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What kinds of things can we see using gravity, that we may not otherwise be able to see? Pamela will fill us in on the Great Attractor, etc!
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Announcer: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser Cain: Astronomy Cast, Episode 402, Gravity Eyes: See the Invisible with the Force. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos. We’ll help you understand not only what we know, but how we know what we know. My name is Fraser Cain, I’m the publisher of Universe Today and with me is Dr. Pamela Gay, a professor at Southern Illinois University, Edwardsville, and the director of CosmoQuest. Hey, Pamela, how you doing?
Pamela Gay: I’m doing well. Like so much of the U.S., I’m getting snowed on today and snow is always really exciting.
Fraser Cain: Really? Oh, it’s so warm here; it’s like 13 degrees, it’s beautiful, sunny –
Pamela Gay: That’s Celsius, everyone. That’s Celcius.
Fraser Cain: Yeah, we’re done with winter, we’re into spring. This is, you know, the groundhog didn’t see his shadow so it’s smooth sailing from here on out.
Pamela Gay: See, our groundhog stopped hibernating yesterday. Like, she was out eating the grass and then today she’s getting snowed on. So we basically went from being like 15, 20 degrees Celsius to being 15, 20 degrees Fahrenheit and yeah, groundhogs weren’t meant for that.
Fraser Cain: Just to be clear, groundhog, this is the Punxsutawney groundhog, which is obviously the most accurate method of weather forecasting known to man. Take that, NASA, NOAA.
Pamela Gay: Whereas my groundhog is named George.
Fraser Cain: Really? So do you detect, do you do your weather forecasting with your local groundhog and not the official Punxsutawney Phil groundhog?
Pamela Gay: So Kyle goes strictly, my husband, goes strictly by the Pennsylvania groundhog and I simply hope that ours doesn’t die and that’s about where we’re at, and other that, I like satellites. I really do like my weather to come from satellites. I’m kind of weird that way.
Fraser Cain: Yeah, we don’t believe in astrology and we don’t believe in groundhogs.
Pamela Gay: Well, no. I believe in groundhogs existing, I just don’t believe their forecasts.
Fraser Cain: I’m not sure they’re even real. But let’s get on with this episode of Astronomy Cast.
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Fraser Cain: So we stand on the brink of a potential announcement of direct detection of gravitational waves, as predicted by Einstein, but there are plenty of things we can detect with gravity, like planets, black holes, and other hidden objects. So today, let’s get into it. All right, Pamela. So, yeah, this is not a total coincidence. We are riding the wave of a potential prediction of direct detection of gravitational waves. So what’s the rumor mill churning right now as we’re recording?
Pamela Gay: So the rumor mill, as I understand it, is first there was Lawrence Krauss tweeted something and it was right after he got a text message or an email in his phone and then really quickly undid that, and this was followed by a bunch of people working with LIGO being asked to please lock away some days in March; don’t go anywhere, we need you all here. I’ve heard rumors of a media event. I haven’t seen an invitation to the press yet. And there’s always this sort of back-of-the-mind, well Krauss really doesn’t work for LIGO and LIGO has kind of been in the news with Chris Ott a lot lately, so there could be a whole lot of stuff going on here.
Fraser Cain: Right, so if we’re lucky, by the time you’re hearing this, maybe –
Pamela Gay: No, it’ll be after that, hopefully.
Fraser Cain: After you’re hearing this, but shortly after you hear this, there will be the announcement of the direct detection of gravitational waves as predicted by Einstein. And if we’re unlucky, there will be some other interesting event that is cool, but it’s not, you know, the big Holy Grail, and if we’re super unlucky, everyone’s just getting all the signals wrong and there’s nothing to be said and it’s just business as usual.
Pamela Gay: Or there’s nothing scientific to be said.
Fraser Cain: Yeah, like –
Pamela Gay: It would awesome if they got LISA fully funded and were announcing that, but I don’t think that’s what it is.
Fraser Cain: That would, yeah, that would be pretty great. Yeah, or like another upgrade to LIGO being developed, or as you – I mean, you mentioned this a little, like last week, that maybe like they’re gonna throw in the towel on LIGO. It’s just impossible. It can’t be done.
