Thoughts on Tachyon Cosmology

Abstract

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 r\^ole 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 possiblity that the tachyon was important in a possible pre-inflationary Open-String Era preceding our present Closed String Era.