Finding a way to unify conflicting scientific models into a “Theory of Everything” is essentially a Holy Grail of physics. Now a NASA observatory has conducted one of the first experimental tests of these theories, scouring the sky for evidence of a hypothetical particle that could tie the universe together.
The Standard Model of particle physics does a pretty good job of explaining the universe on the nano scale, but when it’s brought up to the macro scale it kind of falls apart. For instance, one of the biggest holes is that it doesn’t cover gravity.
At the other end of the scale, Albert Einstein’s general theory of relativity has proven resilient in describing the physics of massive objects like planets, stars, galaxies, and galaxy clusters. Unfortunately it also begins to break down when faced with the weirdness at the quantum scale.
Bringing these two disparate realms together under one umbrella has eluded science for decades. In fact, the search for a Unified Theory of Everything consumed much of the working lives of some of the top minds of recent times, such as Einstein and Stephen Hawking.
One of the leading contenders is known as “string theory.” In a nutshell, the idea goes that every particle that we may think of as just a “dot” is actually the tip of a one-dimensional string. Properties of the particle, such as its mass and charge, are determined by how that string vibrates.
There are many different versions of string theory, with some of them more likely than others. The problem is, it’s hard to find any supporting evidence, leading many scientists to dismiss the pursuit as pointless. After all, the scientific method doesn’t like ideas that are untestable.
But there may yet be clues out there. String theory predicts the existence of hypothetical particles called axions, which would have very tiny masses and, strangely, may convert into photons when passing through magnetic fields. The reverse may also be true under other circumstances. And these particles may be just the things to blow the case wide open.
An international team has now used NASA’s Chandra X-ray Observatory to search for signs of these axions in the Perseus cluster, some 240 million light-years from Earth. If there are axions there, they should create distortions in the X-ray energy as they convert into photons.
“While it may sound like a long shot to look for tiny particles like axions in gigantic structures like galaxy clusters, they are actually great places to look,” says David Marsh, co-author of the study. “Galaxy clusters contain magnetic fields over giant distances, and they also often contain bright X-ray sources. Together these properties enhance the chances that conversion of axion-like particles would be detectable.”
The team studied more than five days’ worth of data gathered by Chandra on the Perseus cluster. Specifically, the telescope was measuring the strength of X-rays produced at different energies as material falls into the supermassive black hole at the center of the cluster. If axions were there, distorting that light as they convert into photons, Chandra should be able to spot them.
Unfortunately, it didn’t. While this may suggest that axions don’t exist, the team also says that the particles could just have higher masses than they tested for, or perhaps they don’t convert into photons quite as easily as expected.
“Our research doesn’t rule out the existence of these particles, but it definitely doesn’t help their case,” says Helen Russell, co-author of the study. “These constraints dig into the range of properties suggested by string theory, and may help string theorists weed their theories.”
This isn’t the only axion-hunting experiment to return a null result. Others have tried to detect their electromagnetic oscillations in plasma chambers, their effect on the spins of neutrons, or their ability to create magnetic fields in places where there should be none. Together, these findings may help narrow down the search.
If they are ever discovered, axions could solve many different physics problems. Not only would they be evidence of string theory, but they could explain dark matter, and why there’s far more matter than antimatter in the universe.
The new research was published in the Astrophysical Journal.
Source: NASA
