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 theâ€¦ (More)

It was suggested that observations of the solar system exclude massive gravity, in the sense that the graviton mass must be rigorously zero. This is because there is a discontinuity in the linearizedâ€¦ (More)

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 precisionâ€¦ (More)

We generalize a recently proposed mechanism for the origin of primordial metric perturbations in inflationary models. Quantum fluctuations of light scalar fields during inflation give rise toâ€¦ (More)

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 modelsâ€¦ (More)

The non-linearity of Einstein's equations makes it possible for small-scale matter inhomogeneities to affect the Universe at cosmological distances. We study the size of such effects using a simpleâ€¦ (More)

The Particle Data Group gives an upper bound on the photon mass m < 2 x 10(-16) eV from a laboratory experiment and lists, but does not adopt, an astronomical bound m < 3 x 10(-27) eV, both of whichâ€¦ (More)

We describe five different ways of obtaining the Salpeter formula for the plasma corrections to fusion rates; this formula is valid at the center of the sun with insignificant errors (âˆ¼percent).â€¦ (More)

We show that causality constrains the sign of quartic Riemann corrections to the Einstein-Hilbert action. Our constraint constitutes a restriction on candidate theories of quantum gravity.

We show that flows of pure electromagnetic energy are subject to instabilities familiar from hydrodynamics and plasma physicsâ€”Kelvin-Helmholtz and screw instabilities. In the framework of force-freeâ€¦ (More)