View source for Observational cosmology

{{Short description|Study of the origin of the universe (structure and evolution)}}
{{Physical cosmology|expanded=experiments}}
'''Observational cosmology''' is the study of the structure, the evolution and the origin of the [[universe]] through [[observation]], using instruments such as [[telescope]]s and [[cosmic ray]] detectors.

==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.

==Modern observations==
Today, observational cosmology continues to test the predictions of theoretical cosmology and has led to the refinement of cosmological models. For example, the observational evidence for [[dark matter]] has heavily influenced theoretical modeling of [[structure formation|structure]] and [[galaxy formation]]. When trying to calibrate the Hubble diagram with accurate [[supernova]] [[standard candle]]s, observational evidence for [[dark energy]] was obtained in the late 1990s. These observations have been incorporated into a six-parameter framework known as the [[Lambda-CDM model]] which explains the evolution of the universe in terms of its constituent material. This model has subsequently been verified by detailed observations of the cosmic microwave background, especially through the [[WMAP]] experiment.

Included here are the modern observational efforts that have directly influenced cosmology.

===Redshift surveys===
{{main|Redshift survey}}
With the advent of automated [[telescope]]s and improvements in [[astronomical spectroscopy|spectroscopes]], a number of collaborations have been made to map the universe in [[redshift]] space. By combining redshift with angular position data, a redshift survey maps the 3D distribution of matter within a field of the sky. These observations are used to measure properties of the [[large-scale structure of the cosmos|large-scale structure]] of the universe. The [[Great Wall (astronomy)|Great Wall]], a vast [[supercluster]] of galaxies over 500 million [[light-year]]s wide, provides a dramatic example of a large-scale structure that redshift surveys can detect.<ref>{{citation|doi=10.1126/science.246.4932.897|pmid=17812575|title=Mapping the Universe|journal=Science|volume=246|issue=4932|pages=897–903|year=1989|last1=Geller|first1=M. J.|last2=Huchra|first2=J. P.|bibcode=1989Sci...246..897G|s2cid=31328798}}</ref>
[[File:HSCSDMmap2018.gif|thumb|3D visualization of the dark matter distribution from the Hyper Suprime-Cam redshift survey on [[Subaru Telescope]] in 2018<ref>{{Cite web |last=Duffy |first=Jocelyn |date=October 2, 2018 |title=Hyper Suprime-Cam Survey Maps Dark Matter in the Universe |url=https://www.cmu.edu/news/stories/archives/2018/october/dark-matter-survey.html |archive-url=https://web.archive.org/web/20220412012749/https://www.cmu.edu/news/stories/archives/2018/october/dark-matter-survey.html |archive-date=April 12, 2022 |access-date=December 7, 2022 |website=Carnegie Mellon University}}</ref>]]

The first redshift survey was the [[CfA Redshift Survey]], started in 1977 with the initial data collection completed in 1982.<ref>See the official CfA [http://cfa-www.harvard.edu/~huchra/zcat/ website] for more details.</ref> More recently, the [[2dF Galaxy Redshift Survey]] determined the large-scale structure of one section of the Universe, measuring ''z''-values for over 220,000 galaxies; data collection was completed in 2002, and the final [[data set]] was released 30 June 2003.<ref>{{Cite journal|doi=10.1111/j.1365-2966.2005.09318.x|title=The 2dF galaxy redshift survey: Power-spectrum analysis of the final dataset and cosmological implications|author=Shaun Cole|author2=et al. (The 2dFGRS Collaboration)|journal=Mon. Not. R. Astron. Soc.|volume=362|issue=2|pages=505&ndash;34|date=2005|doi-access=free |arxiv= astro-ph/0501174|bibcode = 2005MNRAS.362..505C |s2cid=6906627}} [http://msowww.anu.edu.au/2dFGRS/ 2dF Galaxy Redshift Survey homepage] {{Webarchive|url=https://web.archive.org/web/20070205010241/http://msowww.anu.edu.au/2dFGRS/ |date=2007-02-05 }}</ref> (In addition to mapping large-scale patterns of galaxies, 2dF established an upper limit on [[neutrino]] mass.) Another notable investigation, the [[Sloan Digital Sky Survey]] (SDSS), is ongoing {{As of|2011|lc=on}} and aims to obtain measurements on around 100 million objects.<ref>[http://www.sdss.org/ SDSS Homepage]</ref> SDSS has recorded redshifts for galaxies as high as 0.4, and has been involved in the detection of [[quasar]]s beyond ''z'' = 6. The [[DEEP2 Redshift Survey]] uses the [[Keck telescopes]] with the new "DEIMOS" [[spectrograph]]; a follow-up to the pilot program DEEP1, DEEP2 is designed to measure faint galaxies with redshifts 0.7 and above, and it is therefore planned to provide a complement to SDSS and 2dF.<ref>{{cite conference|title=Science objectives and early results of the DEEP2 redshift survey|author=Marc Davis|author2=et al. (DEEP2 collaboration)|date=2002|book-title=Conference on Astronomical Telescopes and Instrumentation, Waikoloa, Hawaii, 22–28 August 2002|arxiv=astro-ph/0209419}}</ref>

