Editing Edwin Hubble (section)

==Discoveries==
===Universe goes beyond the Milky Way galaxy===
[[File:100inchHooker.jpg|thumb|upright=1.2|The {{convert|100|in|m|adj=on}} [[Mount Wilson Observatory#100-inch Hooker telescope|Hooker telescope]] at Mount Wilson Observatory that Hubble used to measure galaxy distances and a value for the rate of [[expansion of the universe]].]]
Edwin Hubble's arrival at Mount Wilson Observatory, California, in 1919 coincided roughly with the completion of the {{convert|100|in|m|adj=on}} [[Mount Wilson Observatory#100-inch Hooker telescope|Hooker Telescope]], then the world's largest. At that time, the prevailing view of the cosmos was that the universe consisted entirely of the [[Milky Way]] galaxy.

[[File:Hubble Tuning Fork diagram.svg|left|thumb|upright=1.5|Hubble's [[Hubble sequence|classification scheme]]]]
Using the Hooker Telescope at [[Mount Wilson Observatory|Mount Wilson]], Hubble identified [[Cepheid variable]]s, a [[Cosmic distance ladder#Standard candles|standard candle]] discovered by [[Henrietta Swan Leavitt]].<ref name="Henrietta"/> Comparing their [[Apparent magnitude|apparent luminosity]] to their intrinsic luminosity gives their distance from Earth.<ref>A Science Odyssey: People and Discoveries.</ref><ref>1929: Edwin Hubble Discovers the universe is expanding.</ref> Hubble found Cepheids in several [[Spiral galaxy#Spiral nebula|nebulae]], including the [[Andromeda Galaxy|Andromeda Nebula]] and [[Triangulum Galaxy|Triangulum Nebula]]. His observations, made in 1924, proved conclusively that these nebulae were much too distant to be part of the Milky Way and were, in fact, entire galaxies outside the Milky Way galaxy; thus, today they are no longer considered [[nebula]]e.

This was first [[Hypothesis|hypothesized]] as early as 1755 when [[Immanuel Kant]]'s ''General History of Nature and Theory of the Heavens'' appeared. Hubble's hypothesis [[Great Debate (astronomy)|was opposed]] by many in the astronomy establishment of the time, in particular by [[Harvard University]]{{En dash}}based [[Harlow Shapley#The Great Debate of 1920|Harlow Shapley]]. Despite the opposition, Hubble, then a thirty-five-year-old scientist, had his findings first published in ''[[The New York Times]]'' on {{nowrap|November 23}}, 1924,<ref name="SharovNovikov1993">{{cite book|last1=Sharov|first1=Aleksandr Sergeevich|last2=Novikov|first2=Igor Dmitrievich|title=Edwin Hubble, the discoverer of the Big Bang universe|url=https://books.google.com/books?id=ttEwkEdPc70C&pg=PA34|access-date=December 31, 2011|date=1993|publisher=Cambridge University Press|isbn=978-0-521-41617-7|page=34}}</ref> then presented them to other astronomers at the January 1, 1925, meeting of the [[American Astronomical Society]].<ref name="MB2">{{cite book |author=Bartusiak |first=Marcia |url=https://books.google.com/books?id=7XojzXh4_KEC&q=The+Day+We+Found+the+Universe |title=The Day We Found the Universe |date=2010 |publisher=Random House Digital, Inc |isbn=9780307276605 |pages=x–xi}}</ref> Hubble's results for the Andromeda galaxy were not formally published in a [[Peer review|peer-reviewed]] [[scientific journal]] until 1929.<ref>{{cite journal|first1=E. P.|last1=Hubble|author-link=Edwin Hubble|title=A spiral nebula as a stellar system, Messier 31|journal=[[The Astrophysical Journal]]|volume=69|page=103|date=1929|bibcode=1929ApJ....69..103H|doi=10.1086/143167|doi-access=free}}</ref>