Pamela Gay: Well, I don’t think they’re going to announce they’re throwing in the towel. That’s too many jobs. But there’s been so much going on in terms of LISA, in terms of sexual harassment, in terms of rumored detections. We don’t know. We just know there’s something coming, but they’re not the only gravitational news in town. We’ve had the Planet Nine theoretical detection by folks also at Caltech. So apparently, if you want to detect things using anything but light, Caltech is the place to be right now. So we have Michael Brown and his team –
Fraser Cain: ‘Pluto killer,’ Mike Brown.
Pamela Gay: Gave us another ninth planet, he taketh and he giveth.
Fraser Cain: Giveth, yeah.
Pamela Gay: And then I saw more announcements yesterday about planets being, not planets, about galaxies being discovered behind the disk of the Milky Way and that just sort of reminded me of the Great Attractor. So there’s so much in our universe that we only see with gravity and heck, dark matter is one of the major constituents of the unseen universe.
Fraser Cain: All right, we’ll start with dark matter then. So, I mean, we’ve done a bunch of shows on dark matter, but let’s you know, reiterate of course here that we know dark matter exists purely by detecting its gravity.
Pamela Gay: Exactly, and this is something that goes back to Vera Rubin’s work well before I was born, before a lot of us were born. She, while working on her doctoral dissertation, was studying the rotation of galaxies and then folks has also been studying the rotation of gas clouds in the outer wings of our own galaxy, and when you put all of this data together, galaxies don’t rotate the way one would expect based on the visible mass, based on what we see. And this is continued as we’ve pushed what we’re able to see out into redder wavelengths in the radio and bluer wavelengths out through the gamma rays. We don’t actually detect any of the stuff that we’re looking for with gamma rays, but still.
So the more we look with light, we still haven’t been able to account for all of the matter that orbital dynamics for how galaxies rotate, including our own galaxy. We just can’t account for anything, so that tells us there’s a bunch of matter that we can’t see. We named it dark matter because we lack originality.
Fraser Cain: But that was the first, that was the first hint of it, but then astronomers have come up with much better techniques to map it out with tremendous accuracy.
Pamela Gay: Right, so fast-forward until just the past decade and just within the history of this show, we’ve begun to use techniques looking at how basically what appeared to be voids and empty space seemed to warp background light by shifting the path of the light with gravity. All of these minor shifts add up to gravitational lensing events that tell us where all of the hidden matter, at least where a lot of the hidden matter is located. We’ve seen this in colliding galaxies and galaxy clusters. The Bullet Cluster was the first big detection. We’ve seen that repeated several times. Now we have surveys like COSMOS that have looked through basically tubes through space and measured everything up in that region and looked at how the density of dark matter and the clustering of dark matter has changed over the evolution of the universe.
Fraser Cain: Yeah, and so it’s hilarious, right? We don’t even know what this stuff is, but we can use it as a telescope to probe and understand its distribution and location and even to see things that are further away, that we couldn’t normally see. But thanks to the dark matter, you can actually use it as a telescope, which is just amazing.
Pamela Gay: And what Fraser is talking about is massive gravitational lenses, where we used often the central galaxy and galaxy clusters, the big old cd galaxies, to gravitationally lens background objects like quasars and the combined visible mass and gravitational mass, well that’s a whole lot of mass that can do a whole lot of light bending, and in some cases we can actually catch both – things that are otherwise so – we couldn’t see them, or catch things that we didn’t see it happen in the original go, so we catch it as it comes back around on the bent path towards Earth.
Fraser Cain: Yeah, okay so that’s sort of like the one way that we use gravity to sense our surroundings. Let’s talk another one. I think let’s talk about Planet Nine, and this is you know, we introed this a bit, which is Mike Brown giveth and Mike Brown taketh away. He took away Pluto, but he helped maybe give us a whole ‘nother planet that’s way bigger than Pluto.
Pamela Gay: Right, exactly. So this isn’t the first time this has happened. When we look at the paths of planets, we can see anomalies that either have to be accounted for in the history of the solar system, so like, how did Uranus end up spilled over on its side, or they have to be accounted for by another mass being out there yanking things around. So when we were first trying to understand the orbit of Uranus, there was this idea that maybe there was another massive, or at least as massive as ice giants get, another massive planet out there that was pulling on Uranus’s orbit. And folks made a bunch of different predictions and there was both a British team and a French team that observationally went looking based on these mathematical predictions, and that’s how we found Neptune.