===Cosmic microwave background experiments===
{{main list|List of cosmic microwave background experiments}}
{{excerpt|Cosmic microwave background#Discovery}}
{{excerpt|Cosmic microwave background#Cosmic origin|hat=no}}
{{excerpt|Cosmic microwave background#Progress on theory|hat=no}}
{{excerpt|Cosmic microwave background#COBE|hat=no}}
{{excerpt|Cosmic microwave background#Precision cosmology|hat=no}}
{{excerpt|Cosmic microwave background#Observations after COBE|hat=no}}

===Telescope observations===

====Radio====
The brightest sources of low-frequency radio emission (10&nbsp;MHz and 100&nbsp;GHz) are [[radio galaxy|radio galaxies]] which can be observed out to extremely high redshifts. These are subsets of the [[active galaxy|active galaxies]] that have extended features known as lobes and jets which extend away from the [[galactic nucleus]] distances on the order of [[megaparsec]]s. Because radio galaxies are so bright, astronomers have used them to probe extreme distances and early times in the evolution of the universe.

====Infrared====
Far [[infrared]] observations including [[submillimeter astronomy]] have revealed a number of sources at cosmological distances. With the exception of a few [[infrared window|atmospheric window]]s, most of infrared light is blocked by the atmosphere, so the observations generally take place from balloon or space-based instruments. Current observational experiments in the infrared include [[NICMOS]], the [[Cosmic Origins Spectrograph]], the [[Spitzer Space Telescope]], the [[Keck Interferometer]], the [[Stratospheric Observatory For Infrared Astronomy]], and the [[Herschel Space Observatory]]. The next large space telescope planned by NASA, the [[James Webb Space Telescope]] will also explore in the infrared.

An additional infrared survey, the [[Two-Micron All Sky Survey]], has also been very useful in revealing the distribution of galaxies, similar to other optical surveys described below.

====Optical rays (visible to human eyes)====
Optical light is still the primary means by which astronomy occurs, and in the context of cosmology, this means observing distant galaxies and galaxy clusters in order to learn about the [[Large-scale structure of the universe|large scale structure]] of the Universe as well as [[galaxy evolution]]. [[Redshift survey]]s have been a common means by which this has been accomplished with some of the most famous including the [[2dF Galaxy Redshift Survey]], the [[Sloan Digital Sky Survey]], and the upcoming [[Large Synoptic Survey Telescope]]. These optical observations generally use either [[photometry (astronomy)|photometry]] or [[spectroscopy]] to measure the [[redshift]] of a galaxy and then, via [[Hubble's law]], determine its distance modulo redshift distortions due to [[peculiar velocities]]. Additionally, the position of the galaxies as seen on the sky in [[celestial coordinates]] can be used to gain information about the other two spatial dimensions.

Very deep observations (which is to say sensitive to dim sources) are also useful tools in cosmology. The [[Hubble Deep Field]], [[Hubble Ultra Deep Field]], [[Hubble Extreme Deep Field]], and [[Hubble Deep Field South]] are all examples of this.

====Ultraviolet====
See [[Ultraviolet astronomy]].

====X-rays====
See [[X-ray astronomy]].

====Gamma-rays====
See [[Gamma-ray astronomy]].

===Cosmic ray observations===
See [[Cosmic-ray observatory]].

