What is cosmic inflation and how does it solve the horizon and flatness problems?

Answer 1

First, let's define terms.

The "horizon problem" is the fact that the observable Universe is too uniform. If one does calculations based on simple Big Bang Cosmology -- ie, that the present rate of expansion ("Hubble Expansion") is the rate that has existed since the start of time -- then our Universe did not have enough time to reach the mass density uniformity we see today. It's as if we, while planning to make some iced tea, placed a pan of boiling water a meter away from a pitcher of ice, turned away for one second, and then found the water in both containers was now at room temperature. You'd have every right to ask, "How did they reach equilibrium that quickly?"

The "flatness problem" is the fact that the mass density of our Universe is SO CLOSE to the amount that permits stars to form. A Universe more dense than ours would have collapsed in on itself in less than a billion years, and a Universe less dense would have resulted in matter so thin that galaxies would have never formed. In both cases, no heavy elements would have formed in rings around stars, meaning no life could exist. The ratio of the actual mass density and the density that would permit life in our Universe is called Ω. Again, if one does calculations based on simple Big Bang Cosmology, Ω can differ from one by less than 10^-16 (yes, that's a decimal point followed by FIFTEEN ZEROES!), or no life should exist in our Universe.

Inflation simply states that, prior to the Hubble Expansion we now see, our Universe was sufficiently dense for a long enough time that it could reach mass equilibrium, which was then followed by a rapid expansion ("Inflation"), which was then followed by Hubble Expansion.

If one does the calculations based on an inflationary model, one finds that Ω can differ from one by almost any amount, but will eventually reach near unity when inflation ceases. From then on, Hubble Expansion can continue with Ω being the number we see.

Inflation solves other problems as well; but it also makes predictions about what we would see in our present Universe. Specifically, we would see anisotropies in the Cosmic Microwave Background Radiation of a certain size, but no larger. So far, all observations of the CMBR fail to rule out the Inflation Model.

Answer 2

Cosmic inflation is a theory in cosmology that suggests the universe underwent a rapid expansion in the early stages, smoothing out irregularities and making it flat. This expansion solves the horizon problem by allowing distant regions of the universe to come into contact and equilibrate before separating, leading to a more uniform cosmic microwave background. The flatness problem is alleviated by the rapid expansion causing the overall curvature of the universe to approach zero over time.