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The spin diffusion and damped oscillations are studied in the collision of two spin polarized clouds of cold atoms with resonant interactions. The strong density dependence of the diffusion coefficient leads to inhomogeneous spin diffusion that changes from central to surface spin flow as the temperature increases. The inhomogeneity and the smaller finite trap size significantly reduce the spin diffusion rate at low temperatures. The resulting spin diffusion rates, spin drag and initial damped oscillations are compatible with measurements at low to high temperatures for resonant attractive interactions but are incompatible with a metastable ferromagnetic phase., Comment: 3 pages, 1 figure; Phys. Rev. Letters, 2012

Itinerant ferromagnetism in cold Fermi gases with repulsive interactions is studied applying the Jastrow-Slater approximation generalized to finite polarization and temperature. For two components at zero temperature a second order transition is found at $ak_F\simeq0.90$ compatible with QMC. Thermodynamic functions and observables such as the compressibility and spin susceptibility and the resulting fluctuations in number and spin are calculated. For trapped gases the resulting cloud radii and kinetic energies are calculated and compared to recent experiments. Spin polarized systems are recommended for effective separation of large ferromagnetic domains. Collective modes are predicted and tri-critical points are calculated for multi-component systems., Comment: 8 pages, 6 figures

Universality and crossover is described for attractive and repulsive interactions where, respectively, the BCS-BEC crossover takes place and a ferromagnetic phase transition is claimed. Crossovers are also described for optical lattices and multicomponent systems. The crossovers, universal parameters and phase transitions are described within the Leggett and NSR models and calculated in detail within the Jastrow-Slater approximation. The physics of ultracold Fermi atoms is applied to neutron, nuclear and quark matter, nuclei and electrons in solids whenever possible. Specifically, the differences between optical lattices and cuprates is discussed w.r.t. antiferromagnetic, d-wave superfluid phases and phase separation., Comment: 50 pages, 15 figures. Contribution to Lecture Notes in Physics "BCS-BEC crossover and the Unitary Fermi Gas" edited by W. Zwerger

Many-body effects on superfluidity and transition temperatures are calculated for optical lattices and uniform systems with ultracold multi-component Fermi gases. The induced interactions depend sensitively on the interactions between the multi-components and their densities. The s- and d-wave pairing gaps and critical temperatures are calculated for optical lattices at low (dilute) filling as well as near half filling to leading orders in the interaction strength with and without induced interactions. They can deviate strongly from dilute two-component systems and affect the phase diagram. In two dimensional optical lattices the induced interactions are singular at half filling and strongly affect s- and d-wave superfluidity., Comment: 4 pages, multi-component extensions

Antiferromagnetic, Mott insulator, d-wave and gossamer superfluid phases are calculated for 2D square lattices from the extended Hubbard (t-J-U) model using the Gutzwiller projection method and renormalized mean field theory. Phase separation between antiferromagnetic and d-wave superfluid phases is found near half filling when the on-site repulsion exceeds $U\ga7.3t$, and coincides with a first order transition in the double occupancy. Phase separation is thus predicted for 2D optical lattices with ultracold Fermi atoms whereas it is inhibited in cuprates by Coulomb frustration which instead may lead to stripes. In a confined optical lattice the resulting density distribution is discontinuous an with extended Mott plateau which enhances the antiferromagnetic phase but suppresses the superfluid phase. Observation of Mott insulator, antiferromagnetic, stripe and superfluid phases in density and momentum distributions and correlations is discussed.

