The magnetism of the entire universe is 2.5 billion times weaker than a fridge magnet, according to a new analysis.
鈥淭he magnetic field is weak by that standard, but it鈥檚 a lot of energy. There鈥檚 as much energy in the Milky Way in magnetism as there is in starlight,鈥� says Bryan Gaensler at the University of Toronto in Canada.
As light passes through a magnetic field, the Faraday effect causes the light鈥檚 orientation, or polarisation, to rotate. Gaensler and his colleagues used this to measure the strength of the magnetic field that permeates the intergalactic medium, the parts of space between the galaxies.
Advertisement
They searched through a catalogue of polarised light sources in the universe. Conveniently, these often occur in pairs because galaxies often have a supermassive black hole at the centre shooting out twin jets of radiation in opposite directions. 鈥淭hey鈥檙e like two searchlights that light up the magnetic field,鈥� Gaensler says. They used 317 of these pairs to confirm the strength of the magnetic field in those spots.
Some of the magnetism we see in that light comes from the matter in the galaxy itself, but some is added as light travels to us through the intergalactic medium. To separate out this input, the team compared 5111 distant pairs of polarised light that were both coming from the same direction, but with one that was double or triple the distance of the nearer source.
The longer you travel through space, the bigger the Faraday effect. So, they subtracted the magnetic field measurements from the nearer sources to calculate the contribution from the intergalactic medium. They found that the听magnetism of the universe amounts to 40 nanogauss 鈥� a fridge magnet would be about 100 gauss.
That figure may not tell the whole story, says听Avi Loeb at Harvard University 鈥� we need more measurements to confirm.鈥淔araday rotation happens along the line of sight, so if the magnetic field reverses you wouldn鈥檛 see it,鈥� he says. 鈥淭hat could cancel out some of what we measure as magnetism.鈥�
Tessa Vernstrom at CSIRO in Australia, who led the study, says the team made an assumption about the number of these reversals to do their calculations.听 鈥淭his makes the estimate a lower limit, in that the reversal scale may be much smaller, which would increase the magnetic field limit by an order of magnitude or more,鈥� she says.
Gaensler says that future measurements could help us figure out how the whole universe became magnetic in the first place 鈥� a trait that is key for forming stars and galaxies.
Reference:听
