Editing Observational cosmology (section)

===Hubble's law and the cosmic distance ladder===
{{main|Hubble's law|cosmic distance ladder}}

Distance measurements in astronomy have historically been and continue to be confounded by considerable measurement uncertainty. In particular, while [[stellar parallax]] can be used to measure the distance to nearby stars, the observational limits imposed by the difficulty in measuring the minuscule parallaxes associated with objects beyond our galaxy meant that astronomers had to look for alternative ways to measure cosmic distances. To this end, a [[standard candle]] measurement for [[Cepheid variable]]s was discovered by [[Henrietta Swan Leavitt]] in 1908 which would provide [[Edwin Hubble]] with the rung on the [[cosmic distance ladder]] he would need to determine the distance to [[spiral nebula]]. Hubble used the 100-inch [[Hooker Telescope]] at [[Mount Wilson Observatory]] to identify individual [[star]]s in those [[galaxy|galaxies]], and determine the distance to the galaxies by isolating individual Cepheids. This firmly established the spiral nebula as being objects well outside the Milky Way galaxy. Determining the distance to "island universes", as they were dubbed in the popular media, established the scale of the universe and settled the Shapley-Curtis debate once and for all.<ref>"Island universe" is a reference to speculative ideas promoted by a variety of scholastic thinkers in the 18th and 19th centuries. The most famous early proponent of such ideas was philosopher [[Immanuel Kant]] who published a number of treatises on astronomy in addition to his more famous philosophical works. See Kant, I., 1755. ''Allgemeine Naturgeschichte und Theorie des Himmels'', Part I, J.F. Peterson, Königsberg and Leipzig.</ref>

[[File:Look-back time by redshift.png|thumb|The [[lookback time]] of extragalactic observations by their redshift up to z=20.<ref name="Pilipenko">S.V. Pilipenko (2013-2021) [https://arxiv.org/abs/1303.5961 "Paper-and-pencil cosmological calculator"] arxiv:1303.5961, including [https://code.google.com/archive/p/cosmonom/downloads Fortran-90 code] upon which the citing chart is based.</ref>]]

In 1927, by combining various measurements, including Hubble's distance measurements and [[Vesto Slipher]]'s determinations of [[redshift]]s for these objects, [[Georges Lemaître]] was the first to estimate a constant of proportionality between galaxies' distances and what was termed their "recessional velocities", finding a value of about 600&nbsp;km/s/Mpc.<ref name="Lem27" /><ref name="LemvsHubble_vdBergh" /><ref name="LemvsHubble_Block" /><ref name="LemvsHubbleReich" /><ref name="LemvsHubbleLivio11" /><ref name="LemvsHubbleLivioRiess2013" /> He showed that this was theoretically expected in a universe model based on [[general relativity]].<ref name="Lem27" /> Two years later, Hubble showed that the relation between the distances and velocities was a positive correlation and had a slope of about 500&nbsp;km/s/Mpc.<ref name="Hubble1929" /> This correlation would come to be known as ''[[Hubble's law]]'' and would serve as the observational foundation for the [[metric expansion of space|expanding universe theories]] on which cosmology is still based. The publication of the observations by Slipher, Wirtz, Hubble and their colleagues and the acceptance by the theorists of their theoretical implications in light of Einstein's [[General theory of relativity]] is considered the beginning of the modern science of cosmology.<ref>This popular consideration is echoed in ''[[Time Magazine]]'s'' listing for Edwin Hubble in their [[Time 100]] list of most influential people of the 20th century. [[Michael Lemonick]] recounts, "He discovered the cosmos, and in doing so founded the science of cosmology." [https://web.archive.org/web/20000815215251/http://www.time.com/time/time100/scientist/profile/hubble.html]</ref>