Thoughts on tachyon cosmology

After a pedagogical review of elementary cosmology, I go on to discuss some obstacles to obtaining inflationary or accelerating universes in M/string theory. In particular, I give an account of an old ‘no-go theorem’ to this effect. I then describe some recent ideas about the possible role of the tachyon in cosmology. I stress that there are many objections to a naive inflationary model based on the tachyon, but there remains the possibility that the tachyon was important in a possible pre-inflationary ‘open string era’ preceding our present ‘closed string era’. PACS numbers: 98.80.Jk, 11.25.−w