Now, there were similar predictions that Neptune’s orbit was screwed up because of another planet, which is what originally sent us hunting for Pluto, except it turns out that you can kind of like account for Neptune in other ways so that wasn’t necessary. But we found Pluto, so cool nonetheless, except Pluto wasn’t the massive planet that folks were looking for. It was a little blob of ice. And even back in the days after Clyde Tombaugh had first found it, people were like, “This is not a planet.” And low and behold, fast-forward until ten years ago, and Michael Brown started finding lots of other objects that are close in size to Pluto, are similar orbital characteristics, they’re out in the Kuiper belt out there, and folks were like, “Yep, Pluto’s not a planet. Demote.” And so that’s how Michael Brown got his name ‘Pluto Killer.’
Well, this year, in looking at a bunch of the different objects that he detected, many of which still have, well, zip codes. So we have the orbit Sedna, of 2012 GB174, 2012 VP113, 2013 RF98, 2004 VN112, and 2007 TG422. All of these un-sexily named objects, well Sedna has a cool name, but all of their orbits are pointed in one direction out of our solar system and so these are highly elliptical orbits. In better English terms, they’re flattened ovals, where the sun is closer to one side of the oval than the other. And all of those ovals are pointing off in one direction. And to keep all of them pointed off in one direction it implies that there must be something counter-balancing their orbit. And we know that Pluto and Neptune, for instance, have a resonance that will cause them never to collide, even though their orbits vaguely overlap. And similar rules seem to indicate there must be something out there that’s keeping all of these objects from, well having orbits that are more evenly spread about the sun. And running models, doing the maths, the entire team came up with there must be a ninth planet out there, something that is many times the size of the Earth, and this was theoretical work that was done by Konstantin Batygin.
Fraser Cain: Right, and so the – and this isn’t the first time these predictions have been made. I remember there was a, was it a Spanish team? There was a Mexican team that had done some prediction. It was a Mexican astronomer, yeah, had done some predictions and sort of made these same kind of calculations that there was either a Mars sized object that’s relatively close or a Neptune sized object that’s much further out.
Pamela Gay: And this has to do a lot with the Kuiper belt cliff, this observed lack of Kuiper belt objects out past 55 AU.
Fraser Cain: Yeah, and another part of it is this sort of idea that, you know, why do we get some of these long period comets? There’s got to be something out there that’s interacting with the Kuiper belt system, with the Oort cloud system out there, really far out, farther away than we can see. And so this is the point, right, which is that the prediction is made, with thanks to gravity, for the actual observational predication is made with a telescope, right. Now astronomers need to follow up with some kind of observatory to actually find this thing. It should theoretically be visible within the most sensitive telescopes, within James Webb or Hubble, things like that.
Pamela Gay: And the trick for finding this object is catching it in the act of moving because, well it’s reflecting sunlight so it’s going to appear, in a lot of ways, like a solar-type thing, depending on exactly how it reflects the different colors of light. It may end up having a slightly different black body depending on how it reflects light. But if we don’t catch that sucker moving, we’re not gonna notice that it’s really in our own solar system, so there needs to be repeat observations in the same area with high sensitivity, with high astrometric accuracy that are staggered in time and we’re just not quite there yet. This is the kind of thing where, assuming LSST has the right parts of the sky in its field of view.
Fraser Cain: Right, that’s the large synoptic survey telescope and this is the, this is gonna be, I think, you know, one of the most important observatories, most important scientific tools that’s ever been built.
Pamela Gay: Yes.
Fraser Cain: And I’m gonna keep sort of hyping everybody about this thing because it’s such an amazing observatory. It’s gonna scan the whole sky at a very high –
Pamela Gay: Visible in the Southern Hemisphere.
Fraser Cain: In the Southern Hemisphere, at a very high level of resolution within, I think, four days. It’s gonna have mapped out the whole sky. It’s gonna find asteroids, supernova. It’s gonna find – you know, we were talking to an astronomer. He figures thousands of supernovae because it’s gonna be able to observe all of these galaxies very carefully. So you’re gonna have this, we’re gonna see this world of motion that astronomers have never really been that, you know, have never had a chance to really peer into unless it was just like luck, happened to find a supernova or something.
