Mysterious ultra-high energy gamma rays, the oldest light in the universe, what is the connection between the two?

Ultra-high-energy gamma rays may reveal how the universe evolved very early

This image shows the Milky Way disk (red) and the mysterious gamma-ray signal (purple circle). Credit: NASA Goddard Space Flight Center

Astronomers have discovered an unknown, unexpected gamma-ray signal that originates from outside our galaxy.

NASA and University of Maryland cosmologist Alexander Kashlinsky discovered the strange gamma-ray signal after searching 13 years of data from NASA's Fermi Telescope.

"This was a completely serendipitous discovery," Kashlinsky said in a statement. "We found a signal that was much stronger than we expected, outside the area of ​​the sky we were looking in."

Even stranger is the fact that this gamma ray is related to one of the greatest unsolved mysteries in the universe - the source of the ultra-high-energy particles in the universe.

The team believes the gamma rays are high-energy particles, or cosmic rays, made up mostly of protons, neutrons and atomic nuclei.

These ultra-high-energy cosmic rays (UHECRs) carry more than 10 billion times more energy than regular gamma rays, and their origin remains a mystery—a mystery that the gamma-ray signal seems to have solidified.

Hunt for 'cosmic fossils' leads to unexpected discovery of gamma rays

The properties of the newly discovered gamma rays are very similar to a certain property of cosmic background radiation (CMB).

The CMB is the oldest light in the universe, created by an event that occurred 380,000 years after the universe was born. Before that, the universe was a hot soup of particles through which light could not travel naturally.

Around this time, the universe cooled to the point where electrons and protons could bind together to form primordial atoms. The sudden disappearance of electrons left room for photons to travel freely.

At this moment, the universe suddenly changed from opaque to transparent, and these freely traveling photons are the CMB that humans are now observing.

As the universe continues to expand, these photons begin to lose their energy, and now have a temperature of -270 degrees Celsius (2.78 Kelvin).

The CMB was discovered in May 1964 by two radio engineers, Arno Penzias and Robert Wilson, who found that there was a microwave radiation that enveloped the Earth, similar to a background - hence the name microwave background radiation. They thought at the time that the temperature of the CMB should be the same in all directions, but in the 1990s, NASA's COBE satellite challenged this model, discovering that the temperature of the CMB varies slightly in different directions.

COBE found that in the direction of Leo, the CMB is 0.12% hotter than the average temperature, and in the opposite direction, the CMB is 0.12% cooler than the average temperature, and its flow is also less.

This "two-level" law is believed to be caused by the movement of the solar system itself, which is 370 kilometers per second relative to the CMB. However, if the solar system is moving in one direction in the universe, then all other starlight should show a "two-level" property similar to the CMB, but this speculation has not been confirmed. Astronomers are looking for other evidence that the "two-level" nature of the CMB is the movement of the solar system.

"Such measurements are important because the differences in the properties of the CMB's 'two phases' may help us understand the very early universe, perhaps even up to a hundred billionth of a second after its birth," said team member Fernando Atrio-Balandra, a theoretical physicist at the University of Salamanca in Spain.

One or Two Cosmic Mysteries?

The team turned to Fermi and its Large Area Telescope (LAT), which conducts multiple surveys a day, and the LAT, which runs continuously for many years. The researchers hoped that the LAT data might reveal the "two-level" nature of the gamma-ray signal.

Because of special relativity and the high energies of gamma rays, this "two-level" property should be five times more pronounced than in the CMB. The team found it, but not in the way they expected.

"We found a 'double' of gamma rays, but its maximum is in the southern sky, far from the CMB maximum, and its intensity is 10 times greater than we expected," said Chris Schlander, an astrophysicist at Catholic University and a member of the team. "While this is not what we were looking for, we suspect it is related to a similar property of high-energy cosmic rays."

High-energy charged particles comprising UCHERs are "dual-polar" with the CMB, which was first discovered in 2017 by the Pierre Auger Observatory in Argentina.

Although these charged particles are deflected by the magnetic fields of the Milky Way and other celestial bodies as they travel toward the Earth, and the direction of deflection depends on the strength of the magnetic field, the "poles" of UCHERs are still located very close to the newly discovered gamma-ray signals.

The team believes that because of this similarity in position, the UCHERs and the newly discovered gamma-ray signals are likely related, especially considering that an unknown source could be producing both signals (UCHERs and the newly discovered gamma-rays) at the same time.

Astronomers now want to study where high-energy photons and particles come from to determine their source (or sources) and whether the two are related, representing the same problem or two completely different problems.

The team's findings were presented at the 243rd meeting of the Astronomical Society and the team published a paper in The Astrophysical Journal.

BY: Robert Lea

FY: Chen Li

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