Fermionic atoms in 2D optical lattices and electrons in HTc cuprates may both be described by the Hubbard model. However, if Coulomb frustration is responsible for the striped phases in 2D cuprates the phase diagrams will differ markedly. Two representative scenarios are described by a simple stripe model without phase separation and a mean field model with phase separation in the absence of Coulomb frustration. When Coulomb frustrated both models display antiferromagnetism (AF) and stripe phases with d-wave superfluidity, whereas neutral atoms in optical lattices will only do so in the stripe model. Radii and densities of the various phases in harmonically confined optical lattices are calculated for the two models and have very different Mott plateaus and density discontinuities. We discuss observation of antiferromagnetic, stripe and superfluid phases in density and momentum distributions and correlations from time-of-flight experiments., Comment: 8 pages, 3 figures

We calculate level densities and pairing gaps for an ultracold dilute gas of fermionic atoms in harmonic traps under the influence of mean field and anharmonic quartic trap potentials. Super-shell structures, which were found in Hartree-Fock calculations, are calculated analytically within periodic orbit theory as well as from WKB calculations. For attractive interactions, the underlying level densities are crucial for pairing and super-shell structures in gaps are predicted., Comment: 4 pages, 2 figures.

Phases of Bose or Fermi atoms in optical lattices confined in harmonic traps are studied within the Thomas-Fermi approximation. Critical radii and particle number for onset of Mott insulator states are calculated and phase diagrams shown in 1D, and estimated for 2 and 3D. Methods to observe these and novel phases such as d-wave superconductivity is discussed. Specifically the collective modes are calculated., Comment: Revised and extended. To appear in PRA

The Bose-Hubbard model is extended to include nearest and far neighbor interactions and is related to the fractional quantum Hall effect (FQHE). Both models may be studied in optical lattices with quantum gases. The ground state is calculated for the extended Bose-Hubbard model with strong repulsive interactions (weak hopping). Incompressible Mott insulator states are found at rational filling fractions compatible with the principal and secondary FQHE filling fractions of the lowest Landau levels observed experimentally. It is discussed to which extent these states at fractional filling survive or undergoes a Mott insulator transition to a superfluid as hopping terms are included., Comment: Revised version, to appear in PRA

First and second sound speeds are calculated for a uniform superfluid gas of fermi atoms as a function of temperature, density and interaction strength. The second sound speed is of particular interest as it is a clear signal of a superfluid component and it determines the critical temperature. The sound modes and their dependence on density, scattering length and temperature are calculated in the BCS, molecular BEC and unitarity limits and a smooth crossover is extrapolated. It is found that first and second sound undergo avoided crossing on the BEC side due to mixing. Consequently, they are detectable at crossover both as density and thermal waves in traps., Comment: To appear in Phys. Rev. A

The thermodynamic potential is calculated for a uniform superfluid gas of fermi atoms from the mean field BCS equations including corrections from induced interactions, Hartree-Fock energies and quasiparticle selfenergies. The entropy, specific heat and sound modes are calculated as function of temperature, density and interaction strength from the BCS to the unitarity limit and around the BCS-BEC crossover. The second sound speed is of particular interest as it is a clear signal of a superfluid component and it determines the critical temperature., Comment: 4 pages, 2 figures

Strong evidence for pairing and superfluidity has recently been found in atomic Fermi gases at the BCS-BEC crossover both in collective modes and RF excitation energies. It is argued that the scale for the effective pairing gaps measured in RF experiments is set by the lowest quasiparticle in-gap excitation energies. These are calculated at the BCS-BEC crossover from semiclassical solutions to the Bogoliubov-deGennes equations. The strong damping of the radial breathing mode observed in the BCS limit occur when the lowest quasiparticle excitation energies coincide with the radial frequency, which indicates that a coupling between them take place., Comment: revised version: including damping, resonance position typo corrected

The collective mode frequencies in isotropic and deformed traps are calculated for general polytropic equation of states, $P\propto n^{\gamma+1}$, and expressed in terms of $\gamma$ and the trap geometry. For molecular and standard Bose-Einstein condensates and Fermi gases near Feshbach resonances, the effective power $\gamma\simeq0.5-1.3$ is calculated from Jastrow type wave-function ans\"atze, and from the crossover model of Leggett. The resulting mode frequencies are calculated for these phases around the BCS-BEC crossover., Comment: Revised version to be published in PRL

