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| Captured images of magnetic fields following the spiral pattern of a galaxy called NGC 1068 NASA/SOFIA; NASA/JPL-Caltech/Roma Tre Univ. |
The idea that magnetism helped shape the universe has been dismissed by
scientists for decades, but now new experiments involving plasma that is
hotter than the sun are prompting a rethink.
THERE are few places on Earth where conditions get as extreme as they do at
the National Ignition Facility near Los Angeles. At its heart, 192 lasers
are trained on a gold cylinder roughly the size of an AA battery. As the
beams converge, the temperature in the test chamber leaps to 100 million oC,
hotter than the centre of the sun.
The facility was built to investigate the possibility of harnessing nuclear
fusion, which promises unlimited clean energy. But earlier this year,
researchers announced that its powerful lasers have also been directed at a
different kind of big question – what shaped the universe?
The cosmos is a beautiful place. At the largest scales, a vast web of matter
is woven throughout space. Zoom in and you see galaxies cluster in billowing
clouds, while the individual galaxies themselves come in a wondrous array of
shapes, including elegant spirals like that of our Milky Way.
For decades, it has been thought that only gravity has what it takes to
sculpt such wonders. Now, hot on the heels of a slew of intriguing galactic
observations, laser-fired experiments are throwing up hints that we may have
wrongly dismissed the influence of another force.
Magnetism has always been considered too weak to be a cosmic sculptor. But
those behind the latest results claim that in the white heat of the test
chamber, they have caught a glimpse of how this forgotten force can be
turbocharged. If so, we might have to find a new place for magnetism,
alongside gravity, in our picture of how the cosmos came to look the way it
does.
What we know about gravity and the structure of the universe began to take
shape at pretty much the same time. Back in the early decades of the 20th
century, astronomers such as Edwin Hubble were beginning to observe the true
scale and structure of the cosmos. About the same time, Albert Einstein
published his monumental theory of gravity, general relativity. Initially,
the theory seemed to fit the observations like a glove and so scientists
thought the structure of the universe must be down to gravity alone.
But as observations improved, inconsistencies cropped up. One of the most
famous came in the 1930s, when astronomer Fritz Zwicky showed that galaxies
in one cluster were moving so quickly that they ought to fly apart instead
of being trapped in orbit around one another. He suggested that some form of
“dunkle Materie“, or dark matter, must exist there – something we can’t see,
but which generates extra gravity to help the cluster stick together. From
then on, dark matter became an essential prop for gravity theorists, even
though no one has ever directly detected it.
In the middle of the 20th century, a different view of what was shaping the
universe emerged, courtesy of physicist Hannes Alfvén. Gravity had made the
early running because, although it is a relatively weak force, it acts over
vast distances and pulls on all matter. Magnetism wasn’t on the table, being
a more limited force that only affects electrically charged particles.
Alfvén pointed out, however, that much of the stuff in the universe is in a
state of matter called plasma, a gas made of charged particles. He suggested
that the force exerted on plasma by magnetism ought to be at least
comparable with the effect of gravity on other matter. Magnetic fields must,
he reckoned, play an important – perhaps even dominant – role in shaping the
cosmos.
Alfvén’s backers began devising hypothetical magnetism-based solutions to
several cosmic conundrums, including how spiral galaxies get their shape.
But there were always two big problems for those in magnetism’s corner.
First, it was hard to test the idea because, at the time, there was no
practical way of observing magnetic fields in the wider universe. Second,
and more fundamentally, a magnetic field would have to acquire extraordinary
strength to play a role in shaping galaxies and no one had any idea of how a
sufficiently strong field could be formed.
How magnetic fields are generated
To create a magnetic field, you first need a dynamo, a churning region of
charged, electrically conducting material. This is what happens inside
Earth: liquid metal circulates to produce the magnetic field that surrounds
our planet. A dynamo made of plasma could certainly have formed in the early
universe. The trouble is, any such dynamo would be relatively small fry,
generating magnetic fields far too weak to do any real galactic shaping.
Something would have had to somehow amplify those nascent fields many times
– and no one had any sensible suggestions for how that could have happened.
