Cosmology in Crisis? Confronting the Hubble Tension

[Music] thank you thank you so much and welcome to this exploration of the biggest mysteries of the big bang you now we learned way back in 1929 that space is expanding and ever since we have been trying to understand the rate of cosmic expansion as this gives us insight into so many things into the age of the universe the future of the universe and perhaps even more importantly the rate gives us the capacity to refine our mathematical theories right to to test their capacity to describe the Cosmos on the largest of scales and for a while things were going really well right in recent times just about all of our observations they they were converging on the same rate of cosmic expansion and that rate was perfectly in line with our theoretical understanding so all was good but then what has happened yet more recently is that our observations have become so refined that a kind of mismatch has begun to surface between the expansion rate measured directly right by looking out in space to relatively nearby objects and in that way measuring the rate of expansion versus the expansion rate that we can get by measurements of the early universe and extrapolating those measurements to current times to figure out what the expansion rate should be according to that approach and the mismatch between these two approaches it's called the Hubble tension and some have actually imagined that this tension this slight mismatch might be the crack in modern cosmology that leads us to a completely different understanding of the origin and the history of our universe now the spoiler alert is many of us think that this tension will ultimately be res solved it will give us a deeper understanding of cosmology but likely not a revolution in our thinking about the origin of the universe and our guest for this conversation is someone who has spent a lot of time and a lot of energy and a lot of effort trying to figure out the expansion rate of the universe and trying to resolve this tension and so it gives me great pleasure to Welcome to our stage Wendy fredman who is the John and Marian Sven University professor in astronomy and astrophysics at the University of Chicago and a fellow of the Royal Society her research in observational cosmology and our current projects involve measurements of the Hubble constant thank you for being here thank you for having me so how long have you spent trying to figure out the expansion rate of the universe well since I'm only 42 I guess I started when I was two so it's been a dominant issue in your professional life to figure out the expansion rate of space is that a fair assessment that's a fair assessment I began pretty much when the Hubble Space Telescope was launched or the years leading up to that and became actively involved in this field and then I gave it up for a little while turned to other things but it became interesting again when this tension surfaced right yeah and just as a as a quick spoiler alert for where we are going your thinking right now is that things will ultimately resolve and there's going to be a deeper understanding but not we're going to have to throw away past understanding I I don't see evidence at the moment for an overthrow of our standard Theory I think we don't know where this will ultimately resolve but I think the challenges are severe enough that we need much better data to ascertain at the one in a million chance that this is wrong and that and something to don't see that in the DAT at the moment so what I wanted to do is just to put this issue of the Hubble tension in context is take a step back and go through the history of attempts to measure the expansion rate over time and see how that number has changed from the earliest days right up to the present and then discuss where you think we're going to go say in the next few years so taking a step back of course it all ultimately comes from the work of Albert Einstein who gave us in his beautiful paper on the general theory of relativity well maybe we should just read through the paper together right now in German yes exactly that that would be challenge for me for sure but the basic idea how do you I mean I usually describe it you know using the traditional analogies of the trampoline or rubber surface I mean do you have a favorite way of thinking about what Einstein taught us Einstein taught us that uh space is curved and and the way I like to think about it is the the famous saying that matter is what tells space how to curve and space tells matter how to move so Einstein solved this problem that Newton had of action at at a distance or that we needed a prime mover to keep planets in orbit about the Sun and and so Einstein really married those things in a way that changed our perception of of yeah we have this nice little visual I think really emphasizes what you were just saying you know this idea that you know matter is doing just what you said it's curving the environment and then the curved environment is causing the planets to move in particular trajectory so it's a beautiful geometric way of thinking about the fabric of space now when we learn in 1915 theoretically at least that space and time can warp and curve in 1919 observationally confirmed through the famous Eclipse observations then others start to begin to think about applying