building on Guth’s suggestion soon proposed, a peculiar kind
of 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.
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
QuestfortheUltimateNatureofReality.
Atissueiswhatwemeanby‘BigBang’.Thereissome
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 observableuniverse, 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 ultra-fast 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.
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 recognise
the possibility of a multiverse, has proposed that inflation
happens naturally in a wide range of situations — it’s aCOSMIC 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.ESA / PLANCK COLLABORATION
http://www.skyandtelescope.com.au 31The laws of physics are symmetrical
forwards and backwards, and yet, we can
stir milk into coffee and scramble eggs,
but can’t un-stir or unscramble them.