{"id":31048,"date":"2022-04-21T00:00:00","date_gmt":"2022-04-21T00:00:00","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/?post_type=purple_issue&p=31048"},"modified":"2022-04-29T14:12:03","modified_gmt":"2022-04-29T14:12:03","slug":"pioneers-of-dark-matter","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/2022\/04\/21\/pioneers-of-dark-matter\/","title":{"rendered":"Pioneers of dark matter"},"content":{"rendered":"\n
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<\/p>\n<\/div><\/div>\n\n

Pioneers of dark matter<\/h2>\n\n

The term \u2018dark matter\u2019 was coined a century ago this month. Govert Schilling <\/strong>selects seven scientists who shed light on astronomy\u2019s biggest mystery<\/p>\n\n

Dark matter is what makes the Universe tick. It represents 85 per cent of the material content of our cosmos. Through its gravity, it has enabled the formation of cosmic structure, and it keeps galaxies and galaxy clusters from flying apart. Astronomers have mapped dark matter\u2019s distribution by studying gravitational lensing \u2013 the bending of starlight by massive objects in space \u2013 but no one has ever seen the mysterious stuff, as it doesn\u2019t emit, absorb or reflect light. In this article we take a look at seven of the leading voices who helped progress the quest to understand dark matter.<\/span><\/p>\n\n

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JACOBUS KAPTEYN (1851\u20131922)<\/h3>\n\n\n\n

The originator of the term \u2018dark matter\u2019<\/strong><\/p>\n\n\n\n

\"\"<\/figure><\/div>\n\n\n\n

Dutch astronomer Jacobus Kapteyn made an early mention of the term \u2018dark matter\u2019 in his Astrophysical <\/em>Journal <\/em>paper on the structure of our Milky Way Galaxy. The paper was published on 1 May 1922, a few weeks before Kapteyn died.<\/p>\n\n\n\n

One of 15 children, young Jacobus was raised in a private boarding school run by his parents. In 1878, he was appointed professor of astronomy at the University of Groningen, but he lacked the money to buy a proper telescope. Instead, he joined forces with Scottish astronomer David Gill, who photographed the southern sky from Cape Observatory in South Africa.<\/p>\n\n\n\n

Using a manual plate-measuring machine, Kapteyn spent five-and-a-half years meticulously measuring the positions of 454,875 individual stars \u2013 an impressive undertaking that led to the <\/span>Cape <\/em>Photographic <\/em>Durchmusterung <\/em>(CPD) \u2013 the largest and most accurate stellar catalogue of the time. Later, Kapteyn studied the structure of the Milky Way, working with American astronomer George Ellery Hale at Mount Wilson Observatory in California. In his final paper, he presented a model that is now known as the \u2018Kapteyn Universe\u2019: a relatively small Milky Way with the Sun close to its centre, and nothing beyond its outer edge.<\/p>\n\n\n\n

Although this model would turn out to be completely wrong, Kapteyn realised that studying the motions of stars would reveal the total mass of the system. In his May 1922 paper he wrote: \u201cIt is incidentally suggested that when the theory is perfected it may be possible to determine the amount of dark matter from its gravitational effect.\u201d The riddle of dark matter was born. <\/p>\n\n\n\n

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The Kapteyn Universe, 1922, in which he looked at the motions of stars to study the Milky Way\u2019s total mass<\/figcaption><\/figure><\/div>\n<\/div><\/section>\n\n
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Zwicky suggested that huge amounts of dark matter were preventing the galaxies in the Coma Cluster (pictured) from flying apart <\/figcaption><\/figure><\/div>\n\n