Near Feshbach resonances, $n|a|^3\gg 1$, systems of Bose and Fermi particles become strongly interacting/dense. In this unitary limit both bosons and fermions have very different properties than in a dilute gas, e.g., the energy per particle approach a value $\hbar^2n^{2/3}/m$ times an universal many-body constant. Calculations based upon an approximate Jastrow wave function can quantitatively describe recent measurements of trapped Bose and Fermi atoms near Feshbach resonances. The pairing gap between attractive fermions also scales as $\Delta\sim\hbar^2n^{2/3}/m$ near Feshbach resonances and is a large fraction of the Fermi energy - promising for observing BCS superfluidity in traps. Pairing undergoes several transitions depending on interaction strength and the number of particles in the trap and can also be compared to pairing in nuclei., Comment: Revised version extended to include recent molecular BEC-BCS results

Pairing gaps for fermionic atoms in harmonic oscillator traps are calculated for a wide range of interaction strengths and particle number, and compared to pairing in nuclei. Especially systems, where the pairing gap exceeds the level spacing but is smaller than the shell splitting $\hbar\omega$, are studied which applies to most trapped Fermi atomic systems as well as to finite nuclei. When solving the gap equation for a large trap with such multi-level pairing, one finds that the matrix elements between nearby harmonic oscillator levels and the quasi-particle energies lead to a double logarithm of the gap, and a pronounced shell structure at magic numbers. It is argued that neutron and proton pairing in nuclei belongs to the class of multi-level pairing, that their shell structure follows naturally and that the gaps scale as $\sim A^{-1/3}$ - all in qualitative agreement with odd-even staggering of nuclear binding energies. Pairing in large systems are related to that in the bulk limit. For large nuclei the neutron and proton superfluid gaps approach the asymptotic value in infinite nuclear matter: $\Delta\simeq 1.1$ MeV., Comment: 11 pages, 5 figures

We calculate the elementary excitations and pairing of a trapped atomic Fermi gas in the superfluid phase. The level spectra and pairing gaps undergo several transitions as the strength of the interactions between and the number of atoms are varied. For weak interactions, the Cooper pairs are formed between particles residing in the same harmonic oscillator shell. In this regime, the nature of the paired state is shown to depend critically on the position of the chemical potential relative to the harmonic oscillator shells and on the size of the mean field. For stronger interactions, we find a region where pairing occur between time-reversed harmonic oscillator states in different shells also., Comment: Slightly revised version: Mistakes in equation references in figures corrected. Accepted for Phys. Rev. A

We calculate energy levels of two and three bosons trapped in a harmonic oscillator potential with oscillator length $a_{\mathrm osc}$. The atoms are assumed to interact through a short-range potential with a scattering length $a$, and the short-distance behavior of the three-body wave function is characterized by a parameter $\theta$. For large positive $a/a_{\mathrm osc}$, the energies of states which, in the absence of the trap, correspond to three free atoms approach values independent of $a$ and $\theta$. For other states the $\theta$ dependence of the energy is strong, but the energy is independent of $a$ for $|a/a_{\mathrm osc}|\gg1$., Comment: 4 pages, 3 figures

A review of current neutron star masses and radius measurements is given and the resulting constraints on modern equation of states for nuclear matter. Phase transitions in particular to quark matter is discussed and the novel color superconducting mixed phases., Comment: 14 pages. Revised version

The shell structures for weakly interacting fermions in harmonic oscillator traps at zero temperature undergo several transitions depending on the number of particles in the trap and their interaction strength. Calculations of the one and two particle spectra give the pairing gaps, and the many particle state is constructed by the seniority scheme. For sufficiently few particles, $N\la 10^4$, the pairing field exceeds the mean-field splitting of single particle levels at the Fermi surface and the greater number of almost degenerate states due to the SU(3) symmetry in the harmonic oscillator shell lead to {\it supergaps}. Deformation and rotation are also discussed., Comment: 4 pages, revised version, to appear in Phys.Rev.Lett

