A physics experiment may have uncovered the mysterious force behind the expansion of the universe

Representative image. | Photo credit: iStock Images

What the universe is made up of at the most fundamental levels is a question that continues to both fascinate and frustrate scientists. The discovery of atoms, once thought to be nature’s smallest building blocks, paved the way for further study of subatomic particles such as electrons, protons and neutrons, which were found to be themselves made of particles. even smaller.

As our understanding of these particles grew, the Standard Model of particle physics was born. But after looking around the universe and measuring how widely galaxies are distributed and interact with each other, we soon realized that everything described in the Standard Model was only 5% of the matter in the universe.

What is the rest made up of, you ask? Well, we don’t really know, but about 27% of the universe, scientists are convinced, appears to be made up of something called dark matter – matter that doesn’t reflect any light but behaves similarly to normal matter. , exerting a force of gravity on everything around it.

As for the elements that make up the remaining 68% of the universe, our first clue came in the 1990s when scientists discovered that distant supernovae were fainter than their models had predicted. As more and more evidence came in, a new form of energy was theorized, one that exhibits properties that no other type of matter or radiation possesses – dark energy.

There’s next to nothing we can truly determine about the true nature of dark energy, but the measurements reveal one thing: it is remarkably uniform and constant across the universe, behaving fundamentally differently than other types of energy. energy.

It seems to be driving the expansion of the universe but, curiously, even as the volume of the universe increases, the energy density (of dark energy) remains constant. It is almost as if there is something uniformly present in space, the nature of which does not depend on anything residing in space. This prompted scientists to understand that dark energy could be a vast field that pervades the entire universe or even an inherent feature of spacetime itself.

A less explored theory though is that dark energy could be made up of particles we haven’t yet discovered, and that’s where the XENON1T experiment comes in. The XENON series of experiments were originally intended to search for theoretical dark matter particles that scientists call WIMPs. (massive particles interacting weakly).

Thousands of feet below Italy’s Gran Sasso mountain is a sensor-lined tank of 3.2 metric tons of pure xenon (chemically inert), forming a detector of unknown particles that can fly through the tank, causing ripples. The idea is that as these WIMPs move through xenon, they may occasionally hit a xenon nucleus, causing a brief flash of light that could be detected – what is called nuclear recoil.

But after years of not detecting nuclear recoil, scientists realized they could recalibrate their detector to look for electron recoil – phenomena where unknown particles collide with electrons rather than nuclei. of xenon. The researchers studied the first year’s worth of XENON1T data expecting to find 232 such setbacks – all caused by background contamination. But the experiment gave 285 – an excess of 53. It was confusing. What were these additional signals?

These findings have now led scientists to formulate three theories. The first of these – and obviously the most mundane – is that the result was an experimental hazard likely to wither away with improved accuracy and stats. The second is that some sort of unaccounted background contamination – possibly tritium in the water – may be causing these excess electron recoils.

The third, of which scientists say they are 95% certain, is the first-ever detection of an unknown type of dark energy particle that would require contortions of current particle physics theory. It’s clearly the most exotic of the three theories, but it may draw attention to a particular idea known as chameleon dark energy – a notion of a particle whose density is obscured in regions of the matter-rich spaces, but more noticeable in emptier regions. .

At the moment, however, this is only conjecture. A 95% confidence interval in physics is, after all, far from sufficient, and it is entirely possible that as the experimental framework refines, these findings may never happen again. But if substantiated, it would represent a giant step towards answering one of the greatest existential questions that have plagued humanity.

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