The world’s astronomers are increasingly probing the mystery of where the enormous magnetic fields that permeate our universe come from –from Earth to Mars to the Milky Way to intergalactic voids and beyond to the darkest, most remote regions of the cosmos.
The Death of Mars
A half a billion years ago, Mars magnetic field that protected an ocean as deep as the Mediterranean Sea was switched off. An impact basin deep enough to swallow Mount Everest in Valles Marineris reveals what might be the results of an ancient asteroid the size of Pluto colliding with the Red Planet, switching off its magnetic field, bathing the Red Planet in harmful radiation, and eroding its atmosphere by particles streaming from solar winds. Today, Mars is a frigid desert world with a carbon dioxide atmosphere 100 times thinner than Earth’s.
A strong magnetic field had probably played an important role in protecting the atmosphere from the solar wind and keeping the planet wet and habitable. “Venus and Mars have negligible magnetic fields and do not support life, while Earth’s magnetic field is relatively strong and does,” said exobioexplorer Sarah McIntyre at Australia National University. “We find most detected exoplanets have very weak magnetic fields, so this is an important factor when searching for potentially habitable planets.”
Milky Way Center
An iconic new image of the Milky Way’s violent center (below) from the Murchison Widefield Array, a radio telescope in the Western Australian outback, shows huge golden filaments that indicate enormous magnetic fields –what our galaxy would look like if human eyes could see radio waves.
“An Unseen Magnetic Soul”
Anytime astronomers figure out a new way of looking for magnetic fields in ever more remote regions of the cosmos, inexplicably, they find them, observes Natalie Wolchover in Quanta about the invisible magnetic field lines that loop and swirl through intergalactic space like the grooves of a fingerprint, an unseen “magnetic soul”.
“Magnetism is primordial,’ she writes, “tracing all the way back to the birth of the universe. In that case, weak magnetism should exist everywhere, even in the “voids” of the cosmic web — the very darkest, emptiest regions of the universe. The omnipresent magnetism would have seeded the stronger fields that blossomed in galaxies and clusters.”
In 2019, astronomers discovered 10 million light-years of magnetized space spanning the entire length of a “filament” of the cosmic web, part of the massive web that fills much of space, connecting two galaxy clusters dubbed Abell 0399 and Abell 0401 that are slowly colliding with each other.
The Cosmic Web
“We are just looking at the tip of the iceberg, probably,” said Federica Govoni of the National Institute for Astrophysics in Cagliari, Italy, who led the first detection using the Low-Frequency Array (LOFAR) radio telescope to observe the bridge of radio-emitting plasma extending between the two galaxy clusters that are approaching a merger. The results imply that intergalactic magnetic fields connect the two clusters and challenge theories of particle acceleration in the intergalactic medium.
“To understand the nature of this extended radio source is a real challenge, since the maximum distance that these relativistic electrons can cover during their radiative life is much smaller than the size of the radio filament connecting Abell 0399 and Abell 0401,” Govoni told The Daily Galaxy. “Therefore,” she expains, “a mechanism responsible for the acceleration of electrons along the entire filament must exist. Different theories on the acceleration of particles in the intergalactic medium can therefore be investigated.”
The cosmic web, shown here in a computer simulation, illustrates massive filaments of galaxies separated by giant voids.
Primordial magnetism might also help resolve another cosmological conundrum known as the Hubble tension, observes Wolchover, pointing out that it is “probably the hottest topic in cosmology.”
Primordial Magnetism in the early Universe may Resolve the Hubble Tensio
Currently, the expansion rate of the local Universe is 73 km/s/Mpc according to the measured recession velocities of and distances to nearby galaxies. According to this Hubble constant, a galaxy that is 10 megaparsecs (33 million light years) away is redshifted by 730 km/s, while a more distant galaxy that is 100 megaparsecs away (330 million light years) is moving away from us at 7,300 km/s. Meanwhile, the Hubble constant inferred from the Cosmic Microwave Background (CMB) as measured by the Planck satellite implies a lower Hubble constant of 67 km/s/Mpc, assuming the early Universe has since evolved to the present day according to standard cosmological models. Although the difference is only 9%, both of these Hubble constants are measured to roughly 2% precision, and so there is statistically significant tension between the two values. The addition of non-standard physics to the cosmological models, such as a primordial magnetism in the early Universe, may potentially resolve this tension.
“While the Hubble Constant is constant everywhere in space at a given time, it is not constant in time.” explains Chris Fassnacht, professor of physics at UC Davis about the current crisis in cosmology, or “tension”, in understanding the rate of expansion of the universe —known as the “Hubble Constant”—since the Big Bang, a central part of the quest to discover the origins of the universe.
“Dark Energy is Incredibly Strange”
Some physicists meanwhile propose dark energy is a ‘fifth’ force beyond the four already known – gravitational, electromagnetic, and the strong and weak nuclear forces. However, researchers think this fifth force may be ‘screened’ or ‘hidden’ for large objects like planets, making it difficult to detect.
“Dark energy is incredibly strange, but actually it makes sense to me that it went unnoticed,” said Noble Prize winning physicist Adam Riess, in an interview with The Atlantic. “I have absolutely no clue what dark energy is. Dark energy appears strong enough to push the entire universe – yet its source is unknown, its location is unknown and its physics are highly speculative.”
In a paper posted online in April and under review with Physical Review Letters, the cosmologists KarstenJedamzik and Levon Pogosian, a professor of physics at Simon Fraser University in Canada, propose that weak magnetic fields in the early universe would lead to the faster cosmic expansion rate seen today.
