cosmic inflation

The Big Bang model works beautifully from the first second onward, but it leaves nagging puzzles about the very beginning. Why is the cosmic microwave background the same temperature in opposite directions of the sky, when those regions were seemingly too far apart to have ever exchanged heat? Why is space so precisely flat? Inflation answers both with one bold stroke: an instant of stupendously fast expansion right at the start, blowing a tiny patch up to enormous size in a flash.

Cosmic inflation proposes that between about 10^-36 and 10^-32 seconds after the beginning, the universe underwent accelerated, nearly exponential expansion, a(t) roughly proportional to e^(Ht), growing by a factor of at least e^60 (a trillion trillion times) in that fleeting interval. It is thought to be driven by a scalar field, the inflaton, temporarily trapped in a state of high, nearly constant potential energy that behaves like a huge cosmological constant. This smooths and flattens space (solving the horizon and flatness problems), dilutes any unwanted relics like magnetic monopoles, and, crucially, stretches microscopic quantum fluctuations to cosmic size, seeding the density ripples we see in the CMB.

Inflation's great success is quantitative: it predicts a nearly, but not exactly, scale-invariant spectrum of primordial fluctuations, and the CMB confirms exactly the slight tilt predicted. Yet it is a framework rather than a single theory: the identity of the inflaton is unknown, there are many competing models, and a clean prediction, a specific background of primordial gravitational waves, has not yet been detected. Inflation explains the initial conditions the Big Bang has to assume, but it does not by itself explain the ultimate origin.