Hubble's findings fundamentally changed the scientific view of the universe. Supporters state that Hubble's discovery of nebulae outside of our galaxy helped pave the way for future astronomers.<ref>{{cite news |date=2002 |title=life in the universe Astronomy Encyclopedia. Philip's. Credo Reference |location=London, England}}</ref> Although some of his more renowned colleagues simply scoffed at his results, Hubble published his findings. This published work earned him an award titled the American Association Prize and five hundred dollars from Burton E. Livingston of the Committee on Awards.<ref name="Gale E. Christianson 1996" />

Hubble also devised the most commonly used [[Galaxy morphological classification|system for classifying galaxies]], grouping them according to their appearance in photographic images. He arranged the different groups of galaxies in what became known as the Hubble sequence.<ref>{{cite book |author1=Block |first=David L. |url=https://books.google.com/books?id=HQh9hoLfoHoC&q=Hubble+sequence&pg=PA149 |title=Toward a new millennium in galaxy morphology |author2=Puerari |first2=Ivacircnio |author3=Stockton |first3=Alan |date=2000 |publisher=Springer |isbn=9780792361855 |pages=146–150}}</ref>

===Redshift increases with distance===
Hubble went on to estimate the distances to 24 extra-galactic nebulae, using a variety of methods. In 1929, Hubble examined the relationship between these distances and their [[Radial velocity|radial velocities]] as determined from their [[redshift]]s. All of his estimated distances are now known to be too small, by up to a factor of about 7. This was due to factors such as the fact that there are two kinds of Cepheid variables or confusing bright gas clouds with bright stars.<ref name=Kirshner/> However, his distances were more or less proportional to the true distances, and combining his distances with measurements of the redshifts of the galaxies by [[Vesto Slipher]], and by his assistant [[Milton L. Humason]], he found a roughly linear relationship between the distances of the galaxies and their radial velocities (corrected for solar motion),<ref name="Hubbles Law" /> a discovery that later became known as Hubble's law.

This meant that the greater the distance between any two galaxies, the greater their relative speed of separation. When interpreted that way, Hubble's measurements on 46 galaxies lead to a value for the [[Hubble's law|Hubble constant]] of 500&nbsp;km/s/Mpc, which is much higher than the currently accepted values of 74&nbsp;km/s/Mpc<ref name="gaiariess2018">{{cite journal |last1=Riess |first1=Adam G. |last2=Casertano |first2=Stefano |last3=Yuan |first3=Wenlong |last4=Macri |first4=Lucas |last5=Bucciarelli |first5=Beatrice |last6=Lattanzi |first6=Mario G. |last7=MacKenty |first7=John W. |last8=Bowers |first8=J. Bradley |last9=Zheng |first9=WeiKang |last10=Filippenko |first10=Alexei V. |last11=Huang |first11=Caroline |last12=Anderson |first12=Richard I. |title=Milky Way Cepheid Standards for Measuring Cosmic Distances and Application to Gaia DR2: Implications for the Hubble Constant |arxiv=1804.10655|journal=The Astrophysical Journal |date=2018 |volume=861 |issue=2 |pages=126 |doi=10.3847/1538-4357/aac82e |language=en |issn=0004-637X|bibcode=2018ApJ...861..126R |s2cid=55643027 |doi-access=free }}</ref><ref name="guardianhubbleconstant">{{cite news|last1=Devlin|first1=Hannah|title=The answer to life, the universe and everything might be 73. Or 67|url=https://www.theguardian.com/science/2018/may/10/the-answer-to-life-the-universe-and-everything-might-be-73-or-67|access-date=May 13, 2018|work=the Guardian|date=May 10, 2018|language=en}}</ref> (cosmic distance ladder method) or 68&nbsp;km/s/Mpc<ref name="2018planckcosmos">{{cite journal |title=Planck 2018 results. VI. Cosmological parameters |url=https://www.cosmos.esa.int/web/planck/publications#Planck2018 |journal=Astronomy and Astrophysics |access-date=July 18, 2018|bibcode=2020A&A...641A...6P |author1=Planck Collaboration |last2=Aghanim |first2=N.|author2-link=Nabila Aghanim |last3=Akrami |first3=Y. |last4=Ashdown |first4=M. |last5=Aumont |first5=J. |last6=Baccigalupi |first6=C. |last7=Ballardini |first7=M. |last8=Banday |first8=A. J. |last9=Barreiro |first9=R. B. |last10=Bartolo |first10=N. |last11=Basak |first11=S. |last12=Battye |first12=R. |last13=Benabed |first13=K. |last14=Bernard |first14=J. -P. |last15=Bersanelli |first15=M. |last16=Bielewicz |first16=P. |last17=Bock |first17=J. J. |last18=Bond |first18=J. R. |last19=Borrill |first19=J. |last20=Bouchet |first20=F. R. |last21=Boulanger |first21=F. |last22=Bucher |first22=M. |last23=Burigana |first23=C. |last24=Butler |first24=R. C. |last25=Calabrese |first25=E. |last26=Cardoso |first26=J. -F. |last27=Carron |first27=J. |last28=Challinor |first28=A. |last29=Chiang |first29=H. C. |last30=Chluba |first30=J. |display-authors=29 |year=2020 |volume=641 |pages=A6 |doi=10.1051/0004-6361/201833910 |arxiv=1807.06209 |s2cid=119335614 }}</ref><ref>{{cite journal|title=Large Magellanic Cloud Cepheid Standards Provide a 1% Foundation for the Determination of the Hubble Constant and Stronger Evidence for Physics Beyond LambdaCDM|journal = The Astrophysical Journal|volume = 876|issue = 1|pages = 85|first1=Dan|last1=Scolnic|first2=Lucas M.|last2=Macri|first3=Wenlong|last3=Yuan|first4=Stefano|last4=Casertano|first5=Adam G.|last5=Riess|date=March 18, 2019|arxiv = 1903.07603|doi = 10.3847/1538-4357/ab1422|bibcode = 2019ApJ...876...85R|s2cid = 85528549 | doi-access=free }}</ref> ([[Cosmic microwave background|CMB method]]) due to errors in their distance calibrations.

