Back in the early years of the universe, right after the Big Bang, everything blasted away from everything else. We can still see the light from that blast, by observing very faraway parts of the universe where light takes billions of years to reach our telescopes. And we can measure how fast things were moving in those faraway spotsBased on that speed, we can calculate how fast the universe should be expanding today.
But when astronomers have tried to directly measure how fast the universe is expanding today — a more difficult task, because everything is farther apart now — things seem to be moving faster than those calculations would predict. And a new paper, based on highly detailed observations taken using the Hubble Space Telescope, appears to confirm that finding: Everything is moving about 9 percent too fast.
And still, nobody knows why.
Earlier observations of that increased speed still had a 1 in 3,000 chance that astronomers were wrong, which is considered pretty high for an astrophysics result. This new paper improves astronomers' confidence, with just a 1 in 100,000 chance of being based on an observational error. It's due for publication in the April 25 issue of The Astrophysical Journal Letters, and is available on the preprint server arXiv.
"This mismatch has been growing and has now reached a point that is really impossible to dismiss as a fluke. This is not what we expected," lead author Adam Riess, a Johns Hopkins University Nobel laureate and astrophysicist, said in a statement.
The researchers relied on the same tool that astronomer Edwin Hubble used to show that the universe was expanding back in 1929: a class of pulsing stars called cepheids.
Cepheids, the astronomer Henrietta S. Leavitt had shown in a 1908 paper in the journal Annals of the Harvard College Observatory, pulse in direct proportion to their brightness. That means that astronomers can figure out exactly how bright a cepheid should be based on how fast it's pulsing. Then, by seeing how dim it looks from Earth, they can tell how much light it's lost along the way, and thus how far away it is.
To measure the rate of the universe's expansion, astronomers check the distance to cepheids in nearby and faraway galaxies. But that's usually a slow task to do precisely, with the Hubble able to precisely measure just one distant cepheid at a time. The researchers developed a method to allow the space telescope to "drift" as it images the stars, imaging more than one at the same time and drastically increasing the precision of their overall distance measurement.
What they found directly contradicts predictions made based on observations from the European Space Agency's Planck satellite, which measured the speed of the universe 380,000 years after the Big Bang.
So what does it mean that the universe is almost certainly moving too fast?
"This is not just two experiments disagreeing," Riess said. "We are measuring something fundamentally different. One is a measurement of how fast the universe is expanding today, as we see it. The other is a prediction based on the physics of the early universe and on measurements of how fast it ought to be expanding. If these values don't agree, there becomes a very strong likelihood that we're missing something in the cosmological model that connects the two eras."
Riess doesn't know what the missing thing is, but for now, he plans to keep refining his measurements.
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