Astrophysicist explains big GRAVITATIONAL WAVE discovery! Are they NEW PHYSICS or merging SMBHs?

for the first time ever astrophysicists have found evidence for gravitational waves there are light years long with really low frequencies these are ripples through space itself caused by cataclysmic events and massive objects disturbing the space around them now back in 2017 we saw the first ever detection of gravitational waves and these ones were more like kilometers long with much higher frequencies and we knew that those gravitational waves were coming from the merger of two black holes just a few tens of times heavier than the Sun but this month with all this new evidence that's been published a much longer wavelength gravitational waves we're not a hundred percent sure this time what's causing them our best guess is that it's pairs of supermassive black holes that are spiraling around each other on their way to merging when I say supermassive I mean anywhere from a million to tens of billion times more massive than the Sun but from the data released this month it looks like that might not be the whole story but there could be some new physics here that we could learn so it's incredibly exciting this result literally all me and my astrophysics colleagues can talk about so first of all if you're new here hi this is the channel where we dive into all of those astrophysics details but hopefully keep it at a level that's accessible to everybody in this video we're going to chat about how the evidence for these gravitational waves has been found with Pulsar timing arrays how these gravitational waves are different to the ones detected by ligo the gravitational wave detector we have here on Earth what the data shows us why it's slightly different from what we'd expect for supermassive black holes orbiting each other and also what other new physics we could learn here so let's start with how have they actually done this we need to break down what is a pulsar timing array so starting with pulsars which are spinning neutron stars these these are dead stars made entirely of neutrons they're formed in Supernova from the cause of massive stars when they die gravity crushes in on the core of the star that was there before and takes all of the protons and electrons in the atoms that were there before and forces them together to become neutrons and then packs them in as tight as they can physically go in one big Neutron crystal that's incredibly dense the neutron star that's left behind actually inherits all the spin of the star that was there before but because it's much denser it ends up spinning way faster just like when an ice skater pulls in their arms to make themselves spin faster they also have huge magnetic fields and what that does is produces a beam of radial light out of the poles of the Pulsar which as it spins then acts like a lighthouse sweeping that beam Across the Universe which we then detect as pulses of light hence pulsar which can happen anywhere from like a pulse every second or so to a pulse every millisecond or so that's how fast pulsars spin they're essentially like clocks you can set your watch to those pulses and they are incredibly precise which brings me to timing essentially what you do when you're studying these pulsars is you record the exact time of arrival of the pulses of the Pulsar for many many years but you also do it for many pulsars as well hence the array part of Pulsar timing array you keep an eye on lots of pulsars that are distributed across our galaxy The Milky Way in an array all separated by different distances now because pulsars are so regular like ticking clocks you know like what the rhythm of pulses should be from all of those pulsars across the Galaxy but if that Rhythm changes slightly you know that there's been a difference in the time between Pulses from a number of given pulsars and because speed times time equals distance and these are traveling at the speed of light which is a constant if the time changes then the distance is also changed as well and that can either be the distance between you know Earth and a pulsar or you know like a pair of pulsars in your array either way you're essentially triangulating the positions of these pulsars which should be fixed but aren't so what could have changed them well a gravitational wave stretching and squashing space itself now I know that a big point of confusion for people that I've you know seen being discussed on social media is how these gravitational waves that have been detected by Pulsar timing arrays different to the ones that were detected back in 2017. you know the first ones that we ever detected with ligo a detector here on Earth that uses lasers pointed down kilometer long arms to look for again a change in distance so how is this different well it's all to do with the size of your gravitational wave detector so the separation between the things that you're measuring the distance of the ligo it's pointing a laser down a big tunnel reflecting it off a mirror and bouncing back again that's around about a kilometer long with the Pulsar timing array it's literal light years separations between your pulsars and that means you're sensitive to different frequencies of gravitational waves but ligo they're much higher frequencies with wavelengths around about tens of kilometers or so and we know that those kind of gravitational waves come from the merger of two black holes that are around about sort of 10 up to 100 times the mass of the Sun but when you pulse our timing array the separation between your pulsars is Light Years so you're sensitive to gravitational waves of much low frequencies nanohertz in fact