Three Decades of Space Telescope Observations Converge on a Precise Value for the Hubble Constant
Science history will record that the search for the expansion rate of the universe was the great Holy Grail of 20th century cosmology. Without any observational evidence for space expanding, contracting, or standing still, we wouldn't have a clue to whether the universe was coming or going. What's more, we wouldn't have a clue about its age either – or in fact if the universe was eternal.
The first act of this revelation came when, a century ago, American astronomer Edwin Hubble discovered myriad galaxies outside of our home galaxy, the Milky Way. And, the galaxies weren't standing still. Hubble found that the farther a galaxy is, the faster it appears to be moving away from us. This could be interpreted as the uniform expansion of space. Hubble even said that he studied the galaxies simply as "markers of space." However he was never fully convinced of the idea of a uniformly expanding universe. He suspected his measurements might be evidence of something else more oddball going on in the universe.
For decades after Hubble, astronomers have toiled to nail down the expansion rate that would yield a true age for the universe. This required building a string of cosmic distance ladders assembled from sources that astronomers have a reasonable confidence in their intrinsic brightness. The brightest, and therefore farthest detectable milepost markers are Type Ia supernovae.
When the Hubble Space Telescope was launched in 1990 the universe's expansion rate was so uncertain that its age might only be 8 billion years or as great as 20 billion years.
After 30 years of meticulous work using the Hubble telescope's extraordinary observing power, numerous teams of astronomers have narrowed the expansion rate to a precision of just over 1%. This can be used to predict that the universe will double in size in 10 billion years.
The measurement is about eight times more precise than Hubble's expected capability. But it's become more than just refining a number to cosmologists. In the interim the mystery of dark energy pushing the universe apart was discovered. To compound things even further, the present expansion rate is different than it is expected to be as the universe appeared shortly after the big bang.
You think this would frustrate astronomers, but instead it opens the door to discovering new physics, and confronting unanticipated questions about the underlying workings of the universe. And, finally, reminding us that we have a lot more to learn among the stars.
Find additional articles, images, and videos at HubbleSite.org
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