Arguments about what, if any, role magnetism played in shaping the cosmos
simmered for decades. But by the 1980s, with no answers to these two
problems, magnetism was deemed to have lost. Gravity really was the one true
sculptor of the universe. “Cosmic magnetism is generally the last physical
mechanism that anybody talks about,” says astronomer
Enrique Lopez Rodriguez
at Stanford University in California.
Which isn’t to say gravity can explain every detail of the structure of the
universe. One puzzle involves galaxy clusters, which, as well as the
galaxies themselves and (presumably) some dark matter, contain relatively
empty areas called the intracluster medium, in which there is nothing but
plasma. This plasma emits X-rays, which we can measure from Earth and so
infer its temperature. Since the late 1990s, astronomers have been finding
that the plasmas inside galaxy clusters are inexplicably hot, at 10 million
oC. According to gravitational physics, the gas should have radiated away
that heat long ago.
Detecting magnetic fields in space
Although this particular puzzle didn’t immediately turn astronomers back
towards magnetism, some researchers have recently been wondering if we were
too hasty to dismiss the force so completely from cosmology. One thing that
has changed since the 1980s is our ability to look for magnetic fields in
the universe. Take NASA’s Stratospheric Observatory for Infrared Astronomy
(SOFIA) instrument, an infrared telescope housed in a converted jumbo jet
that can soar high into the atmosphere. It climbs above the water vapour in
the air, which absorbs infrared light and stymies most infrared observations
attempted from the ground.
When cosmic dust grains find themselves in a magnetic field, they line up
like a picket fence, which polarises any infrared light passing through
them. Lopez Rodriguez happened to be working with SOFIA five years ago when
the researchers were commissioning a new instrument that could pick up these
signals and so reveal magnetic fields. He suggested they observe the spiral
galaxy NGC 1068, the core of which was known to be a source of polarised
infrared light. In the first 30 minutes of the observation, they saw
something extraordinary: the magnetic field was clearly following the spiral
pattern of the galaxy (pictured above). Gravity didn’t predict anything like
this. “I was like, whoa, what is going on here?” says Lopez Rodriguez. To
find out if this was a fluke, they looked at 20 other nearby galaxies. “So
far, every single one has a large-scale magnetic field permeating the whole
galaxy,” says Lopez Rodriguez. And these fields all follow the shape of the
spiral arms too.
Other telescopes have seen similar things. In 2020, Yelena Stein, now at the
German Aerospace Centre in Cologne, and her colleagues used the Very Large
Array – a radio telescope in New Mexico – to study the spiral galaxy NGC
4217. They detected a large-scale magnetic field permeating the galaxy.
On their own, these observations are hardly conclusive. The magnetic fields
could be a side effect of the spiral shape rather than a cause of it. And
the reasons for ruling out magnetic fields as cosmic sculptors weren’t just
that we hadn’t seen them, but that we didn’t understand how they could be
sufficiently amplified.
Turbulent dynamo
Now, however, that second objection might be crumbling too. Ever since the
mid-1950s, when geophysicist Stanislav Braginsky wrote down his equations of
fluid motion in a plasma, researchers have been interested in the role of
turbulence – chaotic changes in pressure and flow – in the generation of a
magnetic field. One idea that emerged was that turbulence within the plasma
could affect the properties of the magnetic field generated. Turbulence is
inherently complicated and it was impossible to understand what effects it
might have until the advent of modern computer simulations. But these have
shown that a “turbulent dynamo”, as it is known, should hugely boost a
magnetic field’s strength. Still, these effects would only become easy to
observe in a plasma heated to extreme temperatures – the sort present during
the early universe – which meant this was a tough hypothesis to
experimentally test.
Step forward Jena Meinecke at Oxford University, who has for years been
investigating the turbulent dynamo as part of an international team of
plasma physicists. “It’s the holy grail, in some ways, of plasma physics,”
she says. The emergence of large laser laboratories has been a godsend
because they are the only places capable of creating a plasma hot enough to
get close to the conditions where a turbulent dynamo can be studied.
