COBE, gravitational waves, inflation and extended inflation

Abstract

Abstract We analyse the implications for inflationary models of the cosmic microwave background (CMB) anisotropy measured by COBE. Vacuum fluctuations during inflation generate an adiabatic density perturbation, and also gravitational waves. The ratio of these two contributions to the CMB anisotropy is given for an arbitrary slow-roll inflaton potential. Results from the IRAS/QDOT and POTENT galaxy surveys are used to normalise the spectrum of the density perturbation on the scale 20 h −1 Mpc, so that the COBE measurement on the scale 10 3 h −1 Mpc provides a lower bound on the spectral index n . For “power-law” and “extended” inflation, gravitational waves are significant and the bound is n > 0.84 at the two-sigma level. For “natural” inflation, gravitational waves are negligible and the constraint is weakened to n > 0.70, at best marginally consistent with a recent proposal for explaining the excess clustering observed in the APM galaxy survey. Many versions of extended inflation, including those based on the Brans-Dicke theory, are ruled out, because they require n ⪅ 0.75 in order that bubbles formed at the end of inflation should not be observed now in the CMB.