which means that you have a gravitational wave with the wavelength of tens of light years that means between one crest of the wave and the next you're weighing 10 20 30 years so you can only detect this type of gravitational wave on these huge scales and they're very different to the gravitational waves that we detect with ligo that is different as visible light is to radio light and in the same way they can tell us different things about the universe so this is why Pulsar timing arrays are so useful and it's also why people have been trying to do it for the past 20 years or so it's why they've had to actually observe pulsars for 20 odd years because you have to wait that long before one of these incredibly long gravitational waves actually fully passes through your array of pulsars now there's been many efforts on going around the world to do this and they're now all part of what's known as the international Pulsar timing array but they all individually released their data and Analysis last week first up you've got nanograph the North American nanohertz Observatory for gravitational waves which use the Arecibo telescope in Puerto Rico and still uses the green Bank telescope in West Virginia and the US to monitor their pulsars then you've got the parks Pulsar timing array using the parks radio dish in New South Wales in Australia then the European Pulsar timing array which uses five of the biggest radio telescopes across Europe who combined their data already with the Indian Pulsar timing array which uses the giant meter Wave radio telescope in Western India and finally you've also got the Chinese Pulsar timing array which used the 500 meter aperture spherical radio telescope in Southwest China which is honestly an absolute monster of a telescope now they have all claimed to have found a gravitational wave background signal with their Pulsar timing arrays so not an individual cataclysmic event like what ligo detects with you know the merger of two black holes instead they've got evidence for sort of like a a hum or like a general Symphony of just many gravitational waves of these very long wavelengths that could be coming from many different sources it's an incredible achievement one decades in the making that people declared you know was it even possible as well decades ago it's a true proof of concept that we can do this as well and we can actually detect this background signal of gravitational waves I really enjoyed this analogy from Katie Mack on Twitter she said the Earth is a ship on a cosmic sea and every once in a while we're hit by a big wave and we know something went by but now for the first time we can start to see the choppiness of the entire ocean and we're learning what else lives in our sea now the evidence they have comes from the fact that this background hormones background signal causes correlations in your Pulsar timing array data specifically something known as the hellings Downs correlation which essentially says that if you pick any pair of pulsars in your array and there's been a gravitational wave that's passed between them then the delay caused by that gravitational waves the time that the pulses arrive that delay should be correlated with the distance between your Pulsar so let's look at the data I'm showing the nanograph data here not for any particular reason except for the fact that the plots are neater and I enjoy that and we've got two ways of representing the correlations that you'd expect on the left hand side we've got what's known as the power Spectrum so the correlation in the data is shown by the blue line and the sort of shaded region around it is your uncertainty and then if there's a hellings Downs correlation you'd expect that line to fall in the gaps of the gray violin points so the blue line goes through those gaps and that's the evidence for the gravitational wave background signal there's one more line on this plot is this black dashed line but I'll get back to that in a second first let's look at the right hand side here on the x-axis you've got that separation of your pulsars against how strong the correlation is in that time time delay and it's potted for many different pairs of pulsars at different separations the data again is shown in the blue points now the black dash line shows the shape that you'd expect from the hellings Downs correlation and it's a pretty good fit and it's yet more evidence for that gravitational wave background signal it wasn't just Nano grab that saw this though all the other Pulsar timing array collaborations saw this two varying different levels of significance and sort of good fit so here's the parks Pulsar timing array data here's the European and the Indian Pulsar timing array data combined and then the Chinese Pulsar timing array data and uh now just a little side note because I've got some gossip for you all right so apparently in the collaborations themselves there was a little bit of a disagreement between people on what threshold they should pick before declaring that this result was statistic mystically significant I the result had a very low probability of you know just being Randomness in the data that number or threshold is referred to as Sigma the Greek letter Sigma and in astronomy we use the threshold three sigma so something has a 1 in 400 chance of just being Randomness in your data if your statistical significance is over three sigma you're like yep I got my result I can claim that that's real whereas in particle physics they have a higher threshold you have to reach what's known as five Sigma so a one in two million chance that it could just be Randomness in your data that's created your signal and apparently