Pamela Gay: They’re estimating upwards of 10,000 transient objects every single night, which includes the supernova, which includes flare stars, which includes all of the little variable things that you otherwise wouldn’t have noticed. It includes asteroids; it includes potentially Kuiper belt objects. So LSST is just gonna systematically find, well it’s primary mission is to find everything that might hit us, but beyond that it’s going to give us vast insights into the background universe around us.
Fraser Cain: So, that’s great. Now, we’ve talked about planets in our solar system, but there’s another method of using gravity to find planets in other solar systems, right? There’s two; there’s gravitational microlensing and there’s the rate of velocity method, which is all about gravity.
Pamela Gay: Right. So one of the cool things that we did actually trying to find dark matter initially, was there were a couple of different gravitational microlensing experiments set up. One of the cooler ones in terms of names was the MACHO Project, there was also OGLE, and MACHO was out there pointed vaguely towards the Magellanic clouds and while it was monitoring all of these stars, it was looking to see if any of the stars would suddenly get significantly brighter in light, in a way that was characteristic of a foreground object closer to us passing in front of this background galaxy, and as it passed between us observing that background star, it would gravitationally bend light from that star that otherwise wouldn’t have reached us, to come toward us, make the thing appear much brighter.
The hope was, well maybe we’d find a whole bunch of dim white dwarfs, a whole bunch of rogue stellar mass black holes, neutron stars. A bunch of condensed matter objects that would, in their happy degenerate state, account for all of the dark matter out there. Well, we didn’t find that, but what we did find is, well there are stars out there that do microlens things and sometimes those stars have planets and you’ll get a large microlensing event from the star and a smaller microlensing event from a planet in orbit around it that just happens to line up just right.
Fraser Cain: Yeah, and even like moons potentially, you know, you’re gonna get, that’s still in the works, and what I love about this technique is that amateur astronomers even participate in helping to find these things because the difference in brightness is so significant that even a fairly small, you know, a right equipped telescope can see these things.
Pamela Gay: And exactly how much microlensing we see, whether or not we see microlensing from that giant moon or not, depends on the sensitivity of the equipment, but the star microlensing a background star, easily detectable. In fact, the MACHO project engaged a whole bunch of southern hemisphere scientists, and by scientists, I mean guys in their backyard, women in their driveways pointing their telescopes up and letting us know what they were seeing.
Fraser Cain: Yeah, citizen scientists, our favorite kind.
Pamela Gay: Exactly, exactly.
Fraser Cain: Okay, great. So, let’s talk about the rate of velocity, because again, you’re really watching the gravity move a star around.
Pamela Gay: And this is, again, something that we’ve done an entire episode on. One of the great ways that we started finding planets was by very carefully measuring the rate at which stars are either moving toward the Earth or away from the Earth. If nothing else was intervening you’d expect this to be pretty much a constant velocity, but the thing is, if there’s a planet going around the star, the entire system is orbiting around a central point as a center of mass. And as the star goes around and around that center of mass, its motion isn’t a lot. In fact, it might only be moving at the same rate that a human being might be moving and so you’re looking at a meter per second velocity shift and it’s not moving all that far either. It’s not like the Earth that has a 1 AU radius circle that it goes around each year. But those small, sometimes meter per second motions are entirely visible by Earth-bound spectroscopes and so we look for that little wiggle in the motion and the velocity of the star that’s causes by its planet or planets yanking it around.
Fraser Cain: One of my favorite stories in using gravity to find something was the search for the supermassive black hole at the heart if the Milky Way because once again, you’ve got this gravitational interaction with this object that’s four million times the mass of our sun, with stars nearby it and they are buzzing around it like comets.
Pamela Gay: And this is work being done by Andrea Ghez, who is at UCLA, and she’s using the Keck telescope and a technique called drizzle, where you take high-speed images that, because they’re so fast, the light doesn’t have time to get smeared out by the atmosphere. Then you have to co-align to take into account that one image, the atmosphere will have moved the stars one direction, another image the atmosphere will have moved the light from the stars another direction. So you co-align all of these images and then over days, weeks, months, and now, well more than a decade, she has been able to build up the motion of the innermost visible stars in our galaxy and see these things going at significant fractions the speed of light as they zip around an extremely dense object that, at this point, we have to say is a supermassive black hole.
Fraser Cain: Yeah, if not, it’s something that sure is kind of like a supermassive black hole. If you don’t believe in black holes, fine. Explain that.
