The discrepancy between how fast the universe appears to be increasing and the rate at which we expect it to expand has been a persistent anomaly for scientists.
Cosmologists base their expectation of the expansion rate--called the Hubble constant--on measurements of radiation emitted shortly after the Big Bang. This radiation reveals the elements of the early universe.
Cosmologists then plug the details into their version of cosmic evolution and run the version forward to look at how fast space is increasing today.
However, the prediction has fallen short. When cosmologists look at astronomical items along with pulsating stars and exploding supernovas, they see a universe that's increasing faster, with a larger Hubble constant.
Recently, theorists have been busy imagining new cosmic components that, when delivered to the usual version, might rev up the universe's expected growth rate, making it suit observations.
"Discovering anomalies is the fundamental way that science makes progress," said Avi Loeb, a cosmologist at Harvard University, in Quanta Magazine.
The following are the factors that could be increasing cosmic expansion:
ALSO READ: The Universe Is Expanding Faster Than Expected: Here's New Proof
Decaying Dark Matter
The standard model of cosmology incorporates all of the familiar varieties of matter and radiation and their interactions. Invisible substances known as dark energy and dark matter, which collectively make up some 96% of the cosmos, are included.
Because so little is known about those elements, they are perhaps the place to begin tampering with the standard model. "That's what you have at your disposal to change the expansion rate of the universe," Loeb said.
In a paper published in Physical Review D, Loeb and other researchers considered a dark matter that decays directly into a lighter particle and a massless particle called a dark photon. As increasingly dark matter deteriorated over time, they reasoned, the gravitational pull could have lessened. As a result, the growth of the universe might have sped up, relieving the Hubble tension.
But making small modifications like this to the same old cosmological model can have undesirable knock-on effects. "It's [effortless] to come up with all kinds of slight modifications," said Marc Kamionkowski, a theoretical physicist at Johns Hopkins University. However, he said it's hard to do so without ruining the model's perfect fit with a wealth of other astronomical observations.
ALSO READ: Universe Might Not Be Expanding the Same Way in All Directions, New Study Suggests
Inconstant Dark Energy
Ever since the discovery in 1998 that the expansion of the universe is accelerating, cosmologists have covered repulsive dark energy in their version of cosmic evolution. But its nature remains a mystery. The simplest possibility is that dark power is the "cosmological steady"-- the strength of space itself, with a regular density everywhere. But what if the amount of darkish electricity inside the universe isn't steady?
An extra dose of dark power in the early universe, called early dark energy, should reconcile the conflicting values of the Hubble constant. The outward pressure of this first dark energy would have sped up the universe's enlargement.
"The tricky part is that [early dark energy] can't really stick around; it has to go away quickly," said Lisa Randall, a particle physicist and cosmologist at Harvard University.
Theorists have recommended different exotic forms of dark energy that also exchange as the universe ages. Such dark energy-dominated periods are thought to "occur occasionally throughout the history of the universe," Kamionkowski said.
Modified Gravity
Albert Einstein's concept of gravity implies how space expands as a result. This means that apart from changing or adding cosmic substances to the model, there's another way physicists can reconcile it with the discovered the expansion. "You can imagine that Einstein's equations are not correct," Loeb said.
William Barker, a doctoral student at the University of Cambridge, attempted to find a concept of "modified gravity" as he stumbled across a way to solve the Hubble tension. Barker saw a modified-gravity version that was "capable of behaving as if there were extra radiation in the early universe." He added that the radiation strain could have elevated the cosmic expansion rate.
In a preprint submitted to Physical Review D in March, Barker and co-authors said more analysis is needed to describe how systems like galaxies and clusters evolved.
Wait and See
Even with extra freedom, most of the nonstandard models best reduce the Hubble tension rather than getting rid of it. Scientists are expecting a faster cosmic growth price than the standard model. Still, it's nonetheless not quite fast enough to match observations of supernovas and other astronomical objects.
In the coming years, the Euclid telescope and others will meticulously map how gravity and dark energy have created cosmic evolution. Meanwhile, gravitational waves emitted from colliding neutron stars provide a new way to measure the Hubble constant.
The new statistics will rule out a number of those novel solutions to the Hubble tension. However, new cracks within the model can also appear. "There is a little bit of a sense of wait and see unless someone has [a perfect idea'," said Randall.
