Last year, XENON1T was the most sensitive detector of dark matter in the world. It wasn't dark matter. Maybe neutrinos, maybe solar axions or radioactive pollution in detectors.
A different group of physicists now has a different solution. They believe the signal might not be dark matter but dark energy. This is an important step in the search for the mysterious force.
We are not aware of dark energy. Dark matter is the name that we give to mass that we cannot directly detect. It is inferred because of the extra gravity we cannot account for. About 5 percent of the Universe's normal matter is stars, black holes and planets. Dark matter is around 21 percent.
Dark energy is the remaining 74%. It has not been possible to detect it; instead, we infer that it exists in the Universe's rapid expansion. We call it dark energy. Something is causing the Universe to expand faster than we can explain.
"Despite both components being invisble, we know much more about dark matter, as its existence was suggested as early at the 1920s while dark energy was only discovered in 1998," stated Sunny Vagnozzi, cosmologist at the Kavli Institute of Cosmology at Cambridge University.
"Large-scale experiments such as XENON1T were designed to detect dark matter directly by looking for signs of dark material 'hitting' normal matter. But dark energy is more difficult to find."
The XENON1T tank is filled with 3.2 million tons of ultra-pure xenon liquid and equipped with photomultiplier tubes. Researchers can see the tiny flashes of electroluminescence that occurs when particles interact. This is known as electron recoil.
We know that most of these events are caused by known particle interactions so we can get a good idea of how many should be occurring as background noise. This number is approximately 232 15 per annum. Instead, XENON1T detected 285 incidents between February 2017 and February 2018.
Scientists found that the most plausible explanation was a hypothetical particle called solar-axions. This type of particle was first proposed in the 1970s to answer the question of why strong nuclear forces follow what is called charge-paritysymmetry. However, most models don't believe they do.
However, there is a problem. If the Sun can produce Axions then all stars should be able to do so. The observed heat loss in very warm stars puts strict limits on axion interactions between subatomic particles.
Vagnozzi and his group set out to investigate the possibility of dark energy being responsible for the excess. Although dark energy is still a mystery, most physical models of dark energies result in an unknown fifth force beyond gravity and electromagnetism.
The accelerated expansion of our Universe can only be detected at very large scales. Gravity works on local scales. Any dark matter model that suggests a fifth power would need to explain why it isn't evident in our astronomical area.
Vagnozzi and his colleagues developed a method based on chameleon screening. This avoids the confusion of explaining why we don’t see the fifth power by assuming that it’s too slow-ranged in dense environments such as ours.
Vagnozzi stated that the chameleon screening prevents dark energy particles from being produced in dense objects. This avoids the problems associated with solar axions.
"It also allows you to decouple the events that occur in the local dense Universe from those occurring on larger scales where the density is extremely high."
The results indicated that dark energy particles emanating from a strong magnetic region of Sun, called the tachocline, could have produced the signal seen in the XENON1T data. This explanation is preferred to the background-only explanation with a confidence level of 2.5 sigma.
However, it's not as strong as the solar-axions explanation which had a confidence of 3.5 Sigma; or even neutrinos and radioactive pollution which both had a confidence of 3.2 Sigma.
This alternative solution does not have the same thorny problems as the other. The paper by the researchers suggests that it may be possible for XENON1T to have detected dark energy in a direct way.
This is only if the signal was genuine. It is necessary to detect another signal before we can make that determination. XENON1T is currently being upgraded so we will have to wait.
The paper is a starting point for new ideas, even if the signal doesn’t appear in the next observation run.
Vagnozzi stated that it was surprising that the excess could have been caused by dark matter rather than dark energy. It's truly special when things work together like this.
The research was published in Physical Review D.
