Sky & Telescope - February 2019 - 19

ESA / PL A NCK COLL A BOR ATION

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 converted, eventually, to ordinary matter and radiation. As weird
as it sounds, a sub-microscopic patch of space became our
vast observable universe.
Inﬂation 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 ﬁlls space is considered negative, while the repulsive force driving inﬂation 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 ﬁrst, there was general agreement that the Big Bang
happened ﬁrst, and then a tiny fraction of a second later,
inﬂation began. Another fraction of a second later inﬂation
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 inﬂation as something that happened before the Big Bang. "If we take inﬂation
seriously, then we need to start correcting people claiming
that inﬂation happened shortly after our Big Bang, because it
happened before it, creating it," wrote MIT physicist Max Tegmark 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 inﬂation. And inﬂation wasn't much of a bang. Tegmark
describes it as a cold little swoosh.
Another problem with the old scenario, ﬁtting inﬂation
in after the Big Bang, is that it was hard to explain how this
inﬂationary 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 making any assumptions about a conspiracy to stop inﬂation, you
end up with the prospect that it would end in some places,
but it would keep going in others. Since inﬂation is an ultrafast expansion of space itself, the still-inﬂating parts would
grow huge, dwarﬁng what we thought was the universe. And
that monstrously large, still-inﬂating 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 universe" as some call it, this inﬂating 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 inﬂating
and non-inﬂating regions at a given time, one must use a different deﬁnition of time that includes both events inside the
pocket and outside the pocket at the same time," he says.
But how did inﬂation begin? Cosmologist Andrei Linde
(Stanford University), one of the ﬁrst people to recognize the
possibility of a multiverse, has proposed that inﬂation happens naturally in a wide range of situations - it's a surpris-

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Sky & Telescope - February 2019

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