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By far the most important property of inflationary cosmology is that it produces perturbations, in the form of both density perturbations and gravitational waves. The density perturbations may be responsible for the formation and clustering of galaxies, as well as creating anisotropies in the microwave background radiation. The gravitational waves do not affect the formation of galaxies, but may contribute extra microwave anisotropies on the large angular scales sampled by the COBE satellite [14, 15]. An alternative terminology for the density perturbations is scalar perturbations and for the gravitational waves is tensor perturbations, the terminology referring to their transformation properties.

In this article I will focus on the nature of the predictions from the inflationary cosmology rather than detailed comparison with observations, which was the focus for a series of lectures at this Summer School by Pedro Viana [16].