A team of astronomers at the University of Arizona has found that some types of supernovae are much more diverse than thought, suggesting that the universe is not expanding at nearly the rate we thought.

The team found that type Ia supernovae — which are considered very uniform and predictable to the point that cosmologists use them as beacons to explore the universe — can be categorized into different populations, according to a Business Standard report.

Rather than having random differences, Ia supernovae can actually be separated into two groups, and the group that is in the minority near us is actually the majority far away when the universe was much younger. The assumption previously was that if you keep going further out, type Ia supernovae are all the same, but the study has found that that’s not the case, said Peter Milne, an astronomer at the University of Arizona.

This throws into question just how fast the universe is expanding, and whether dark energy is really pulling apart the universe at a faster and faster rate.

This theory stems from previous observations that resulted in the 2011 Nobel Price for Physics when University of Arizona scientists discovered that many supernovae were somewhat fainter than expected, suggesting they have moved farther from Earth than they would if the universe was expanding at the same rate all the time.

Milne and his team observed a large sample of type Ia supernovae in ultraviolet and visible light for their study, and then they combined those observations with those of the Hubble Space Telescope and NASA’s Swift satellite.

They found that color differences between the two groups of supernovae can explain some of the perceived acceleration, meaning that there isn’t as much acceleration as previously believed.