Recent Hubble Space Telescope results have deepened one of the greatest mysteries of astronomy.
Astronomers know that the universe is expanding and the expansion is accelerating. Sometimes you hear news reports claiming that the universe is "growing faster than we thought". But that is not quite how it is. The rate of expansion, referred to as the Hubble constant, is subject to an important discrepancy: its value changes due to the way scientists measure it. New results of the Hubble Space Telescope have now "raised the discrepancy beyond a plausible level of coincidence," according to a paper to be published in the Astrophysical Journal.
As the space between the stars and galaxies grows, scientists have thought of several ways to measure the rate of expansion. One method calculates the extent based on the farthest radiation that our experiments can see as a cosmic microwave background. Others have used information from supernovae to calculate the rate. Both methods have measured an expansion rate of about 67.7 kilometers per second per megaparsek – that is, the universe grows 3.67 million light years faster by 67.7 kilometers per second.
Other measurements do not match. Scientists using the Hubble Space Telescope calculated the Hubble constant using a recent high-accuracy measurement of the distance to a nearby satellite galaxy called the Large Magellanic Cloud, as well as new observations of 70 cepheid variables, a type of pulsating star. The pulsation rate and brightness of the Cepheids are close enough together to calculate their distance. Along with other improvements, they calculated the expansion of the universe at 74 kilometers per second per megaparsek.
Basically – as scientists continue to look away, the universe seems to expand more slowly than the local universe.
The new measurement makes the discrepancy officially so big that it is not plausible that statistical data fluctuations fluctuate statistically. In addition, other tests seem to show that the deviation is not caused by errors in the measurements. This means that the experiments may measure a feature of the universe that is not explained by the most widely accepted theory of cosmology.
It's hard to say what's really going on, but the next step is clear. "A continuous pursuit of precision in the determination of [the Hubble constant] … is required to move from the discovery of a difference to a diagnosis of the source," write the authors of the study.
Scientists are already pursuing new ways to measure the value of Hubble's constant, especially using colliding neutron stars and the gravitational waves they create in space themselves. By calculating the distance to collide with the gravitational waves and the speed at which the stars go back to the collision light, physicists have another way to calculate the value of the Hubble constant. So it's not the Universe expanding faster or slower than we thought before.
Instead, the discrepancy in these measurements may ultimately reveal a whole new aspect of the universe that scientists are currently in the dark about.