Symmetry is a tidy and attractive idea that falls apart in our untidy Universe. Indeed, since the 1960s, some kind of broken symmetry has been required to explain why there is more matter than antimatter in the Universe — why, that is, that any of this exists at all. But pinning down the source behind this existential symmetry violation, even finding proof of it, has been impossible. Yet in a new study in the Monthly Notices of the Royal Astronomical Society, astronomers found the first evidence of this necessary violation of symmetry at the moment of creation. They studied a whopping million trillion 3D galactic quadruplets in the Universe and discovered that the Universe at one point preferred one set of shapes over their mirror images.
This image from Webb reveals never-before-seen details of the Stephan’s Quintet. Image credit: NASA / ESA / CSA / STScI.
Parity symmetry violation points to an infinitesimal period in our Universe’s history when the laws of physics were different than they are today, with enormous consequences for how the Universe evolved.
The finding, established with a high level of statistical confidence, has two primary consequences.
First, this parity violation could only have imprinted itself on the future galaxies during a period of extreme inflation in the earliest moments of the Universe, confirming a central component of the Big Bang theory of the origin of the cosmos.
Parity violation would also help answer perhaps the most crucial question in cosmology: why is there something instead of nothing?
That’s because parity violation is required to explain why there is more matter than antimatter, an essential condition for galaxies, stars, planets and life to form in the way they have.
“I’ve always been interested in big questions about the Universe,” said University of Florida’s Professor Zachary Slepian.
“What is the beginning of the Universe? What are the rules under which it evolves? Why is there something rather than nothing? This work addresses those big questions.”
Parity symmetry is the idea that physical laws shouldn’t prefer one shape over its mirror image.
Scientists usually use the language of handedness to describe this trait, because our left and right hands are mirror images we are all familiar with.
There is no way to rotate your left hand in 3D to make it look like your right hand, which means they are always distinguishable from one another.
Parity violation would mean that the Universe does have a preference for either left- or right-handed shapes.
To discover the Universe’s handedness, Professor Slepian and colleagues imagined all the possible combinations of four galaxies connected by imaginary lines in space.
This makes for a 3D object called a tetrahedron, like a lopsided pyramid — the simplest shape that has a mirror image.
They defined right- and left-handed galactic tetrahedrons based on how galaxies were connected to their closest and farthest partners in these imaginary shapes.
Their method required analyzing a trillion imaginary tetrahedrons for each of a million galaxies, a mind-boggling number of combinations.
So the authors developed sophisticated mathematical formulas that allowed the immense calculations to be performed in a reasonable period. It still required a considerable amount of computational power.
The technical aspects of the analysis make it difficult to say whether the Universe prefers right-handed or left-handed shapes, but they saw clear evidence that the cosmos does have a preference.
They established their finding with a degree of certainty known as 7 sigma, a measure of how unlikely it is to achieve the result based on chance alone.
The finding can’t yet explain how we ended up with this crucial abundance of matter.
The ‘how’ will require new physics going beyond the Standard Model, which explains our current Universe.
But the new results do strongly suggest that there was an asymmetry at the earliest moments of the Big Bang.
Now the race is on for scientists to produce a theory that can explain the mirror-image preference of the Universe and the excess of matter.
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Jiamin Hou et al. 2023 Measurement of parity-odd modes in the large-scale 4-point correlation function of Sloan Digital Sky Survey Baryon Oscillation Spectroscopic Survey twelfth data release CMASS and LOWZ galaxies. MNRAS 522 (4): 5701-5739; doi: 10.1093/mnras/stad1062
