What Is Dark Matter? Axions Could Be the Key to Solving the Universe’s Greatest Puzzle

The story of how a strange hypothetical particle became a contender for stellar dark matter.

What Is Dark Matter?
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Conundrums pervade physics, and they are, in some ways, what keeps it going. These perplexing problems encourage a quest for the truth. But, of all the conundrums, I’d say two of them are certainly priority A.

To begin with, when astronomers gaze up at the sky, they see stars and galaxies moving away from our planet and from each other in every direction. The expanding cosmos resembles a bubble inflating up, which is how we know it’s expanding. But something isn’t adding up.

There doesn’t appear to be enough matter floating around in space — stars, particles, planets, and everything else — for it to expand at such a rapid rate. To put it another way, the universe is expanding much faster than our physics predicts, and it’s getting faster as you read this. This brings us to the second issue.

According to the best estimates, galaxies are spinning so fast as everything zips about that the spirals should act like out-of-control merry-go-rounds tossing metal horses from the ride. There doesn’t appear to be enough material in the universe to hold them all together. Despite this, the Milky Way does not appear to be migrating apart.

So… what’s going on?

Physicists refer to “missing” things pushing the cosmos outward as dark energy, and parts holding galaxies together as dark matter, presumably in a halo-like form. They don’t interact with visible light or matter, thus they’re effectively invisible. Dark matter and dark energy account for nearly all of the universe’s mass and energy.

“It may well consist of one or more types of fundamental particle… while the part or all of it might consist of macroscopic lumps of some invisible form of matter, such as black holes,” the authors of a recent review published in the journal Science Advances wrote.

Dark matter, whether it exists in black holes or not, is completely elusive. Scientists have chosen a few suspects from the cosmic lineup in order to unlock its secrets, and one of the most sought particles is a strange little speck known as the axion.

The axions’ wide-eyed hypothesis

You’ve probably heard of the Standard Model, which is essentially the holy grail of particle physics and is constantly being strengthened. It explains how each and every particle in the universe functions.

Some “particle physicists are restless and dissatisfied with the Standard Model since it has many theoretical inadequacies and leaves many crucial experimental concerns unsolved,” according to the Science Advances assessment. For us, it immediately leads to a dilemma involving a well-known scientific principle known as CPT invariance. The physics riddles keep on coming.

CPT invariance states that when it comes to C (charge), P (parity), and T (time), the cosmos must be symmetrical. As a result, it’s also known as CPT symmetry. It asserts that the cosmos would remain the same if everything had the opposite charge, was left-handed instead of right-handed, and went through time backwards instead of forwards.

CPT symmetry seemed unshakable for a long time. Then there was 1956.

To cut a long storey short, scientists discovered something that breaks the CPT symmetry’s P component. It’s known as the weak force, and it governs events such as neutrino collisions and the fusing of elements in the sun. Everyone was taken aback, perplexed, and terrified.

CPT symmetry is used in nearly every fundamental physics idea.

Researchers found the weak force, which also violated C symmetry, about a decade later. Things were starting to come apart. Physicists can only hope and pray that even if P is broken… and CP is broken… CPT isn’t broken. Perhaps the trio is only required by weak forces to maintain CPT symmetry. Fortunately, this notion appears to be right. Despite C and CP blips, the weak force follows the entire CPT symmetry for some reason. Phew.

But there’s a problem. You’d think that if weak forces violate CP symmetry, strong forces would as well, right? They don’t, and physicists have no idea why. This is known as the strong CP problem, and it’s here that things start to get interesting.

The strong force is observed by neutrons, which are uncharged particles within atoms. Furthermore, their neutral charge means they violate T symmetry, allowing for simplification. And, according to the study, “if we uncover something that violates T symmetry, it must also violate CP symmetry in such a way that the combination CPT is not violated.” But… that’s strange. The strong CP problem prevents neutrons from doing so.

As a result, the axion concept was born.

Roberto Peccei and Helen Quinn, physicists, proposed adding a new dimension to the Standard Model years ago. It involved a field of ultralight particles known as axions, which solved the strong CP problem and hence relaxed neutron requirements. According to the report, Axions’ method seemed to work so well that it became the “most common solution to the strong CP problem.” It was nothing short of a miracle.

To be clear, axions are still hypothetical, however, consider the following scenario. Physicists have introduced a new particle to the Standard Model, which depicts the universe as specks. What does this signify for the rest of the world?

The key to dark matter?

Axions would be “cool,” or travelling very slowly through space, according to the Peccei-Quinn theory. “The presence of [dark matter] is deduced from its gravitational effects,” the researchers write, “and astrophysical measurements suggest that it is ‘cool.'”

“There are experimental top limitations on how strongly [the axion] interacts with visible matter,” the article adds.

In other words, axions that assist explain the strong CP problem appear to have theoretical features that are similar to dark matter. Very well done.

CERN, the European Council for Nuclear Research, which runs the Large Hadron Collider and is leading the charge for antimatter research, also emphasises the importance of antimatter research “One of the most intriguing properties of axions is that they could be produced in large numbers in a natural way soon after the Big Bang. This population of axions would still exist today and possibly make up the universe’s dark matter.”

So there you have it. Axions are one of physics’ trendiest topics since they appear to explain so much. Those sought-after pieces, though, are still hypothetical.

Also Read: Time Travel Theories Possibilities | The Arrow of Time Concept

Will we ever find axions?

Will we ever find axions?

Scientists have been looking for axions for 40 years.

According to the authors of a recent assessment published in Science Advances, the majority of these searches “primarily utilise the action field interaction with the electromagnetic fields.”

CERN, for example, created the Axion Search Telescope, a piece of equipment designed to detect a trace of particles produced in the sun’s core. There are tremendous electric fields inside our star that might conceivably interact with axions — assuming they’re there at all.

However, the effort has faced a number of significant obstacles thus far. For one thing, “the particle mass is not theoretically foreseeable,” according to the authors, which means we have no idea what an axion would look like.

Scientists are still looking for them, assuming a wide range of masses in the process. However, recent evidence suggests that the particle’s energy is likely between 40 and 180 micro electron volts. At around 1 billionth the mass of an electron, that’s unimaginably minuscule.

“The axion signal is also likely to be very limited… and exceedingly low due to very weak couplings to Standard Model particles and fields,” the team says. In other words, even though microscopic axions make every effort to alert us to their presence, we may miss them. Their cues could be so subtle that we wouldn’t perceive them.

Despite these obstacles, the axion hunt continues. Most scientists say that they must exist somewhere, but when it comes to adequately describe dark matter, they appear to be too good to be true.

The study authors underline that “most experimental approaches assume that axions compose 100 per cent of the dark matter halo,” suggesting that there may be a possibility to “probe into axion physics without relying on such an assumption.”

What if axions are merely one chapter of dark matter history, despite being the star of the show?

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