Sky & Telescope - September 2021 - 27

THE CMB This view of the cosmic microwave background comes from the final data release of the Planck mission. The CMB's tiny temperature
fluctuations correspond to regions of slightly different densities extant in the universe when it first became transparent to light. These regions became
the seeds of future cosmic structure. Orange is warmer. (Due to projection effects, the fluctuations are distorted closer to the poles.)
fields, as well as the interactions among all these elements. It
is a virtual lab specifically designed to study the early universe
by witnessing how changes in initial conditions can lead to
different outcomes.
We've run simulations to see how the plasma environment
would have changed from just before the electroweak phase
transition through the QCD transition, depending on the
conditions we start off with. Along the way, we played with
things such as varying how fast the plasma sloshed around
and removing all dark matter, to see how that changed the
system's behavior.
Based on our simulations, dark matter seems to stabilize
the primordial plasma, keeping density perturbations from
becoming too large and sending our universe to a premature
death. We've also found that the dynamics of relativistic
plasma don't match what would happen in a
non-relativistic plasma; given these differences,
magnetic fields might more easily arise
in relativistic plasmas. If the stirring created
by the phase transitions is strong enough, the
electroweak and QCD phase transitions may
be enough to produce the magnetic fields we
expect in the early universe. If the stirring is
weak, however, initial magnetic fields from
inflation may have been necessary.
How Will We Really Know?
To steal a phrase from one of my fellow
scientists: Theory without experiment is
philosophy. It's not enough to calculate what
the early universe was like - we need to see
our predictions verified by the cosmos we see
around us. This requires observation.
But we started out by saying that the CMB only shows us
what the universe was like when it was 380,000 years old. We
need an even older signal.
Primordial gravitational waves are our best candidate
100 GAUSS
Strength of a
fridge magnet
10−6 GAUSS
Strength of the
magnetic field in a
Milky Way-like galaxy
10−15 GAUSS
Strength of the
magnetic field in
intergalactic space
(values approximate)
for that signal. These are waves in the fabric of spacetime
that we think were produced in the first few instants of the
universe by inflation and the turbulent plasma of the phase
transitions. They may have left an imprint on the CMB (S&T:
Oct. 2013, p. 22) or even a background echo that we could
directly detect with future gravitational wave observatories,
such as pulsar-timing arrays or the planned LISA spacecraft.
These waves would have been faint, less than 5% of the tiny
temperature fluctuations seen in the CMB. Still, they might
be " visible " with the right technology.
The payoffs for observing these waves would be significant,
since they could tell us precisely what
the conditions were in the early universe.
Knowing the waves' sizes and strengths
would enable us to distinguish between the
different scenarios that I and other researchers
explore, because these waves are part of
the conditions that explain magnetogenesis.
Right now, all we have from observations are
upper limits, and the waves might be too tiny
to measure. But if we can predict the characteristics
of these waves and if they can be
detected, we will finally know the universe's
earliest moments.
¢ Physicist DAVID GARRISON is a professor
and associate dean at the University of Houston-Clear
Lake. He founded the university's
physics program.
sk yand tele scope .o r g * SEPTEMBER 2021 27
ESA / PLANCK COLLABORATION

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Sky & Telescope - September 2021

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