this idea not say to the curvature around the Sun or the Earth but to the entirety of the cosmos and people have in mind are like fredon and lametra and it's always nice to kind of see these folks because you know they were the real pioneers of the subject and interesting my understanding is that Einstein resisted this idea of an expanding Universe well he actually he said your mathematics is correct but your physics is atrocious yeah he was very down on it yeah now if Einstein would have said that to me I think it would have crumbled but fredman and uh and lamro who actually was the direct recipient of that particular comment this Jesuit priest who I guess approached Einstein at the 1927 solv conference with this calculation and got that reception from Einstein they stayed at it yeah and then of course it was observers right yeah well you have to remember that at the time we didn't have any evidence that the Universe was expanding right so what Einstein did originally in 1915 1917 was to force the universe to be static because we didn't know anything about the universe beyond the extent of the Milky Way galaxy yeah and that didn't come until later and so there was Alexander Freedman working out solutions to Einstein's equations and saying well wait a minute they're expanding and compacting solutions and in fact the universe wouldn't be stable uh with this cosmological constant yeah and then the Metra 5 years later independently worked that out but he also used data that Edwin Hubble had published a few galaxies but actually was vesto slier Data wasn't it it was both both he he did have a couple of of hubbles data points for the distances right and it was a big leap and he didn't plot the diagram right um which Hubble in 1929 then showed that there was this really uh very apparent yeah in fact I think we can even show a picture of the data so I think the the graph that you're referring to maybe the one where we see here this relationship between how far away a galaxy is and how fast it is moving but to get to that data there was one essential problem that Hubble needed to resolve which is how do you figure out how far away things are when you're looking in your telescope and this is an issue that we still face today we have more sophisticated tools but can you take us through the work you know I think it's nice to pull out the history here you know one individual in particular Henrietta Swan leit who doesn't really get a lot of air time when we when we talk about but vital contribution everything we do now in cosmology rests on her fundamental contribution can you just give us a feel for what that is yeah so at the turn of the last century we could map and astronomers have spent a lot of time mapping positions on the sky of of stars but you don't know the third dimension how far away is a star and so is the Star Bright because it happens to be nearby or is it faint because it's far away and there was just no way to know that and except for some very nearby stars and what henrieta levit did was she was studying stars in our nearby what we know is a Galaxy the large melenic cloud and its companion the small melenic cloud and she noticed that there were stars that were changing in brightness so most stars in our lifetimes if we look up at the sky they don't change they're they're boring they just they don't change and and what she noticed that is that over time scales of a couple days to maybe a hundred days yeah they were brightening and dimming in fact can we show I think we actually have a some of the data if we can bring that up so this is one photographic plate and then I guess she noticed that if you looked at the same region of the sky ey and if you overlay these and compare see in that upper right area when you go from one photograph to another there's one star that's bright and dark and bright and dark so that that's what she yeah so at the time the detector that astronomer used were these large photographic glass plates yes so they would take a series of these over many many nights and then she would compare the different nights and she could chart out how they were brightening and fading in brightness and what she noticed what she discovered was that there was a correlation very strong relationship between how bright the star was and how fast it was changing in its Luminosity yeah so maybe you can take us through this little visual of that so so so so these Stars these Sephia variables they're super giant Stars they're maybe 10,000 times as bright as the sun and their outer atmospheres are actually moving in and out and in and out and so as the atmosphere is is changing the the star is brightening as it's expanding and then uh the start turns around and then starts to fade again yeah and so the the mean brightness is what we now measure and that is correlated with the period of variation how long it takes to go through this cycle and is it so is it that the brighter the star is the longer that mean period between effectively it's just the larger Stars take longer to go through their oscillations and so the brighter the star the yeah the slower it's it's moving and so why is that vital to have that correlation so that provided the means then of assessing what this third dimension was so if we can measure how bright these seids are say nearby by some geometric technique so we know