Yet the reason for the redshift remained unclear. Georges Lemaître predicted on theoretical grounds based on Einstein's equations for [[general relativity]] the [[redshift]]-distance relation, and published observational support for it, two years before the discovery of Hubble's law.<ref name="nature.com">{{cite journal| title = Lost in translation: Mystery of the missing text solved Mario Livio ''Nature'' 479, 171–173 (10 November 2011)| journal = Nature| date = November 2011| volume = 479| issue = 7372| pages = 171–173| doi = 10.1038/479171a| last1 = Livio| first1 = Mario| pmid = 22071745| s2cid = 203468083| doi-access = free}}</ref> Although he used the term "velocities" in his paper (and "apparent radial velocities" in the introduction), he later expressed doubt about interpreting these as real velocities. In 1931, he wrote a letter to the Dutch cosmologist [[Willem de Sitter]] expressing his opinion on the theoretical interpretation of the redshift-distance relation:<ref name="Kirshner">{{Cite journal |author=Kirshner |first=Robert P. |date=January 6, 2004 |title=Hubble's diagram and cosmic expansion |journal=Proceedings of the National Academy of Sciences |volume=101 |issue=1 |pages=8–13 |bibcode=2004PNAS..101....8K |doi=10.1073/pnas.2536799100 |pmc=314128 |pmid=14695886 |doi-access=free}}</ref>

{{blockquote|Mr. Humason and I are both deeply sensible of your gracious appreciation of the papers on velocities and distances of nebulae. We use the term 'apparent' velocities to emphasize the empirical features of the correlation. The interpretation, we feel, should be left to you and the very few others who are competent to discuss the matter with authority.}}

Today, the "apparent velocities" in question are usually thought of as an increase in [[Comoving and proper distances|proper distance]] that occurs due to the [[expansion of the universe]]. Light traveling through an expanding metric will experience a Hubble-type redshift, a mechanism somewhat different from the [[Doppler effect]], although the two mechanisms become equivalent descriptions related by a [[Coordinate system#Transformations|coordinate transformation]] for nearby galaxies.

