Sky & Telescope - September 2021 - 26

Magnetogenesis
some quantity, such as the time it takes for the block to reach
the bottom of the incline or the speed it has when it reaches
the bottom. But students who repeat this experiment in the
lab often find that the real-world results are at least a little
different from their calculations. Later, they learn to add friction
. . . only to find they had also neglected air resistance.
We see this process play out with physics problems at all
levels. Over time, as we grow as physicists, we learn which
elements of a problem are " negligible " and which are " essential. "
We accept the (supposedly) small errors that result
from neglecting insignificant factors. This approach tends
to work well, especially for linear systems, those with simple
proportional relationships. For more complex nonlinear
systems, the situation is more complicated, and an entire science
has developed around understanding nonlinear dynamics
and chaos. In addition, the idea of emergence, in which
the behavior of the whole is significantly different than the
summed behavior of its parts, implies that focusing on isolated
subsystems may not be the best way to understand the
early universe.
The problem with our picture of the early universe is that
we constructed it by looking at its components in isolation.
For example, we can extrapolate the size of density perturbations
in the early universe from how big the temperature
variations are in the CMB, but these calculations will be
several orders of magnitude off if we don't account for how
much the phase transitions disrupted the plasma. How magnetic
fields and gravitational waves impacted the development
of the universe is still a mystery. Also, what is dark matter,
and what role does it play in all this?
SOUPED-UP PHYSICS LAB This is one of the end-cap calorimeters
for the gigantic ATLAS experiment at the Large Hadron Collider. The calorimeters
detect hadrons emitted when proton beams smash together.
ATLAS scientists conduct a wide range of studies, from investigating
the Higgs boson to recreating the early universe's conditions. Notice the
hardhatted people for scale.
One of the issues cosmologists grapple with is magnetogenesis,
the origin of the first magnetic fields. Magnetic fields
are extremely important in plasma physics because they act
like puppet strings, directing how plasma's charged particles
move. For example, large nuclear fusion projects and
advanced space propulsion systems use magnetic fields to
control plasmas. Because the early universe was filled with
plasma - and, even today, plasma makes up most visible mass
- magnetic fields should be extremely important to how the
universe evolved.
In addition, we see magnetic fields all over the universe,
from stars to galaxies. Although moving charged particles can
create tiny magnetic fields from scratch, they easily cancel
one another out. It's difficult to generate coherent fields on
the scale of galaxies and clusters without a seed field and a
dynamo mechanism to amplify it. So whatever caused magnetogenesis
may ultimately be the same thing that created
magnetic fields in every galaxy in the universe. We want to
know where they came from and when they first developed.
Researchers predict that a magnetic field as weak as 10−20
gauss on a scale of a few million light-years would have
been enough to spark the modern universe's fields. This
is extremely tiny compared to what we see on Earth: Our
planet's magnetic field is about 0.5 G at the surface, and a
fridge magnet is 100 G. Galaxies themselves have fields on
the order of 10−6 G.
But there's no consensus yet as to how the first magnetic
fields came to exist. Something must have set charged particles
moving in the right way to create a field. Some scientists
think magnetogenesis happened during inflation; others, during
one of the phase transitions. Still others point to much
later in cosmic history, when the first stars formed.
My own research focuses in part on the development
of magnetic fields during the phase transitions. A major
objective of my team's work is to study the evolution of our
universe without introducing any more assumptions than
are absolutely necessary. We do this by essentially building
tiny universes inside a computer and watching them develop.
Some of these universes seem to have the potential to grow
into something that looks like ours, while others fizzle out
quickly. We've developed software that can model most of the
physics one may expect to encounter in the early universe.
Our code can solve Einstein's equations of general relativity
as well as the relativistic plasma physics equations for a compact
early universe, all at the same time - something that
was long impossible before recent advances in high-performance
computing. It also uses a special time-stepping routine
so it can simulate large or small portions of time, depending
on the physical situation. It took a lot of testing to ensure the
code could correctly simulate things like expansion, shocks,
and gravitational waves.
We simulate a relativistic plasma containing every species
of particle thought to exist during that time in history, plus
dark matter, gravitational waves, a cosmological constant,
and perturbations in temperature, density, and magnetic
26 SEPTEMBER 2021 * SKY & TELESCOPE
CLAUDIA MARCELLONI / ATLAS EXPERIMENT © 2007 CERN
Sky & Telescope - September 2021

Table of Contents for the Digital Edition of Sky & Telescope - September 2021
