Editing Observational cosmology (section)

==Early observations==
The science of [[physical cosmology]] as it is practiced today had its subject material defined in the years following the [[Shapley-Curtis debate]] when it was determined that the [[universe]] had a larger scale than the [[Milky Way galaxy]]. This was precipitated by observations that established the [[size of the universe|size]] and the dynamics of the cosmos that could be explained by [[Albert Einstein]]'s [[General Theory of Relativity]]. In its infancy, cosmology was a speculative science based on a very limited number of observations and characterized by a dispute between [[Steady-state model|steady state]] theorists and promoters of [[Big Bang]] cosmology. It was not until the 1990s and beyond that the astronomical observations would be able to eliminate competing theories and drive the science to the "Golden Age of Cosmology" which was heralded by [[David Schramm (astrophysicist)|David Schramm]] at a [[United States National Academy of Sciences|National Academy of Sciences]] colloquium in 1992.<ref>Arthur M. Sackler Colloquia of the National Academy of Sciences: Physical Cosmology; Irvine, California: March 27–28, 1992.</ref>

===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>

===Nuclide abundances===
{{main|cosmochemistry|astrochemistry}}
Determination of the [[Abundance of the chemical elements#Abundance of elements in the Universe|cosmic abundance of elements]] has a history dating back to early [[spectroscopy|spectroscopic]] measurements of light from astronomical objects and the identification of [[emission line|emission]] and [[absorption line]]s which corresponded to particular electronic transitions in [[chemical element]]s identified on Earth. For example, the element [[Helium]] was first identified through its spectroscopic signature in the [[Sun]] before it was isolated as a gas on Earth.<ref>''The Encyclopedia of the Chemical Elements'', page 256</ref><ref>''Oxford English Dictionary'' (1989), s.v. "helium". Retrieved December 16, 2006, from Oxford English Dictionary Online. Also, from quotation there: Thomson, W. (1872). ''Rep. Brit. Assoc.'' xcix: "Frankland and Lockyer find the yellow prominences to give a very decided bright line not far from D, but hitherto not identified with any terrestrial flame. It seems to indicate a new substance, which they propose to call Helium."</ref>

Computing relative abundances was achieved through corresponding spectroscopic observations to measurements of the elemental composition of [[meteorite]]s.
<!--A compilation of results is found [[Cosmochemical Periodic Table of the Elements in the Solar System|here]].-->

===Detection of the cosmic microwave background===
{{main|Discovery of cosmic microwave background radiation}}
[[File:WMAP 2012.png|thumb|the CMB seen by WMAP]]
A [[cosmic microwave background]] was predicted in 1948 by [[George Gamow]] and [[Ralph Alpher]], and by Alpher and [[Robert Herman]] as due to the hot [[Big Bang]] model. Moreover, Alpher and Herman were able to estimate the temperature,<ref>{{cite journal | last1 = Gamow | first1 = G. | year = 1948 | title = The Origin of Elements and the Separation of Galaxies| journal = Physical Review | volume = 74| issue = 4 | page=505 | doi=10.1103/physrev.74.505.2|bibcode = 1948PhRv...74..505G }}{{cite journal | last1 = Gamow | first1 = G. | title = The evolution of the universe | journal = Nature|volume= 162 | issue = 4122 | pages = 680–2 |year=1948| doi=10.1038/162680a0| pmid = 18893719 |bibcode = 1948Natur.162..680G | s2cid = 4793163 }} {{cite journal | first1= R. A. |last1= Alpher |first2= R. |last2=Herman|title=On the Relative Abundance of the Elements | journal = Physical Review | volume = 74 | issue = 11| page = 1577 | doi=10.1103/physrev.74.1577|bibcode = 1948PhRv...74.1577A |year= 1948 }}</ref> but their results were not widely discussed in the community. Their prediction was rediscovered by [[Robert Dicke]] and [[Yakov Zel'dovich]] in the early 1960s with the first published recognition of the CMB radiation as a detectable phenomenon appeared in a brief paper by [[Soviet Union|Soviet]] astrophysicists [[A. G. Doroshkevich]] and [[Igor Dmitriyevich Novikov|Igor Novikov]], in the spring of 1964.<ref>{{cite journal |author=A. A. Penzias |year=1979 |title=The origin of elements. |url=https://www.nobelprize.org/prizes/physics/1978/penzias/lecture/ |journal=[[Nobel Prize in Physics|Nobel lecture]] |volume=205 |issue=4406 |pages=549–54 |bibcode=1979Sci...205..549P |doi=10.1126/science.205.4406.549 |pmid=17729659 |access-date=October 4, 2006|url-access=subscription }}</ref> In 1964, [[David Todd Wilkinson]] and Peter Roll, Dicke's colleagues at [[Princeton University]], began constructing a Dicke radiometer to measure the cosmic microwave background.<ref>R. H. Dicke, "The measurement of thermal radiation at microwave frequencies", ''Rev. Sci. Instrum.'' '''17''', 268 (1946). This basic design for a radiometer has been used in most subsequent cosmic microwave background experiments.</ref> In 1965, [[Arno Penzias]] and [[Robert Woodrow Wilson]] at the [[Crawford Hill]] location of [[Bell Telephone Laboratories]] in nearby [[Holmdel Township, New Jersey]] had built a Dicke radiometer that they intended to use for radio astronomy and satellite communication experiments. Their instrument had an excess 3.5&nbsp;K [[noise temperature|antenna temperature]] which they could not account for. After receiving a telephone call from Crawford Hill, Dicke famously quipped: "Boys, we've been scooped."<ref>A. A. Penzias and R. W. Wilson, "A Measurement of Excess Antenna Temperature at 4080 Mc/s," ''Astrophysical Journal'' '''142''' (1965), 419. R. H. Dicke, P. J. E. Peebles, P. G. Roll and D. T. Wilkinson, "Cosmic Black-Body Radiation," ''Astrophysical Journal'' '''142''' (1965), 414. The history is given in P. J. E. Peebles, ''Principles of physical cosmology'' (Princeton Univ. Pr., Princeton 1993).</ref> A meeting between the Princeton and Crawford Hill groups determined that the antenna temperature was indeed due to the microwave background. Penzias and Wilson received the 1978 [[Nobel Prize in Physics]] for their discovery.