WHEN it comes to the cosmos, gravity is the big attraction. The same force that keeps our feet on the ground also shapes the universe. It takes clouds of gas and sculpts them into planets and stars. It fashions hundreds of billions of stars into galaxies, which clump together to form clusters, then superclusters. Yet gravity isn鈥檛 the only player in the game 鈥� another force operates across the cosmic landscape, and that is magnetism.
Magnetic fields stretch for vast distances in the near-nothingness of deep space, even spanning the billions of light years between galaxies. Admittedly, these fields are feeble. A fridge magnet is more than a million times stronger than the weak, all-pervading sea of magnetism in the Milky Way and beyond. That might explain why cosmology has largely ignored magnetism. After all, how could something so puny influence a galaxy?
Times and minds are changing, however. Yes, gravity holds things together, but key physical processes in the universe need magnetism 鈥� from star formation to black holes pumping out high-energy jets. 鈥淚t turns out that many previously unsolved problems in astronomy suddenly make sense once one includes the effects of interstellar magnetism,鈥� says of the University of Sydney in Australia.
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Could the same be true for the universe at large? What makes fields on the galactic scale and beyond so enticing is that they appear to be the legacy of processes that happened shortly after the big bang. What鈥檚 more, most of the visible mass in the universe consists of charged particles whose movements are in thrall to cosmic magnetism as well as gravity. That raises the tantalising possibility that magnetism has played a key role in shaping the universe ever since the start of time.
But before we can be certain, we need answers to some important questions: how exactly did the fields get there and when?
We鈥檝e always known magnetism plays an important part for us close to home. It was German physicist Carl Friedrich Gauss who first measured Earth鈥檚 magnetic field in 1835 with the help of a simple magnet suspended on a string. Now we have a pretty good idea how the sun and Earth generate their fields. As molten iron in Earth鈥檚 outer core (or plasma in the sun鈥檚 case) moves across magnetic field lines, the effect is to induce electric currents. These give rise to a magnetic field that supplements the existing one. Thanks to this dynamo action, a small 鈥渟eed鈥� field can grow into a much larger one.
And a good job too. Earth鈥檚 magnetic field shields us from deadly particles that would otherwise tear away the ozone layer and expose our planet to harmful ultraviolet rays. The sun鈥檚 field protects us too, deflecting yet more deadly particles from beyond the solar system. On a bigger scale, magnetism might even have contributed to the origin of life (see 鈥�Power of the weak鈥�).
Few people expected interstellar space to be magnetic, though. The first proof came in 1949, when US astronomers John Hall and William Hiltner showed that 鈥渟omething鈥� was polarising starlight on its way to us. That something turned out to be cosmic magnetism, lining up interstellar dust grains like tiny compass needles. It was a remarkable discovery, says Gaensler.
Since then, a host of techniques have been developed to measure magnetic fields in the Milky Way, its neighbouring galaxies and beyond. For example, the was made in 2011 by Niels Oppermann and colleagues at the Max Planck Institute for Astrophysics in Garching, Germany. It shows that the field lines follow the galaxy鈥檚 spiral shape. It also confirms that the Milky Way鈥檚 overall magnetic field is a few microgauss (10-6 gauss) 鈥� some 100,000 times smaller than the field at Earth鈥檚 surface.
Astronomers believe that the magnetic fields in spiral galaxies like the Milky Way are amplified and maintained by a dynamo. As the galaxy turns, it moves charged particles across an existing magnetic field, which boosts the field further. 鈥淭he hypothesis is that the galaxy is born with a much weaker field,鈥� says , an astrophysicist at Queens University in Kingston, Canada. 鈥淏ut where did the weak primordial fields that seed the dynamo come from?鈥� he asks.
The first field
For decades, researchers have been trying to solve this conundrum, but their instruments have not been sensitive enough to test any of their theories.
And so the theories have multiplied. One idea is that the first magnetic fields were produced in very early stars and then spread into the interstellar medium via stellar winds or supernova explosions. Another is that when the first galaxies formed, roughly 100 million years after the big bang, the supermassive black holes at their centres generated strong magnetic fields that were subsequently dispersed into the intergalactic medium by powerful jets. A more recent idea is that cosmic magnetism may have been triggered by plasma fluctuations in young galaxies. Once you have weak field, it can be rapidly amplified by the dynamo effect. So the rotation and turbulence of the galaxy鈥檚 interstellar medium (the gas and dust between stars) reinforce the original, feeble field. These processes can double the strength of a field in roughly the time it takes a star or the centre of a black hole to rotate. Such timescales are negligible, compared to the age of the universe, and so a meagre field can quickly reach appreciable strengths.
