How Fast Is the Universe Expanding?

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Please refer to the appropriate style manual or other sources if you have any questions. </div> <div class="label mt-20 mb-10">Select Citation Style</div> <select class="js-citation-format-select form-select"> <option selected value="mla">MLA</option> <option value="apa">APA</option> <option value="chicago">Chicago Manual of Style</option> </select> <div class="citation font-serif border rounded p-15 mt-20" data-authors="Gregersen, Erik" data-authors-initial="Gregersen, E." data-title="How Fast Is the Universe Expanding?" data-published-date="9 Feb. 2023" data-url="https://www.britannica.com/story/how-fast-is-the-universe-expanding" > <div class="citation-text"></div> </div> <button class="js-copy-citation-button mt-20 btn btn-xs btn-outline-blue border shadow-sm pr-10" > <em class="material-icons md-icon ml-n10 my-n5 mr-5" data-icon="file_copy"></em> <span class="js-citation-status-text">Copy Citation</span> </button> </div> </div> </div> <div> <button class="js-share-modal-button js-modal gtm-topic-tool btn btn-xs btn-outline-blue border shadow-sm pr-10" data-modal=".md-share-modal"> <em class="material-icons md-icon ml-n10 my-n5 mr-5" data-icon="share"></em> Share </button> <div class="md-share-modal size-lg d-none qa-share-modal"> <div class="md-modal-body"> <div class="h2 font-serif d-flex align-items-center pb-15 border-bottom"> <em class="material-icons text-blue mr-10" data-icon="share"></em> Share </div> <div class="label my-20">Share to social media</div> <div class="md-social-toolbar-circle d-flex align-items-start inverted" data-value="share" title="How Fast Is the Universe Expanding?" data-url="https://www.britannica.com/story/how-fast-is-the-universe-expanding" > <a class="social-icon facebook justify-content-center d-flex align-items-center align-self-center" data-provider="facebook" href="https://www.facebook.com/BRITANNICA/" target="_blank" rel="noopener"><span>Facebook</span></a> <a class="social-icon x justify-content-center d-flex align-items-center align-self-center" data-provider="x" href="https://x.com/britannica" target="_blank" rel="noopener"><span>X</span></a> </div> <div class="label pt-20 mt-20 mb-5 border-top">URL</div> <a class="font-serif text-truncate d-inline-block" href="https://www.britannica.com/story/how-fast-is-the-universe-expanding">https://www.britannica.com/story/how-fast-is-the-universe-expanding</a> </div> </div> </div> </div> <div class="md-byline module-spacing font-12"> <div class="font-serif font-12"> <span class="written-by text-gray-700"> Written by </span> <div class="editor-popover popover p-0"> <a class="d-block p-20 qa-editor-popup gtm-byline font-12 byline-contributor" href="/editor/Erik-Gregersen/6723" > <div class="editor-title font-16 font-weight-bold">Erik Gregersen</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Erik Gregersen is a senior editor at Encyclopaedia Britannica, specializing in the physical sciences and technology. Before joining Britannica in 2007, he worked at the University of Chicago Press on the...</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Erik Gregersen</span></div> <div class="font-serif font-12 text-gray-700"> <span class="qa-fact-checked-by">Fact-checked by</span> <div class="editor-popover popover p-0"> <a class="d-block p-20 qa-editor-popup font-12" href="/editor/The-Editors-of-Encyclopaedia-Britannica/4419" > <div class="editor-title font-16 font-weight-bold">The Editors of Encyclopaedia Britannica</div> <div class="editor-description font-12 font-serif mt-5 text-black">Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link font-12 "> The Editors of Encyclopaedia Britannica</span></div></div> </div> <article class="topic-content content mt-20"><figure class="md-assembly md-feature-assembly print-false" data-type="1" data-descid="1"><div class="md-assembly-wrapper card-media" data-type="image"><a href="https://cdn.britannica.com/27/238127-050-E0772AD4/galaxy-cluster-Abell-2744.jpg" target="_blank" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center " data-href="*"><img src="https://cdn.britannica.com/27/238127-050-E0772AD4/galaxy-cluster-Abell-2744.jpg?w=600&q=60" alt="Galaxy clusters like Abell 2744 can act as a natural cosmic lens, magnifying light from more distant, background objects through gravity. NASA's James Webb Space Telescope may be able to detect light from the first stars in the universe if they are gravitationally lensed by such clusters. (astronomy, space exploration, galaxies)" data-width="1434" data-height="1600" loading="eager"><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class=""><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div><cite class="credit d-block mt-5">NASA, ESA, J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)</cite></figcaption></figure> <p class="topic-paragraph">The rate at which the universe is expanding is called the <a href="https://www.britannica.com/science/Hubble-constant" class="md-crosslink " data-show-preview="true">Hubble constant</a>, named after astronomer <a href="https://www.britannica.com/biography/Edwin-Hubble" class="md-crosslink " data-show-preview="true">Edwin Hubble</a>, who, with <a href="https://www.britannica.com/biography/Milton-Humason" class="md-crosslink " data-show-preview="true">Milton Humason</a>, showed convincingly that the velocity with which a galaxy was moving away from Earth was proportional to its distance. That is, <em>velocity = Hubble constant × distance</em>. Rearranging this equation gives <em>Hubble constant = velocity ÷ distance</em>, so the Hubble constant is expressed in units of kilometers per second (km/s) per megaparsec (Mpc), where a megaparsec is 3.26 million light-years.</p><span class="MOD1"></span> <p class="topic-paragraph">So what is this constant? How fast is the universe expanding? In 1929 Hubble got a value of about 500 km/s/Mpc. Much more accurate measurements dropped this to about 100 km/s/Mpc by about 1960, but the astronomical community became divided into two camps, one championing 100 km/s/Mpc and the other at 50 km/s/Mpc. Solving this problem became one of the key projects of the <a href="https://www.britannica.com/topic/Hubble-Space-Telescope" class="md-crosslink " data-show-preview="true">Hubble Space Telescope</a> (HST), and in 2001 the HST astronomers came to the conclusion of 72 km/s/Mpc.</p><span class="MOD2"></span> <p class="topic-paragraph">Astronomers were finally happy, but their joy lasted only a few years. Recent accurate measurements of the <a href="https://www.britannica.com/science/cosmic-microwave-background" class="md-crosslink " data-show-preview="true">cosmic microwave background</a> give values of about 68 km/s/Mpc, while measurements using observations like that done by HST still give values of about 72 km/s/Mpc. This disagreement has been called the “Hubble tension” and has not yet been resolved.</p><span class="MOD3"></span> <span class="AM-END-OF-ARTICLE am-inline"></span></article> </div> </div> </div> <aside class="cm-rightrail d-none d-md-block ad-rail"></aside> </div> </div> </div> </div> </main> <div class="grid gx-0"> <div class="col"> <div class="md-loading hidden display-4 font-weight-normal"></div> <div class="text-center my-20 mb-100"> <button class="md-infinite-load-next btn btn-blue" data-value="next-page">Load More</button> </div> </div> <div class="col-md-da-320 hidden-xs"> </div> </div> </div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript>  <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Story","templateName":"COMPANION","pageNumber":1,"pagesTotal":11,"pageId":12934,"pageLength":null,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-133/js/libs/jquery-3.5.0.min.js?v=3.133.48"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-133/dist/story-page.js?v=3.133.48' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"A","adLeg":"A","userType":"ANONYMOUS","pageType":"Story","pageSubtype":"COMPANION","articleTemplateType":null,"gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":false,"hasAiTopQuestions":false,"hasSimplifyButton":false} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>

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