Editing Hubble Space Telescope (section)

==== Age and expansion of the universe ====
Among its primary mission targets was to measure distances to [[Cepheid variable]] stars more accurately than ever before, and thus [[Hubble's law#Using Hubble space telescope data|constrain the value]] of the [[Hubble constant]], the measure of the rate at which the universe is expanding, which is also related to its age. Before the launch of HST, estimates of the Hubble constant typically had [[errors and residuals in statistics|errors]] of up to 50%, but Hubble measurements of Cepheid variables in the [[Virgo Cluster]] and other distant galaxy clusters provided a measured value with an accuracy of ±10%, which is consistent with other more accurate measurements made since Hubble's launch using other techniques.<ref>{{cite journal |last1=Freedman |first1=W. L. |last2=Madore |first2=B. F. |last3=Gibson |first3=B.K. |last4=Ferrarese |first4=L. |last5=Kelson |first5=D. D. |last6=Sakai |first6=S. |last7=Mould |first7=J. R. |last8=Kennicutt |first8=R. C. Jr. |display-authors=etal |date=2001 |title=Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant |journal=[[The Astrophysical Journal]] |volume=553 |issue=1 |pages=47–72 |arxiv=astro-ph/0012376 |bibcode=2001ApJ...553...47F |doi=10.1086/320638 |s2cid=119097691}}</ref> The estimated age is now about 13.7&nbsp;billion years, but before the Hubble Telescope, scientists predicted an age ranging from 10 to 20&nbsp;billion years.<ref>{{Cite web |url=http://www.worldsciencefestival.com/2015/04/25-greatest-hubble-telescope-discoveries-past-25-years/ |title=25 of the Greatest Hubble Telescope Discoveries From the Past 25 Years |publisher=World Science Festival |first=Roxanne |last=Palmer |date=April 24, 2015 |access-date=February 23, 2016 |url-status=dead |archive-url=https://web.archive.org/web/20160306180429/http://www.worldsciencefestival.com/2015/04/25-greatest-hubble-telescope-discoveries-past-25-years/ |archive-date=March 6, 2016}}</ref>

While Hubble helped to refine estimates of the age of the universe, it also upended theories about its future. Astronomers from the [[High-z Supernova Search Team]] and the [[Supernova Cosmology Project]] used ground-based telescopes and HST to observe distant [[supernova]]e and uncovered evidence that, far from decelerating under the influence of [[gravity]], the expansion of the universe is instead [[Deceleration parameter|accelerating]]. Three members of these two groups have subsequently been awarded [[Nobel Prize]]s for their discovery.<ref>{{cite book |title=Cosmology |first=Steven |last=Weinberg |publisher=Oxford University Press |date=2008 |isbn=978-0-19-852682-7}}</ref> The cause of this acceleration remains poorly understood;<ref>{{cite journal |last1=Clifton |first1=Timothy |last2=Ferreira |first2=Pedro G. |date=March 23, 2009 |title=Does Dark Energy Really Exist? |url=http://www.scientificamerican.com/article.cfm?id=does-dark-energy-exist |url-status=live |journal=Scientific American |volume=300 |issue=4 |pages=48–55 |bibcode=2009SciAm.300d..48C |doi=10.1038/scientificamerican0409-48 |pmid=19363920 |archive-url=https://web.archive.org/web/20110928054243/http://www.scientificamerican.com/article.cfm?id=does-dark-energy-exist |archive-date=September 28, 2011 |access-date=June 16, 2009}}</ref> the term used for the currently-unknown cause is [[dark energy]], signifying that it is dark (unable to be directly seen and detected) to our current scientific instruments.<ref>{{cite journal |journal=Science |date=June 20, 2003 |volume=300 |pmid=12817137 |issue=5627 |pages=1896–1897 |doi=10.1126/science.300.5627.1896 |title=Dark Energy Tiptoes Toward the Spotlight |first=Charles |last=Seife|s2cid=42463717 }}</ref>