intrinsically how bright that star is then we can go to other systems the stars are obviously fainter the Luminosity falls off with distance as distance squared in fact and so if we can measure that change in brightness at a given period using her correlation then we automatically by the inverse Square law get the distance to that object and that just opened up a whole new field so now you no longer have to worry about trying to answer the direct question of is it bright because it's close or is it bright because maybe it's far away and it's super bright now you say just measure the the duration for it to go through that cycle and you know how far away it is and these what we call light curves are unique to seids this very sharp rise and brightness and a slow decline so once you've identified them you know you have a sepid you can measure this we now call it the levit law um 100 years later and oh by the way there was a New York Times obituary for Henrietta levit I think in March of this year so it's one of those obituaries the New York Times has recognized that there were women who did interesting things over the last hundred years but they didn't actually notice them at the time backst facto yeah an obituary I did not know that that's wonderful yeah wow and did they consult like with experts like you they did they did they oh that's fantastic that is great nice all right so this gives us a means of determining how far away things are but for Hubble's graph you also need a means of determining how fast they are moving what what Hubble discovered when he eventually realized there were seids in many of these nebuli that had been known for a couple hundred years before that that if he plotted the velocity of the Galaxy as a function of its distance from us there was a correlation in the sense that galaxies that were farther from us were moving faster so that's the velocity part the faster and that came from observations made by an astronomer named vestos slier slier right and what and he didn't really credit slier in that first paper no they're just there are the velocities I'll use them yeah and so but just take us through so the red shift idea just to sort of Go full circle here how do you determine the speed I guess by the degree to which certain spectral lines have been shifted to the red as that distant object is moving away the light that it emits gets stretched and that makes it more toward the red and the degree to what it shifted gives you insight into how fast it's moving away from you right so this eventually became the interpretation of hubbles data y married with Einstein's general relativity that this would give rise to an expansion of the UN of the entire universe Y and so the the red shifts or the velocities come they're they're simple to measure unlike the distances so you you uh different elements have different spectral features at a given wavelength you you measure them in a laboratory and they always will be at that uh position so you can take a spectrum of your Galaxy compare the positions of the spectral line and this difference in the wavelength then gives you a measure of the Velocity so those can be very accurately measured and that's not the problem is you know that's sort of the easy ENT of trying to make these measurements it's not the velocities it's the distances that are hard and so this ultimately does result in in the paper that Hubble writes and we showed the graph from that paper a moment ago which is pretty much the moment when people take seriously this idea of the expansion of the universe verse now with that Hubble then has the capacity to make a prediction for the rate from the data that he's measured and what does he find for the rate of the expansion of space yeah so that that plot that shows velocity versus distance the slope of that relation which is what we now refer to in fact is the Hubble the Metra constant y or law is uh what Hubble found was a value of 500 right and we now know that it's something like 70 so it's way high it was way high but can you just give us a sense we have the number up there so it's in these funny units kilometers per second per megap par just give us a sense of I mean usually we talk about speeds of sort of you know kilometers per second why kilometers per second per Mega Parc yeah so you know it's a unit of velocity and it's a unit of distance he's plotting velocity versus distance and if you notice because there are two units of distance it's actually one over it's inverse time so it gives you a measure of the age of the universe right now we now know that we have to understand what the other components in the universe are how much mass is there how much energy and so on but it's a very good estimate one over the Hubble constant gives you a measure of the age of the universe and that immediately became problematic because it was the wrong age because it was the wrong Hubble conent and the wrong age it was too young it was about Earth would have been older than the universe something like that Hubble got two billion years and even from geological age dating at that time we knew it was at least three or four billion years old right so that was the first age problem right and so pretty quickly my understanding in the history of these ideas people began to realize what was wrong with the details of Hubble's interpretation