In a nutshell, inflation is the term used to describe an extremely short period of turbocharged expansion that happened immediately after the big bang. Moreover, after years of trying, astrophysicists have just reported the first experimental evidence that inflation actually happened. Charles Bennett of Johns Hopkins University in Baltimore and co-workers made the breakthrough after a painstaking analysis of three years of data from the WMAP satellite.
First proposed by the US physicist Alan Guth in 1980, inflation soon became a cornerstone of modern cosmology as it could answer questions about the universe that the big bang could not explain on its own. According to the big bang theory, the universe and everything in it - including time and space, stars, planets and galaxies - came into being in an enormous explosion about 13.7bn years ago. Since then the universe has been expanding and cooling down.
Conclusive evidence for the big bang came in 1965 with the detection of cosmic microwave background - a weak glow of microwaves from all directions often called the "echo of the big bang".
However, aspects of the universe cannot be explained by the big bang alone. For example, the universe is flat, which basically means that parallel lines never meet. This might seem like common sense but Einstein's general theory of relativity - essential for understanding the behaviour of stars and galaxies - allows for space to be curved, either positively, like the surface of a ball, or negatively, like a saddle. Why should the universe be flat rather than curved?
Inflation solved this "flatness problem" by assuming that the baby universe grew very fast - faster than the speed of light - in a very short time just after the big bang. The numbers involved seem incredible: the universe expanded by a factor of around 1060 (one followed by 60 zeros) in about 10-35 seconds (a decimal point followed by 34 zeros and a one). This extraordinary stretching of space explains why the universe is flat.
Inflation also solves two other problems. The horizon problem is the inability of the big bang to explain why parts of the universe never in contact with each other have the same temperature. The big bang should also have created lots of isolated north and south magnetic poles (monopoles), but these have never been detected. Both problems disappear with cosmic inflation.
Experiments such as WMAP measure the temperature of the cosmic background - which is distinctly chilly at about 270 degrees below the freezing point of water. Finding evidence for inflation involved measuring this temperature to an accuracy of better than a millionth of one degree and identifying a particular pattern of waves that could only have been produced by inflation. The next challenge is to learn more of what causes inflation in the first place.
The WMAP results also confirm how little we know about the universe. Over the past decade it has become clear that ordinary matter - such as the stuff that makes up the stars - is only responsible for 4% of the density of the universe. A mysterious substance called "dark matter" makes up a further 22%, and the even more mysterious "dark energy" accounts for the remaining 74%. We may be living in the golden age of cosmology, but there is still much to discover.