[{"command":"openDialog","selector":"#drupal-modal","settings":null,"data":"\u003Cdiv id=\u0022republish_modal_form\u0022\u003E\u003Cform class=\u0022modal-form-example-modal-form ecl-form\u0022 data-drupal-selector=\u0022modal-form-example-modal-form\u0022 action=\u0022\/en\/article\/modal\/7041\u0022 method=\u0022post\u0022 id=\u0022modal-form-example-modal-form\u0022 accept-charset=\u0022UTF-8\u0022\u003E\u003Cp\u003EHorizon articles can be republished for free under the Creative Commons Attribution 4.0 International (CC BY 4.0) licence.\u003C\/p\u003E\n \u003Cp\u003EYou must give appropriate credit. We ask you to do this by:\u003Cbr \/\u003E\n 1) Using the original journalist\u0027s byline\u003Cbr \/\u003E\n 2) Linking back to our original story\u003Cbr \/\u003E\n 3) Using the following text in the footer: This article was originally published in \u003Ca href=\u0027#\u0027\u003EHorizon, the EU Research and Innovation magazine\u003C\/a\u003E\u003C\/p\u003E\n \u003Cp\u003ESee our full republication guidelines \u003Ca href=\u0027\/horizon-magazine\/republish-our-stories\u0027\u003Ehere\u003C\/a\u003E\u003C\/p\u003E\n \u003Cp\u003EHTML for this article, including the attribution and page view counter, is below:\u003C\/p\u003E\u003Cdiv class=\u0022js-form-item form-item js-form-type-textarea form-item-body-content js-form-item-body-content ecl-form-group ecl-form-group--text-area form-no-label ecl-u-mv-m\u0022\u003E\n \n\u003Cdiv\u003E\n \u003Ctextarea data-drupal-selector=\u0022edit-body-content\u0022 aria-describedby=\u0022edit-body-content--description\u0022 id=\u0022edit-body-content\u0022 name=\u0022body_content\u0022 rows=\u00225\u0022 cols=\u002260\u0022 class=\u0022form-textarea ecl-text-area\u0022\u003E\u003Ch2\u003EGravitational waves helping to expose black holes, dark matter and theoretical particles\u003C\/h2\u003E\u003Cp\u003EAlmost 100 years after Einstein predicted their existence as part of his theory of general relativity, \u003Ca href=\u0022https:\/\/horizon-magazine.eu\/article\/gravitational-waves-detected-scientists-announce.html\u0022 target=\u0022_blank\u0022 rel=\u0022noopener noreferrer\u0022\u003Egravitational waves were first detected in 2015\u003C\/a\u003E by scientists working on the Laser Interferometer Gravitational Waves Observatory (LIGO), earning them the Nobel Prize in physics.\u0026nbsp;\u003C\/p\u003E\u003Cp\u003EThe faint disturbances the giant instrument picked up were created by two black holes crashing into one another 1.3 billion light years from Earth. As these two super-heavy objects collided, they deformed space and time.\u003C\/p\u003E\u003Cp\u003E\u2018The deformation propagates out like ripples on a lake,\u2019 explained Professor Paolo Pani, a theoretical physicist at Sapienza University of Rome, Italy. \u2018These are gravitational waves.\u2019\u003C\/p\u003E\u003Cp\u003EAll objects with mass will create their own slight dip in the fabric of spacetime, creating what we call gravity. But only cataclysmic events involving the heaviest objects, such as black holes and neutron stars, can create gravitational waves big enough to be detected on Earth. They radiate out across the universe at the speed of light, passing through almost everything in their path.\u003C\/p\u003E\u003Cp\u003EBut the ability to detect these waves is also now providing astronomers with new ways of looking at the universe. Prof. Pani is leading the \u003Ca href=\u0022https:\/\/cordis.europa.eu\/project\/rcn\/212069\/factsheet\/en\u0022 target=\u0022_blank\u0022 rel=\u0022noopener noreferrer\u0022\u003EDarkGRA\u003C\/a\u003E project in an attempt to use gravitational waves to probe some of the biggest mysteries of the universe, including heavy exotic stars, dark matter and black holes themselves.