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Image credits: ESA/Hubble & NASA
An international team of scientists has measured a new, higher value for the speed at which the universe expands. They used two massive galaxies as "gravitational lenses" to investigate the light from remote objects. The study was led by the Max Planck Institute for Astrophysics in Garching in Germany.
Because of the expansion of space, the distance between galaxies has been increasing since the big bang (13.8 billion years ago). The farther away a galaxy is, the greater the speed at which it moves away from us. The Hubble constant is the ratio between speed and distance. For astronomers, that Hubble constant is an essential unit to determine distances in the universe accurately. And those distances are crucial for estimating how bright or large objects are, which helps astronomers understand how the universe has evolved.
But there is a problem: various ways to measure the value of the Hubble constant do not give precisely the same result. The Hubble constant is shown in kilometers per second per megaparsec (3.3 million light-years). The most accurate determination has been made by analyzing the cosmic background radiation by the WMAP mission, which gave a value of 71 km / s per megaparsec, and by the Planck mission, which came to around 67. The difference is minute but investigating details of the origin and development of the universe is complicated. Other techniques, based on measurements on supernovas, generally yield slightly higher values.
The new study presents a new method to measure the Hubble constant. Researchers looked at how the gravity of two different galaxies diverts the light from objects behind it. The systems work like a gravitational lens. Using smart calculation methods, scientists were able to determine the diameter of these "lenses". With this diameter, they could accurately calculate the distance to the two lenses. Ultimately, this yielded a new, higher value for the Hubble constant.
The astronomers found a value of 82 +/- 8 km / s per megaparsec, indicating that the universe is expanding faster than previously assumed. The uncertainty in the estimate is due to statistical uncertainty as only two gravity lenses were used. Researchers are already working on unleashing their technique on more gravitational lenses, which will reduce uncertainty.
The results seem to confirm that there is a systematic difference between the value of the Hubble constant derived from indirect sources, such as the cosmic background radiation emitted shortly after the Big Bang, and methods based on measurements on objects such as supernovas (or in this case a gravitational lens) which are much younger. According to Léon Koopmans, co-author of the study, this indicates that we do not yet fully understand the physics of the early universe and that the standard model of cosmology may need to be revised.
Sources and further reading: NOVA press release / A measurement of the Hubble constant from angular diameter distances to two gravitational lenses
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