Cold dark matter (CDM) models predict small-scale structure in excess of observations of the cores and abundance of dwarf galaxies. These problems might be solved, and the virtues of CDM models retained, even without postulating ad hoc dark matter particle or field interactions, if the dark matter is composed of ultralight scalar particles (m approximately 10(-22) eV), initially in a (cold) Bose-Einstein condensate, similar to axion dark matter models. The wave properties of the dark matter stabilize gravitational collapse, providing halo cores and sharply suppressing small-scale linear power.

We propose a new mechanism to generate density perturbations in inflationary models. Spatial fluctuations in the decay rate of the inflaton field to ordinary matter lead to fluctuations in the reheating temperature. We argue that in most realistic models of inflation the coupling of the inflaton to normal matter is determined by the vacuum expectation values of fields in the theory. If those fields are light during inflation (this is a generic situation in the minimal models of supersymmetric inflation) they will fluctuate leading to density perturbations through the proposed mechanism. We show that these fluctuations could easily dominate over the ones generated through the standard mechanism. The new scenario has several consequences for inflation model building and observations. The proposed mechanism allows to generate the observed level of density perturbations with a much lower scale of inflation and thus generically predicts a smaller level of gravitational waves. The relation between the slope of the spectrum of the produced density perturbations and the potential of the inflaton field is different from the standard relations obtained in the context of slow roll inflation. Because the field responsible for the fluctuations is not the inflaton, it can have significantly larger self couplings and thus density perturbations could be non-Gaussian. The non-Gaussianity can be large enough to be detectable by CMB and Large Scale Structure observations.

Cosmologically motivated theories that explain small acceleration rate of the Universe via modification of gravity at very large, horizon or super-horizon distances, can be tested by precision gravitational measurements at much shorter scales, such as the Earth-Moon distance. Contrary to the naive expectation the predicted corrections to the Einsteinian metric near gravitating sources are so significant that might fall within sensitivity of the proposed Lunar Ranging experiments. The key reason for such corrections is the van Dam-Veltman-Zakharov discontinuity present in linearized versions of all such theories, and its subsequent absence at the non-linear level ala Vainshtein.

Non-radiating, advection-dominated, accretion flows are convectively unstable in the radial direction. We calculate the two-dimensional (r − θ) structure of such flows assuming that (1) convection transports angular momentum inwards, opposite to normal viscosity and (2) viscous transport by other mechanisms (e.g., magnetic fields) is weak (α ≪ 1). Under such conditions convection dominates the dynamics of the accretion flow and leads to a steady state structure that is marginally stable to convection. We show that the marginally stable flow has a constant temperature and rotational velocity on spherical shells, a net flux of energy from small to large radii, zero net accretion rate, and a radial density profile of ρ ∝ r −1/2 , flatter than the ρ ∝ r −3/2 profile characteristic of spherical accretion flows. This solution accurately describes the full two-dimensional structure of recent axisymmetric numerical simulations of advection-dominated accretion flows. Narayan & Yi (1994;1995) argued that convection was unlikely to significantly modify the structure of ADAFs, essentially because the inflow time of the gas would be shorter than the characteristic convective turnover time (i.e., advection would overwhelm convection). Their analysis 1 Chandra Fellow 2 Note that this is very different from thin accretion disks which can be vertically convective.

We review and analyze the available information on the nuclear-fusion cross sections that are most important for solar energy generation and solar neutrino production. We provide best values for the low-energy cross-section factors and, wherever possible, estimates of the uncertainties. We also describe the most important experiments and calculations that are required in order to improve our knowledge of solar fusion rates. [S0034-6861(98)00704-1]

Gamma-ray burst afterglow polarization is discussed. We Ðnd an observable, up to D10%, polarization, if the magnetic Ðeld coherence length grows at about the speed of light after the Ðeld is generated at the shock front. Detection of a polarized afterglow would show that collisionless ultrarelativistic shocks can generate strong large-scale magnetic Ðelds and conÐrm the synchrotron afterglow model. Nondetection, at the D1% level, would imply that either the synchrotron emission model is incorrect or that strong magnetic Ðelds, after they are generated in the shock, somehow manage to stay undissipated at "" microscopic, ÏÏ skin depth, scales. Analytic light curves of synchrotron emission from an ultrarelativistic self-similar blast wave are obtained for an arbitrary electron distribution function, taking into account the e †ects of synchrotron cooling. The peak synchrotron Ñux and the Ñux at frequencies much smaller than the peak frequency are insensitive to the details of the electron distribution function ; hence, their observational determination would provide strong constraints on blast-wave parameters.

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We extend and reconcile recent work on turbulence and particle heating in advection-dominated accretion flows. For approximately equipartition magnetic fields, the turbulence primarily heats the electrons. For weaker magnetic fields, the protons are primarily heated. The division between electron and proton heating occurs between β ∼ 5 and β ∼ 100 (β ≡ ratio of gas to magnetic pressure), depending on unknown details of how Alfvén waves are converted into whistlers on scales of the proton Larmor radius. We also discuss the possibility that magnetic reconnection could be a significant source of electron heating.