\u003C\/p\u003E\u003Cp\u003EPreviously astrophysicists have been forced to infer the presence of black holes by looking at the behaviour of the material around them. Thought to be the super-heavy remains of collapsed stars, the gravity they produce is so great that not even light escapes. Anything that passes the boundary of a black hole, known as the event horizon, stays there.\u0026nbsp;\u003C\/p\u003E\u003Cp\u003E\u2018This is why we cannot see black holes,\u2019 said Prof. Pani. \u2018Instead we see an absence of light from them. Black holes are a big mystery still.\u2019\u0026nbsp;\u003C\/p\u003E\u003Cp\u003EGravitational waves, however, are allowing scientists like Prof. Pani to view them directly. \u2018They are sort of a messenger of the spacetime around these objects, without using any intermediate,\u2019 he said.\u003C\/p\u003E\u003Cp\u003EBy studying the features of these waves it is possible to obtain information about the mass, rotation, radius and speed of these previously invisible objects. \u2018The goal of our project is to understand the gravitational wave observations from very compact objects, so we can rule out or confirm other types of objects,\u2019 said Prof. Pani.\u003C\/p\u003E\u003Cp\u003E\u003Cblockquote class=\u0022tw-text-center tw-text-blue tw-font-bold tw-text-2xl lg:tw-w-1\/2 tw-border-2 tw-border-blue tw-p-12 tw-my-8 lg:tw-m-12 lg:tw--ml-16 tw-float-left\u0022\u003E\n \u003Cspan class=\u0022tw-text-5xl tw-rotate-180\u0022\u003E\u201c\u003C\/span\u003E\n \u003Cp class=\u0022tw-font-serif tw-italic\u0022\u003E\u0026#039;Dark matter interacts very little with anything else, so is very difficult to test in the lab.\u0026#039;\u003C\/p\u003E\n \u003Cfooter\u003E\n \u003Ccite class=\u0022tw-not-italic tw-font-normal tw-text-sm tw-text-black\u0022\u003EProfessor Paolo Pani, Sapienza University of Rome, Italy\u003C\/cite\u003E\n \u003C\/footer\u003E\n\u003C\/blockquote\u003E\n\u003C\/p\u003E\u003Cp\u003EAccording to general relativity, the merger of two very compact objects \u2013 such as white dwarfs, neutron stars or black holes \u2013 will cause the final object to collapse to form a black hole. But there are alternative theories that predict they could also form objects of a similar mass and radius to black holes, but without an event horizon. These mysterious compact objects would therefore have a surface that would reflect gravitational waves.\u003C\/p\u003E\u003Cp\u003E\u2018If there is a surface, after a merger of the objects, there should be gravitational wave echoes, so a signal that is reflected from the surface,\u2019 Prof Pani explained. It should be possible to detect these echoes in the signals picked up here on Earth.\u0026nbsp;\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EDark matter \u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003EThere is another explanation, however, that would lead to black holes unexpectedly producing echoes or other unexplained gravitational wave features \u2013 they could be sitting in a bath of dark matter, a hypothetical form of matter that has yet to be seen but is thought to account for 85% of all matter in the universe. This too could produce a distinctive tell-tale gravitational wave.\u0026nbsp;\u003C\/p\u003E\u003Cp\u003E\u2018Dark matter interacts very little with anything else, so is very difficult to test in the lab,\u2019 said Prof. Pani. But by looking for distinct signals in the gravitational waves it could allow scientists to \u2018see\u2019 it for the first time.\u003C\/p\u003E\u003Cp\u003ESome gravitational observations can only be explained either by the presence of dark matter, which we cannot see, or by changing our laws of gravity. Professor Ulrich Sperhake, a theoretical physicist at the University of Cambridge, UK, and lead scientist in the \u003Ca href=\u0022https:\/\/cordis.europa.eu\/project\/rcn\/199098\/factsheet\/en \u0022 target=\u0022_blank\u0022 rel=\u0022noopener noreferrer\u0022\u003EStronGrHEP\u003C\/a\u003E project, described gravitational waves as a \u2018new window onto the universe\u2019 that could help us unravel these mysteries.