FRITZ ZWICKY (1898\u20131974) <\/h3>\n\n

Realised that galaxies must be more massive than they appear<\/strong><\/p>\n\n

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Born in Bulgaria to Swiss parents, Fritz Zwicky moved to the US at the age of 27 to assist Nobel scientist Robert Millikan in his studies of solid state physics at the California Institute of Technology. That same institute was home to eminent astronomers like George Ellery Hale, Edwin Hubble and Walter Baade. Before long, Zwicky got hooked on astronomy, and became a stellar hot shot himself, studying supernovae and predicting the existence of neutron stars together with Baade.<\/p>\n\n

Using Mount Wilson\u2019s 2.5m Hooker Telescope, he clocked the velocity of individual galaxies in the Coma Cluster, by measuring their Doppler effect \u2013a minute change of wavelength in the galaxy\u2019s light. The observed velocity spread within the cluster is a measure of the total mass of the cluster.<\/p>\n\n

It turned out that the Coma galaxies were moving much faster than expected on the basis of the cluster\u2019s visible content. To prevent the cluster from flying apart, it would need huge amounts of invisible mass \u2013 \u2018dunkle Materie\u2019 (dark matter), as Zwicky wrote in his 1933 paper in a rather obscure Swiss magazine.<\/p>\n\n

Similar results for our own Galaxy had been obtained a year earlier by Kapteyn\u2019s student Jan Oort (of Oort Cloud fame). And later work by Sinclair Smith at Mount Wilson on the Virgo Cluster led to the same conclusion.<\/p>\n\n

Surprisingly, the dark matter riddle was largely neglected by the astronomical community for a couple of decades.<\/p>\n\n

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Fritz Zwicky, here at Palomar Observatory in the 1930s, referred to \u2018dunkle Materie\u2019 (dark matter) in his research<\/figcaption><\/figure><\/div>\n\n
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VERA RUBIN (1928\u20132016) <\/h3>\n\n

Measured the outskirts of galaxies and found they spin as fast as their centres<\/strong><\/p>\n\n

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As a child, Vera Rubin loved watching the stars. But in 1948, Princeton University still didn\u2019t allow female astronomy graduate students, so she went to Cornell University instead and obtained her PhD at Georgetown University. There, she got a job at the Carnegie Institution of Washington\u2019s Department of Terrestrial Magnetism, where she met instrument builder Kent Ford, who was developing an electronic image tube that would allow the spectroscopic study of faint astronomical objects.<\/p>\n\n

Starting in late 1966, Rubin and Ford regularly drove all the way to Arizona to make spectral measurements of individual glowing gas clouds in the Andromeda Galaxy, mounting the bulky image tube on telescopes at Lowell Observatory, near Flagstaff, and at Kitt Peak National Observatory, near Tucson. They presented their first results at an American Astronomical Society (AAS) meeting in December 1968.<\/p>\n\n

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After discovering the \u2018flat rotation curve\u2019 of M31, Rubin tested more galaxy targets<\/figcaption><\/figure><\/div>\n\n

Over the next decade, it became ever more clear that Andromeda has a \u2018flat rotation curve\u2019: the outer parts of the galaxy rotate as fast as the inner parts, whereas our understanding of gravity says the outer regions should rotate slower if the stars are the only masses present. This suggested there was more to the galaxy than met the eye. \u201cThe conclusion is inescapable that non-luminous matter exists beyond the optical galaxy,\u201d Rubin and Ford wrote in an <\/span>Astrophysical <\/em>Journal <\/em>paper in 1980, together with Norbert Thonnard.<\/p>\n\n

At the time of Rubin\u2019s death in 2016, Princeton astrophysicist Neta Bahcall called her \u201cthe mother of flat rotation curves and dark matter\u201d. Rubin herself never failed to mention the very important supporting observations by radio astronomers, who arrived at the same conclusion.<\/p>\n\n

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JAMES PEEBLES (1935\u2013) <\/h3>\n\n\n\n

Discovered the early Universe doesn\u2019t match up with what we see today<\/strong><\/p>\n\n\n\n

