The universe is strangely magnetic – and a process of runaway expansion at its birth is to blame. New calculations shore up the idea that cosmic inflation stretched out tiny magnetic fields in the infant universe, leading to the magnetism that now surrounds galaxies, galactic clusters and giant voids in space.

To create a magnetic field in stars or a planet, the spins of individual electrons in a magnetic material, such as iron, line up in the same direction. If these then rotate, like liquid iron in Earth’s outer core, it creates a geodynamo, which produces an electric current and in turn a magnetic field.

But the alignment of interstellar dust grains and measurements from radiotelescopes reveal that primordial magnetic fields surround galaxies, galaxy clusters and even cosmic voids – and these cannot be caused by geodynamos.

This magnetic fields are much weaker than Earth’s, measuring at most 10-6 gauss, although if you add up all the ones across the universe, it amounts to a lot of energy, says astrophysicist Gianluca Gregori of the University of Oxford.


Wanted: more magnetism

There are several explanations for where this magnetic energy came from, but all have been dogged by weaknesses. That includes the most popular, which says that quantum electromagnetic fluctuations permeated the nascent universe and got amplified by inflation, the process credited with pushing the infant cosmos apart faster than the speed of light for just a tiny fraction of a second.

This scenario was first suggested in 1988, by Michael Turner at the University of Chicago, and Larry Widrow, now at Queen’s University in Ontario, Canada. The trouble was that their calculations put the field at just 10-50 gauss – much smaller than the magnetism observed today. Their method of solving the riddle required deviating from the standard model of particle physics, which many found unacceptable.

Now Leonardo Campanelli of the University of Bari in Italy has found a way for these fluctuations to seed primordial magnetism without resorting to non-standard physics.

Pointing out that Widrow and Turner only took account of a fraction of the fluctuations during inflation – using all of them led to an infinite amount of energy, which is impossible – Campanelli turns to a mathematical tool called renormalisation.

Quantum trick

This was dreamed up by quantum theorists to deal with the infinite energies that they faced: it provides a mathematical way for turning these fiendish, hypothetical entities into real, finite energies. “It allows us to understand that only a part of this infinite energy is ‘real’ – namely, observable. And the other part is unphysical – not real, not observable, not usable, briefly, not existent,” Campanelli says.

But until now, no one had thought to apply renormalisation to the primordial magnetism problem.

When Campanelli did, he got a much bigger value for the primordial magnetic field – of 10-12 gauss. That’s still smaller than what has been measured, but it is large enough that the exotic physics is no longer needed. Instead, a process of twisting and stretching, caused by the same turbulence that causes galaxies to collide, can amplify the magnetic field to present-day values, Campanelli calculates.

That’s a lot of stretching, but “not unreasonable over the age of the Universe”, says Gregori, who was not involved in the work. “These new results are very interesting because if confirmed, they could pave the way to a full understanding of what is going on,” he adds.

Widrow agrees. “If 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,” he says.

The results are due to be published in Physical Review Letters.

Reference: arxiv.org/abs/1304.6534