We calculate the energy and condensate fraction for a dense system of bosons interacting through an attractive short range interaction with positive s-wave scattering length $a$. At high densities, $n>>a^{-3}$, the energy per particle, chemical potential, and square of the sound speed are independent of the scattering length and proportional to $n^{2/3}$, as in Fermi systems., Comment: 4 pages, 3 figures

Anisotropic transverse flow is studied in Pb+Pb and Au+Au collisions at SPS and RHIC energies. The centrality and transverse momentum dependence at midrapidity of the elliptic flow coefficient v_2 is calculated in the hydrodynamic and low density limits. Hydrodynamics is found to agree well with the RHIC data for semicentral collisions up to transverse momenta of 1-1.5 GeV/c, but it considerably overestimates the measured elliptic flow at SPS energies. The low density limit LDL is inconsistent with the measured magnitude of v_2 at RHIC energies and with the shape of its p_t-dependence at both RHIC and SPS energies. The success of the hydrodynamic model points to very rapid thermalization in Au+Au collisions at RHIC and provides a serious challenge for kinetic approaches based on classical scattering of on-shell particles., Comment: 7 pages incl. 5 figures; submitted to Physics Letters B; Ref. 4 and a few typos corrected; no changes in contents

Event-by-event fluctuations and correlations between particles produced in relativistic nuclear collisions are studied. The fluctuations in positive, negative, total and net charge are closely related through correlations. In the event of a phase transitions to a quark-gluon plasma, fluctuations in total and net charge can be enhanced and reduced respectively which, however, is very sensitive to the acceptance and centrality. If the colliding system experiences strong density fluctuations due, e.g., to droplet formation in a first-order phase transition, all fluctuations can be enhanced substantially. The importance of fluctuations and correlations is exemplified by event-by-event measurement of the multiplicities of $J/\Psi$'s and charged particles since these observables should anti-correlate in the presence of co-mover or anomalous absorption., Comment: revised version to appear in Phys. Rev. C, 5 pages

We calculate the effects of induced interactions on the transition temperature to the BCS state in dilute Fermi gases. For a pure Fermi system with 2 species having equal densities, the transition temperature is suppressed by a factor $(4e)^{1/3}\approx 2.2$, and for $\nu$ fermion species, the transition temperature is increased by a factor $(4e)^{\nu /3-1} \approx 2.2^{\nu-3}$. For mixtures of fermions and bosons the exchange of boson density fluctuations gives rise to an effective interaction, and we estimate the increase of the transition temperature due to this effect., Comment: 4 pages, 3 figures

Motivated by forthcoming experiments at RHIC and LHC, and results from SPS, a review is given of the present state of event-by-event fluctuations in ultrarelativistic heavy-ion collisions. Fluctuations in particle multiplicities, ratios, transverse momenta, rapidity, etc. are calculated in participant nucleon as well as thermal models. The physical observables, including multiplicity, kaon to pion ratios, and transverse momenta agree well with recent NA49 data at the SPS, and indicate that such studies do not yet reveal the presence of new physics. Predictions for RHIC and LHC energies are given. The centrality dependence with and without a phase transition to a quark-gluon plasma is discussed - in particular, how the physical quantities are expected to display a qualitative different behavior in case of a phase transition, and can be signaled by anomalous fluctuations and correlations in a number of observables., Comment: 44 pages, 10 figs., revised, to appear in phys. rep. 2001

This review contains chapters discussing: Energy density fluctionals of nuclear matter, Many-body theory of nucleon matter, Hadronic and quark matter, Mixtures of phases in dense matter, Neutron star observations and predictions., Comment: 33 pages +13 figs., Ann. Rev. Nucl. & Part. Science, 2000