\"\"<\/figure><\/div>\n\n\n\n

Born in Winnipeg, Canada, James Peebles has been at Princeton University since 1958. He was the founder of what is now known as \u2018physical cosmology\u2019 \u2013 studying the birth and evolution of the Universe though theoretical physics.<\/p>\n\n\n\n

In 1973, along with fellow Princeton astrophysicist Jerry Ostriker, Peebles showed that disc galaxies like our Milky Way or Andromeda cannot be stable unless they are embedded in giant halos of dark matter. The two scientists, together with Israel\u2019s Amos Yahil, also made the first reliable estimate of the mass density of the Universe in 1974.<\/p>\n\n\n\n

Peebles studied the cosmic microwave background (CMB) \u2013a relic of the Big Bang \u2013 as this could show him if the small fluctuations in the matter density of the primordial Universe are what would gravitationally evolve into the galaxy clusters and superclusters we see today.<\/span><\/p>\n\n\n\n

His solution: dark matter has to be cold (eg consisting of relatively slow-moving particles), and should hardly experience any interaction with \u2018normal\u2019 (so-called baryonic) matter, other than through gravity. In that case, clumps of dark matter could start to form before the CMB was released. At a later stage, gas would fall into these gravitational wells, leading to the structured Universe we see today. <\/p>\n\n\n\n

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From studying the CMB, Peebles found dark matter was \u2018cold\u2019 or slow-moving<\/figcaption><\/figure>\n<\/div><\/section>\n\n

SANDRA FABER (1944\u2013) <\/h3>\n\n

Collected comprehensive evidence that dark matter was real<\/strong><\/p>\n\n

\"\"<\/figure><\/div>\n\n

In the early 1970s, Harvard PhD student Sandra Faber temporarily worked at Carnegie\u2019s Department of Terrestrial Magnetism in Washington, DC, where she learned about the rotation curve measurements of Rubin and Ford. She studied elliptical galaxies and discovered that these too appear to be embedded in dark matter halos.<\/p>\n\n

After moving to the University of California at Santa Cruz, Faber, together with John Gallagher, wrote a hugely influential review article about dark matter for Annual <\/em>Reviews <\/em>of <\/em>Astronomy <\/em>and <\/em>Astrophysics, <\/em>published in 1979. By presenting all the available evidence, the two authors convinced the scientific community that dark matter was not just a figment of our imagination, but a real, major constituent of the Universe.<\/p>\n\n

Five years later, in 1984, Faber was co-author of yet another landmark paper, this time in Nature. <\/em>With George Blumenthal, Joel Primack and Martin Rees, she described the evolution of a cold, dark matter-dominated Universe. The paper provided a detailed description of the formation of globular star clusters, galaxies and galaxy clusters, and even discussed possible candidates for James Peebles\u2019s \u2018cold dark matter\u2019.<\/p>\n\n

\u201cWe have shown that a universe with [approximately] 10 times as much cold dark matter as baryonic matter provides a remarkably good fit to the observed Universe,\u201d the authors wrote, concluding that the cold dark matter picture \u201cseems to be the best model available and merits close scrutiny and testing\u201d.<\/p>\n\n

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Faber discovered galaxies are surrounded by clouds \u2013 or halos \u2013 of dark matter<\/figcaption><\/figure><\/div>\n\n
<\/div>
\n

ELENA APRILE (1954\u2013) <\/h3>\n\n\n\n

Looks for dark matter not in the sky, but deep underground<\/strong><\/p>\n\n\n\n

\"\"<\/figure><\/div>\n\n\n\n

Italian physicist Elena Aprile was born in Milan, worked at CERN (the European particle physics laboratory near Geneva), and moved to the US in 1983. At Columbia University in New York, she helped to build balloon experiments that used noble gases like argon and xenon to detect these particles, neutrinos and gamma rays.<\/p>\n\n\n\n

