Transcript: Astrometry

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Fraser: Astronomy Cast Episode 182 for Monday March 22, 2010, Astrometry. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we 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. Hi Pamela, how’re you doing?

Pamela: I’m doing well. How are you doing, Fraser?

Fraser: Good. One little piece of news… which is that we’ve had a couple of radio stations ask if they can run Astronomy Cast on the air and want our permission, and… YES! So if you run a radio station, if you know a radio station and you want to use Astronomy Cast, feel free… be our guest. Free… don’t pay ever! Yes, that would be fine with us… would be great if you want to do that… college radio, NPR, or here in Canada on the CBC… CBC–call me! And ABC in Australia… anything. For free… go ahead… play it all you like. Use our content for any purpose whatsoever. Just so that’s all clear. But yeah, if you want to email us and want us to do a little promo for you, no problem. That’d be cool. Alright, so let’s move on with the show. So, astronomers have been cataloging star positions for thousands of years from the first calculations made by Hipparchus to the more recent star catalogs made by the spacecraft named after him. This is astrometry… another way to find our place in the universe. Alright Pamela, well I guess we need to go right back to the earliest age, and I guess at some point, humans realized that there was some kind of rhyme or reason to the position of the stars… that they weren’t going anywhere, that there’s a way to map this. And I think the name that comes to mind is Hipparchus, so how did this all come about?

Pamela: Well, it’s impossible to know exactly when people realized that… well, wow, you see the Plow every night, you see whatever your favorite constellation is year after year after year… always appearing in the same season. Star maps though, those started being made originally by the ancient Babylonians. That’s where we start getting squirrelly names like Zubenelgenubi for different stars and Betelgeuse which leads to many arguments over *betelgice,* *beetlejuice,* whatever…

Fraser: Right, but we have to thank the Babylonians for that name… wow…

Pamela: Yeah, so those names all came down from the ancient Babylonians. But in terms of things we can get our hands on and study easily, well we can’t necessarily get our hands on the work by Hipparchus… not in all cases… but his work, his original star maps from basically 150 B.C., his original work throughout his life, it was used as the base information for the Almagest by Ptolemy, which is perhaps one of the most famous early astronomy books.

Fraser: So, who was Hipparchus?

Pamela: He was a mathematician, he was a geometrist. He was working in the Mediterranean… he was at Rhodes for some of his measurements, he was in Egypt for others. He was a Greek. And as he traveled and as he measured, he worked with others to try to figure out… if I’m seeing this, what are you seeing when you are. It was in putting all these pieces together that Hipparchus was able to make some really amazing discoveries. He was one of the early scientists who based his discoveries not on philosophy, not on shadows on cave walls, but on looking. So he noticed things like… the moon has an obvious change in size over time. When he watched it through what’s called the diopter, he was able to tell that sometimes it was a little bit bigger, sometimes it was a little bit smaller, and this was an interesting discovery back in the days when we thought all orbits were perfect circles.

Fraser: Right, right, I mean the moon changes like 15% from its most distant point to its closest point. It actually changes in brightness, there are some full moons that are a lot brighter than others. And they were able to calculate this.

Pamela: And noticing all this, he sat down and tried to run the math assuming epicycles to try to figure out what’s going on based on the changes in size that he was able to observe.

Fraser: So, epicycles… this is where the moon is orbiting the earth in a perfect circle but then it’s on a little….

Pamela: …tiny circle on top of the perfect circle. So you can imagine it like a bicycle wheel that the moon is attached to the outside of the bicycle wheel and the bicycle wheel is rolling itself around the moon’s perfectly circular orbit. So you end up with the moon essentially doing loop-the-loops around the earth but never crossing over the same point on the loop-the-loop… so just like a bicycle you don’t end up with the rim doing weird things as it rolls across the ground… you always end up with constant movement forward of the tire. The moon constantly moves forward.

Fraser: Right, so he was able to use… by looking at the moon from different places on the earth, working out its distance.

Pamela: And he also used eclipses as a specific way to make sure he got the timing right. So, by having him in one place watching the eclipse and noting… I see the sun blocked out 100% and having somebody else somewhere else looking at the eclipse saying…. I see it blocked out this percent and figuring out what angular shift that must imply. So if you hold your thumb up and you block out a distant object and you blink from one eye to the next, you’ll see your thumb bounce left and right. Well, if you’re at two different points on the earth’s surface, and you look at the moon against the sun, and you see the moon bounce left and right, well you can use geometry to figure out where the moon has to be located.

Fraser: That’s amazing. Amazing they can work out that stuff so long ago.

