BYLINE: Ula Chrobak

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Key takeaways:

Researchers arrive at significantly different values for the Hubble constant – the rate at which the universe is expanding – depending on whether they calculate it mathematically or measure it from observations.An international team of researchers simulated the effect of magnetic fields formed shortly after the Big Bang on the plasma of the early universe.They found these magnetic fields could resolve the Hubble tension and help explain other mysterious magnetic fields.

Astrophysicists at the Department of Energy’s SLAC National Accelerator Laboratory and their collaborators are close to resolving a longstanding cosmological question: Were there significant magnetic fields in the early universe? 

Recently, through computer simulations of the first few hundred thousand years of the universe, the research team discovered the surest evidence yet for the existence of primordial magnetic fields. In findings published in Nature Astronomy, they further report that these magnetic fields can resolve important conflicts in cosmology, including a discrepancy known as the Hubble tension.

“It’s this extra physics that happens to make everything match better,” said co-author Tom Abel, professor of particle physics and astrophysics at SLAC and of physics at Stanford University and a member of the Kavli Institute for Particle Astrophysics and Cosmology.

Mysteries of magnetic fields

Everywhere cosmologists look, they find magnetic fields. Some are easily explained by electrical charges and currents, like how Earth’s molten iron core generates the magnetic field that gives us compass directions. Stars, too, spark magnetic fields through the movement of their hot plasma.

But other magnetic fields are more difficult to explain, including ones detected in galaxy clusters and those lingering in dark voids of space.

“We are not so sure where they come from,” said Karsten Jedamzik, a research scientist with the French National Centre for Scientific Research and lead author of the paper. “Are those magnetic fields due to astrophysical sources, or are they left over from the Big Bang?”

In the 1970s, physicists first posited that primordial magnetic fields may have emerged soon after the Big Bang, over 13 billion years ago. Because the universe is made of charged particles that move around and make currents, which in turn give rise to magnetic fields, physicists were certain that some degree of magnetism must have originated early on. “We know that some magnetic fields must have been produced,” said Levon Pogosian, a professor of physics at Simon Fraser University in Canada and co-author on the paper. “It’s just a question of whether there were enough of them produced to explain astrophysical puzzles.”

Interest in primordial magnetic fields has accelerated in recent years as physicists have sought to solve a problem called the Hubble tension. This tension arises from two different ways of measuring the Hubble constant – the rate at which the universe is expanding – yielding a small, but significant, difference. 

When measured by observing stars and supernovae, the Hubble constant is slightly larger than when it’s calculated based on measurements of the cosmic microwave background (CMB) – light that was released when particles combined to form hydrogen in the very early universe.

“The Hubble tension is the biggest problem in cosmology right now,” said Jedamzik. “History has shown that if you have a model and you have observations that don’t fit each other, this may lead to the discovery of new physics or new features of the universe.”

Clues in the cosmic microwave background

Recently, the team performed the most realistic simulation of primordial magnetic fields to date. At Simon Fraser University’s supercomputer, they ran astrophysical models that simulated the “soup” of the early universe – a hot plasma of electrons, neutrons and photons. A feat of coding, it took the team four years to build and run these three-dimensional simulations.

In the simulations, the physicists varied the level of magnetic force interacting with the plasma. Magnetic forces increase the density of particles, causing them to clump together. In the simulations with greater magnetism, electrons and neutrons found each other and formed hydrogen faster. This led to an earlier “recombination,” the moment at which most of the universe’s particles formed hydrogen, turning the universe transparent and releasing a flash of photons – the source of the CMB.

The researchers then compared patterns in the simulated CMB to the CMB as observed today. They analyzed which level of simulated magnetic forces had generated patterns most statistically similar to observational data.

They found that a light level of magnetism – about 5 to 10 pico‑Gauss, around a trillionth of the strength of a fridge magnet – produced the best match for today’s CMB. Based on that match, they calculated a new Hubble constant – and found that this value closely matched the observed Hubble constant.  Jedamzik and Pogosian had initially made this connection five years ago, but the computer models they used were much simpler. The finding gave the researchers more confidence that magnetic fields could solve the Hubble tension.

“The model with a magnetic field that matches the observations has the beautiful quality that it also matches the Hubble constant today,” said Abel. “Whereas, in the usual analysis that does not consider the influence of magnetic fields, they get a value for the Hubble constant that doesn’t match observations.”

The missing astrophysical puzzle piece

Why does the presence of primordial magnetic fields fix the calculated Hubble Constant? The reason lies in their effect on recombination. When the magnetism increased density, drawing particles together, the universe became transparent sooner. This, in turn, means that something called the “sound horizon” is smaller.

The sound horizon – the distance sound travelled in the time from the Big Bang until the universe became transparent – is a standard ruler used by physicists to measure objects in space. Just like how we can calculate the distance to a building by knowing its height and the angle of the line between us and its tip, cosmologists can triangulate the Hubble constant. They measure the angle of the sound horizon through observation, infer the length of the sound horizon by theoretical calculation, and from there get the distance to the region where the universe became first transparent, which is inversely proportional to the Hubble constant.  Without changing other values, a smaller standard ruler would shrink the size of celestial objects. To recalibrate objects back to their known size, the Hubble constant needs to go up slightly.

“It’s a very simple and mundane explanation for this Hubble tension,” said Abel. Whole new fields have been theorized to explain the cosmological mystery – but the team believes their explanation offers the most straightforward solution.

Not only do primordial magnetic fields explain the Hubble tension, they might also resolve other magnetic mysteries. The same adjustment to cosmological math used to calculate the Hubble constant also provides a foundation for the powerful magnetic fields found in galaxy clusters. “Turns out, the number that we need to fix the Hubble tension is the same one we need to explain galaxy clusters,” said Abel. “It’s a very simple and satisfying solution.”

But this is not a cosmologically closed case. Since the study, scientists have already performed more detailed measurements of the CMB – and now the team’s astrophysical simulations must sharpen to compare the two, said Jedamzik. “We have to match it in precision with theoretical calculations.”

Still, the researchers are allowing themselves some celebration. “As theoretical physicists, we often work on ideas that have a very small chance of being true,” said Pogosian. “This is actually not that far from having a chance to be true, so there is a real thrill for us.”

Funding sources for the research include the DOE Office of Science; the BC DRI Group, Canada; the Digital Research Alliance of Canada; the National Sciences and Engineering Research Council of Canada; and the French National Research Agency.

 

About SLAC

SLAC National Accelerator Laboratory explores how the universe works at the biggest, smallest and fastest scales and invents powerful tools used by researchers around the globe. As world leaders in ultrafast science and bold explorers of the physics of the universe, we forge new ground in understanding our origins and building a healthier and more sustainable future. Our discovery and innovation help develop new materials and chemical processes and open unprecedented views of the cosmos and life’s most delicate machinery. Building on more than 60 years of visionary research, we help shape the future by advancing areas such as quantum technology, scientific computing and the development of next-generation accelerators.

SLAC is operated by Stanford University for the U.S. Department of Energy’s Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time.