Artist's concept of the three steps to the Hubble constant. See an animation. (Credit: NASA, ESA, A. Field/STScI and A. Riess, STScI/JHU.)


A team of astronomers including four from Texas A&M University has discovered that the universe is expanding 5 to 9 percent faster than expected, based on observations with the Hubble Space Telescope.

The surprising result, to be detailed in an upcoming issue of The Astrophysical Journal, represents the most recent advance in a thus-far 11-year study by the SH0ES (Supernova Ho for the Equation of State) Team, founded in 2005 by Nobel Laureate Adam Riess of the Space Telescope Science Institute and The Johns Hopkins University and Texas A&M's Lucas Macri, an associate professor in the Texas A&M Department of Physics and Astronomy and member of the George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy since 2008.

The project aims to improve the measurement of the current expansion rate of the universe -- known as the Hubble constant or Ho after Edwin Hubble, who first measured the expansion of the universe nearly a century ago -- to a level of accuracy and precision that allows a better understanding of its composition and ultimate fate. In this latest round of results, the team reduced the uncertainty to an unprecedented level of only 2.4 percent by developing innovative techniques that improved the precision of distance measurements to faraway galaxies.

"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," Riess said.

The team looked for galaxies containing both Cepheid stars and white-dwarf supernovae. Cepheid stars pulsate at rates that correspond to their true brightness, which can be compared with their apparent brightness as seen from Earth to accurately determine their distance. White-dwarf supernovae, another commonly used cosmic yardstick, are exploding stars that flare with the same brightness and are brilliant enough to be seen from relatively longer distances. By measuring about 2,400 Cepheid stars in 19 galaxies and comparing the observed brightness of both types of stars, the team accurately measured their true brightness and calculated distances to roughly 300 supernovae in far-flung galaxies.

The team compared those distances with the expansion of space as measured by the stretching of light from receding galaxies to calculate an improved Hubble constant value of 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.

See an animation of the cosmic distance ladder, courtesy of NASA, ESA, A. Field (STScI) and A. Riess (STScI/JHU), and find additional information about how the team uses this to reduce Hubble constant uncertainty.

Three Texas A&M Astronomy group members in addition to Macri played a part in the team's refining progress. Postdoctoral fellow Samantha Hoffmann led the analysis of visible-light observations that resulted in the discovery and homogeneous characterization of the Cepheids. Graduate student Wenlong Yuan contributed to those efforts, studying the images of one of those galaxies and also analyzing ground-based infrared images of Cepheids in the Milky Way. Postdoctoral fellow Peter Brown also provided observations of some of the supernovae from NASA's Swift satellite.

Riess notes that this refined calibration presents a puzzle, given that 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 which are 5 and 9 percent smaller, respectively.

"If we know the initial amounts of stuff in the universe 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," Riess said. "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 Hubble 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 the SH0ES 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."

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 -- the backbone of the universe upon which galaxies built themselves up into the large-scale structures seen today -- possesses some weird, unexpected characteristics.

And finally, the speedier universe may be telling astronomers that Einstein's theory of gravity is incomplete.

"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," Macri said.

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 (of which Macri was a junior member) 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, when Macri was a Hubble Postdoctoral Fellow at the National Optical Astronomy Observatory in Tucson.

For additional images with complete captions and more information about the Hubble constant finding as well as Hubble, please see the official press release.

To learn more about Macri's research or Texas A&M astronomy, go to http://astronomy.tamu.edu.

Read related coverage in the Bryan-College Station Eagle.

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About Hubble: The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C. For more information, visit http://www.nasa.gov/hubble.

About Research at Texas A&M University: As one of the world's leading research institutions, Texas A&M is at the forefront in making significant contributions to scholarship and discovery, including that of science and technology. Research conducted at Texas A&M represented annual expenditures of more than $866.6 million in fiscal year 2015. Texas A&M ranked in the top 20 of the National Science Foundation's Higher Education Research and Development survey (2014), based on expenditures of more than $854 million in fiscal year 2014. Texas A&M's research creates new knowledge that provides basic, fundamental and applied contributions resulting in many cases in economic benefits to the state, nation and world. To learn more, visit http://research.tamu.edu.


Contact: Shana K. Hutchins, (979) 862-1237 or shutchins@science.tamu.edu or Dr. Lucas Macri, (979) 314-1592 or lmacri@tamu.edu

Ray Villard

  • This Hubble Space Telescope image shows one of the galaxies in the survey to refine the measurement for how fast the universe expands with time, called the Hubble constant. The galaxy, UGC 9391, contains two types of stars astronomers use to calculate accurate distances to galaxies, a key measurement in determining the Hubble constant. The red circles mark the locations of Cepheid variable stars, while the blue "X" at bottom right denotes the location of supernova 2003du, a special class of exploding star called a Type Ia supernova.. (Credit: NASA, ESA and A. Riess, STScI / JHU.)

  • Dr. Lucas Macri

  • Dr. Samantha Hoffmann

  • Wenlong Yuan

  • Dr. Peter Brown

© Texas A&M University. To request use of any of our photographs for educational use or to view additional options from our archive, please contact the College of Science Communications Office.

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