I won’t fault you for being unfamiliar with dark matter and dark energy. I’m certain most astronomers and astrophysicists would sit along with you. This is because both entities are so elusive that we probably know more about what they’re not compared to what they are.
For some time now, astronomers and astrophysicists have pondered on the anomaly that is the total mass of galaxies. Galaxies are rotating at such high speeds that the mutual gravity that binds the stars inside them should not have been enough to hold a galaxy together; galaxies should have torn themselves apart. Approximating the forces required to keep galaxies together reveals that there appears to be much more mass in a galaxy than it appears to have—that is, the total estimated mass of all matter that we can see doesn’t match up to the value that we expected to come out of the calculations.
It appears that there’s something out there, invisible to the electromagnetic spectrum as we can’t spot it with any of our telescopes and imaging equipment, that accounts for the rest of the “missing mass.” Scientists call this mysterious source of gravity “dark matter.” It’s “dark” in the sense that it doesn’t interact with any part of the electromagnetic spectrum; yet it is definitely there, since the stability of galaxies proves it.
To detect the presence of dark matter, scientists have created detectors such as the XENON1T dark-matter detector, a 1,300-kg (2,800-lb) vessel containing pure liquid xenon (Xe) that’s been buried some 1.5 km (1 mi) below the surface of the Gran Sasso mountains of Italy. This detector was designed to detect even the smallest interactions of any particles that pass through the liquid xenon vat; it was placed underground in a cryostat together with cosmic ray shielding to eliminate as many false positive detections from other free particles as possible.
Last year, however, XENON1T encountered some “unexplained results” in one of its recent runs. It appears that, according to the researchers, the detector instead got a hint of what may be dark energy instead of what it was supposed to detect. Their findings were published in the journal Physical Review D.
Dark energy, in simple terms, is another mysterious cosmic entity that is said to comprise about 68% of the universe. Dark energy is said to exert a repulsive force that is responsible for the accelerating expansion of the universe, as discovered by astronomer Edwin Hubble decades ago.
According to Sunny Vagnozzi, physicist from the Kavli Institute for Cosmology at the University of Cambridge, “large-scale experiments like XENON1T have been designed to directly detect dark matter, […] but dark energy is even more elusive.”
Vagnozzi and team discovered the odd signal as an unexpected excess over the usual background data obtained during one of XENON1T’s runs. These excesses are usually flukes, yet according to Frascati National Laboratories researcher Luca Visinelli, “once in a while, they can also lead to fundamental discoveries.”
Models were conceptualized and designed to try and make sense of what produced the excess in the background; this time around, however, they explored that the excess could be due to dark energy instead of the experiment’s original dark matter target. In order to do so, the scientists looked at gravitational interactions within the structures of the detector, as this tends to be how dark energy interacts with the rest of the universe.
The model that they ended up with used what is known as “chameleon screening;” what came out as a result “shuts down the production of dark energy particles in very dense objects, avoiding the problems faced by solar axions,” according to Vagnozzi. (Solar axions were among the first possible reasons to be proposed as sources of the excess, being hypothetical particles possibly produced in environments like inside our own Sun; however, the amount of axions needed to fulfill the data registered as excess would undermine common understanding of stellar evolution, according to the research team)
“If XENON1T actually saw something, you’d expect to see a similar excess again in future experiments, but this time with a much stronger signal,” according to Visinelli. Vagnozzi aldo added: “It was really surprising that this excess could in principle have been caused by dark energy rather than dark matter. When things click together like that, it’s really special.”
The original excess obtained from measurements still needs to be confirmed with further detections and experiments. As Visinelli put it: “We first need to know that this wasn’t simply a fluke.” Despite this, the calculations they had already obtained using their new model suggest that detection experiments like XENON1T are capable of detecting dark energy.
(To learn more about how dark matter may influence our cosmic neighborhood, check out our piece on how the gravity of a galactic cluster bent incoming light from a supernova.)
References
- Burrage, C., Copeland, E. J., Moss, A., & Stevenson, J. A. (2018). The shape dependence of chameleon screening. Journal of Cosmology and Astroparticle Physics, 2018(01), 056–056. https://doi.org/10.1088/1475-7516/2018/01/056
- Dark matter. (n.d.). CERN. Retrieved October 6, 2021, from https://home.cern/science/physics/dark-matter
- Sci-News. (2021, September 16). XENON1T Experiment May Have Directly Detected Dark Energy. Sci-News. http://www.sci-news.com/astronomy/xenon1t-dark-energy-10074.html
- Vagnozzi, S., Visinelli, L., Brax, P., Davis, A.-C., & Sakstein, J. (2021). Direct detection of dark energy: The XENON1T excess and future prospects. Physical Review D, 104(6), 063023. https://doi.org/10.1103/PhysRevD.104.063023
