spectra are evaluated using solutions of inverse problem on the
basis of the corresponding parameterizations of energy spectra for primary H,
He, O-like and Fe-like nuclei...

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## ‣ Energy dependence of W values for protons in hydrogen

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 22/05/2014
Português

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The mean energy $W$ required to produce an ion pair in molecular hydrogen has
been obtained for protons in the energy range between 1 MeV and 4.5 MeV. The W
values were derived from the existing experimental data on elastic {\it
$\pi^-$p} scattering at the beam energy of 40 GeV. In the experiment, the
ionization chamber IKAR filled with hydrogen at a pressure of 10 at served
simultaneously as a gas target and a detector for recoil protons. For selected
events of elastic scattering, the ionization yield produced by recoil protons
was measured in IKAR, while the energy was determined kinematically through the
scattering angles of the incident particles measured with a system of
multi-wire proportional chambers. The ionization produced by $\alpha$-particles
from $\alpha$-sources of $^{234}$U deposited on the chamber electrodes was used
for absolute normalization of the W values. The energy dependence of $W$ for
protons in H$_2$ shows an anomalous increase of $W$ with increasing energy in
the measured energy range. At the energy of 4.76 MeV, the ionization yield for
alpha particles is by 2\% larger than that for protons.; Comment: 12 pages, 5 figures

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## ‣ Interacting Ghost Dark Energy in Non-Flat Universe

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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

A new dark energy model called "ghost dark energy" was recently suggested to
explain the observed accelerating expansion of the universe. This model
originates from the Veneziano ghost of QCD. The dark energy density is
proportional to Hubble parameter, $\rho_D=\alpha H$, where $\alpha$ is a
constant of order $\Lambda_{\rm QCD}^3$ and $\Lambda_{\rm QCD}\sim 100 MeV$ is
QCD mass scale. In this paper, we extend the ghost dark energy model to the
universe with spatial curvature in the presence of interaction between dark
matter and dark energy. We study cosmological implications of this model in
detail. In the absence of interaction the equation of state parameter of ghost
dark energy is always $w_D > -1 $ and mimics a cosmological constant in the
late time, while it is possible to have $w_D < -1 $ provided the interaction is
taken into account. When $k = 0$, all previous results of ghost dark energy in
flat universe are recovered. To check the observational consistency, we use
Supernova type Ia (SNIa) Gold sample, shift parameter of Cosmic Microwave
Background radiation (CMB) and the Baryonic Acoustic Oscillation peak from
Sloan Digital Sky Survey (SDSS). The best fit values of free parameter at
$1\sigma$ confidence interval are: $\Omega_m^0= 0.35^{+0.02}_{-0.03}$...

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## ‣ Correlation of $\gamma$-ray and high-energy cosmic ray fluxes from the giant lobes of Centaurus A

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/12/2013
Português

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The spectral energy distribution of giant lobes shows one main peak detected
by the Wilkinson Microwave Anisotropy Probe at low energy of $10^{-5}$ eV and a
faint $\gamma$-ray flux imaged by Fermi Large Area Telescope at energy $\geq$
100 MeV. On the other hand, Pierre Auger Observatory associated some
ultra-high-energy cosmic rays with the direction of Centaurus A and IceCube
reported 28 neutrino-induced events in a TeV - PeV energy range, although none
of them related with this direction. In this work we describe the spectra for
each of the lobes, the main peak with synchrotron radiation, and the
high-energy emission with $pp$ interactions. Obtaining a good description of
the main peak, we deduce the magnetic fields, electron densities and the age of
the lobes. Describing successfully the $\gamma$-ray emission by pp interactions
and considering as targets those thermal particles in the lobes with density in
the range $10^{-10}$ to $10^{-4}\, {\rm cm}^{-3}$, we calculate the number of
ultra-high-energy cosmic rays. Although $\gamma$-spectrum is well described
with any density in the range, only when $10^{-4}\, {\rm cm}^{-3}$ is
considered, the expected number of events is very similar to that observed by
Pierre Auger Observatory, otherwise we obtain an excessive luminosity. In
addition...

