Symmetry breaking in ‘galactic tetrahedrons’ linked to parity violation – Physics World

Symmetry breaking in ‘galactic tetrahedrons’ linked to parity violation – Physics World

Time Stamp: June 13, 2023 9:58 AM

JWST galaxies
Parity violation: image of galaxies taken by the James Webb Space Telescope. Astronomers have found evidence that parity violation in the early universe affects the distribution of galaxies today. (Courtesy: NASA/ESA/CSA/JWST)

Astronomers in the US have discovered an unexpected asymmetry in the relative positions of galaxies that are hundreds of millions of light–years apart. The phenomenon could be explained by a breaking of the symmetry of the laws of nature that is believed to have occurred shortly after the Big Bang. As a result, the observation could help explain why there appears to be much more matter than antimatter in the observable universe.

The discovery was made by analysing a database of over one million galaxies observed by the Baryon Oscillation Spectroscopic Survey (BOSS). The research was done by Jiamin Hou and Zachary Slepian at the University of Florida, and Robert Cahn at the Lawrence Berkeley National Laboratory in California, who found the unexpected pattern.

The observation is related to parity symmetry, which applies to the long-range electromagnetic and gravitational interactions in the Standard Model of particle physics. Parity requires that a physical system will behave the same way as its mirror image. Human hands, for example, are mirror images of each other but the laws of physics apply equally to right and left hands.

Parity violation

In the microscopic world, however, parity symmetry can be violated by the weak interaction and possibly by the strong interaction – which both act at very short distances.

The trio explored parity symmetry on a very large scale by drawing lines between quadruplets of galaxies that are separated by distances between 65 million and 500 million light–years. As they showed in a recent paper in Physical Review Letters, the tetrahedrons created by this exercise could then be analysed for evidence of parity violation.

Now, they report the result of such a study, which Slepian describes as a “huge surprise”.

The team defined right and left-handed galactic tetrahedrons based on how galaxies were connected to their closest and farthest partners. They found that there were significantly more galaxies with one type of handedness than the other.

Galactic tetrahedrons

“For any given galaxy distribution we assume that the clustering is invariant under rotation about any galaxy,” explains Slepian. “So, if I’m sitting in one galaxy, I should see that the pattern of clustering is on average the same wherever I rotate my head and look. Yet instead we see an excess of tetrahedra over their mirror images.”

Despite strength of the effect, the reason for this handedness remains a mystery. Gravity is the only known force that can act over the huge distances separating the galaxies, and it should not violate parity. Instead, Slepian says that the asymmetry, “must have been imprinted even earlier in the universe’s history when other forces were at play”.

This takes us all the way back to the period of cosmic inflation, which occurred about 10−33 s after the Big Bang. At this point the universe experienced a brief period of extremely rapid expansion. Physicists believe that quantum fluctuations during inflation have since expanded to become the large-scale structure of the universe. Therefore, any parity violation present during inflation could become imprinted in how galaxies are distributed in the universe 13.7 billion years later.

 The origin of this parity violation remains unknown. “It could have been a new force, or a new particle, acting on a quantum scale at that time,” says Slepian.

Missing antimatter

This potential observation of parity violation in how galaxies are distributed is exciting news. As well as suggesting the existence of physics beyond the Standard Model, could also help solve another of physics’ deepest mysteries: why is there much more matter than antimatter in the universe.

The Standard Model predicts that equal amounts of matter and antimatter should have been formed in the Big Bang. Had that happened, matter and antimatter would have annihilated each other, leaving the universe with neither. Luckily for us there seems to have been an excess of matter left over – a phenomenon called baryogenesis.

It is possible that the mechanism that caused parity violation that led to this latest astronomical observation could also related to baryogenesis.

“There’s a range of mechanisms that can cause parity violation, all pretty speculative,” says Slepian. He cites hypothetical particles called axions, or one of the fundamental forces behaving differently in the high energies of the Big Bang. “While it’s not guaranteed that whatever mechanism is producing this parity violation in the galaxies could also explain baryogenesis, I think there certainly could be a relationship.”

While the existence of this galactic asymmetry has not been established beyond any doubt, the findings provide strong evidence for inflation and physics beyond the Standard Model. However, a systematic error in the data could be responsible for the observation. “I’ll feel a lot better once the same signal is seen in a different dataset taken by a different instrument with different software and different people,” says Slepian.

Slepian, Hou and Cahn are all members of the science team of the Dark Energy Spectroscopic Instrument (DESI) at Kitt Peak National Observatory. It will observe over 35 million galaxies, and the trio intend to use DESI to make further observations to confirm their findings.

The results are described in Monthly Notices of the Royal Astronomical Society.

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