A team of U.S. astronomers has used the NASA/ESA Hubble Space Telescope to make a new measurement of the Hubble constant, the rate at which the Universe is expanding. The results, to be published in the Astrophysical Journal, are forcing the scientists to consider that they may be seeing evidence of something unexpected at work in the Universe.
The team — led by Nobel Laureate Adam Riess, a professor of astronomy and physics at the Johns Hopkins University and a senior member of the science staff at the Space Telescope Science Institute — has been successful in refining the Hubble constant value by streamlining and strengthening the construction of the cosmic distance ladder, which astronomers use to measure accurate distances to galaxies near to and far from Earth.
The astronomers have compared those distances with the expansion of space as measured by the stretching of light from receding galaxies. They then have used the apparent outward velocity of galaxies at each distance to calculate the Hubble constant. But the Hubble constant’s value is only as precise as the accuracy of the measurements.
Scientists cannot use a tape measure to gauge the distances between galaxies. Instead, they have selected special classes of stars and supernovae as cosmic yardsticks or milepost markers to precisely measure galactic distances.
Among the most reliable for shorter distances are Cepheid variables, pulsating stars that brighten and dim at rates that correspond to their intrinsic brightness.
Their distances, therefore, can be inferred by comparing their intrinsic brightness with their apparent brightness as seen from Earth.
The new Hubble result is based on measurements of the parallax (apparent shift of an object’s position due to a change in an observer’s point of view) of eight Cepheids in our Milky Way Galaxy.
These stars are about 10 times farther away than any studied previously, residing between 6,000 light-years and 12,000 light-years from Earth, making them more challenging to measure.
They pulsate at longer intervals, just like the Cepheids observed by Hubble in distant galaxies containing another reliable yardstick, exploding stars called Type Ia supernovae. This type of supernova flares with uniform brightness and is brilliant enough to be seen from relatively farther away.
Previous Hubble observations studied 10 faster-blinking Cepheids located 300 light-years to 1,600 light-years from Earth.
To measure parallax with Hubble, Professor Riess and co-authors had to gauge the apparent tiny wobble of the Cepheids due to Earth’s motion around the Sun. These wobbles are the size of just 1/100 of a single pixel on the telescope’s camera, which is roughly the apparent size of a grain of sand seen 100 miles away.
Therefore, to ensure the accuracy of the measurements, they developed a clever method that was not envisioned when Hubble was launched. They invented a scanning technique in which the telescope measured a star’s position a thousand times a minute every six months for four years.
The authors calibrated the true brightness of the eight Cepheids and cross-correlated them with their more distant blinking cousins to tighten the inaccuracies in their distance ladder.
They then compared the brightness of the Cepheids and supernovae in those galaxies with better confidence, so they could more accurately measure the stars’ true brightness, and therefore calculate distances to hundreds of supernovae in far-flung galaxies with more precision.
Another advantage to the study is that the team used the same instrument, Hubble’s Wide Field Camera 3 (WFC3), to calibrate the luminosities of both the nearby Cepheids and those in other galaxies, eliminating the systematic errors that are almost unavoidably introduced by comparing those measurements from different telescopes.
The new value of the Hubble constant reinforces the disparity with the expected value derived from observations of the early Universe’s expansion, 378,000 years after the Big Bang — the violent event that created the Universe roughly 13.8 billion years ago.
Those measurements were made by ESA’s Planck satellite, which maps the cosmic microwave background, a relic of the Big Bang. The difference between the two values is about 9%.
The new Hubble measurements help reduce the chance that the discrepancy in the values is a coincidence to 1 in 5,000.
Planck’s result predicted that the Hubble constant value should now be 67 km per second per megaparsec (3.3 million light-years), and could be no higher than 69 km per second per megaparsec. This means that for every 3.3 million light-years farther away a galaxy is from us, it is moving 67 km per second faster.
But Professor Riess and colleagues measured a value of 73 km per second per megaparsec, indicating galaxies are moving at a faster rate than implied by observations of the early Universe.
The Hubble data are so precise that astronomers cannot dismiss the gap between the two results as errors in any single measurement or method.
“Both results have been tested multiple ways, so barring a series of unrelated mistakes. It is increasingly likely that this is not a bug but a feature of the Universe,” Professor Riess said.
The team proposes a few possible explanations for the mismatch, all related to the 95% of the Universe that is shrouded in darkness.
One possibility is that dark energy, already known to be accelerating the cosmos, may be shoving galaxies away from each other with even greater — or growing — strength.
This means that the acceleration itself might not have a constant value in the Universe but changes over time.
Another idea is that the Universe contains a new subatomic particle that travels close to the speed of light.
Such speedy particles are collectively called ‘dark radiation’ and include previously-known particles like neutrinos, which are created in nuclear reactions and radioactive decays.
Unlike a normal neutrino, which interacts by a subatomic force, this new particle would be affected only by gravity and is dubbed a ‘sterile neutrino.’
Yet another attractive possibility is that dark matter interacts more strongly with normal matter or radiation than previously assumed.
Any of these scenarios would change the contents of the early Universe, leading to inconsistencies in theoretical models. These inconsistencies would result in an incorrect value for the Hubble constant, inferred from observations of the young cosmos. This value would then be at odds with the number derived from the Hubble observations.
Professor Riess and co-authors don’t have any answers yet to this vexing problem, but they will continue to work on fine-tuning the Universe’s expansion rate.
Adam G. Riess et al. 2018. New Parallaxes of Galactic Cepheids from Spatially Scanning the Hubble Space Telescope: Implications for the Hubble Constant. ApJ, in press; arXiv: 1801.01120
This article is based on text provided by the National Aeronautics and Space Administration.