Using two of the world's most powerful space telescopes – NASA's Hubble and ESA's Gaia – astronomers have made the most precise measurement to date of the universe's expansion rate. Their figure of 73.5 km (45.6 mi) per second per megaparsec lowers the uncertainty level to just 2.2%. However, these results show a mysterious incompatibility with data from Planck, a third observatory.
By combining observations from both Hubble and the Gaia observatory, astronomers have further refined the value for Hubble's constant, the rate at which the universe is expanding from the Big Bang, 13.8 billion years ago. As the measurements have become more precise, however, the team's determination of this law has become more and more at odds with the measurements from another space observatory – ESA's Planck mission – which is producing a different predicted value for the Hubble constant.
The Planck observatory was launched in 2009 – mapping the early universe as it appeared only 360,000 years after the Big Bang – and its final data release was announced this week by ESA. The entire sky is imprinted with the signature of the Big Bang, encoded in microwaves. Planck measured the sizes of ripples in this Cosmic Microwave Background (CMB) that were produced by slight irregularities in the Big Bang fireball. The fine details of these ripples have encoded how much dark matter and normal matter exists, the trajectory of the universe at that time, and other cosmological parameters.
These measurements, which are still being assessed, allow scientists to predict how the early universe would likely have evolved into the expansion rate we can measure today. However, those predictions do not seem to match the new measurements of our nearby, local universe.
With the addition of this new Gaia and Hubble Space Telescope data, we now have a serious tension with the Cosmic Microwave Background data," said Planck team member George Efstathiou of the Kavli Institute for Cosmology in Cambridge, England, who was not involved with the new work.
"The tension seems to have grown into a full-blown incompatibility between our views of the early and late time universe," said team leader and Nobel Laureate Adam Riess of the Space Telescope Science Institute and the Johns Hopkins University in Baltimore, Maryland. "At this point, clearly it's not simply some gross error in any one measurement. It's as though you predicted how tall a child would become from a growth chart, and then found the adult he or she became greatly exceeded the prediction. We are very perplexed."
In 2005, Professor Riess and his team set out to measure the universe's rate of expansion with extreme accuracy. Over the years, they shaved down the uncertainty to unprecedented levels, refining their techniques each time. Now, using the combined power of Hubble and Gaia, their latest results have reduced that uncertainty to just 2.2%.
"Hubble is really amazing as a general-purpose observatory, but Gaia is the new gold standard for calibrating distance," explains Stefano Casertano from the Space Telescope Science Institute. "It is purpose-built for measuring parallax – this is what it was designed to do."
The Hubble constant is named after Edwin Hubble (1889–1953), who discovered that the universe was uniformly expanding in all directions – a finding that gave birth to modern cosmology. Galaxies appear to recede from Earth proportional to their distances, meaning that the farther away they are, the faster they appear to be moving away. This is a consequence of expanding space, not a value of true space velocity. By measuring the Hubble constant over time, astronomers can construct a picture of our cosmic evolution, infer the make-up of the universe, and uncover clues concerning its ultimate fate.
The two major methods of measuring this rate give incompatible results. One method is direct, building a cosmic "distance ladder" from measurements of stars in our local universe. The other method uses the Cosmic Microwave Background to measure the trajectory of the universe shortly after the Big Bang and then uses physics to describe the universe and extrapolate to the present expansion rate. Together, these should provide an end-to-end test of our basic understanding of the so-called "Standard Model" of the universe. However, the pieces don't fit.
Using Hubble and the newly released data from Gaia, Professor Riess' team measured the present rate of expansion to be 73.5 kilometres (45.6 miles) per second per megaparsec. This means that for every 3.3 million light-years farther away a galaxy is from us, it appears to be moving 73.5 kilometres per second faster. However, the Planck results predict the universe should be expanding today at only 67.0 kilometres (41.6 miles) per second per megaparsec. As the teams' measurements have become more and more precise, the gap between them has continued to widen, and is now four times the size of their combined uncertainty.
This provides more evidence for the existence of a "new physics" underlying the foundations of the universe. Some possibilities include the interaction strength of dark matter, or dark energy being even more exotic than previously thought, or some currently unknown particle exerting an influence.
The goal of Riess' team is to work with Gaia to cross the threshold of refining the Hubble constant to a value of only one percent by the early 2020s. Meanwhile, astrophysicists will likely continue to grapple with revisiting their ideas about the physics of the early universe.
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