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‣ Observational Constraints on Dark Energy and Cosmic Curvature

Wang, Yun; Mukherjee, Pia
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...

‣ The High Energy Budget Allocations in Shocks and GRB

Eichler, David; Guetta, Dafne; Pohl, Martin
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...

‣ High-energy fluxes of atmospheric neutrinos

Sinegovskaya, T. S.; Ogorodnikova, E. V.; Sinegovsky, S. I.
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...

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

Wang, Yun; Lovelace, Geoffrey
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...

‣ Energy Theorem for 2+1 dimensional gravity

Menotti, P.; Seminara, D.
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

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

Waxman, Eli
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)

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

Fodor, Z.
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

‣ From Static Potentials to High-Energy Scattering

Steffen, Frank Daniel
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...

‣ Quasinormal modes of black holes absorbing dark energy

He, Xi; Wang, Bin; Wu, Shao-Feng; Lin, Chi-Yong
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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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

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

Duan, ChunGui; Song, LiHua; Wang, ShuoHe; Li, GuangLie
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...

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

Agashe, Kaustubh; Franceschini, Roberto; Hong, Sungwoo; Kim, Doojin
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...

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

Antonsen, Frank
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)

‣ Zero-point quantum fluctuations and dark energy

Maggiore, Michele
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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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...

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

Sahoo, Raghunath; Mishra, Aditya Nath; Behera, Nirbhay K.; Nandi, Basanta K.
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"

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

Younus, Mohammed; Srivastava, Dinesh K.
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

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

Bogner, S. K.; Furnstahl, R. J.; Perry, R. J.; Schwenk, A.
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

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

Visser, Matt
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

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

Harada, Tomohiro; Iguchi, Hideo; Nakao, Ken-ichi
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...

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

Neupane, Ishwaree P.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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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...

‣ The Phenomenology of Elastic Energy Loss

Renk, Thorsten
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