A hundred years ago Einstein abandoned the belief in a privileged reference frame, adopting instead the principle of relativity, which holds that the laws of physics take the same form in all inertial frames of reference. The logical consequences of this idea have been astounding; in this talk I will explore in particular the consequences for gravitation and cosmology. While Newton told us that gravity is created only by masses, relativity implies that if a mass density can create a gravitational field, then so can a pressure. In conventional hot big bang cosmology, the pressure in the early universe was so high that it contributed as much to the gravitational field as did the mass density. Although pressures are normally positive, modern particle theories predict that at high energies there are states with negative pressure, which would create repulsive gravitational fields. Inflationary cosmology is based on the premise that the big bang was propelled by such gravitational repulsion. Inflation can explain a number of features of our universe, including its uniformity, the value of its mass density, and the properties of the faint ripples that are observed in the cosmic background radiation. It even offers a possible explanation for the origin of essentially all the matter in the universe.
Organiser(s): L. Alvarez-Gaume, A. De Rujula / PH-TH