Back in 1998, it was discovered that our universe is not just expanding, but that expansion is accelerating over time. Exactly what is happening is unknown, but astronomers don’t need to understand a thing before they name it. The mysterious thing accelerating the expansion of the universe got called dark energy, and it makes up roughly 70% of the universe’s mass-energy ingredient list. 

Discovering the universe is accelerating apart was weird, but as we’ve worked to understand that expansion, things have gotten somehow weirder. It was discovered that when researchers measure the universe’s expansion parameters using data from the beginning of the universe — using data from the Cosmic Microwave Background — you get one value, but when you measure them from the more modern universe you get a different value. The difference is small enough that everyone blamed observational error for a while, but it’s hit the point where the observers can say with beautiful detail, their results are right, there is a discrepancy, and there is either something wrong with our understanding of the universe, or there is no physics we’re missing.

I’m not sure if a new paper in Physical Review D counts as new physics or a better understanding of old physics, but however you label it, the paper, by Florian Niedermann and Martin Sloth, explains these differences as a result of dark energy undergoing some sort of a phase change as the universe expanded. It’s a quantum process with no direct analogy, but this kind of phase change can maybe be likened to carbon dioxide freezing out to ice or atoms changing their excitation state. This phase change can neatly explain the changes measured. This paper is new, and I can’t tell you if better cosmologists than me will find holes in their theories. It has so far made it past peer review, and this is a start. Maybe, just maybe, a path out of our current confusion has been found.

More Information

University of Southern Denmark press release

“New early dark energy,” Florian Niedermann and Martin S. Sloth, 2021 February 19, Physical Review D