Pamela: And they were able to do it fairly accurately. Now when the eclipse that they were looking at occurred, the sun wasn’t high in the sky so it wasn’t a perfect measurement, but it was good enough to get a lower limit on far away the moon should be.

Fraser: And how big the earth is, and how far away the sun might be… I mean they were pretty close on all that stuff. It’s quite amazing. When you think about the ancient Greeks, and how they thought… they knew a lot… they didn’t think the world was flat…they knew roughly how big the earth was… it’s quite amazing, so anyway, right, but I guess Hipparchus and what we’re doing is most famously named for working out the star positions.

Pamela: Right. He basically sat and created a map of the sky that very carefully tracked what did the stars look like where he was living. Exactly why he did this… it’s thought that it might have been encouraged by an observation of a supernova that made him just want to note down where everything is so that if something else new cropped up he would be able to know it was new. And so using what was called an armillary sphere, a way to very carefully measure the separations on the sky, he wrote down the positions for at least 850 stars. It’s unknown quite what coordinate system he used, but his 850 stars–these formed the foundation for Ptolemy’s work about 300 years later. that basically charted a lot more stars and again formed the foundations for our modern way of looking up at the stars.

Fraser: And just to kind of get into the nitty-gritty here, you talk about using an armillary sphere… what did that look like? How was this tool used to calculate star positions?

Pamela: Well, it was a small device… you might call it a spherical astrolabe. It basically allowed you to mark out where the horizons were, and you had lots of different rings that you could rotate to start figuring out what the angles were. Now I have to admit, to me it looks like a very complicated strange device and I’m not entirely sure how you use it to make measurements, but I think it was a good way of lining things up, and, you know, if this is here and this is here and you can measure the angle off of set known positions, you can start to figure out where things go in the sky. So I don’t think he was actually using it, holding it up in front of his eye and making measurements but was rather using it to make calculations. I know where these things are, I’ve measured this relative to these, therefore this has to be in this location.

Fraser: Right, and so Ptolemy used sort of similar methods, but of course with Ptolemy he had everything orbiting around the earth.

Pamela: Which is a bit problematic and then, epicycles…

Fraser: Yeah, epicycles. Let’s make this more complicated…. But, he produced an even more accurate map… So then what was the next improvement on this process?

Pamela: So, you had Hipparchus working about 150 BC, Ptolemy working about 300 years later, and then while the Europeans were busy with their Crusades, you had the Arabs working very carefully to produce new catalogs. I’m going to destroy this pronunciation… I’m going to apologize as I so often do on this show… There was someone by the name of Abd al-Rahman al-Sufi who worked on a catalog of about 10,000 entries of the sun’s position over the years. And, he was very carefully also noticing when eclipses occurred and it was off of a lot of his work that future work was able to say… ok, we now know how things are changing. We now know how the sun’s position on the sky is changing over time. So, then there was another astronomer, Ulugh Beg, who compiled a catalog of star positions. This time instead of the 800, it was 1019 stars. And this new star, it was probably consistent to less-than-your-pinky’s-width across the sky. He was making fairly precise measurements.

Fraser: Right, and it’s interesting…. A lot of the star names that we use today are Arab names. A lot of the modern names that we use… I mean you talked about some Babylonian ones and some other backgrounds… but a lot of them are Arab names. And if you look at big lists of all the named stars, most of them have Arab names. It’s quite interesting. So then kind of now we get into the modern age where maybe Ptolemy was wrong… maybe not everything does revolve around the earth, right?

Pamela: Well, and here’s the thing… with the early debates on who is orbiting what, it was easy to say on both sides… well, you’re wrong because you’re not fitting the data, or oh, no I’m right… it’s the data that has the errors. If you don’t have extremely precise instruments, you can always blame the data. And it was Tycho Brahe who took the first really amazing set of data where his positions were the most precise ever made. If you hold your thumb up on the sky, it’s 2 degrees across… depending on your thumb… some thumbs are fatter than others. And each of those degrees can be divided up into 60 minutes, each of those minutes can be divided up into 60 seconds, so you’re looking at several hundred seconds spanning across your thumb. Now his measurements were accurate to within 15 to 35 seconds of arc across the sky. Using his data, you could no longer argue with whether your math was right or wrong… either it was right or it was wrong. It had to match the data. That was the source that was the most reliable.

Fraser: And then this star data would then be used, right? So you could then say Saturn was this far away from that star on this date. And that would be the way that you could then start to detect these elliptical orbits, not circular orbits… right?

Pamela: Right. And so you could very precisely say… relative to the sun, relative to the earth, exactly where everything was located in the sky over time. And this is where poor Kepler was left struggling. He was a mathematician. He was very good at what he did, and he was looking for circular orbits… he was looking for perfect circles. He tried inscribing them in crazy geometries, he tried doing all sorts of crazy stuff before finally realizing the data said… and you can’t argue with good data… the data said planets are moving in ellipses.