Ground state energies and superfluid gaps are calculated for degenerate Fermi systems interacting via long attractive scattering lengths such as cold atomic gases, neutron and nuclear matter. In the intermediate region of densities, where the interparticle spacing $(\sim 1/k_F)$ is longer than the range of the interaction but shorter than the scattering length, the superfluid gaps and the energy per particle are found to be proportional to the Fermi energy and thus differs from the dilute and high density limits. The attractive potential increase linearly with the spin-isospin or hyperspin statistical factor such that, e.g., symmetric nuclear matter undergoes spinodal decomposition and collapses whereas neutron matter and Fermionic atomic gases with two hyperspin states are mechanically stable in the intermediate density region. The regions of spinodal instabilities in the resulting phase diagram are reduced and do not prevent a superfluid transition., Comment: extended and revised version, 7 pages including new phase diagram

Color superconductivity in quark matter is studied for electrically charge neutral neutron star matter in $\beta$-equilibrium. Both bulk quark matter and mixed phases of quark and nuclear matter are treated. The electron chemical potential and strange quark mass affect the various quark chemical potentials and therefore also the color superconductivity due to dicolor pairing or color-flavor locking., Comment: 3 pages including 1 figure

The kaon energy in neutron matter is calculated analytically with the Klein-Gordon equation, by making a Wigner-Seitz cell approximation and employing a $K^-N$ square well potential. The transition from the low density Lenz potential, proportional to scattering length, to the high density Hartree potential is found to begin at fairly low densities. Exact non-relativistic calculations of the kaon energy in a simple cubic crystal of neutrons are used to test the Wigner-Seitz and the Ericson-Ericson approximation methods. All the calculations indicate that by $\sim 4$ times nuclear matter density the Hartree limit is reached, and as the Hartree potential is less attractive, the density for kaon condensation appears to higher than previously estimated. Effects of a hypothetical repulsive core in the $K^-N$ potential are also studied., Comment: 4 pages including 2 figures

Recent developments in neutron star theory and observation are discussed. Based on modern nucleon-nucleon potentials more reliable equations of state for dense nuclear matter have been constructed. Furthermore, phase transitions such as pion, kaon and hyperon condensation, superfluidity and quark matter can occur in cores of neutron stars. Specifically, the nuclear to quark matter phase transition and its mixed phases with intriguing structures is treated. Rotating neutron stars with and without phase transitions are discussed and compared to observed masses, radii and glitches. The observations of possible heavy $\sim 2M_\odot$ neutron stars in X-ray binaries and QPO's require relatively stiff equation of states and restrict strong phase transitions to occur at very high nuclear densities only., Comment: Proc. of the 10th Int. Conf. on Recent Progress in Many-Body Theories (MBX), Seattle, 10-15 Sep 1999, World Scientific. 16 pages

After a brief history of neutron stars and supernovae recent developments are discussed. Based on modern nucleon-nucleon potentials more reliable equations of state for dense nuclear matter have been constructed. Furthermore, phase transitions such as pion, kaon and hyperon condensation, superfluidity and quark matter can occur in cores of neutron stars. Specifically, the nuclear to quark matter phase transition and its mixed phases with intriguing structures is treated. Rotating neutron stars with and without phase transitions are discussed and compared to observed masses, radii and glitches. The observations of possible heavy $\sim 2M_\odot$ neutron stars in X-ray binaries and QPO's require relatively stiff equation of states and restricts strong phase transitions to occur at very high nuclear densities only., Comment: Proc. of 6. Colloque Cosmologie, Obs. de Paris, june 16-18, 1999

Motivated by forthcoming experiments at RHIC and LHC, we study event-by-event fluctuations in ultrarelativistic heavy-ion collisions in participant nucleon as well as thermal models. The calculated physical observables, including multiplicity, kaon to pion ratios, and transverse momenta agree well with recent NA49 data at the SPS, and indicate that such studies do not yet reveal the presence of new physics. Finally, we present a simple model of how a first order phase transition can be signaled by very large fluctuations., Comment: final version, 4 pages, to appear in Phys. Lett. B