\u0026nbsp;\u0026nbsp;\u003C\/p\u003E\u003Cp\u003EIf there is all this dark matter hanging around two black holes as they merge, then this would soak up energy. It would mean that in a black hole collision like that detected by LIGO, the gravitational waves would look a bit different than it would without dark matter.\u0026nbsp;\u003C\/p\u003E\u003Cp\u003EOne observational puzzle they could shed light on is why galaxies rotate faster than their size suggests they should. \u2018The speed of rotation is related to the mass that is inside,\u2019 said Prof. Sperhake. So if a galaxy is spinning faster than the mass we can see, there are two possible explanations: we either need to alter our fundamental theories of how gravity works or there is dark matter in the galaxies that we cannot see.\u003C\/p\u003E\u003Cp\u003EAn idea Prof. Sperhake is investigating is to extend Einstein\u2019s general relativity with a new theory, dubbed scalar tensor gravity. This suggests that the universe is filled with an extra field \u2013 similar to a magnetic or electrical field \u2013 that has yet to be detected.\u0026nbsp;\u0026nbsp;\u0026nbsp;\u0026nbsp;\u003C\/p\u003E\u003Cp\u003EIt would mean that the supernova explosion of a dying star would not only be visible as a burst of gravitational waves, but there would be an afterglow of gravitational waves that we might detect. We could direct LIGO to regions of the sky where stars have exploded \u2013 known as supernovae \u2013 to try to detect such an afterglow from the scalar field that may persist centuries after the actual explosion.\u003C\/p\u003E\u003Cp\u003ESeparately, Prof. Sperhake is investigating if dark matter could be explained by theoretical subatomic particles called axions. He is trying to model what the echoes of gravitational waves from black holes might look like if these particles are present.\u0026nbsp;\u003C\/p\u003E\u003Cp\u003E\u2018I would say axions are one of the best candidates for dark matter,\u2019 he said. The next step is to apply his models to the data that LIGO gathers to see if theory and observation are a match.\u0026nbsp;\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EBeautiful theory \u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003EDr Richard Brito joined Prof. Pani\u2019s group in Italy earlier this year as part of his own project, \u003Ca href=\u0022https:\/\/cordis.europa.eu\/project\/rcn\/215102\/factsheet\/en\u0022 target=\u0022_blank\u0022 rel=\u0022noopener noreferrer\u0022\u003EFunGraW\u003C\/a\u003E to use gravitational waves to test the existence of axion particles. But he will also be using them to test Einstein\u2019s theory itself and whether it may be incorrect at vast scales.\u0026nbsp;\u003C\/p\u003E\u003Cp\u003E\u2018If we see objects almost as compact as black holes but without an event horizon, that means that general relativity is wrong at those scales,\u2019 he said.\u0026nbsp;\u0026nbsp;\u003C\/p\u003E\u003Cp\u003EIt could have important everyday implications. The theory of general relativity is crucial to the daily operation of GPS for example. But finding that Einstein\u2019s theory breaks down at large scales does not mean it should be thrown out. Rather, an addendum might be needed.\u003C\/p\u003E\u003Cp\u003E\u2018You\u2019d have a hard time matching the mathematical clarity of Einstein\u2019s theory,\u2019 said Prof. Sperhake. \u2018It is not only amazing because of all the fantastic predictions it does. It has the appeal of being a beautiful theory. And physicists interestingly regard beauty as an important ingredient in a theory.\u2019\u003C\/p\u003E\u003Cp\u003E\u003Cem\u003EThe research in this article was funded by the EU. 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