Matter Contributions to the Expansion Rate of the Universe

We consider the effect of various particles on the cosmic expansion rate relative to that of the graviton. Effectively massless fermions, gauge bosons and conformally coupled scalars make only minuscule contributions due to local conformal invariance. Minimally coupled scalars can give much stronger contributions, but they are still sub-dominant to those of gravitons on account of global conformal invariance. Unless effectively massless scalar particles with very particular couplings exist, the leading effect on the expansion rate is furnished solely by the graviton. An upper bound on the mass of such scalar particles is obtained.Comment: 14 pages, plain TeX, 7 Postscript files, uses psfig.st

Quantum Gravity Slows Inflation

We consider the quantum gravitational back-reaction on an initially inflating, homogeneous and isotropic universe whose topology is $T^3 \times \Re$. Although there is no secular effect at one loop, an explicit calculation shows that two-loop processes act to slow the rate of expansion by an amount which becomes non-perturbatively large at late times. By exploiting Feynman's tree theorem we show that all higher loops act in the same sense.Comment: 19 pages, plain TeX, 1 Postscript file, uses psfig.sty, revised June 1996 for publication in Nuclear Physics

Pair creation and axial anomaly in light-cone QED2QED_2

The 1+1 dimensional massive Dirac equation is solved exactly in light-cone coordinates for $x^+ > 0$ and $x^- > -L$, in the presence of an arbitrary $x^+$ dependent electric field. Our solution resolves the ambiguity at $p^+ = 0$. We also obtain the one loop rate for pair production for a positive electric field, compute the expectation values of the vector and axial vector currents, and recover the well known anomaly $e^2 E/\pi$ in the divergence of the latter. A final intriguing result is that the theory seems to exhibit a phase transition in the limit of infinite $L$.Comment: 33 pages, 2 figures, LaTeX 2 epsilon, revised for publicatio