and his measurements and the Hubble constant that speed of expansion began to immediately come down right it wasn't that immediate it it was um really not until the 1950s when an astronomer by the name of Walter B who was working at the same Observatory where hubba was yeah now know it as the Carnegie observatory in fact I spent most of my career there sure and he Walter B had discovered that there were two different populations of stars one was a young population and the other was an older population and it turned out that the kind of seph there were two kinds of seids belonging to each of these populations and the kind of seids that had been available to Hubble at the time were the wrong kind so when B came or he interpreted it the wrong way right he had no idea they were seids but they were they they were doing the bright dark bright dark thing but they were not fitting into the pattern that this other category would right and and B's realization led to a doubling of the distances or a having of the Hub con I said then now we're down to 250 km per second per me that happened in the early 1950s so there was actually quite a a block of time years or maybe even 30 years um and then what was the the next Insight that brought us even closer the next Insight was that some of the stars that Hubble had used so there weren't enough galaxies that he could measure seids in nearby they're hard to measure we can talk about that in a while and so he used what he thought were the brightest stars in these galaxies and he said okay if we compare the brightest stars in this galaxy they're probably similar to the brightest stars in another galaxy and and that way he could again use the inverse Square law of light to get a distance but it turned out that those Stars what he thought were stars were actually regions of ionized gas that we call H2 regs and they just they wrong so that brought the value down to maybe 75ish and then people started to argue about whether it was 50 or 100 and there were Decades of a debate is the Hub constant 50 or 100 and that meant that we didn't know the age or the size of the universe to better than a factor of two so if the uh Hubble constant was 100 then the age was 10 billion years if it was 50 it was 20 billion years and I don't know about you but if someone said you're you know 100 or 50 it's kind of a difference it's you look different yeah so it it was very annoying for astronomers and so this then takes us up to the more modern story of really trying to refine this and you mentioned that you began to really focus upon these ideas with did did you say with the launch of the Hubble Space leading up to Hubble Hubble was supposed to be launched in 1986 and then I started in about 1984 right and then there was the Challenger accident in H delayed till 199 that was 86 I guess yeah so that sort of changed things significantly so what then was your approach were you following in the tradition that we've outlined or was it very different I was very fortunate to come along at a time when the traditional detector astronomers had used these glass astronomical plates were superseded by electronic solid state devices charge couple devices that we now have in our handheld phones uh but they became available at astronomical observatories and they had a huge advantage in that they were much more sensitive than the photographic plates they were linear and what I mean by that is photographic plates saturated when things were bright and then they were just wrong yeah and so you could actually whatever photons were coming to us from these Stars linearly correlated with the electronic signal that you measured from your detector and they were sensitive to Red wavelength LS and that turned out to be very important because the photographic plates were only sensitive at Blue wavelengths which just turns out to be unfortunately the place where there are dust particles that are generated in the atmospheres of stars they are between us in these seids and they scatter the blue light so it just turns out that the size of these dust grains is comparable to the wavelength of blue light so it gets scattered and absorbed and you get the wrong answer you're measuring the wrong it but the red wavelengths which are longer barely see the dust and so you can make a much more accurate measurement and correct for the presence of so that's what I did in the 1980s in anticipation of of Hubble and I guess this this work in particular is a culmination of a lot of those studies of yours can you just yeah it looks like hubbles in the general structure that's hubbles plot right it's velocity versus distance but this was what came to be known as the Hubble key project so before Hubble was flown this big debate which had been ra raging about the the size of the universe one of the and probably the main motivation for building Hubble was to resolve that debate it was like a flagship science project and in fact the size of the primary mirror from Hubble was set by the ability to detect seate variables in a nearby cluster to us it's called the Virgo cluster and it was felt if you couldn't go out as far as Virgo you would not solve this I see problem and so the head of the Space Telescope Science Institute at the time Ricardo geone put together a committee and said well what are the most important projects that Hubble