A clue to one of the biggest questions in cosmology — why regular matter, rather than antimatter, survived to fill the universe — may have been found in data from a NASA space telescope.
A new study suggests that gamma-rays (high-energy light) detected by the Fermi Gamma-ray Space Telescope show signs of the existence of a magnetic field that originated mere nanoseconds after the Big Bang. In addition, the researchers on the new study speculate that the magnetic field carries evidence of the fact that there is far more matter than antimatter in our universe.
The detection of the signal in the Fermi data is currently too weak to be claimed as a "discovery," and no other solid evidence of an early-universe magnetic field exists. But if the signal bears out and the researchers' speculations withstand scrutiny, the work could help scientists understand why the observable universe is made primarily of matter and not antimatter. [The Gamma Ray Universe: Photos by the Fermi Telescope]
Matter vs. antimatter
It's easy to take matter for granted. The stuff that makes up our planet and everything on it — as well as our sun and all the other visible objects in the universe — never seems to be at risk of disappearing in an instant. But around the time our universe was born, there may have been just such an instant — a moment when matter won out and something called antimatter did not.
Cosmologists think the universe started with equal parts matter and antimatter; when matter and antimatter collide with great force, they annihilate each other. So, what happened to most of the antimatter (it still exists in the universe, but in very small quantities)? Why did matter dominate? It's one of the biggest questions plaguing modern science.
Tanmay Vachaspati, a professor of physics at Arizona State University and his colleagues think they have found a clue to this mystery. They say that a signal in the Fermi gamma-ray data suggests an overwhelming production of matter, but not antimatter, in the early universe. They detailed their findings in a paper published online today (May 14) in the journal Monthly Notices of the Royal Astronomical Society.
A universal magnetic field
The team claims to have identified a sort of "twisting" of the gamma rays that the Fermi telescope detects, and the researchers say the detection of this twisted gamma-ray signal is verified in their paper.
Vachaspati and his colleagues' interpretation of what that signal means boils down to this: The twisted gamma-rays are evidence of a magnetic field that has been present in the universe since less than a second after the Big Bang. This magnetic field has a left-hand orientation, and that is evidence of the overwhelming production of matter in the early universe, as antimatter would have produced a right-hand orientation, they said. [Most Amazing Gamma Ray Sources in the Universe]
There are many particle-physics events that must occur for this magnetic field to leave an imprint on the gamma-rays, the researchers .
Scientists don't know for sure if this kind of "primordial" magnetic field exists in our universe. There have been magnetic fields observed in some galaxies and galaxy clusters that could be magnifications of a magnetic field that already existed in the universe, and to demonstrate that it exists would be a fascinating discovery, scientists say.
The discovery of this left-hand signal was first reported by Vachaspati and colleagues in a paper published in 2014.
"We were kind of cautious, and we didn't want to make a big deal of it, because we thought maybe the signal would go away with more data or more analysis," Vachaspati said. "And then, in [the new paper], we used more data and did other kinds of analysis. And the signal is still there."
But the signal may not be a "discovery" quite yet.
In analyzing statistical data from instruments like the Fermi telescope, there is always a chance that a signal could arise purely by chance. The odds of this occurring are measured by a value called sigma. A result with 1 sigma has roughly 1-in-3 odds of arising purely by chance (not a very good bet).
The signal detected by Vachaspati and colleagues has a 3-sigma uncertainty, or about 0.3 percent odds that it has appeared purely by chance. This may seem good, but in particle physics, most signals are not officially called a "discovery" until they have a 5-sigma value (1-in-2-million chance that the signal is a purely random fluctuation).
Tonia Venters, a researcher at NASA Goddard Space Flight Center who works with Fermi telescope data, said it's important to practice caution.
"Our field has seen many results at [2- and 3-sigma] significances come and go, so we tend to be rather skeptical when faced with even a 3-sigma result (0.3% probability of occurring by chance)," Venters told Space.com in an email. "To us, a 3-sigma result is interesting enough to wait for more data, but not enough to generate much excitement.”
It should be noted that there are other ways to judge the validity of a signal, and sigma is not always the best metric to use. However, it often serves as a good way to quickly evaluate the strength of a result. Vachaspati said he puts more weight on the fact that certain predictions made about the signal in the first paper were confirmed in the new analysis.
The next step, Vachaspati said, is to continue to look for the signal in more Fermi telescope data. The collaboration is expected to release new data this year. He will discuss the work with colleagues from around the world at a monthlong conference on cosmological magnetic fields this June and July.
"I think the most important part is that we're seeing a suspicious signal in the data, and then the rest is kind of one step at a time," Vachaspati said. "We think the most likely candidate for why this is happening is the magnetic field. And then, if it is the magnetic field, then it seems most likely to me it's going to be this matter-antimatter asymmetry.
"But people have different ideas, so that part becomes more theoretical," he added. "The interesting thing is that there seems to be a signal."