Having a constant named after you ensures a hallowed place in astronomical history, and we can assume that Edwin Hubble would have been delighted with our continuing studies of the constant that bears his name. It was Hubble who showed that the velocity of distant galaxies as measured by their Doppler shift is proportional to their distance from the Earth. But what would the man behind the Hubble Constant have made of the ‘Hubble Bubble’? It’s based on the idea that our region of the cosmos is surrounded by a bubble of relatively empty space, a bubble some eight-billion light years across that helps account for our observations of the universe’s expansion.
The theory goes something like this: We assume that the Hubble Constant should be the same no matter where it is measured, because we make the larger assumption that our planet does not occupy a special position in the universe. But suppose that’s wrong, and that the Earth is near the center of a region of extremely low density. If that’s the case, then denser material outside that void would attract material away from the center. What we would see would be stars accelerating away from us at a rate faster than the more general expansion of the universe.
The Hubble Bubble is an ingenious notion, one of the ideas advanced as an alternative to dark energy to explain why the expansion of the universe seems to be accelerating. If you had to choose between a Hubble Bubble and a mysterious dark energy that worked counter to gravity, the Bubble would seem a safer choice, given that it doesn’t conjure up a new form of energy, but the observational evidence for the Bubble is lacking, and now the idea has been hobbled by new work by Adam Riess (Space Telescope Science Institute) and colleagues.
Deflating the Bubble
Riess used data from the new Wide Field Camera 3 (WFC3) aboard the Hubble Space Telescope to measure the Hubble Constant to a greater precision than ever before. The value the team arrived at — 73.8 kilometers per second per megaparsec — means that for every additional megaparsec (3.26 million light years) a galaxy is from Earth, it appears to be moving 73.8 kilometers per second faster away from us. The uncertainty over the figure for the universe’s expansion rate in the new observations has now been reduced to just 3.3 percent, reducing the error margin by a full 30 percent over the previous Hubble measurement in 2009.
The new precision is thanks to Wide Field Camera 3, which helps the scientists study a wider range of stars to eliminate systematic errors introduced by comparing the measurements of different telescopes. The team compared the apparent brightness of Type Ia supernovae and Cepheid variable stars to measure their intrinsic brightness and calculate the distances to Type Ia supernovae in distant galaxies. Riess calls WFC3 the best ever flown on Hubble for such measurements, adding that it improved “…the precision of prior measurements in a small fraction of the time it previously took.” Further WFC3 work should tighten the Constant even more, and even better numbers should be within range of the James Webb Space Telescope.
This work is significant because as we tighten our knowledge of the universe’s expansion rate, we restrict the range of dark energy’s strength. The Bubble theory arose because scientists were looking for ways around a dark energy that opposed gravity. But the consequences of a Hubble Bubble are clear — if it’s there, the universe’s expansion rate must be slower than astronomers have calculated, with the lower-density bubble expanding faster than the more massive universe that surrounds it. Riess and team have tightened the Hubble Constant to the point where this lower value — 60 to 65 kilometers per second per megaparsec — is no longer tenable.
Lucas Macri (Texas A&M University), who collaborated with Riess, notes the importance of the study:
“The hardest part of the bubble theory to accept was that it required us to live very near the center of such an empty region of space. This has about a one in a million chance of occurring. But since we know that something weird is making the universe accelerate, it’s better to let the data be our guide.”
Riess, you may recall, is one of the co-discoverers of the universe’s accelerating expansion, having demonstrated that distant Type Ia supernovae were dimmer than they ought to be, an indication of additional distance that had to be the result of faster than expected expansion. Meanwhile, it’s extraordinary to realize how much the Hubble instrument has helped us pin down the value of the Hubble Constant, which had seen estimates varying by a factor of two before the telescope’s 1990 launch. This NASA news release points out that by 1999, the Hubble telescope had refined the value of the Hubble Constant to an error of about ten percent. Riess’ new work continues the 80-year measurement of this critical value and promises more to come.
The paper is Riess et al., “A 3% Solution: Determination of the Hubble Constant with the Hubble Space Telescope and Wide Field Camera 3,” Astrophysical Journal Vol. 730, Number 2 (April 1, 2011). Abstract available.