In the 1930s, Hubble was involved in determining the distribution of galaxies and [[shape of the universe|spatial curvature]]. These data seemed to indicate that the universe was [[Euclidean geometry|flat]] and homogeneous, but there was a deviation from flatness at large redshifts. According to [[Allan Sandage]],
{{blockquote|Hubble believed that his count data gave a more reasonable result concerning spatial curvature if the redshift correction was made assuming no recession. To the very end of his writings, he maintained this position, favouring (or at the very least keeping open) the model where no true expansion exists, and therefore that the redshift "represents a hitherto unrecognized principle of nature."<ref>{{cite journal | last1 = Sandage | first1 = Allan | year = 1989 | title = Edwin Hubble 1889–1953 | url = http://antwrp.gsfc.nasa.gov/diamond_jubilee/1996/sandage_hubble.html| journal = The Journal of the Royal Astronomical Society of Canada | volume = 83 | issue = 6 }}</ref>}}

There were methodological problems with Hubble's survey technique that showed a deviation from flatness at large redshifts. In particular, the technique did not account for changes in luminosity of galaxies due to [[Galaxy formation and evolution|galaxy evolution]]. Earlier, in&nbsp;1917, [[Albert Einstein]] had found that his newly developed theory of general relativity indicated that the universe must be either expanding or contracting. Unable to believe what his own equations were telling him, Einstein introduced a [[cosmological constant]] (a&nbsp;"[[Wiktionary:fudge factor|fudge factor]]") to the equations to avoid this "problem". When Einstein learned of Hubble's redshifts, he immediately realized that the expansion predicted by general relativity must be real, and in later life, he said that changing his equations was "the biggest blunder of [his] life".<ref>{{cite web|title=Cosmological Constant|author=Public Broadcasting Station (PBS)|publisher=PBS.org|access-date=May 29, 2011|url=https://www.pbs.org/wnet/hawking/strange/html/strange_cosmo.html|archive-date=June 4, 2011|archive-url=https://web.archive.org/web/20110604145438/http://www.pbs.org/wnet/hawking/strange/html/strange_cosmo.html|url-status=dead}}</ref> In fact, Einstein apparently once visited Hubble and tried to convince him that the universe was expanding.{{Citation_needed|date=November 2024}}

Hubble also discovered the [[asteroid]] [[1373 Cincinnati]] on August 30, 1935. In 1936 he wrote ''The Observational Approach to Cosmology'' and ''The Realm of the Nebulae'' which explained his approaches to extra-galactic astronomy and his view of the subject's history.

In December 1941, Hubble reported to the [[American Association for the Advancement of Science]] that results from a six-year survey with the Mount Wilson telescope did not support the expanding universe theory. According to a ''Los Angeles Times'' article reporting on Hubble's remarks, "The nebulae could not be uniformly distributed, as the telescope shows they are, and still fit the explosion idea. Explanations which try to get around what the great telescope sees, he said, fail to stand up. The explosion, for example, would have had to start long after the earth was created, and possibly even after the first life appeared here."<ref name="LATimes">{{cite journal|title=Savant Refutes Theory of Exploding Universe – Mt. Wilson Astronomer Reports Results of Long Searching With 100-Inch Telescope|journal=The Los Angeles Times |date=December 31, 1941 |page=10|url=https://www.newspapers.com/newspage/160887084/}}</ref><ref name="Harnisch">{{cite web |author=Harnisch |first=Larry |date=December 31, 2011 |title=Hubble: No Evidence of 'Big Bang' Theory |url=https://ladailymirror.com/2011/12/31/hubble-no-evidence-of-big-bang-theory/ |website=Los Angeles Daily Mirror ([[WP:NEWSBLOG]])}} (Shows legible photo of the article.)</ref> (Hubble's estimate of what we now call the Hubble constant would put the Big Bang only 2 billion years ago.)