Like a Living Organism
Astrophysicists at Johns Hopkins led by Nobel Laureate, Adam Riess, say researchers need to find conclusive evidence of primordial magnetism that appears to be everywhere, is the missing agent that shaped the universe.
“Everyone knows it’s one of those big puzzles,” said Pogosian. “But for decades, there was no way to tell whether magnetism is truly ubiquitous and thus a primordial component of the cosmos, so cosmologists largely stopped paying attention.”
Magnetism “is a little bit like a living organism,” said Torsten Enßlin, a theoretical astrophysicist at the Max Planck Institute for Astrophysics, “because magnetic fields tap into every free energy source they can hold onto and grow. They can spread and affect other areas with their presence, where they grow as well.”
“Free energy is energy that can be used to perform some work,” Enßlin explains in an email to The Daily Galaxy. “For example the ordered motion of a gas stream can certainly be regarded as free energy, but the random thermal motion of its atoms not necessarily as such. Magnetic fields that are frozen into an ionized gas are able to extract kinetic energy from it and convert this into further magnetic energy to grow exponentially in strength, up to a level at which their own forces are starting to alter the gas flow pattern significantly. This dynamo mechanism maintains magnetic fields in many astrophysical bodies, from planets, over stars, to galaxy and galaxy clusters.”
“This ability to grow lets magnetic fields appear a bit like living organisms,” Enßlin observes, “as they can harvest free energy to maintain themselves. Furthermore, one needs a seed magnetic field for this growth, similar as one needs some seed organisms to grow an ecosystem. Of course this analogy has its limitations, as magnetic fields for example do not have a complex genetic code that gets copied from one generation to the next. Nevertheless, if one focuses on magnetization of an ionized gas in turbulent motion, one sees that this can spread and grow very much like microorganisms in an environment with nutrition.”
The Last Word
“Rather than saying ‘magnetic fields lead to the faster cosmic expansion,’ it would be more accurate to say that accounting for the magnetic fields would lead to a ‘prediction’ of a faster cosmic expansion, Levon Pogosian explains, replying to an email from The Daily Galaxy. “Let me elaborate on this point: The Hubble tension” refers to the disagreement between the value of the Hubble constant (i.e. the current expansion rate of the universe) measured directly and the value predicted by the Lambda Cold Dark Matter (LCDM) model fit to the Cosmic Microwave Background data. Magnetic fields introduce a new ingredient into the LCDM model, such that, the fit to the CMB data results in a larger value of Hubble Constant.”
“Why does this happen?” Pogosian explains: “Cosmic Microwave Background (CMB) is the oldest light in the universe we can see. This light was released some 400,000 years after the Big Bang, when the universe turned from opaque to transparent. This transition happened when electrons and protons started to bind to form neutral hydrogen atoms. Cosmologists refer to this period as the epoch of recombination.”
“The CMB has been measured with exquisite accuracy,” he notes in his email, “allowing us to measure all free parameters of the LCDM model very precisely. Once these parameters are known, one should be able to predict all the observed properties of the universe, with the current expansion rate being one of them.
“The LCDM parameters one measures from CMB depend strongly on when the CMB was released. If recombination happened at an earlier time, the predicted H0 would be larger. That’s where magnetic fields help.
“Inhomogeneous primordial magnetic fields, if present just prior and during recombination, would compress the proton-electron plasma in little pockets, making it clumpy. As a result of this clumping, the protons and electrons would be forming hydrogen more efficiently and recombination would occur at an earlier time, which is what LCDM needs to predict a higher Hubble Constant.”
“Interestingly,” Pogosian concludes in his email, “the strength of the magnetic field one needs to relieve the H0 tension, around 0.05 nano-Gauss in comoving units, happens to be of just the right order to also explain all the other sightings of cosmic magnetic fields. Needless to say, this is very exciting!”
“An earlier completion of recombination would mean that the comoving sound horizon at the time CMB was emitted was smaller. The angular size of that sound horizon is the best measured quantity in cosmology. With this angular size fixed, a smaller sound horizon would mean that the comoving distance to the epoch of recombination is shorter. This, in turn, would require the current rate of expansion to be larger.”
New Discovery of Magnetic Fields in Cosmic Filaments
Shane O’Sullivan, a radio astronomer that works on understanding the properties of magnetic fields that pervade the Universe and co-PI of the LOFAR Magnetism Key Science Project (MKSP), wrote The Daily Galaxy to announce: “Excitingly, we have just discovered the signature of magnetic fields in intergalactic cosmic filaments, using measurements of ~1,000 galaxies with the LOFAR radio telescope. This first detection by our team is described in Carretti et al. (2022), where we find an average field strength of ~3 picoTesla (about a billion times weaker than a fridge magnet). This discovery is only the beginning, and soon we will be able to trace the evolution of these fields back in time to understand the origin of cosmic magnetism in the very early Universe. ”
Image credit: Combined radio/optical image of galaxy IC 342, using data from both the VLA and the Effelsberg telescope. Lines indicate the orientation of magnetic fields in the galaxy. R. Beck, MPIfR; NRAO/AUI/NSF; graphics: U. Klein, AIfA; Background image: T.A. Rector, University of Alaska Anchorage and H. Schweiker, WIYN; NOAO/AURA/NSF.
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Federica Govoni, Torsten Enßlin, Levon Pogosian, Shane O’Sullivan and The Hidden Magnetic Universe Begins to Come Into View/Quanta and Science