Image collected by the Hubble Space Telescope of an object called NGC 2623, which is made up of two galaxies in the final stages of merging together. (Credit: ESA\/Hubble & NASA)<\/p>\n
Scientists at the University of Colorado Boulder may have solved a pressing mystery about the universe\u2019s gravitational wave background<\/a>.<\/p>\n That\u2019s the name for the ripples in space and time that move constantly through the cosmos and \u201cjiggle us almost like Jell-O,\u201d according to CU Boulder astrophysicist Julie Comerford.<\/p>\n The study, published recently in \u201cThe Astrophysical Journal,\u201d<\/a> reveals new insights into the evolution of the universe\u2014namely, how smaller galaxies may have coalesced over billions of years to form larger and more complex galaxies like the Milky Way.<\/p>\n Comerford explained that, at any one time in the universe, countless galaxies are in the process of merging.<\/p>\n Each of those galaxies has an aptly named supermassive black hole at its center. As galaxies merge, these black holes spin around each other, whipping in circles until they eventually smack together. The resulting collisions create waves in space and time that are so subtle humans never feel them.<\/p>\n Artist’s depiction of the gravitational wave background. (Credit: NANOGrav collaboration; Aurore Simonet)<\/p>\n \u201cYou can picture lots of people in a swimming pool,\u201d said Comerford, lead author of the new study and professor in the Department of Astrophysical and Planetary Sciences<\/a> at CU Boulder. \u201cThey\u2019re all creating their own waves, and the waves overlap. That\u2019s what the gravitational wave background is like.\u201d<\/p>\n In 2023, several international collaborations, including the North American Nanohertz Observatory for Gravitational Waves<\/a> (NANOGrav) experiment, reported that they had detected the gravitational wave background<\/a> for the first time.<\/p>\n There was just one problem: Based on the groups\u2019 measurements, those waves were much larger than scientists had estimated. No one knew why.<\/p>\n In the new study, Comerford and study co-author Joseph Simon, a former postdoctoral researcher at CU Boulder, may have found the explanation.<\/p>\n Using observations of real galaxies and computer simulations the team discovered something that researchers hadn\u2019t accounted for: When a smaller supermassive black hole merges with a larger one, the smaller black hole seems to gain a lot of mass.<\/p>\n That extra mass makes a difference. Just like swimmers doing cannonballs in a pool, larger supermassive black holes produce larger gravitational waves.<\/p>\n \u201cWe had a prediction for what the gravitational wave background should be, and what NANOGrav found was larger than expected,\u201d Comerford said. \u201cIt was a surprise and a fun new puzzle to figure out.\u201d<\/p>\n Growth spurts<\/p>\n Supermassive black holes, like galaxies themselves, come in all sorts of sizes. Some of these celestial objects are truly humongous, with a mass equal to billions of Earth\u2019s suns. Others are still big, but slightly less so, with a mass millions of times larger than the sun.<\/p>\n For years, many scientists studying the gravitational wave background didn\u2019t believe those smaller black holes mattered, Comerford said. They were too little, the thinking went, to make a meaningful contribution to the gravitational wave background.<\/p>\n Comerford and Simon weren\u2019t so sure.<\/p>\n In part, that\u2019s because galaxy mergers can be messy affairs. When two galaxies come together, gas from those galaxies begins to funnel toward the supermassive black holes at their centers. This gas forms a doughnut-shaped cloud outside the black holes spiraling around each other. Some of that gas falls back into the black holes and makes them larger in the process.<\/p>\n But previous simulations suggested something surprising: The black holes in a merging pair may not grow at the same pace.<\/p>\n \u201cThe more massive black hole sits closer to the center of the doughnut where there isn\u2019t much gas,\u201d Comerford said. \u201cThe smaller black hole is further out, so it\u2019s closer to where the gas is.\u201d<\/p>\n The beginning<\/p>\n That difference in growth rates, or what the scientists call \u201cpreferential accretion,\u201d could matter a lot.<\/p>\n In the current study, Comerford designed a detailed set of equations capturing the physics of how galaxies merge. The group then adjusted those equations to make smaller black holes grow 10% more than larger ones.<\/p>\n That single tweak was enough to make estimates of the gravitational wave background line up with measurements from the NANOGrav experiment.<\/p>\n \u201cThey start out little, but because the little ones grow the most, they shouldn\u2019t be discounted,\u201d Comerford said. \u00a0<\/p>\n She noted that the study doesn\u2019t completely solve the mystery: Her team has launched a new effort to observe real galaxies in the act of merging to see if their physics line up with what the simulations found.<\/p>\n The effort, she said, is part of a larger push to understand some of the most fundamental questions about the universe. That includes how \u201cprimordial\u201d galaxies at the dawn of the universe, which were tiny and made up mostly of gas, may have built the gigantic black holes that exist today.<\/p>\n \u201cI\u2019ve spent my career studying supermassive black holes, and we don\u2019t even know how they form,\u201d Comerford said.<\/p>\n","protected":false},"excerpt":{"rendered":"Image collected by the Hubble Space Telescope of an object called NGC 2623, which is made up of…\n","protected":false},"author":2,"featured_media":273679,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[77],"tags":[18,19,17,133],"class_list":{"0":"post-273678","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-science","8":"tag-eire","9":"tag-ie","10":"tag-ireland","11":"tag-science"},"share_on_mastodon":{"url":"https:\/\/pubeurope.com\/@ie\/115858228289779675","error":""},"_links":{"self":[{"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/posts\/273678","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/comments?post=273678"}],"version-history":[{"count":0,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/posts\/273678\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/media\/273679"}],"wp:attachment":[{"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/media?parent=273678"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/categories?post=273678"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.europesays.com\/ie\/wp-json\/wp\/v2\/tags?post=273678"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}