How the Universe Got Big

A team of radio astronomers, working in Antarctica, where the air is clear and dry, have found the first direct evidence for the theory of cosmic inflation. That’s the theory about the origin of the universe first stated by the physicist Alan Guth in 1980.

Here’s some background from an article Guth wrote in 1997 for Beam Line, the magazine of the Stanford Linear Accelerator Center (now the SLAC National Accelerator Laboratory):

Although it is called the “Big Bang theory,” it is not really the theory of a bang at all. It is only the theory of the aftermath of a bang. It elegantly describes how the early Universe expanded and cooled, and how matter clumped to form galaxies and stars. But the theory says nothing about the underlying physics of the primordial explosion. It gives not even a clue about what banged, what caused it to bang, or what happened before it banged. The inflationary universe theory, on the other hand, is a description of the bang itself, and provides plausible answers to these questions and more.

Guth explains that in order for the universe we observe to have begun with a Big Bang, the early universe must have been extremely uniform and have had a precise density. However:

The classical form of the Big Bang theory requires us to postulate, without explanation, that the primordial fireball filled space from the beginning. The temperature was the same everywhere by assumption, not as a consequence of any physical process….

[In addition] the initial values of the [universe’s] mass density and expansion rate are not predicted by the theory, but must be postulated. Unless we postulate that the mass density at one second just happened to have a value between 0.999999999999999 and 1.000000000000001 times the critical density [the boundary value between a universe that will expand forever and one that will eventually collapse], the Big Bang theory will not describe a universe that resembles the one in which we live…

Although the properties of the Big Bang are very special, we now know that the laws of physics provide a mechanism that produces exactly this sort of a bang. The mechanism is known as cosmic inflation.

The National Accelerator Laboratory issued a press release today:

Instead of the universe beginning as a rapidly expanding fireball, Guth theorized that the universe inflated extremely rapidly [faster than the speed of light] from a tiny piece of space and became exponentially larger in a fraction of a second.

For inflation to occur, the universe must have been in a state that allowed a sudden change to release enormous energy, creating an expanding universe almost from nothing. The process was apparently a kind of delayed phase transition, as when water is supercooled below its natural freezing point and then, because of some disturbance, suddenly freezes, generating heat.

However, as Guth immediately realized, certain predictions in his scenario contradicted observational data. In the early 1980s, Russian physicist Andrei Linde modified [the theory so that it] generated predictions that closely matched actual observations of the sky.

The new observations reported today are the first evidence of the existence of gravity waves. These are ripples in spacetime originally predicted by Albert Einstein. The radio astronomers working in Antarctica found traces of these ancient gravity waves by analyzing the cosmic background radiation left over from the Big Bang. Andre Linde reacted to the news: “These results are a smoking gun for inflation, because alternative theories do not predict such a signal. This is something I have been hoping to see for 30 years.”

Future Nobel Prize-winner Alan Guth offered this summary in 1997:

While it may be too early to say that inflation is proved, I claim that the case for inflation is compelling. It is hard to even conceive of an alternative theory that could explain the basic features of the observed Universe. Not only does inflation produce just the kind of special bang that matches the observed Universe, but quantum fluctuations during inflation could have produced non-uniformities which served as the seeds of cosmic structure [in particular, the existence of galaxies].

Physicists doubted whether Guth’s theory would ever be proven. With today’s announcement, cosmic inflation is a big step closer to becoming settled science.

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