there was a lot of disagreement about whether they should wait for three sigma or for Five Sigma because the Nano grab result was at four Sigma significance so a one in 20 000 chance that the signal they've detected is just Randomness in their data so take from that what you will but you know I'm an astrophysicist I'm a three sigma gal so I thought it was great to see the detection of this signal in the data of course the big question is what is actually causing this background hum of gravitational waves across the universe that we're detecting with these Pulsar timing arrays well let's go back to that black dash line that I ignored before on the left hand side of this plot this is what you'd expect the correlation in the data to look like if this background hum is due just to pairs of supermassive black holes spiraling around each other and so if that's the case the data in the blue shade aided region should be close to the black dashed line which for lower frequencies it is but above tens of nanohertz also they start to move away from each other and all of a sudden that doesn't look to be a good fit so what is going on here why did all the you know big media Outlets report it as most likely to be these supermassive black holes orbiting each other well we know that there is a supermassive black hole at the center of every Galaxy and we know that galaxies merge together and that process is very common so pairs of supermassive black holes should also be common especially after these Galaxy mergers although we actually don't know if it's possible for the supermassive black holes to actually merge because we don't know of any process that can cause them to lose enough of the rotational energy that's keeping them in orbit around each other to actually like shrink their distance and merge we know of a process that can do it when they're quite far apart still but when they get within a distance or around about three light years or so we don't know of any process that the supermassive black holes could actually lose energy to eventually merge this is actually called the the last the final parsec problem could probably make a whole video about it actually if you want me to do that you pop a comment below but aside from that the concept of pairs of supermassive black holes is adding nothing new to our current models of the universe we're pretty sure they've got to exist and so that is always going to be the first point of call going with something you know to describe what you found and so the nanograph team looked into this too they showed that models of these supermassive black hole binaries could reproduce this background signal that they'd seen but not quite perfectly enough to be above that threshold to be statistically significant like what we talked about before just to compare this again with ligo it was practically a perfect match of model from general relativity to the data that they recorded so it was very clear back in 2017 that those gravitational waves were from the merger of two less massive black holes whereas in this case it doesn't seem to be obvious at all one option that the nanograph team actually investigated was okay maybe it is that background hum but there's also maybe some pair of supermassive black holes that are nearby in a nearby Galaxy that are just louder than that background hum and that sort of offsetting your data slightly they did find some weak evidence that there might be two possible pairs of supermassive black holes nearby but eventually ended up dismissing them both so that leads us instead to taking a big leap into the unknown oh starting to consider some new physics that might explain this background signal so these are completely hypothetical ideas that we have no evidence for yet but if you run the maths they should be able to generate these very low frequency long wavelength gravitational waves so for example inflation which is a period of insanely rapid expansion of the universe we're talking exponential increase where in just a tiny tiny tiny fraction of a second the universe expanded by a factor of a hundred trillion trillion so if you had any just tiny Quantum fluctuations on the individual particle levels before inflation those fluctuations can be Amplified until they're you know huge mega structures in the universe which can then generate these long wavelength gravitational waves another option is types of cosmic strings so these are permanent wrinkles in space itself that are a relic of again the very early days of the universe but now after the universe has expanded they can be the size of the entire Universe even if they're only the width of a single proton but again they'll be big enough to generate these very long wavelength gravitational waves then you've also got phase transitions so that's Mata transforming from one phase to another as the universe cools so when it was very hot and dense you would have just had the most basic building block of matter ever which is just a quark right the most basic subatomic particle but is the universe called you could then bind those quarks together to make protons the best atoms could form for the very first time that would have been a huge cataclysmic event in the very early days seconds of the universe's life and again it's thought that could generate these very long wavelength gravitational waves so again the nanograph team looked at this they modeled the gravitational wave background signal you would expect from a number of possible new physics models and compared How likely they were to these pairs of supermassive black hole binaries just being responsible for the background signal that's what's shown in the blue