When, in 2011, she learned about the British ZELPIN-experiment in the Boulby Mine in Yorkshire \u2013a xenonbased detector to search for dark matter particles \u2013 she switched gears and started her own dark-matter experiment, working with Brown University\u2019s Richard Gaitskell, among others.<\/p>\n\n\n\n

In 2010, the group\u2019s first full-scale detector was installed in the Gran Sasso Tunnel, in the Italian Apennines, shielded from cosmic rays and other possible disturbances. Over the past decade, Aprile\u2019s team has constructed ever larger and more sensitive versions of their detector. (Gaitskell now leads his own US project in an old gold mine in South Dakota.) XENONnT, as the current version of the detector is called, became operational last year. Like its predecessors, it searches for the tiny flashes of light that are produced when a dark matter particle slams into a xenon nucleus \u2013a very rare interaction.<\/span><\/p>\n\n\n\n

So far, no convincing dark matter signal has been observed, but Aprile doesn\u2019t give up. Plans for an even larger experiment, called Darwin, are on the drawing board.<\/p>\n\n\n\n

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The XENON dark matter project: (left) the water tank; and (right) the three-storey service building<\/figcaption><\/figure>\n<\/div><\/section>\n\n
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By studying the decline in gravitational forces in the outer parts of a spiral galaxy like NGC 2403 (pictured), scientists can challenge theories of dark matter<\/figcaption><\/figure><\/div>\n\n

MORDEHAI MILGROM (1946\u2013) <\/h3>\n\n

Questioned whether we really do need dark matter after all<\/strong><\/p>\n\n

\"\"<\/figure><\/div>\n\n

When Israeli particle physicist Mordehai Milgrom visited Princeton in 1980, he learned about flat rotation curves and the riddle of dark matter for the first time. But instead of theorising about hypothetical particles, Milgrom asked himself: What if our ideas about gravity are wrong?<\/p>\n\n

In the summer of 1983, he published his MOND theory (Modified Newtonian Dynamics) in The <\/em>Astrophysical <\/em>Journal. <\/em>According to MOND, the strength of the gravitational force declines at a much slower rate with distance (not as 1\/r2<\/sup> , but as 1\/r) in weak gravity environments, like the outer parts of galaxies. This relatively simple (albeit rather ad hoc) adaptation of Newton\u2019s laws perfectly explains the flat rotation curves found by Rubin, Ford and their radio astronomy colleagues, without any need for mysterious dark matter.<\/p>\n\n

Despite many attempts, no one has yet been able to falsify MOND, which is not to say that the theory doesn\u2019t face any problems. For example, Milgrom and his followers still need at least some (baryonic) dark matter in clusters of galaxies, and they haven\u2019t fully succeeded in formulating an elegant \u2018relativistic\u2019 version of their theory.<\/p>\n\n

Nevertheless, MOND\u2019s successes in explaining galactic rotation curves are impressive and Milgrom could possibly be on to something. If so, our century-long search for dark matter may have been unrealistic, and Milgrom might end up in the astronomy history books as the pioneer of a new era in our understanding of the Universe. Only time will tell.<\/span><\/p>\n\n

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The rotation curve of galaxy NGC 2403. Milgrom was able to fit the square data points almost perfectly using MOND (Modified Newtonian Dynamics) without the need for dark matter<\/figcaption><\/figure><\/div>\n\n
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\"\"<\/figure><\/div>\n<\/div>\n\n\n\n
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Govert Schilling\u2019s <\/strong>book The Elephant in the Universe <\/em>is published on 31 May by Harvard University Press<\/p>\n<\/div>\n<\/div>\n\n