Pamela Gay: And these things are getting a couple of times the distance of Pluto from the sun away from this thing in the center of our Milky Way and pretty much the only way we know to shove that much mass into that small of a volume is to create something that looks, smells, tastes like a supermassive black hole.
Fraser Cain: Yeah, totally.
Pamela Gay: Again, don’t taste the science.
Fraser Cain: Don’t taste the supermassive black holes. So another sort of interesting aspect of the Milky Way is that, you know, is this idea that there is, if you look out beyond the Milky Way, sort of above and below, it really appears like all of the objects around you, all of the galaxies that are in our local area are all kind of sliding towards something.
Pamela Gay: And this isn’t true if you look in all directions, but if you look in the direction of like the Hydra constellation and the Centaurs constellation, if you look towards that part of the disk of the Milky Way – if you look straight through the disk, all you see is dust, gas, and foreground stars. The dust and gas of our galaxy, it’s bad. It likes to block out background light, but if you look directly above and directly below, you’re entirely right. All of these galaxies seem to have an extra component of their motion that is pointing towards something lined up directly behind the disk of the Milky Way.
Now, we’re starting to get hints of what that thing looks like as we move into redder and redder wavelengths that are able to pass through that gas and dust, but this thing, this Great Attractor behind the disk was initially detected solely through the motions of galaxies that seemed, well, it looked like there was something calling them their way and we called that thing the Great Attractor.
Fraser Cain: Yeah, and you’re exactly right, that before it was like a big mystery but now thanks to the infrared telescopes, the amount that you can’t see that’s actually blocked by the Milky Way is getting smaller and smaller and you can really start to see that there’s this galaxy cluster that’s just in that area. So it’s just a galaxy cluster, something freaky.
Pamela Gay: Yeah, yeah, it’s called the, I’m gonna mispronounce this, it’s the Laniakea Supercluster and it’s tens of thousands of times the mass of our Milky Way.
Fraser Cain: Crazy. So, there’s a few other things that I think we look at in terms of gravity. Man, there’s actually just – we could literally do seven shows on this. You know, we talked about gravity waves, so we can detect moons in orbit around Saturn within the rings because of the way they interact with their gravity with the ring particles and you get these amazing waves and crazy rippley structures in the rings themselves.
Pamela Gay: And so, in general, any time you have basically a wave going through different fluid media and it turns out that you can treat the rings of Saturn as though they were fluid. Within different fluid media, the interface between two different fluids, you can end up with different waves being created by the force of gravity between these two fluids and the interactions. It’s really complicated, ugly math, but the way it looks has actually cropped up in some really neat ways in science fiction novels and what we see is curvy, weird, awesome geometric shapes in the rings of Saturn that are being driven by the gravity from these moons.
Fraser Cain: Yeah, amazing! These tenders, these moons that tend to the rings. Just a few more things, think about some of the tidal interactions that we see between galaxies that allows us to chart, like we get to look back through their past by seeing how they look today, especially if they’re totally messed up.
Pamela Gay: And this is again, one of those ways that we start to be able to see where is the invisible stuff at the front edge of a collision starting to have an effect. We will see galaxies that are still clearly fairly far apart, but are already starting to distort one another as you see, well the big gravity from one object that’s only partly visible and invisible having an effect on another object, that again, it’s getting dominated by the dark matter.
Fraser Cain: So why don’t we wrap this up, Pamela. I got one last, final word and this was a press release that was released today when we were recording this, on February 8th. “On Thursday, scientists will provide an update on the search for gravitational waves.”
Pamela Gay: Okay, so it is going to be gravitational waves and they’ve now done the press call.
Fraser Cain: Yeah, so big thanks to Ellot Avaron for digging that up. I think we can call it probably gravitational waves detected, but we’ll see what happens on Thursday.
Pamela Gay: I, you know, how many sigmas could it be?
Fraser Cain: You’re right, you’re right, you’re right. Gravitational waves not detected. Gravitational waves ruled out, could be possible too.
Pamela Gay: No, I’m gonna say it’s gonna be a low sigma detection that they announce that keeps people scratching their heads for a few more months, or years, or decades. Think about Higgs.
Fraser Cain: Right, that sounds great. Well, thanks Pamela. Hopefully we can talk about it next week.
Pamela Gay: Okay, okay.
Fraser Cain: Thanks, Pamela.
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