The Gravitational Wave Background - Sixty Symbols

what's going on you're holding some papers what's the big news in your world yeah so the big news uh in my world is they've discovered they've seen or they think they've seen gravitational wave background so this is a series of experiments a bunch of experiments but a bunch of different Pulsa timing arays think they've seen a background of gravitational waves and this this will be the first time people have seen anything like this this was in July and it's really taken the taken the field by storm since then it all happened on one day all of the different Pulsar timing array experiments put out papers which just arrived in our inbox that morning and they all had essentially the same picture which was that they'd all seen this background of gravitational waves which is something people had theorized about existing for a long time but suddenly was seen how can they make this clim oh great yeah so they have the most ingenious uh observation uh that we live on earth and surrounding us we have our galaxy in our galaxies we have these uh pulsers several pulsers these pulsers are very fast rotating neutron stars and they have this beam of light like a lighthouse and once in a while the light crosses our path and amongst these pulsers there are some pulsers which are actually millisecond so that means in a second they rotate thousand times and there are huge objects in the sky as it's quite fast rotation that's crazy fast isn't it super crazy fast okay and then they are very very stable clocks they're almost in fact until a decade ago uh p where these millisecond pulsers were the best clock that we ever had okay so they are great time Keepers and then imagine a gravitational wave passing through them and a gravitational wave is actually a stretch and compression of space and it's a special kind of stretch and compression which is different from a soundwave which is also a stretch and compression of space it's just that in Soundwave compresses and stretches a medium let's say air water gravitational wave can stretch and compress the fabric of SpaceTime okay the second thing second difference is that it's what we call a quadripolar compression which means uh that uh if a gravitational wave is passing like that then it will compress the space in One Direction then it will stretch simultaneously in the orthogonal perpendicular Direction now if we are on earth and we have bunch of uh pulsers around them we know they timing pretty well that great clocks as the gravitational wave passes through them let's say the gravit W stretches in that direction so couple of pulsers which are like this they get stretch like that and the other two pulsers which are like on the other side perpendicular to them they get compressed towards Earth so you would see that this Lighthouse signal which is quite a good clock it comes to you from these two directions quicker than expected and the other two directions slower than expected and that's a tail tail signature of gravitation waves so they have absorbed these pulsers for about 15 years and they have seen seen a yearly variation in this kind of systematic pattern so we've seen gravitational waves before but the ones we've seen by ligo and Virgo are a few kilometers long in terms of their wavelength and this one is slightly as long so it's a completely different game what could have produced them so definitely not the same things as ligo and Virgo they're colliding black holes aren't they exactly right these colliding black holes they're a few kilometers wide themselves and maybe a few kilometers apart or tens of kilometers so there's no way they can make a wave on Lighty scales there's a bunch of different possibilities and one possibility is just a massive massive black hole a super massive black hole which is self is let's say a billion times bigger than the previous black holes we've seen and so pairs of those colliding could potentially cause this signal but there's a bunch of other predictions as well which could cause this signal so the waves that we've seen are a background it's not like we've seen them coming from a particular direction um they they look more like the waves on the sea not like a tidal wave but like in general there's waves people have thought that it might be a bit like the COS microwave background where we have light coming from from all directions from the from the early big bang and this would be like a similar thing but in gravitational waves so we have a gravitational wave background it's kind of like the the waves of the sea stretching and squeezing SpaceTime quarks in the early Universe at very very high temperatures uh in in the very early Universe just after the hot big bang are expected to be um free and kind of move around freely happily but then as the universe cooled at some point those quarks got got trapped together into hadrons into protons and into neutrons and that happened at kind of a moment uh in time when the universe was about 10 to 12 Celsius or kelvin or whatever uh obviously roughly the same at that point if this happened in a kind of dramatic way if if all the hadrons when they were first produced uh happened in what's called A first order phase transition then then that could have produced gravitational waves or would have produced gravitational waves and those could uh be the ones we're seeing today um so so that's another possibility another leading candidate to explain the signal uh the signal is bit too steeper than what black hole models expect so we want some kind of signal which goes Steep and high in amplitude they can also caused by large Quantum fluctuations in the early universe so we see all these galaxies today in the universe and we are very s that all these galaxies are formed by tiny Quantum fluctuations in the early universe which was created by inflation once in a while inflation can create large Quantum fluctuations especially large Quantum fluctuations of small wavelength and when these fluctuations uh in the later Universe they start to push around the plasma a lot once in a while the plasma can collapse to form primordial black holes and the overall effects would be you have this compressed Plasma in the early universe and any kind of compressed plasma