Using the most recent realistic effective interactions for nuclear matter with a smooth extrapolation to high densities including causality, we constrain the equation of state and calculate maximum masses of rotating neutron stars. First and second order phase transitions to, e.g., quark matter at high densities are included. If neutron star masses of $\sim 2.3M_\odot$ from quasi-periodic oscillations in low mass X-ray binaries are confirmed, a soft equation of state as well as strong phase transitions can be excluded in neutron star cores., Comment: Replaced with revised version, 7 pages, 3 figs. To appear in Ap. J. Lett

The electrical conductivity has been calculated in the early universe at temperatures below as well as above the electroweak vacuum scale, $T_c\simeq 100$GeV. Debye and dynamical screening of electric and magnetic interactions leads to a finite conductivity, $\sigma_{el}\sim T/\alpha\ln(1/\alpha)$, at temperatures well below $T_c$. At temperatures above, $W^\pm$ charge-exchange processes -- analogous to color exchange through gluons in QCD -- effectively stop left-handed charged leptons. However, right-handed leptons can carry current, resulting in $\sigma_{el}/T$ being only a factor $\sim \cos^4\theta_W$ smaller than at temperatures below $T_c$., Comment: 6 pages including 1 figures. Proc. of SEWM, dec. 98

The nuclear overlap zone in non-central relativistic heavy ion collisions is azimuthally very asymmetric. By varying the angle between the axes of deformation and the transverse direction of the pair momenta, the suppression of $J/\Psi$ and $\Psi'$ will oscillate in a characteristic way. Whereas the average suppression is mostly sensitive to the early and high density stages of the collision, the amplitude is more sensitive to the late stages. This effect provides additional information on the $J/\Psi$ suppression mechanisms such as direct absorption on participating nucleons, comover absorption or formation of a quark-gluon plasma. The behavior of the average $J/\Psi$ suppression and its amplitude with centrality of the collisions is discussed for SPS, RHIC and LHC energies with and without a phase transition., Comment: Revised and extended version, new figures

Elliptic flow and HBT are studied for non-central relativistic nuclear collisions. Azimuthal asymmetries show up in both elliptic flow and HBT radii and are calculated in both collisionless and hydrodynamic limits relevant for peripheral and central collisions respectively. Determining the reaction plane and measuring the HBT radii as function of the angle between the reaction plane and the particle momenta can determine the physical quantities as source sizes, deformations, emission times, duration of emission and opacities. Comparison to SPS data and predictions for RHIC and LHC energies are given. The centrality dependence with and without a phase transition to a quark-gluon plasma is discussed - in particular, how the physical quantities are expected to display a qualitative different behavior in case of a phase transition., Comment: Revised version, to appear in Phys. Rev. C

The nuclear overlap zone in non-central relativistic heavy ion collisions is azimuthally very asymmetric. By varying the angle between the axes of deformation and the transverse direction of the pair momenta, the transverse HBT radii oscillate in a characteristic way. It is shown that these oscillations allow determination of source sizes, deformations as well as the opacity and duration of emission of the source created in any non-central high energy nuclear collisions. The behavior of the physical quantities with centrality of the collisions is discussed --- in particular changes caused by a possible phase transition to a quark-gluon plasma., Comment: Revised version, to appear in Phys. Rev. Letters

We discuss possible experimental signatures of forming a Quark-Gluon plasma in high energy nuclear collisions. In first order phase transitions such as the chiral symmetry restoration supercooling may lead to density fluctuations such as droplet formation and hot spots. These will lead to rapidity fluctuations event-by-event and triggering on such fluctuations will have distinct effects for HBT radii., Comment: 10 pages, proc. of "Advances in QCD", Minnesota, april 16-19, 1998

The kaon energy in a nuclear medium and its dependence on kaon-nucleon and nucleon-nucleon correlations is discussed. The transition from the Lenz potential at low densities to the Hartree potential at high densities can be calculated analytically by making a Wigner-Seitz cell approximation and employing a square well potential. As the Hartree potential is less attractive than the Lenz one, kaon condensation inside cores of neutron stars appears to be less likely than previously estimated., Comment: 5 pages, Proc. of "Nuclear Astrophysics", Hirschegg jan. 11-17, 1998