鈥淭here is mounting evidence that the first galaxies have magnetic fields similar in strength to the Milky Way鈥檚鈥�
The trouble is that young galaxies whose light comes to us from far away should have much smaller fields than nearby ones. But astronomers find mounting evidence that microgauss fields existed in these early galaxies too. Either the dynamo mechanism is stronger, or more tantalisingly, the seed fields were generated within a blink of the big bang.
Boom time
Widrow and his colleague at the University of Chicago suggested just such a scenario in 1988. They proposed that primordial magnetic fields were created in the first fleeting moments after the big bang and amplified by inflation, when the budding cosmos expanded faster than light. The large galaxy structures that we observe today were caused by quantum fluctuations in energy during this period. Widrow and Turner showed that inflation could also amplify fluctuations in the electromagnetic field in such a way as to leave the entire universe with a dose of magnetism.
For that idea to work, the pair had to make changes to the famous equations formulated by James Clerk Maxwell to describe electrical and magnetic fields, and they introduced an exotic particle called the axion. 鈥淭he idea was ad hoc and aspects of the proposal troubled particle theorists,鈥� admits Widrow. Their calculations also put the seed magnetic field at 10-50 gauss, meaning a powerful dynamo would be required to reach the level of magnetism we observe now.
Nevertheless, Widrow and Turner鈥檚 idea proved to be a starting point for other investigators. 鈥淭heir theory was the very first suggesting the production of magnetic fields during inflation,鈥� says at the University of G枚ttingen in Germany. 鈥淚t marked a cornerstone in our understanding.鈥�
Earlier this year, physicist Leonardo Campanelli of the University of Bari in Italy explained how these fluctuations may have seeded primordial magnetism without resorting to non-standard physics. He used a mathematical trick called renormalisation, something particle theorists already use to tame infinities that render their equations useless. 鈥淣o one had thought to apply renormalisation to the primordial magnetism problem,鈥� says Campanelli.
His work arrives at a much larger value for the primordial magnetic field of 10-12 gauss. That鈥檚 still smaller than the 10-6 gauss value measured in intergalactic space. But this background field, into which the first stars and galaxies are born, is enough to be amplified to present-day values, he says.
Widrow is impressed with . 鈥淚f the calculations in this paper hold up, then it makes large-scale magnetic fields a natural and expected outcome of inflation, rather than something that requires exotic, if not controversial, modifications to the laws of physics,鈥� he says.
Others pinpoint problems with any theory that generates a cosmos-wide magnetic field during inflation or soon afterwards. That鈥檚 because it might have been almost wiped out during an era known as the dark ages.
For the first 378,000 years of its existence the universe was so hot that there were no atoms, only electrons, nuclei and photons. This swirling cauldron of electrical charge was the perfect place to brew a stronger magnetic field, reinforcing the seed field present soon after inflation.
As the universe expanded, it cooled enough for protons to capture electrons and become neutral hydrogen atoms. As they combined, the particles allowed a rush of radiation to flood the universe 鈥� this is the famous cosmic microwave background (CMB).
Then came the dark ages 鈥� so called because nothing around at the time emitted light. The only source of radiation came from hydrogen atoms, which released radio waves with a distinctive wavelength of 21 centimetres.
Perhaps problematically for cosmic magnetism, the numbers of charged particles plummeted. During the dark ages, there was just one free electron or proton for every 10,000 hydrogen atoms. Because magnetic fields rely on the movement of electrons and protons, some researchers think that the seed field might have winked out at this point.
The dark ages lasted until the first sources of light appeared in the universe. As these stars and galaxies formed, they released huge amounts of radiation that stripped electrons from the hydrogen atoms. This epoch of reionisation lasted roughly a billion years and meant the universe was awash with magnetic-field friendly electrons and protons.
We are not yet sure how well cosmic magnetism fared during these tumultuous times. However, after decades of exotic theories, some answers might be forthcoming.
