This happens more often in science than you think. Astronomers have just made a completely unexpected discovery. While looking for a structure in the gamma rays that come to us from the universe, they discovered a completely different signal.
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After the Big Bang, when our universe became cold enough to form the first atoms, it also released a flash of light. What remains today, 13 billion years later, is what astronomers call the cosmic microwave background (CMB). At first glance it looks the same in all directions. But upon closer inspection, it shines more in the direction of the constellation Leo and less in the opposite direction. We learned about 0.12% in the early 1990s from NASA's Cosmic Background Explorer (Cobe) mission. A nothing. But a nothing that astronomers are very interested in.
The cosmic microwave background holds a secret…
Researchers believe that we owe this structure of the so-called dipolar diffuse cosmic background to the movement of our solar system at the heart of our galaxy, the Milky Way. We move there at around 370 kilometers per second. Therefore, the same type of dipolar structure should also be observed at wavelengths other than the wavelength at which the CMB is most pronounced, namely microwaves.
The structure of the diffuse cosmic background in gamma rays?
To verify this, NASA astronomers analyzed thirteen years of data returned by the Fermi Space Telescope. It observes our sky in the gamma ray range. And it revealed something completely unexpected to the researchers. “A much stronger signal and in a different region of the sky” than sought.
In The Astrophysical Journal Letters, astronomers report that they have clearly identified a “gamma dipole.” But its peak is far from that of the CMB. In the southern sky. Its magnitude is ten times larger than what researchers would have expected if it were due to the movement of our solar system.
“Such a measurement is important because a discrepancy with the size and direction of the cosmic microwave background dipole could give us insight into the physical processes that occurred at the beginning of the universe, perhaps when it was less than a trillionth of a second,” explains Fernando Atrio- Barandela, professor of theoretical physics at the University of Salamanca (Spain), in a NASA press release.
A gamma signal that coincides with another mysterious signal
The astronomers find that the gamma signal recorded points strangely in a similar direction to another unexplained feature produced by some of the rarest and most energetic cosmic particles ever discovered. And with almost identical amplitude. This other signal was observed several years ago. About 7% more gamma rays, or ultra-high energy particles, come from this region of our sky than average.
According to scientists, the two phenomena could therefore be linked. Still unknown sources could produce both gamma rays and very high-energy cosmic rays. To be clear, astronomers need to either identify these still-mysterious sources or propose alternative explanations for these two phenomena.