==Future observations==

===Cosmic neutrinos===
{{main|Cosmic neutrino background}}
It is a prediction of the [[Big Bang]] model that the universe is filled with a [[Cosmic neutrino background|neutrino background radiation]], analogous to the [[cosmic microwave background radiation]]. The microwave background is a relic from when the universe was about 380,000 years old, but the neutrino background is a relic from when the universe was about two seconds old.

If this neutrino radiation could be observed, it would be a window into very early stages of the universe. Unfortunately, these neutrinos would now be very cold, and so they are effectively impossible to observe directly.

===Gravitational waves===
{{main|Gravitational wave background}}

== See also ==

* [[Big Bang]]
* [[Cosmic background radiation]]

==References==
{{reflist|33em|refs=

<ref name="Lem27">
{{cite journal
 |last=Lemaître |first=G. |author-link=Georges Lemaître
 |title=Un univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques
 |date=1927
 |journal=[[Annales de la Société Scientifique de Bruxelles A]]
 |volume=47 |pages=49–56
 |bibcode=1927ASSB...47...49L
}} Partially translated in {{Cite journal
 |last=Lemaître |first=G.
 |title=Expansion of the universe, A homogeneous universe of constant mass and increasing radius accounting for the radial velocity of extra-galactic nebulae
 |date=1931
 |journal=[[Monthly Notices of the Royal Astronomical Society]]
 |volume=91 |issue= 5|pages=483–490
 |bibcode=1931MNRAS..91..483L
 |doi=10.1093/mnras/91.5.483
|doi-access=free
 }}</ref>

<ref name="LemvsHubble_vdBergh">
{{cite journal
 |last=van den Bergh |first=S.
 |date=2011
 |title=The Curious Case of Lemaitre's Equation No. 24
 |journal=[[Journal of the Royal Astronomical Society of Canada]]
 |volume=105 |issue=4 |pages=151
 |arxiv=1106.1195
 |bibcode=2011JRASC.105..151V
}}</ref>

<ref name="LemvsHubble_Block">
{{cite book
 |last=Block |first=D. L.
 |chapter=Georges Lemaître and Stigler's Law of Eponymy
 |date=2012
 |editor1-last=Holder |editor1-first=R. D.
 |editor2-last=Mitton |editor2-first=S.
 |title=Georges Lemaître: Life, Science and Legacy
 |series=Astrophysics and Space Science Library
 |volume=395 |pages=89–96
 |arxiv=1106.3928
 |bibcode=2012ASSL..395...89B
 |doi=10.1007/978-3-642-32254-9_8
 |isbn=978-3-642-32253-2
|s2cid=119205665
 }}</ref>

<ref name="LemvsHubbleReich">
{{cite journal
 |last=Reich |first=E. S.
 |date=27 June 2011
 |title=Edwin Hubble in translation trouble
 |url=http://www.nature.com/news/2011/110627/full/news.2011.385.html
 |journal=[[Nature News]]
 |doi=10.1038/news.2011.385
|url-access=subscription
 }}</ref>

<ref name="LemvsHubbleLivio11">
{{cite journal
 |last=Livio |first=M.
 |date=2011
 |title=Lost in translation: Mystery of the missing text solved
 |journal=[[Nature (journal)|Nature]]
 |volume=479 |issue=7372 |pages=171–173
 |bibcode=2011Natur.479..171L
 |doi=10.1038/479171a
 |pmid=22071745
|s2cid=203468083
 |doi-access=free
 }}</ref>

<ref name="LemvsHubbleLivioRiess2013">
{{cite journal
 |last1=Livio |first1=M.
 |last2=Riess |first2=A.
 |date=2013
 |title=Measuring the Hubble constant
 |journal=[[Physics Today]]
 |volume=66 |issue=10 |pages=41
 |bibcode=2013PhT....66j..41L
 |doi=10.1063/PT.3.2148
}}</ref>

<ref name="Hubble1929">
{{cite journal
 |last=Hubble |first=E.
 |date=1929
 |title=A relation between distance and radial velocity among extra-galactic nebulae
 |journal=[[Proceedings of the National Academy of Sciences]]
 |volume=15 |issue=3 |pages=168–73
 |bibcode=1929PNAS...15..168H
 |doi=10.1073/pnas.15.3.168
 |pmid=16577160
 |pmc=522427
|doi-access=free
 }}</ref>
}}

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[[Category:Observational cosmology| ]]
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[[Category:Observational astronomy]]

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