POSSIBLE SOLUTION TO THE HORIZON PROBLEM - MODIFIED AGING IN MASSLESSSCALAR THEORIES OF GRAVITY

An early MAD (massively aged and detained) epoch during which the Univ ersity becomes older than in the standard model is proposed as a possi ble new resolution to the horizon problem. This scenario differs from inflation in that there is no period of vacuum domination required and no entropy violation. Extensions of Einstein gravity which allow the Planck mass m(Pl) to change with time as the Universe evolves may prov ide such a MAD resolution to the horizon problem: in a cosmology where the gravitational constant G = m(Pl)2 is not in fact constant, the Un iverse may be older at a given temperature than in the standard hot bi g bang model. Thus, larger regions of space could have come into causa l contact at that temperature. This opens the possibility that large r egions became smooth without violating causality. We discuss in this p aper theories of gravity in which the gravitational constant is replac ed with a function of a massless scalar field. We first consider the o riginal Brans-Dicke proposal and then address more general scalar theo ries. However, this resolution to the smoothness problem can more gene rally be a feature of any physics which allows the Planck mass to vary with time. Solutions to the equations of motion during the radiation dominated era for Brans-Dicke gravity and more general massless scalar theories of gravity are presented. In particular, we study the evolut ion of the field PHI which determines the Planck mass at any given tim e, PHI(t) = m(Pl)(t)2, in the absence of a potential for PHI. We find that, regardless of initial conditions, the Planck mass evolves toward s an asymptotic value m(Pl) = PHI1/2 . For both a Brans-Dicke cosmolog y and a more general scalar theory, our observable Universe could fit inside a region causally connected at some high temperature T(c) prior to matter-radiation equality if there is a large disparity between th e early value of the Planck mass and the Planck mass today; specifical ly, our causality condition is that m(Pl)/m(Pl)(T0) greater than or si milar to T(c)/T0, where m(Pl)(T0) = M0 = 10(19) GeV is the Planck mass today and T0 is the temperature of the cosmic background radiation to day. Still, an additional mechanism is required to drive the Planck ma ss to the value M0 before the Universe cools below a temperature of T0 is similar to 2.74-degrees K. A mechanism capable of anchoring the Pl anck mass fast enough will necessarily accelerate the cosmological exp ansion and thus involves important dynamics. We suggest possible mecha nisms to anchor the Planck mass and complete this MAD model.