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 ʹno-go theoremʹ 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 ʹopen string eraʹ preceding our present ʹclosed string eraʹ.