is a great source for gravitational waves so you would also see gravitational waves because of compression of plasma and these are what we call scalar induced gravitational waves gravitational waves induced by scalar density fluctuations or sound waves they're probably one of the best candidates that fit the data so that they best fitting model so far and this a topic that I work on so I'm also super interested and excited about it is every theorist looking at this data and thinking oh this fits my idea and everyone's trying to shape it to fit their idea would have been nice but in this paper uh in the paper where they announced that what kind of new physics can fit this model they do a comparison between different theories and they say which Theory fits better and which doesn't and there are some theories which fit better than other theories which don't fit that good so there is actually a hierarchy of comparison between different theories and uh what we have seen is Cosmic super strings and these uh scalar induced gravitational waves they fit the best definitely better than supermassive black holes at least 50 times better so there's a factor they compute says the probability of explaining the data better than purely supermassive black holes and for these models some of them are 50 60 factors higher so quite well explaining so the pictures show the signal this picture for example shows the signal in green and the prediction from super massive black holes in blue and you can see they don't quite agree but they're not that far apart either this plot at the beginning is partly what motivates other new physics explanations like this Quark had transition you can see this this blue area has quite large errow bars and really we need to understand super massive black holes better if we want to to know if they can explain the signal if these waves are passing through everywhere all the time and they're powerful enough to affect Pulsar why aren't they affecting you and me why don't we feel it why don't we get ripped apart by it like it seems like these are oh well they're powerful but gravity is a weak Force so I'm being held as a as one body through electromagnetic force like my muscle Etc that's much much stronger than gravity so if a gravitational wave passes through me I will get distorted just like the pulsers uh but I won't feel it because it's negligible compared to My overall size however if you look at Earth and pulser they're not held by electromagnetic force they are H they just floating around in space freely so a little bit of change in their distance due to gravitational waves can be caught by a delay in a signal arrival time so that's how you can use big scale Galactic scale experiments to cast them I know you're a theorist this is a result of some experimental work as well clearly yeah is this like a Triumph for the experimentalists or is this like a great moment for the theorists who's lording it over who at the moment um yeah good one I guess this is a Triumph of the experimentalists and the theorists are just hoping that the experimentalist can do yeah I mean as we get to the 16 and the 20year data set hopefully we'll have a bit more data on like the spectral shape of this or the type of waves the amplitude of these waves and so on and then we'll be able to tell which theory is best uh but for now um it's difficult to tell like the theorists have lots of predictions and the experimentalists are leading the way what's the next step in this area of research is it going to come from theorists do you need more experiments what are we all waiting for now what's the next hurdle yes from both sides first of all we have this observational tool they have this amazing set of 68 millisecond pulsers so they can test the timing even better and better over the time as theorist of early Universe uh we want to figure out all the different processes which can give rise to a gravitational wave signal which matches with what we have seen okay and there are several possible scenarios uh which would uh emit gravitational waves in the early Universe when Universe was younger than 1 second old much younger than 1 second or so it's a really early Universe in fact all of those topics are pursued here uh and and and some of us walk actively on this topic so we have to figure out what the signal is expected from a theory so that we can test directly I'm interested in the early Universe I'm interested in particle physics I want to know what the universe is made of at the smallest scales like what a quarks made of anything like these kind of questions and with the Large Hadron Collider you can only Collide stuff so hard and eventually you run out of being able to turn the energy up and if we want to find out more about particle physics we're going to have to look now to the early Universe to cosmology and with light we're stuck at the cosmic microwave background because before that time at higher energies in the cosmic microwave background the universe was opaque so light couldn't pass through it but with gravitational waves gravitational waves pass freely through even the densest things they pass through neutron stars for example so we have like a direct signal from way higher energy things going on and possibly direct signals that can tell us about what the universe is made of at the smallest scales these what 60 yard Pulsar being used as an instrument like this was pretty clever isn't it like yeah incredibly clever I think it was Steven deweer that came up with this like absolutely amazing idea so inspired I don't know I guess it's the first time we've had a bit of experimental kit that's so much bigger than the earth do check out the links on screen and in the video description for more on the topics we've covered today gravitational waves large hron colada all that good stuff and if you think you'd like to study or work in the school of physics at the University of Nottingham well there's a link in the video description about that as well they start going so rapidly around one another they begin to approach the speed of light in fact something like 60% of the speed of light

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Id: S85D-rdTyCw

Length: 11min 40sec (700 seconds)

Published: Tue Nov 28 2023