Cosmic microwave background probes models of inflation.

Inflation creates both scalar (density) and tensor (gravity wave) metric perturbations. We find that the tensor mode contribution to the CMB anisotropy on large-angular scales can only exceed that of the scalar mode in models where the spectrum of perturbations deviates significantly from scale invariance (e.g., extended and power-law inflation models and extreme versions of chaotic inflation). If the tensor mode dominates at largeangular scales, then the value of ∆T/T predicted on 1 is less than if the scalar mode dominates, and, for cold dark matter models, b > 1 can be made consistent with the COBE DMR results. The recent COBE DMR [1] measurements of large-angular-scale anisotropy in the cosmic microwave background (CMB) provide important experimental support for the hot big bang model. Perhaps the most striking conclusion to be drawn from the COBE DMR data is that it is consistent with a scaleinvariant spectrum of primordial density (scalar) perturbations extending well outside the horizon at the epoch of last scattering. A scale-invariant spectrum is consistent with inflation, which predicts perturbations generated by quantum fluctuations [2], and also with models that generate perturbations by classical effects, such as theories with cosmic strings, textures, global monopoles, and non-topological excitations. Inflation also produces a spectrum of gravity waves (tensor metric fluctuations) with wavelengths extending beyond the horizon, providing a possible means for distinguishing it from the other scenarios. Recently it was even speculated that the anisotropy detected by the COBE DMR might be largely due to inflation-produced tensor rather than scalar perturbations [3]. In this Letter, we show that tensor dominance of the CMB quadrupole anisotropy is indeed possible for a class of inflationary models. We find that the ratio of tensor to scalar contributions is directly tied to the rate of inflationary expansion and the “tilt” of the spectrum of density perturbations away from scale invariance. Models that permit tensor dominance include extended inflation, power-law inflation and extreme versions of chaotic inflation. While the COBE DMR results alone cannot distinguish tensor from scalar perturbations, we show how additional measurements on small-angular scales may distinguish the two. We also discuss the implications for large-scale structure. CMB temperature anisotropies on large-angular scales (> ∼ 1) are produced by metric fluctuations through the Sachs-Wolfe effect [4]. These tem-