Sky & Telescope - September 2021 - 23

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What Was the Early Universe Like?
Normally, we study our universe by looking out into the vast
expanse. Because we have shown the speed of light to be
constant and finite, we assume that light has traveled at a
consistent speed since the beginning of time. So the further
out in space we look, the further back in time we are seeing.
Light's finite speed has allowed us to make many discoveries,
like the fact that our universe appears to be expanding and
that it was much smaller, hotter, and denser in the past.
This knowledge, however, implies that we can only look
so far into the past because at some point, the universe must
have been too hot and dense for light to freely travel without
scattering. We identify this surface of last scattering as the
birth of the CMB, which suffuses our universe.
To know what happened even earlier in our universe's
history, we must extrapolate backwards in time. In order to
do so, we must make certain assumptions, and we need to
be completely clear as to which assumptions we are making.
First off, we assume that the laws of nature essentially work
the same now as they did when the universe was young. Second,
we assume that no particular part of the universe was
special - that is, that the physical conditions were basically
the same everywhere.
Beyond these two fairly standard assumptions, we also
tend to make often-overlooked assumptions. For example, we
understand the dynamics of fluids and laboratory plasmas
fairly well, so we tend to assume that the highly energetic
plasma in the early universe behaved similarly. We also
ELECTRO WEAK
PHASE
TRANSITION
Higgs boson
appears
Electroweak
force splits
into weak and
electromagnetic
forces
10-12
10-10
10-5
QUARK EPOCH
QCD PHASE
TRANSITION
Neutrons
and protons
form
Neutrons
and protons
too hot to
combine
together
1
10 102
1
Atomic
nuclei
form
Atoms
form
Universe
becomes
transparent
1010
1013
1020
assume that the fundamental physics we study in the lowgravity,
vacuum conditions at the Large Hadron Collider
(LHC) functioned the same way when the universe was
hotter and denser, and gravity was much stronger. In addition,
we assume that ripples in spacetime called gravitational
waves didn't play a major role in the evolution of
our universe.
None of these assumptions may be right. Even so, we
can still develop a good idea of what the early universe was
like. Using our most basic understanding of the laws of
nature, developed through years of theory and experimentation,
we can turn back the clock and compress the cosmos.
As we do so, the universe changes from a collection
of stars and galaxies into a cloud of neutral atoms not yet
bound into structures by gravity. Going back even further,
the particles would have been too hot to form neutral
atoms and instead would have resembled a plasma, a
sea of charged electrons and atomic nuclei. The soup
of charged particles would have reacted strongly to any
magnetic fields present, so the dynamics of this plasma
would have been much different than that of a collection
of neutral atoms. This particle soup filled the universe just
prior to the CMB's release.
Going back further, to when the universe was less than
10 seconds old and the average temperature was more
than a billion kelvin, the plasma's energy would have been
so high that it's impossible to model the motion of its particles
without using Einstein's special theory of relativity.
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Sky & Telescope - September 2021

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