can do Hubble were to fall in the ocean what would we not solve and he was afraid that if he left it up to astronomers because we'd been waiting decades for a space telescope that they would divide up the pieces into you know small little pieces and big projects wouldn't get done I see so he set aside time for these key projects and this was the number one key project so we set out to measure seate variables in a couple of dozen galaxies same method as Hubble had used and take us through then what we can learn from the actual graph can you bring that graph back up thank you so if we look at this again we're we're plotting velocity as a function of distance the the galaxies that are moving faster are also farther away yeah but in fact if you look at the first tick mark in the upper graph so let's look at the upper part for the first bit Hubble's observations fit in the first tick mark that's it that's all as far as he could go yeah so we're now able to go much farther out in distance and we can use other methods the points in green are what we call type 1 a supern noi and we can see them almost across the visible universe so we can measure this expansion rate much farther out and that has advantages we can talk about but we were able to use these charge couped devices on Hubble to get to correct for this dust this problem that Hubble had and the answer that you're getting then roughly is 72 km per second per meapar so sort of a refining of the numbers from before but now I want to turn to another approach so this is making use of these particular kinds of stars whether sephius or 1 Supernova there's another approach based on something else the microwave background radiation which is this heat left over from The Big Bang I think many people are familiar with just in a quick nutshell how does one use that to measure the expansion rate yeah those observations are really spectacular so this is the glow The Remnant radiation from The Big Bang Yeah and it's now possible to measure tiny little differences in the temperature across the sky and I mean tiny they're like one 1,000th of a percent tiny fluctuation temperature and also now you can measure polarization and you can fit the spectrum of those fluctuations and so you measure temperature differences across the sky and you fit a model which is now what we call our standard model which has dark matter Dark Energy yep and four other parameters and then you can infer what the expansion rate would be today extrapolating these early Universe measur a very predictive model to what would happen and it's not extrapolating out into Infinity it's model predicts this with really high precision and it's a an Exquisite fit to this spectrum you look at it by eye and you think oh my goodness this is really spectacular and they get a value of 67.4 with a Precision of better than 1% right so smaller than the 72 or 73 that we get from right so we get this mismatch between those early Universe based measurements and the somewhat later measurements and has this been concerning you is this the kind you know in some sense you're measuring the expansion rate of space and it only differs by you know you know 5 10% that's pretty good right but not good enough no now I think it isn't good enough but it's also important to recognize it's pretty amazing we're using very different techniques right here are stars that are pulsating and stars that are exploding we're measuring them locally then we compare to you know early on in the universe and they agree that well so we never took a moment to say wow we're understanding something here that's pretty good agreement right but as you were saying in the introduction this measurement offers an opportunity to really test our current models of cosmology and if you can measure this accurately then you really learn about the evolution of the universe so last 10 minutes obviously what I'd like to explore is can we resolve this tension sort of what your thinking is and I know you like to emphasize a difference that doesn't always get sufficient emphasis which is precision versus accuracy so we have this little graphic which I think you gave us at some level can you just tell us what we should learn from what we're seeing here yeah if if you look at the Target here you can hit the target and be very precise but you're going to miss the bullseye yeah and and so by Precision we mean we can make a measurement over and over and over and over right if I want to measure the size of the laptop or the tablet you're holding I can take a ruler and I can measure it maybe it's a little small once it's a little big but if I average over very many measurements I'll get very close to the right answer but if there's something that is systematic in the sense of for example the dust that I was just talking about will always make your Galaxy look farther away yeah make the Stars fainter and no matter how many times you make the measurement you're still going to have that problem bubble had that problem so it we've been told and people have referred to this current epic is the Epic of precision cosmology and what I like to say is I hope we're in an epic of accurate cosmology because they're very different and we need to we need to improve both now a key thing then is to not be influenced by the dust or at least not to have it bias your measurements in