22 July 2014 sky & telescope
Big Bang Breakthroughsee in the universe today. But it’s still early days, and the
BICEP team members, when asked at the press confer-
ence, said they do not claim to have discovered running.From a Series of Successes...
In the 34 years since Alan Guth proposed it, infl ation has
been a classic example of the scientifi c process at work.
Puzzling observations led to a theory, which made test-
able new predictions, which proved to be true, which led
to refi nements of the theory and further predictions.
Guth announced the theory in 1980 as the fi rst physi-
cally detailed, by-the-bootstraps mechanism for producing
an entire universe from practically nothing. Guth drew
upon the idea of a Grand Unifi ed Field breaking apart
very early into the strong and weak nuclear forces and
electromagnetism. He found that a tiny bit of space could
self-expand by at least a factor of 10^24 in 10–32 second or
less, while fi lling with ultradense material drawing on the
Grand Unifi ed Field’s decay. The regular Big Bang takes
over after that. Interestingly, the BICEP2 discovery puts
the energy scale of infl ation well into the Grand Unifi ed
Field range. That may not be a coincidence.
This scenario solved other crucial paradoxes of the
original Big Bang. One was the fl atness problem, or why
the cosmic allotment of matter and energy was fi ne-
tuned with fantastic precision to balance between a fast
early recollapse (a “Big Crunch”) and a fast early expan-sion away to practically nothing (a “Big Chill”). Infl a-
tion also solved the horizon problem, or how very distant
regions on opposite sides of the sky today can look nearly
alike even though in an uninfl ated Big Bang, nothing can
ever have had any common infl uence on them at all.
Then, starting in 1982, infl ation scored perhaps the
grandest scientifi c home run of our time. Cosmologists
had long struggled to fi nd a way that today’s lumpy, web-
like cosmic structure — galaxies, galaxy clusters — could
arise from the extremely smooth material that emerged
from the Big Bang. What were the gravitational seeds
that grew to become today’s lumps? Nothing worked.
Infl ation, physicists realized, gave the stunning answer.
Infl ation took the microscopic, purely random quantum
fl uctuations in the smooth material very early and blew
them up almost instantly to nearly the masses of galax-
ies. The normal workings of gravity took over from there,
giving us the universe we see today. The numbers worked
perfectly. In the subsequent 22 years, virtually every new
cosmic discovery has confi rmed this picture to greater
and greater precision.
Infl ation also predicts that the early quantum seeds
should have had a size distribution that was nearly equal
at all scales, but not quite. The “running” eff ect required
by infl ation coming to a halt should have given a slight
tilt to the quantum seeds’ spectrum of sizes. Theorists
predicted that the “spectral index” of this tilt, a value