points and then also they compared like a combo of some new physics plus the supermassive black hole binaries shown by the red points the stat they're using is called the Bayes factor and essentially the higher that number is the more likely that model is to explain the data than just the supermassive black hole binaries alone and when that value starts to get in the like 10 to 100 range is when people start to pay attention so you can see there's an inflation related model and a cosmic string related model that are some of the strongest candidates for new physics seeming to fit better with that gravitational wave background signal than the supermassive black hole binaries alone which is very interesting this opens the door to studying the very early Universe before gravitational waves the only thing we had to study the universe with was light right whether that was visible light radio light x-ray UV whatever it was and so we've only been able to study the universe back to when it was just 370 000 years old with the cosmic microwave background the oldest light in the universe the first light given out but it's looking like with gravitational waves will have the opportunity to see beyond that to the very first seconds fractions of the seconds of the universe's life that is a game changer it's nothing definitive yet because there's a lot of assumptions and caveats that go into you know building these new models of physics and generating what we think the gravitational wave background signal can look like it even goes into you know the supermassive black hole binary models as well so things could still change but there is one more test that you can do and that is to test how isotropic the gravitational wave background signal is I is it the same in all the directions that you look or is it slightly different when you look over there versus when you look over there because if it's something to do with the early universe that's imprinted you know due to inflation and then sort of like magnified to huge scales then it should be the same in every single direction that you look it should be isotropic but if the signal is coming from these pairs of supermassive black holes these binaries that are orbiting each other then it should be different in the different directions that you look because it'll depend on how many binary pairs are over there compared to over here it should be an isotropic and so of course the nanograph team did that test too and they couldn't find any evidence for anisotropy it seems as if the gravitational wave background signal that they've detected is isotropic it's the same in all directions suggesting it's some new physics but as always we can't get excited just yet because nanograph for example they only used 67 pulsars across the Milky Way ideally you need more pulsars to test this properly which is lucky because we have all of these individual collaborations that are now ready to combine their data sets so nanograph the European PTA the Indian PTA the parks PTA and the Chinese PTA will all combine to give us not only more pulsars in the array but crucially more separations between pulsars and if you have that the hope is that they'll be better constraints and better statistics on fitting these models to the data you'd hopefully be able to confirm that gravitational wave background signal at much higher confidence so go over that five Sigma threshold level that we talked about before and do more tests to determine what scenario explains this gravitational wave signal best so why this is such a big deal and why ozash business are so excited is because this really does open the door to a whole new realm of studies you know a new way of studying the universe and not only that but hopefully being able to study a part of the universe's history that we've never been able to before those first few seconds of the universe's life that we know so little about we're really only going off what our theoretical models can tell us but either way it's either going to be down to the supermassive black hole binaries in which case we're going to learn so much about how supermassive black holes and galaxies form and evolve together and which came first the galaxy of stars or the supermassive black hole and the Galaxy stars formed around it or we're gonna learn some brand new physics in the next couple of years distance between Earth and a pulsar Pulsar or it can be the distance between you know two pairs of pulsars in two pairs to a pet to Port sales spaces Hardware it's harder between Earth and a pulsar or you know two individual pulsars in your array of pulsars I said pulsars so many times that we have no evidence for yeah but if you run the maths they could generate these low frequency I think I just like blanked out there I don't know what I did I was like that was a very long pause my brain just like call what is this hello phone call was asking me to be on Sky News but not talking about this talking about Virgin Galactic or something and I'm like I'm I'm not your person sorry I'm trying to think if I've covered it all like there were so many papers to read it took me so long to put this video together I feel like I've been in this room a very long time going slightly Stir Crazy

Channel: Dr. Becky

Views: 621,499

Keywords: pulsar timing array, gravitational waves, gravity, einstein, black holes, supermassive black holes, SMBHs, merging, merger, pulsars, neturon stars, jocelyn bell burnell, dr becky, becky smethurst, rebecca smethurst, NASA, nanograv, EPTA, India PTA, chinese PTA, parkes PTA, parkes radio dish, arecibo

Id: BUmJxZ7PQzw

Length: 24min 42sec (1482 seconds)

Published: Thu Jul 06 2023