Inflationary cosmology [Guth 1981, Linde 1982a, Albrecht & Steinhardt 1982] has in recent years had a number of dramatic successes. The inflationary predictions of a flat Universe and nearly scale-invariant primordial density perturbations with Gaussian initial conditions [Guth & Pi 1982, Hawking 1982, Linde 1982b, Starobinsky 1982, Bardeen et al. 1983] have been found to be consistent with a series of increasingly precise cosmic microwave background (CMB) experiments [Miller et al. 1999, de Bernardis et al. 2000, Hanany et al. 2000, Halverson et al. 2002, Mason et al. 2002]. Theorists discuss open-Universe and alternative structure-formation models, such as topological defects, with far less frequency than they did just three years ago.
Historically, when experimental breakthroughs confirm a particular theoretical paradigm and eliminate others, progress can be made at the edges - i.e., precision tests of the new standard model. In the case of inflation, a number of important questions should be addressed. For example, what is the physics responsible for inflation? What is the energy scale of inflation? In particular, we really do not understand why the simple slow-roll model of inflation - really no more than a toy model - works so well. Might deviations from the simplest model expected in realistic theories lead to small deviations from the canonical predictions of inflation? For example, is the density of the Universe precisely equal to the critical density? Are there deviations from scale invariance on small distance scales that arise as a consequence of the end of inflation? Might there be some small admixture of entropy perturbations in addition to the predominant adiabatic perturbations? Are there small deviations from Gaussian initial conditions?
A variety of forthcoming CMB experiments will test the flatness of the Universe with additional precision and determine the primordial spectrum of perturbations with increasing accuracy. CMB experiments and galaxy surveys and weak-lensing maps that determine the mass distribution in the Universe today will test Gaussian initial conditions. Our understanding of galactic substructure may shed light on the end of inflation. Experimentalists are beginning to contemplate programs to detect the unique polarization signature due to inflationary gravitational waves.
Here, I briefly review several new probes of possible relics of inflation; namely inflationary gravitational waves, non-Gaussianity, and galactic substructure.