Evidence of magnetic field amplification
Meinecke’s team, led by Gianluca Gregori at the University of Oxford, did
its first laser experiment in 2018. At the Omega Laser Facility at the
University of Rochester in New York, the researchers created a small
“forest” of targets made of foils and grids on poles that they calculated
would disrupt the plasma in a turbulent way. The laser then zapped a capsule
of deuterium, a heavier form of hydrogen, and turned it into a plasma,
which, thanks to the grids of foil nearby, swirled with turbulence. The
researchers saw a rapid amplification of the plasma’s magnetic field –
the first ever glimpse of a turbulent dynamo in action.
Emboldened, the researchers wanted to see the effect in its full glory,
which meant using a more powerful laser to drive the turbulence even harder.
That was what brought them to the world’s most powerful laser apparatus, the
National Ignition Facility at Lawrence Livermore National Laboratory in
California, with its 192 laser beams. It is rare for scientists to be
granted time on the laser for experiments on anything other than fusion, but
Meinecke’s team secured permission and got to work.
The researchers’ first “shot day” came later in 2018. They performed a
similar experiment to the one at Omega, watching with X-ray cameras to see
what happened. They were expecting a sharp increase in the plasma’s
temperature, but instead the cameras captured a patchwork of hot and cold
spots. “It was like looking at a Dalmatian,” says Meinecke. “I showed it to
the team and they said ‘there’s no way that can be the result.'” But the
experiment was redone and it was always the same.
That was when the penny dropped. The turbulent dynamo effect the researchers
had created was so strong that the resultant magnetic field was trapping
particles inside certain regions of the plasma. It was enough to reduce the
flow of heat by a factor of 100, creating the hot and cold spots. The team
has spent years double and triple-checking the results and only
went public with them a few months ago. Plasma physicist Patrick Diamond at the University of California, San
Diego, says this is “a significant step forward”. He points out that his own
field, nuclear fusion, relies on strong magnetic fields to trap energy and
heat, but it does so using strong, uniform fields. “This is the first case
where you see this reduction in thermal diffusivity as a consequence of a
chaotic or turbulent magnetic field,” he says.
This unexpected ability of the magnetic fields to trap heat could solve the
puzzle of the inexplicably hot intracluster medium in galaxy clusters.
Meinecke and her team are proposing that the magnetic field created by a
turbulent dynamo could hold plasmas in place, and suppress the diffusion of
heat for billions of years. “The magnetic field contains enough energy that
it can tell the matter how to move,” says Gregori – just as gravity does.
Indeed, when they first saw that Dalmatian-like pattern of hot and cold
spots in their turbulent plasma, they were struck by how similar it looked
to the structures we see in galaxy clusters.
Could magnetism replace dark matter?
All this is breathing new life into Alfvén’s argument that magnetism helped
sculpt the universe. It is tempting to ask if this could even banish the
need for dark matter. That is a step too far for Stein, though. “I think
it’s not that we need less dark matter or more dark matter, it’s just that
we need to understand how these processes work,” she says. In other words,
astronomers need to start taking magnetic fields a lot more seriously.
It is a challenge Lopez Rodriguez is willing to accept. He, too, is
sceptical that magnetic fields in spiral galaxies are strong enough to
replace dark matter, but the truth is that no one knows what modifying
effects they may have. His first investigation is into why magnetic fields
seem to ubiquitously follow the spiral pattern of their host galaxies.
According to gravity theorists, spiral galaxies are explained by something
called density wave theory. It posits that as matter orbits the centre of a
galaxy, slightly more dense regions slow down other patches of passing
matter, enhancing the first region’s density, sparking star formation and
delineating the spiral arms. The one thing it doesn’t predict is why a
large-scale magnetic field would follow this structure so precisely, but
then neither does any dynamo theory. “Yet we see it everywhere,” says Lopez
Rodriguez.
To find out what is going on, he will soon begin a three-year programme of
computer modelling, running simulations of galaxy formation with and without
the fields to see what comes out. If all goes to plan, he will finally have
some answers and perhaps we will at last know how the cosmos was chiselled
into shape.