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## ‣ A Dark Energy Model Characterized by the 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

Quantum mechanics together with general relativity leads to the
K\'arolyh\'azy relation and a corresponding energy density of quantum
fluctuations of space-time. Based on the energy density we propose a dark
energy model, in which the age of the universe is introduced as the length
measure. This dark energy is consistent with astronomical data if the unique
numerical parameter in the dark energy model is taken to be a number of order
one. The dark energy behaves like a cosmological constant at early time and
drives the universe to an eternally accelerated expansion with power-law form
at late time. In addition, we point out a subtlety in this kind of dark energy
model.; Comment: Latex, 8 pages with one figure;v2: some minor changes and a few
references added;v3: some explanations on the result added; v4: the version
to appear in PLB

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## ‣ Quantum states with negative energy density in the Dirac field and quantum inequalities

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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Energy densities of the quantum states that are superposition of two
multi-electron-positron states are examined. It is shown that the energy
densities can be negative only when two multi-particle states have the same
number of electrons and positrons or when one state has one more
electron-positron pair than the other. In the cases in which negative energy
could arise, we find that the energy is that of a positive constant plus a
propagating part which oscillates between positive and negative, and the energy
can dip to negative at some places at for a certain period of time if the
quantum states are properly manipulated. It is demonstrated that the negative
energy densities satisfy the quantum inequality. Our results also reveal that
for a given particle content, the detection of negative energy is an operation
that depends on the frame where any measurement is to be performed. This
suggests that the sign of energy density for a quantum state may be a
coordinate-dependent quantity in quantum theory.; Comment: Revtex,9 pages, no figures, a couple of typos corrected

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## ‣ On the High Energy Emission of the Short GRB 090510

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Português

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Long-lived high-energy (>100MeV) emission, a common feature of most Fermi-LAT
detected gamma-ray burst, is detected up to \sim 10^2 s in the short GRB
090510. We study the origin of this long-lived high-energy emission, using
broad-band observations including X-ray and optical data. We confirm that the
late > 100 MeV, X-ray and optical emission can be naturally explained via
synchrotron emission from an adiabatic forward shock propagating into a
homogeneous ambient medium with low number density. The Klein-Nishina effects
are found to be significant, and effects due to jet spreading and magnetic
field amplification in the shock appear to be required. Under the constraints
from the low-energy observations, the adiabatic forward shock synchrotron
emission is consistent with the later-time (t>2s) high-energy emission, but
falls below the early-time (t < 2s) high energy emission. Thus we argue that an
extra high energy component is needed at early times. A standard reverse shock
origin is found to be inconsistent with this extra component. Therefore, we
attribute the early part of the high-energy emission (t< 2s) to the prompt
component, and the long-lived high energy emission (t>2s) to the adiabatic
forward shock synchrotron afterglow radiation. This avoids the requirement for
an extremely high initial Lorentz factor.; Comment: 29 pages...

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## ‣ Decoupling Dark Energy from Matter

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 22/04/2009
Português

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

We examine the embedding of dark energy in high energy models based upon
supergravity and extend the usual phenomenological setting comprising an
observable sector and a hidden supersymmetry breaking sector by including a
third sector leading to the acceleration of the expansion of the universe. We
find that gravitational constraints on the non-existence of a fifth force
naturally imply that the dark energy sector must possess an approximate shift
symmetry. When exact, the shift symmetry provides an example of a dark energy
sector with a runaway potential and a nearly massless dark energy field whose
coupling to matter is very weak, contrary to the usual lore that dark energy
fields must couple strongly to matter and lead to gravitational
inconsistencies. Moreover, the shape of the potential is stable under one-loop
radiative corrections. When the shift symmetry is slightly broken by higher
order terms in the Kahler potential, the coupling to matter remains small.
However, the cosmological dynamics are largely affected by the shift symmetry
breaking operators leading to the appearance of a minimum of the scalar
potential such that dark energy behaves like an effective cosmological constant
from very early on in the history of the universe.; Comment: 22 pages...

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## ‣ Superconducting dark energy

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 10/04/2015
Português

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

Based on the analogy with superconductor physics we consider a
scalar-vector-tensor gravitational model, in which the dark energy action is
described by a gauge invariant electromagnetic type functional. By assuming
that the ground state of the dark energy is in a form of a condensate with the
U(1) symmetry spontaneously broken, the gauge invariant electromagnetic dark
energy can be described in terms of the combination of a vector and of a scalar
field (corresponding to the Goldstone boson), respectively. The gravitational
field equations are obtained by also assuming the possibility of a non-minimal
coupling between the cosmological mass current and the superconducting dark
energy. The cosmological implications of the dark energy model are investigated
for a Friedmann-Robertson-Walker homogeneous and isotropic geometry for two
particular choices of the electromagnetic type potential, corresponding to a
pure electric type field, and to a pure magnetic field, respectively. The time
evolution of the scale factor, matter energy density and deceleration parameter
are obtained for both cases, and it is shown that in the presence of the
superconducting dark energy the Universe ends its evolution in an exponentially
accelerating vacuum de Sitter state. By using the formalism of the irreversible
thermodynamic processes for open systems we interpret the generalized
conservation equations in the superconducting dark energy model as describing
matter creation. The particle production rates...

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