My first Magazine Sky & Telescope - 02.2019 | Page 21

COSMIC MICROWAVE
BACKGROUND The
Planck spacecraft
mapped the relict
radiation from the Big
Bang phase with an
angular resolution of
5 arcminutes, or 1 / 12 °.
The small changes in
temperature correspond
to density variations that
became the seeds of
cosmic structure.
energy that creates a repulsive force drove this exponential
expansion. Instead of thinning out, the density of this energy
remained the same. And so as space grew, the total amount
of this energy swelled to enormous proportions and was con-
verted, eventually, to ordinary matter and radiation. As weird
as it sounds, a sub-microscopic patch of space became our
vast observable universe.
Infl ation seems to defy the law of conservation of energy
— Guth himself famously quipped that it’s the ultimate free
lunch — but it’s all perfectly compatible with the rules laid
out in Einstein’s general relativity. Energy can be positive or
negative. The gravitational energy that fi lls space is consid-
ered negative, while the repulsive force driving infl ation is
considered positive. So you can start out with zero energy
and get a whole lot of both positive and negative energy, says
cosmologist Anthony Aguirre (University of California, Santa
Cruz). Because these two add up to zero, the conservation law
isn’t violated (S&T: Nov. 2006, p. 36).
At fi rst, there was general agreement that the Big Bang
happened fi rst, and then a tiny fraction of a second later,
infl ation began. Another fraction of a second later infl ation
ended, starting the hot, dense phase of the universe that
expanded into our universe of space, stars, and upwards of
2 trillion galaxies.
But now many cosmologists refer to infl ation as some-
thing that happened before the Big Bang. “If we take infl ation
seriously, then we need to start correcting people claiming
that infl ation happened shortly after our Big Bang, because it
happened before it, creating it,” wrote MIT physicist Max Teg-
mark in his 2014 book, Our Mathematical Universe: My Quest
for the Ultimate Nature of Reality.
At issue is what we mean by “Big Bang.” There is some
disagreement over what, exactly, the term refers to these days,
says Matthew Johnson (York University and the Perimeter
Institute for Theoretical Physics, Canada). Many agree with
Tegmark that “Big Bang” should refer only to the hot, dense
state of matter that expanded and cooled into our observable
universe, and not to any notion of an absolute beginning.
This terminology can help separate the part of cosmology
that’s backed with strong evidence — the hot Big Bang and
what came afterwards — from the more speculative notion
of infl ation. And infl ation wasn’t much of a bang. Tegmark
describes it as a cold little swoosh.
Another problem with the old scenario, fi tting infl ation
in after the Big Bang, is that it was hard to explain how this
infl ationary phase could come to a neat and tidy end. “There’s
this side effect that once you get it going, it’s very hard to stop
everywhere at once,” says Aguirre. If you want to avoid mak-
ing any assumptions about a conspiracy to stop infl ation, you
end up with the prospect that it would end in some places,
but it would keep going in others. Since infl ation is an ultra-
fast expansion of space itself, the still-infl ating parts would
grow huge, dwarfi ng what we thought was the universe. And
that monstrously large, still-infl ating part would continue
to spawn new, independent universes like bubbles in a foam
(S&T: Dec. 2012, p. 20).
From a perspective outside of our bubble, or “pocket uni-
verse” as some call it, this infl ating space was busy giving rise
to other bubble universes long before ours came into being in
an ever growing multiverse. You just have to consider time in
a broader frame, says Aguirre. “To see the mixture of infl ating
and non-infl ating regions at a given time, one must use a dif-
ferent defi nition of time that includes both events inside the
pocket and outside the pocket at the same time,” he says.
But how did infl ation begin? Cosmologist Andrei Linde
(Stanford University), one of the fi rst people to recognize the
possibility of a multiverse, has proposed that infl ation hap-
pens naturally in a wide range of situations — it’s a surpris-
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