By now you may be one of the millions of people who have seen the viral YouTube video that shows physicist Chao-Lin Kuo of Stanford University telling his colleague Andrei Linde, "Five sigma, as clear as day, r of 0.2."
To most people, Linde's reaction -- one of delighted disbelief -- may have seemed incomprehensible. How can anybody get so excited about the words "r of 0.2"? Yet those words represented one of the most dramatic discoveries of modern times (if confirmed). In the simplest terms, what Kuo meant was that they have discovered direct evidence, as clear as one could hope for, that the event known as "cosmic inflation" really happened.
Cosmic inflation describes a phase lasting a tiny fraction of a second in the universe's existence, in which the universe expanded at a faster-than-light speed from a speck much, much smaller than an atom to about the size of a grapefruit. The theory for this extraordinary process, originally formulated by physicists Alan Guth (currently at MIT) and Andrei Linde (to whom Kuo was delivering the news), suggested that the stupendous expansion happened when the universe was about one trillionth of a trillionth of a trillionth of a second old! In other words, this was what truly "banged" in the Big Bang. (See "What Did Go 'Bang' in the Big Bang?")
Now are you surprised that Linde reacted the way he did?
The signature that Kuo was talking about came from the BICEP2 telescope at the South Pole:
The BICEP2 telescope is in the foreground; in the background is the South Pole Telescope (credit: Steffen Richter/Associated Press).
The inflationary model predicted that the explosive expansion would have generated certain ripples -- gravity waves -- predicted to exist by Einstein's theory of general relativity. These waves stretch space in one direction and squeeze it in another, leaving what is known as a "B-mode polarization" imprint in the cosmic microwave background (the whirling patterns seen below):
Map of the so-called "B-modes," the imprints left by gravitational waves on the cosmic microwave background (from arXiv:1403.3985 by the BICEP2 Collaboration).
BICEP2 detected these B-modes at a statistically significant level. (The "five sigma" that Kuo referred to means that there is only a 1-in-35-million probability of the result occurring by chance.) Inflation is also what created all the matter and radiation in our universe, and some versions of inflation theory predict the existence of a multiverse -- a huge ensemble of universes. (See "How Can We Tell If a Multiverse Exists?")
So with that simple knock on Linde's door, Chao-Lin Kuo was bringing the news that we may have witnessed the birth of the cosmos.