Phase transitions in neutron stars due to formation of quark matter, kaon condensates, etc. are discussed with particular attention to the order of these transitions. Observational consequences of phase transitions in pulsar angular velocities are examined., Comment: 5 pages, proc. of Hirschegg meeting, jan. 11-17, 1998

The freeze-out of hot and dense hadronic matter formed in relativistic nuclear collisions is probed by HBT interferometry of identical pions, kaons, etc. Coulomb repulsion/attraction of positive/negative particles show up at small particle momenta and is also very sensitive to the freeze-out conditions. The source sizes and times freeze-out are extracted from $\pi^-/\pi^+$ spectra and HBT radii and compared., Comment: 4 pages, proc. of QM'97, Tsukuba, Japan

As rotating neutron stars slow down, the pressure and the density in the core region increase due to the decreasing centrifugal forces and phase transitions may occur in the center. We extract the analytic behavior near the critical angular velocity $\Omega_0$, where the phase transitions occur in the center of a neutron star, and calculate the moment of inertia, angular velocity, rate of slow down, braking index, etc. For a first order phase transition these quantities have a characteristic behavior, e.g., the braking index diverges as $\sim(\Omega_0-\Omega)^{-1/2}$. Observational consequences for first, second and other phase transitions are discussed., Comment: 5 pages, one figure included, revtex latex style

Coulomb effects on $\pi^\pm$ and $K^\pm$ spectra in relativistic nuclear collisions are investigated. At collision energies around 1 GeV the ratio of $\pi^-$ to $\pi^+$ is enhanced several times at low transverse momenta but less at ultrarelativistic energies. We describe the ratios at SIS, AGS and SPS energies with simple analytic models as well as more elaborate numerical models incorporating the expansion dynamics. The Coulomb effect depends on the properties of the source after the violent collision phase and provides information on source sizes, freeze-out times, and expansion velocities. Comparison with results from HBT analyses are made. Predictions for $\pi^\pm$ and $K^\pm$ at RHIC and LHC energies are given., Comment: 22 pages including 12 figures. Added figure and revisions

We discuss the possibility that the sources for gamma ray bursts are hot neutron stars at cosmological distance scales. The temperature of such stars would be $T \sim 1 \MeV$. Such hot stars can produce an electromagnetic blast wave provided that the ratio of baryon and photon numbers $N_{B}/N_{\gamma} \le 10^{-6}$. The typical time scale for such blasts, the total luminosity, and correlation of gamma ray energy with time of arrival are shown to be roughly consistent with observation. The spectrum of photons also appears to be consistent with known data., Comment: 19 pages. 1 postscript figure included. Uses LaTeX2e and epsf package

Bose-Einstein correlations in relativistic heavy ion collisions and their dependence on the freeze-out condition in hydrodynamic models is compared to time-like freeze-out, where particles are emitted away only from the surface, i.e. space- vs. time-like freeze-out. The corresponding HBT radii are calculated for the two models emphasizing the difference in the outward HBT radius., Comment: Proc. of hydrodynamics workshop, may 12-16, Trento

We discuss the complex phase generated in three pion correlation functions. The lowest order contribution to the phase is of order $q^2 R/K$, where $q$ is a typical relative momentum, $K$ is a typical center of mass momentum and $R$ is a typical radius parameter. This contribution is of purely kinematic origin. At next order we find a generic contribution of order $(qR)^3$ which is a result of odd modifications to the source emission function. We argue, that the scale for typical HBT correlations in ultrarelativistic heavy ion collisions is $q/K \ll qR \sim 1$, so that the third order correction actually dominates the phase in the experimentally relevant momentum range. We study in detail such contributions which arise from source asymmetries generated by flow, the source geometry and resonance decays., Comment: 9 pages, revtex, epsfig, 3 figures included, manuscript also available at http://www.nbi.dk/~vischer/publications.html