Editing Hubble's law (section)

=== Earlier measurements ===
For the original 1929 estimate of the constant now bearing his name, Hubble used observations of [[Cepheid variable]] stars as "[[Cosmic distance ladder#Standard_candles|standard candles]]" to measure distance.<ref name=Allen/> The result he obtained was {{val|500|u=km/s|up=Mpc}}, much larger than the value astronomers currently calculate. Later observations by astronomer [[Walter Baade]] led him to realize that there were distinct "[[stellar population|populations]]" for stars (Population I and Population II) in a galaxy. The same observations led him to discover that there are two types of Cepheid variable stars with different luminosities. Using this discovery, he recalculated Hubble constant and the size of the known universe, doubling the previous calculation made by Hubble in 1929.<ref>Baade, W. (1944) The resolution of Messier 32, NGC 205, and the central region of the Andromeda nebula. ApJ 100 137–146</ref><ref>Baade, W. (1956) The period-luminosity relation of the Cepheids. PASP 68 5–16</ref><ref name=Allen>{{cite web|last=Allen|first=Nick|title=Section 2: The Great Debate and the Great Mistake: Shapley, Hubble, Baade|url=http://www.institute-of-brilliant-failures.com/section2.htm|website=The Cepheid Distance Scale: A History|access-date=19 November 2011|archive-url=https://web.archive.org/web/20071210105344/http://www.institute-of-brilliant-failures.com/section2.htm|archive-date=10 December 2007|url-status=dead}}</ref> He announced this finding to considerable astonishment at the 1952 meeting of the [[International Astronomical Union]] in Rome.

For most of the second half of the 20th century, the value of {{math|''H''{{sub|0}}}} was estimated to be between {{val|50|and|90|u=km/s|up=Mpc}}.

The value of the Hubble constant was the topic of a long and rather bitter controversy between [[Gérard de Vaucouleurs]], who claimed the value was around 100, and [[Allan Sandage]], who claimed the value was near 50.<ref name="Overbye"/> In one demonstration of vitriol shared between the parties, when Sandage and [[Gustav Andreas Tammann]] (Sandage's research colleague) formally acknowledged the shortcomings of confirming the systematic error of their method in 1975, Vaucouleurs responded "It is unfortunate that this sober warning was so soon forgotten and ignored by most astronomers and textbook writers".<ref name=":0">{{Cite book |last=de Vaucouleurs |first=G. |title=The cosmic distance scale and the Hubble constant |publisher=Mount Stromlo and Siding Spring Observatories, Australian National University |year=1982}}</ref> In 1996, a debate moderated by [[John N. Bahcall|John Bahcall]] between Sidney van den Bergh and Gustav Tammann was held in similar fashion to the earlier Shapley–Curtis debate over these two competing values.

This previously wide variance in estimates was partially resolved with the introduction of the [[Lambda-CDM model|ΛCDM]] model of the universe in the late 1990s. Incorporating the ΛCDM model, observations of high-redshift clusters at X-ray and microwave wavelengths using the [[Sunyaev–Zel'dovich effect]], measurements of anisotropies in the [[cosmic microwave background]] radiation, and optical surveys all gave a value of around 50–70&nbsp;km/s/Mpc for the constant.<ref name=Myers1999>{{cite journal |title=Scaling the universe: Gravitational lenses and the Hubble constant |last=Myers |first=S. T. |date=1999 |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=96 |issue=8 |pages=4236–4239 |doi=10.1073/pnas.96.8.4236 |doi-access=free |pmid=10200245 |pmc=33560|bibcode=1999PNAS...96.4236M }}</ref>