a way that will take you off the bullseye to the wrong spot and that is where a new machine that everyone is familiar with who follows these developments at all the James Webb Space Telescope so you know this little graphic here is showing the different wavelengths that the famous observational tools are sensitive to and so tell us why James Webb was chosen to be sensitive to the particular part of the electromagnetic spectrum that we've highlighted here yeah so Hubble which we've been using up until now of course is sensitive to Optical wavelengths which our eyes are sensitive to and a little bit sensitive to the infrared but James web is optimized now for the infrared so again this example of dust it's a perfect tool to get us you know much more accurate observation that are not sensitive to the dust and their other effects like chemical composition but it's resolution it's it's a larger mirror and the resolution goes inversely as the size of the Mir the bigger the telescope the better resolution you have so these beautiful comparisons between Hubble and James web really I think get to the heart of of what this machine can offer so this is like a stellar Nursery where new stars are being born and the left image is the optical and the right is the infrared and you can see many more stars even though there's a lot of dust there and if we go to seids so I gather this is a Hubble image this is a Hubble image of a seph do you see it yeah and then if we take a look at what James web does a lot better A lot better right and so as you have begun to make use of this more pristine data how has the value of the Hubble parameter begun to change or not yeah we're we're using three different techniques we applied to the James web telescope to not just use seids because each method will have its own kind of systematics but we use Stars called red giants and there are other stars carbon Stars so they're independent measures to the same galaxies right and what we're finding with this higher resolution data is that the Hubble constant is coming down and and the different methods are agreeing remarkably well so this this plot here I guess begins to show that so just tell us what we're seeing here so we're seeing on the left the curve which is labeled plunk that's the European satellite that made measurements of these fluctuations in the temperature of the background radi that's the early Universe type measure early universe that gives about 67 Y and with a very high Precision so it has a very tall Peak compared to the others on the right is the measurements from the Hubble space t scope the blue curve there that's giving a value of 73 or so and then in the middle is what's coming now from James web from the three methods that I just mentioned and it's in pretty good agreement with the plank measurements I.E the standard model of cosmology it still overlaps with the sepian measurements yep so I wouldn't say we're done yet but I think we're we may be at a turning point where we're seeing that things you don't need to change the standard model it's not a Sigma or one in a million CH chance that this would be wrong there's this beautiful graphic that again you're deeply familiar with that kind of summarizes in some sense the whole subject you know over the past I don't know since the year 2000 and so it's a wonderful in progress success story as these measurements are again converging to a value that the theory understands well and the different observations are converging upon so is the message that standard cosmology is currently alive and well and we just have to keep pressing on to make these measurements ever better I I think that we have a model that is pretty spectacular in many ways there's this dark matter we don't yet know what it is there's dark energy that we fundamentally don't understand yet so I think we're all hoping that there will be something that will either break it or give us some insight into what is this model what is it but I am not so certain that it's the Hubble tension that's going to lead us in that direction is what I so is there part of you that's disappointed yeah I think it'd be fun to find you know evidence for Something Completely new and different yeah but I think the history of the subject again shows how difficult this is and we don't rule it out but I think the evidence you extraordinary claims require extraordinary evidence and we're not there and and so we certainly don't require it well it's a it's a wonderful episode you know going back to the pioneers of the subject at the turn of the 20th century and who knows maybe you'll still turn up that crazy anomaly that requires us to rethink things but so far it's all going swimmingly well so thank you so much for this conversation thanks a lot [Music] [Music]

Channel: World Science Festival

Views: 36,309

Keywords: Brian Greene, space telescope, sunshield, Goddard Space Flight Center, Near InfraRed Camera, pictures of the universe, World, Science, Festival, Big Ideas Series, New York City, exoplanets, SpaceX, astrophysics, cosmology, Astrochemistry, Official NASA Broadcast, #UnfoldTheUniverse, Wendy Freedman, Cosmic Expansion, Hubble, Microwave Background Radiation, James Webb Space Telescope, The Future of Cosmology, Resolve Hubble Tension

Id: CQ-4-MWa7Xw

Length: 36min 25sec (2185 seconds)

Published: Fri Jul 12 2024