By combining observations from several telescopes that probe the universe at different eras in cosmic history, researchers will be able to trace the evolution of magnetism (see diagram). Knowing how the magnetic fields looked in the infant universe and evolved will help us to constrain models of their origins.
According to astronomer at the University of Manchester in the UK, next year could see the first analysis of the measurements of cosmic magnetism taken by the satellite, which has been studying the CMB. If primordial fields did exist when the universe was 378,000 years old, their 鈥渋mprint鈥� should be the on the CMB.
Telescopes pull together
Joining the Planck scientists are radio astronomers at the Low-Frequency Array for radio astronomy (LOFAR), whose antennas stretch over five European countries. Plus researchers working on two instruments in the Australian outback 鈥� the Australian Square Kilometre Array Pathfinder (ASKAP) and the Murchison Widefield Array. They are looking for radio synchrotron emissions: radio waves emitted by cosmic-ray electrons spiralling around the field lines.
LOFAR is designed especially to measure longer wavelengths, so it will be able to probe regions of weaker magnetic fields 鈥� such as between galaxies 鈥� and explore the question of how far from the galactic disc the magnetic fields extend. It will also be able to probe fields in galaxies that formed early on in the universe.
Gaensler, one of the leaders of ASKAP鈥檚 project on cosmic magnetism, is confident about being able to identify which of the many theories is correct. 鈥淲e鈥檒l know the answer within two years,鈥� he says.
If they find evidence of strong magnetic fields in proto-galaxies, this would favour the idea that the magnetism was started by shock fronts or plasma fluctuations in young galaxies, says at the Max Planck Institute for Radio Astronomy in Bonn, Germany. However, if the first fields are found near galactic nuclei, this may favour early stars or early galactic dynamo action.
Greater observing power is on its way, too, in the shape of the gigantic radio telescope, the Square Kilometre Array (SKA) in Australia and South Africa. This complex of thousands of radio antennas will allow researchers to study magnetic fields at 10 times the resolution available today. The SKA is set to make its first measurements in the early 2020s. It will probe the epoch of reionisation in an effort to identify the first objects to appear in the universe. It will also be searching for early magnetic fields. 鈥淭he SKA will allow us to measure the intensity and polarisation of radio waves with unprecedented sensitivity,鈥� says , an astrophysicist at the University of Saskatchewan in Saskatoon, Canada.
If the SKA finds that strong fields existed around the first objects, the primordial idea may get support, says Beck. It would indicate that magnetic fields preceded the formation of galaxies 鈥� and may have had an effect on their evolution. And in this case, help could come from Planck or the next generation of CMB satellites that researchers would like to build.
In a decade or so, once measurements from all these telescopes and satellites have been analysed, our cosmic maps will have to be redrawn. 鈥淢ost of the numerical simulations of gas dynamics during the cosmological evolution of galaxies ignore magnetic fields,鈥� says Harvard University astronomer . 鈥淭he next frontier will be to incorporate them, along with cosmic rays, and try to see what effects these components have on galaxies.鈥�
When we understand how the invisible hands of gravity and magnetism operate across the cosmos, then we will know how our universe really works.
Power of the weak
It would take 10 million Milky Ways to stick a shopping list to your refrigerator door 鈥� that鈥檚 how incredibly weak our galaxy鈥檚 magnetic field is. It is still enough to influence the motion of charged particles called cosmic rays, bending their paths and even trapping them inside our galaxy for millions of years.
Without magnetic fields, cosmic rays would fly out of the Milky Way shortly after they are made, points out . The consequences would be profound. 鈥淐osmic rays are an important component of the Milky Way. They ionise gas deep in the proto-planetary discs. They are important for mutations in biology on Earth. In short, they are an important fact of life,鈥� he says.
Indeed, life鈥檚 big break could be the work of high-energy cosmic rays getting their oomph from being slingshot by magnetic fields. It seems that these particles initiate the chemistry that forms sugars, amino acids and other molecular building blocks of life found in dense gas clouds (麻豆传媒, 19 October, p 42). Despite this, we don鈥檛 know for sure where cosmic rays come from because cosmic magnetic fields deflect them. By studying the fields, says Loeb, we will find clues about the origins of cosmic rays and solve a very important mystery.
This article appeared in print under the headline 鈥淭he forgotten force鈥�
