In response to the usual mannequin of cosmology, that growth fee ought to be the identical irrespective of the way you measure it. However lately, scientists have discovered that the growth fee of the early universe and that of the more moderen universe range by roughly 9%.
That discrepancy, or rigidity, “has provoked heated debates within the cosmology group about whether or not this distinction could possibly be attributable to systematic errors or whether or not it’s a sign of recent physics,” Teixeira and her co-authors wrote. Their mannequin posits that in a world the place darkish vitality and darkish matter work together, what gave the impression to be a disaster attributable to disparate growth charges as an alternative turns into one thing to be anticipated.
A Darkish Dimension
If darkish vitality and darkish matter do work together, it may imply that they’ve a standard origin, Khoury mentioned.
Ongoing work in string concept — the concept our universe, at its smallest, most elementary degree, is made up of vibrating strings — suggests a means these ideas may be related. In 2019, constructing on the thought in string concept that darkish vitality is of course various, Vafa and two collaborators concluded that the mass of darkish matter particles may range over time. This led them to suggest in 2022 that darkish matter and darkish vitality may share a hyperlink with a so-called darkish dimension.
Cumrun Vafa, a physicist at Harvard College, thinks the mass of darkish matter might range over time.
String concept posits the existence of six or seven further dimensions along with our acquainted three dimensions of house and one in every of time. All of those dimensions are considered as small as bodily attainable — near the Planck scale (10-35 meters) — however the researchers proposed that the darkish dimension could possibly be considerably larger than the others — on the order of a micron (10-6 meters).
Gravitons, theoretical particles that impart the pressure of gravity, may leak into this enlarged darkish dimension. In the event that they did, they’d choose up mass and turn into what are referred to as darkish gravitons. These huge gravitons would reside at midnight dimension, however their gravitational results could possibly be felt in different dimensions, permitting them to meet the function that’s usually ascribed to darkish matter.
On this situation, “there’s a very pure coupling between darkish vitality and darkish matter,” mentioned Georges Obied, a physicist on the College of Chicago. Adjustments within the measurement of the darkish dimension would have an effect on each darkish vitality and darkish matter.
In a July 2025 paper, Obied and Vafa, together with Alek Bedroya of Princeton and David Wu of Harvard, discovered that the situation proposed in 2019 was in line with the DESI information. Their mannequin predicts that the power of darkish vitality and the mass of darkish matter will lower over time and that the speed at which darkish vitality adjustments will probably be proportional to its vitality density. As a result of astrophysical measurements inform us that the vitality density of darkish vitality is awfully small, “it gained’t change quick,” Vafa mentioned.
“It’s not shocking that we didn’t see it till now,” he mentioned, as a result of the speed of change is so tiny. “We needed to wait your complete age of the universe to detect one thing that small.”
If darkish matter is coupled to darkish vitality, darkish matter particles may work together with each other via a brand new, long-range pressure that’s separate from gravity. The excellent news, Obied mentioned, is that “there could possibly be astrophysical methods of testing this.”
Coincidentally, two physicists — Marc Kamionkowski, now at Johns Hopkins College, and Michael Kesden, now on the College of Texas at Dallas — have already regarded into one such check. In a paper revealed in 2006, they imagined a sequence of occasions wherein two galaxies come shut to one another and the gravity of 1 galaxy tugs on the different. If darkish matter has a stronger gravitational attraction to different darkish matter than it does to atypical matter, a particular form of “tidal tail” — an prolonged stream of stars, fuel, and dirt — would type behind one of many galaxies.
Kesden and Kamionkowski regarded for this impact and didn’t discover it, which enabled them to set an higher sure on the attainable power of the additional enticing pressure. The higher sure was about 20 instances larger than the quantity Vafa’s staff proposed, so the expected worth fell properly throughout the observational sure. “It’s fascinating that we at the moment are discovering connections between that pretty summary work and observational and experimental work,” Kamionkowski mentioned.
The truth that a prediction based mostly on calculations from string concept roughly agreed with the astrophysical proof doesn’t affirm the validity of those “stringy” fashions. However any correspondence between string concept and experiment is gratifying to Vafa, who has spent the previous 4 many years attempting to wrest the speculation from the purely conceptual realm to the purpose of producing testable predictions.
Obied, one in every of Vafa’s former Ph.D. college students, is pushed by the identical aim. Understanding darkish vitality and darkish matter and the connection they might share is a frightening drawback, he mentioned. “And I believe it’s very useful to come back at this in numerous methods” — be it from observational cosmology, from particle physics, or from string concept.
“That is how folks ought to do science,” Obied mentioned. “I imply, it’s the job of theoretical physicists to discover every little thing that’s attainable, to get all the probabilities on the desk. And ultimately, the information will assist us determine.”
