When astronomer Edwin Hubble discovered nearly 100 years ago that the universe was uniformly expanding in all directions, the finding was a big surprise. Then, in the mid-1990s, another shocker occurred: astronomers found that the expansion rate was accelerating perhaps due to a repulsive property called "dark energy." Now, the latest measurements of our runaway universe suggest that it is expanding faster than astronomers thought. The consequences could be very significant for our understanding of the shadowy contents of our unruly universe. It may mean that dark energy is shoving galaxies away from each other with even greater – or growing – strength. Or, the early cosmos may contain a new type of subatomic particle referred to as "dark radiation." A third possibility is that "dark matter," an invisible form of matter that makes up the bulk of our universe, possesses some weird, unexpected characteristics. Finally, Einstein's theory of gravity may be incomplete.
These unnerving scenarios are based on the research of a team led by Nobel Laureate Adam Riess, who began a quest in 2005 to measure the universe's expansion rate to unprecedented accuracy with new, innovative observing techniques. The new measurement reduces the rate of expansion to an uncertainty of only 2.4 percent. That's the good news. The bad news is that it does not agree with expansion measurements derived from probing the fireball relic radiation from the big bang. So it seems like something's amiss – possibly sending cosmologists back to the drawing board.
Astronomers using NASA's Hubble Space Telescope have discovered that the universe is expanding 5 percent to 9 percent faster than expected.
"This surprising finding may be an important clue to understanding those mysterious parts of the universe that make up 95 percent of everything and don't emit light, such as dark energy, dark matter, and dark radiation," said study leader and Nobel Laureate Adam Riess of the Space Telescope Science Institute and The Johns Hopkins University, both in Baltimore, Maryland.
The results will appear in an upcoming issue of The Astrophysical Journal.
There are a few possible explanations for the universe's excessive speed. One possibility is that dark energy, already known to be accelerating the universe, may be shoving galaxies away from each other with even greater – or growing – strength.
Another idea is that the cosmos contained a new subatomic particle in its early history that traveled close to the speed of light. Such speedy particles are collectively referred to as "dark radiation" and include previously known particles like neutrinos. More energy from additional dark radiation could be throwing off the best efforts to predict today's expansion rate from its post-big bang trajectory.
The boost in acceleration could also mean that dark matter possesses some weird, unknown characteristics. Dark matter is the backbone of the universe upon which galaxies built themselves up into the large-scale structures seen today.
And finally, the speedier universe may be telling astronomers that Einstein's theory of gravity is incomplete.
Riess' team made the discovery by refining the universe's current expansion rate to unprecedented accuracy, reducing the uncertainty to only 2.4 percent. The team made the refinements by developing innovative techniques that improved the precision of distance measurements to faraway galaxies.
These measurements are fundamental to making more precise calculations of how fast the universe expands with time, a value called the Hubble constant. The improved Hubble constant value is 73.2 kilometers per second per megaparsec. (A megaparsec equals 3.26 million light-years.) The new value means the distance between cosmic objects will double in another 9.8 billion years.
This refined calibration presents a puzzle, however, because it does not quite match the expansion rate predicted for the universe from its trajectory seen shortly after the big bang. Measurements of the afterglow from the big bang by NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency's Planck satellite mission yield predictions for the Hubble constant that are 5 percent and 9 percent smaller, respectively.
"We know so little about the dark parts of the universe, it's important to measure how they push and pull on space over cosmic history," said Lucas Macri of Texas A&M University in College Station, a key collaborator on the study.
Added Riess: "If we know the initial amounts of stuff in the universe, such as dark energy and dark matter, and we have the physics correct, then you can go from a measurement at the time shortly after the big bang and use that understanding to predict how fast the universe should be expanding today. However, if this discrepancy holds up, it appears we may not have the right understanding, and it changes how big the Hubble constant should be today."
Comparing the universe's expansion rate with WMAP, Planck, and the Hubble Space Telescope is like building a bridge, Riess explained. On the distant shore are the cosmic microwave background observations of the early universe. On the nearby shore are the measurements made by Riess' team using Hubble.
"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," Riess said. "But now the ends are not quite meeting in the middle and we want to know why."
The Hubble observations were conducted by the Supernova H0 for the Equation of State (SH0ES) team, which works to refine the accuracy of the Hubble constant to a precision that allows for a better understanding of the universe's behavior.
Riess' team made the improvements by streamlining and strengthening the construction of the cosmic distance ladder, which astronomers use to measure accurate distances to galaxies near and far from Earth. The team compared those distances with the expansion of space as measured by the stretching of light from receding galaxies. They used these two values to calculate the Hubble constant.
Among the most reliable of these cosmic yardsticks for relatively shorter distances are Cepheid variables, pulsating stars that dim and fade at rates that correspond to their true brightness. Their distances, therefore, can be inferred by comparing their true brightness with their apparent brightness as seen from Earth.
The researchers calibrated this stellar yardstick using a basic tool of geometry called parallax, the same technique that surveyors use to measure distances on Earth. With Hubble's sharp-eyed Wide Field Camera 3 (WFC3), they extended the parallax measurements farther than previously possible, across the Milky Way galaxy, to reach distant Cepheids.
To calculate accurate distances to nearby galaxies, the team looked for galaxies containing both Cepheid stars and another reliable yardstick, Type Ia supernovae, exploding stars that flare with the same brightness and are brilliant enough to be seen from relatively longer distances. So far, Riess' team has measured about 2,400 Cepheid stars in 19 of these galaxies, representing the largest sample of such measurements outside the Milky Way. By comparing the observed brightness of both types of stars in those nearby galaxies, the astronomers could then accurately measure their true brightness and therefore calculate distances to roughly 300 Type Ia supernovae in far-flung galaxies.
Using one instrument, WFC3, to bridge the Cepheid rungs in the distance ladder, the researchers eliminated the systematic errors that are almost unavoidably introduced by comparing measurements from different telescopes. Measuring the Hubble constant with a single instrument is like measuring a hallway with a long tape measure instead of a single 12-inch ruler. By avoiding the need to pick up the ruler and lay it back down over and over again, you can prevent cumulative errors.
The SH0ES Team is still using Hubble to reduce the uncertainty in the Hubble constant even more, with a goal to reach an accuracy of 1 percent. Current telescopes such as the European Space Agency's Gaia satellite, and future telescopes such as the James Webb Space Telescope (JWST), an infrared observatory, and the Wide Field Infrared Space Telescope (WFIRST), also could help astronomers make better measurements of the expansion rate.
Before Hubble was launched in 1990, the estimates of the Hubble constant varied by a factor of two. In the late 1990s the Hubble Space Telescope Key Project on the Extragalactic Distance Scale refined the value of the Hubble constant to within an error of only 10 percent, accomplishing one of the telescope's key goals. The SH0ES team has reduced the uncertainty in the Hubble constant value by 76 percent since beginning its quest in 2005.