Photos: NASA\/ESA\/M. JEE AND H. FORD (JOHNS HOPKINS UNIVERSITY), SCIENCE PHOTO LIBRARY, NASA\/ESA AND THE HUBBLE HERITAGE TEAM (STSCI\/AURA), CALTECH OPTICAL OBSERVATORIES, AIP EMILIO SEGR\u00c8 VISUAL ARCHIVES\/RUBIN COLLECTION, TOMMY NAWRATIL\/CCDGUIDE.COM, SIPA US\/ ALAMY STOCK PHOTO, ESA AND THE PLANCK COLLABORATION, CARNEGIE INSTITUTION FOR SCIENCE, NASA\/ESA\/JPL-CALTECH\/YALE\/CNRS, ELENA APRILE\/ COLUMBIA UNIVERSITY, XENON COLLABORATION, JOHANNES SCHEDLER\/CCDGUIDE.COM, WEIZMANN INSTITUTE OF SCIENCE, HINDAWI<\/p>\n","protected":false},"excerpt":{"rendered":"

Seven key scientists who shed light on the cosmos\u2019s biggest mystery<\/p>\n","protected":false},"author":24,"featured_media":31030,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"ub_ctt_via":"","purple_page_number":"60","purple_custom_meta_purple_page_number":"60","purple_seq_number":"1","purple_custom_meta_purple_seq_number":"1","purple_source_article":"article_60-1.xml","purple_custom_meta_purple_source_article":"article_60-1.xml","purple_source_issue":"May-2022","purple_custom_meta_purple_source_issue":"May-2022","purple_external_id":"May-2022-60-1","purple_custom_meta_purple_external_id":"May-2022-60-1","purple_issue_code":"|0000086552||","purple_custom_meta_purple_issue_code":"|0000086552||","purple_android_product":"com.im.skyatnight.204","purple_custom_meta_purple_android_product":"com.im.skyatnight.204","purple_ios_product":"com.im.skyatnight.204","purple_custom_meta_purple_ios_product":"com.im.skyatnight.204","purple_web_product":"","purple_custom_meta_purple_web_product":"","purple_publication_id":"075fab74-0a21-4201-866a-899d6c41c40c","purple_migrated":"","kt_blocks_editor_width":""},"categories":[21],"tags":[88,14],"featured_image_src":"https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/553feba8-1e9a-468e-944a-09f8d6366f3b.jpg","author_info":{"display_name":"importmanagerhub@sprylab.com","author_link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/author\/importmanagerhubsprylab-com\/"},"acf":{"readingTimeMinutes":"12","apple_news_title":""},"uagb_featured_image_src":{"full":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/553feba8-1e9a-468e-944a-09f8d6366f3b.jpg",1735,2048,false],"thumbnail":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/553feba8-1e9a-468e-944a-09f8d6366f3b-150x150.jpg",150,150,true],"medium":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/553feba8-1e9a-468e-944a-09f8d6366f3b-254x300.jpg",254,300,true],"medium_large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/553feba8-1e9a-468e-944a-09f8d6366f3b-768x907.jpg",768,907,true],"large":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/553feba8-1e9a-468e-944a-09f8d6366f3b-868x1024.jpg",800,944,true],"1536x1536":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/553feba8-1e9a-468e-944a-09f8d6366f3b-1301x1536.jpg",1301,1536,true],"2048x2048":["https:\/\/c01.purpledshub.com\/uploads\/sites\/77\/2022\/04\/553feba8-1e9a-468e-944a-09f8d6366f3b.jpg",1735,2048,false]},"uagb_author_info":{"display_name":"importmanagerhub@sprylab.com","author_link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/author\/importmanagerhubsprylab-com\/"},"uagb_comment_info":0,"uagb_excerpt":"Seven key scientists who shed light on the cosmos\u2019s biggest mystery","_links":{"self":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts\/31048"}],"collection":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/users\/24"}],"replies":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/comments?post=31048"}],"version-history":[{"count":9,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts\/31048\/revisions"}],"predecessor-version":[{"id":31639,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/posts\/31048\/revisions\/31639"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media\/31030"}],"wp:attachment":[{"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/media?parent=31048"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/categories?post=31048"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/wp-json\/wp\/v2\/tags?post=31048"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}