At the beginning of time, all the matter in the universe was compressed into an infinitesimally small point. That tiny speck of everything then exploded and formed the universe.
In some sense, it's still exploding, expanding at an accelerating rate.
In the past, scientists have looked to the radiation left behind from the Big Bang — its smoking gun — to calculate what the rate of the expanding universe ought to be today.
But new evidence, soon to be published in The Astrophysical Journal, suggests these estimates may be wrong, or at least incomplete. New observations from the Hubble Space Telescope have indicated that the universe may be expanding 5 to 9 percent faster than predicted by the Big Bang.
Using the Hubble, scientists from across the US were able to painstakingly measure the distance to stars and supernovae in many galaxies. They then used this data to refine what's known as the "Hubble constant," the rate by which the universe expands, as measured by direct observations.
But when this new "Hubble constant" was compared with the estimates from the Big Bang inferences, the numbers just didn't match.
“You start at two ends, and you expect to meet in the middle if all of your drawings are right and your measurements are right,” Adam Riess, the Nobel laureate at the Space Telescope Science Institute and Johns Hopkins University, who led the project, explained Thursday in a statement. “But now the ends are not quite meeting in the middle and we want to know why.”
Add this to the long list of questions physicists still have about the universe
The prediction based on the Big Bang "should match our measurement," Lucas Macri, a Texas A&M physicist and one of the study's co-authors, tells me. "If they don't … there must be a physical reason why these two things are not agreeing."
So what accounts for the discrepancy?
Either there's something about the Big Bang that previous estimates have not accounted for or there are factors that come into play after the Big Bang that scientist don't yet understand.
Macri highlights four possible explanations.
The first is related to the Big Bang.
1) "We're seeing evidence of a previously unknown subatomic particle that was abundant right after the Big Bang (a.k.a. 'dark radiation')," he says. "If you change the assumptions about what was in the primordial soup, things will have shifted a bit."
The other possibilities are related to "dark energy" and "dark matter," the substances that make up most of the universe yet can't be directly observed.
2) Dark energy — the mysterious force that opposes gravity and is causing the universe to accelerate — "is growing in strength and 'pushing' galaxies apart faster than it did before," he says.
3) Dark matter — matter that we can't see but that is theorized to exist and make up most of the matter in the universe — "is even weirder than we thought."
Or it could not so simply be:
4) "Our theory of gravity is incomplete."
He also mentions that their results aren't set in stone. "There’s one chance in 1,000 that we got this measurement by accident," he says. Physics requires a one in 4 million chance for results to be considered truth. More observations will need to be made.
Macri says he and other researchers will know more soon, especially if they get to use the James Webb Space Telescope, which will replace Hubble in the year 2018. The James Webb will be able to look much deeper into space than Hubble and can refine the Hubble constant estimate further. "A modest amount of time with James Webb will allow us to make a very significant improvement on our measurement," Macri says.
Overall, he says, it's important to know the exact rate of universal expansion because it will yield a more accurate age of the universe. "To get the age of the universe you need to have the Hubble constant," he says. Right now the uncertainty of their estimate is 2.4 percent, which is the best yet. But not good enough.