Fraser: It’s really interesting, because it’s like these star maps are so important for every other piece of astronomy. Without these star maps we would have no way to know the truth about the way the universe functions…. about the fact that the sun is the center of the solar system, and it’s this background information… somebody had to build this background map that you could then chart everything against. If you didn’t have that map, no other kind of astronomy was going to be possible without it. So, they’re the unsung heroes…

Pamela: And what’s amazing is just how they did these things… so Brahe basically had a room where he had a device that could slide up and down, but only in one coordinate… it’s what is called a mural quadrant. And he waited for the earth to rotate… and as the earth rotated it carried things in and out of his field of view, allowing him to very precisely, knowing that his object was secure, see this is definitely separated from this by this amount… this is definitely separated from this other thing by this amount… And, this gave him one part of the sky, though… there was still so much more of the sky waiting to be discovered. So since then, we’ve been working to try and pull together the entire sky into one coherent catalog. And this is where it starts to get tricky. You have things getting carried over Poland where Brahe’s working, you have things pulled over Arizona where later-people started working, you had people working in South Africa… working in Australia… all using very precise instruments… instruments that said, relative to where I’m located, this object has this angle in the sky and passed as high overhead as it could at this time… this next one passed over a few minutes later… you need time, you need position. And now I have one very accurate stripe, another very accurate stripe, and then you have to figure out how to align the stripes of sky you very carefully matched.

Fraser: Right, and so they were building… a series of astronomers came, one after another, building more and more detailed star maps, filling in these holes in the sky. But I think the big question that they had to answer was how far away are these stars… I mean we can make a very accurate sphere around the earth and position all the stars on it, but a truer understanding of our place in the universe is to create a true 3-D map where you know the accurate distance to each one of these stars… so how did that happen?

Pamela: We had hints of how to do it as early back as Hipparchus. We knew that using parallax–if you shift yourself from the left to the right… north on the planet to south on the planet… you can see nearby objects shift… like the moon. It was thought if some of the stars are closer than some of the other stars, won’t you see them shift left to right?

Fraser: Oh, like over the course of a year, so when you’re on one side of the earth’s orbit around the sun you’re looking at the sun from one point of view, and then you wait six months, you’re on the other side of the earth’s orbit and you’d see the star from another point of view. And the close stars should be wiggling back and forth against the background stars.

Pamela: And people started trying to make these measurements as early as the 1500s when we finally started getting good telescopes with good fields of view. But none of those early instruments were quite good enough. James Bradley made the first really solid attempt in 1729, and unfortunately what he was able to instead discover was light suffers aberration by our atmosphere, the planet is wobbling a little bit, it has nutation in its axis, and so he very carefully cataloged 3222 stars and didn’t find parallax. But, when Frederick Vessel was working in the 1800s, he built on Bradley’s work. As he made his very careful–with even better optics–measurements, he was finally able to measure the stellar parallax of star 61 Cygni that we now know has a parallax of .3 arc seconds. So, an arc second–just to give you an even clearer idea of what it is–take a piece of hair, yank it out of your head, hold it at taut at arm’s length, and the width of the piece of hair… that’s one arc second on the sky.

Fraser: So it’s a third of a piece of hair held at arm’s length and he was able to measure that distance in movement, through the wiggly, jiggly atmosphere…

Pamela: And he had to wait for the earth to move six months to measure that shift compared to other nearby stars that were more distant.

Fraser: And this was a big parallax, right?

Pamela: This was a huge parallax, and it’s not an easy set of measurements to make, and they made it. Now what we talk about is milliarcseconds. The Tycho catalog looked at parallaxes of 20 to 30 milliarc seconds. So here you’re starting to look at 2/100, 3/100 of an arc second of a shift. It’s complicated work, but we’re able to do it. This is giving us a 3-dimensional understanding of nearby objects.

Fraser: So I guess the story comes around… there’s the Hipparchus satellite, launched about 20 years ago.

Pamela: And the Hipparchus mission built on that Tycho catalog. It looked at 2 1/2 million stars and published in 2000, it was again able to make these milliarcsecond measurements of parallax. It did 2 things… it didn’t just measure the parallax, it also measured the radial motion of the star. So it started to be able to give us distinct… these things aren’t just shifting due to the earth’s motion, but they’re actually moving across the sky due to their own orbital motion. That starts to give you a full picture of… well, this thing is orbiting this way, and this other thing is orbiting this other way… and now do we not only know how far away things are, but we know how fast they’re moving across the sky. Now it’s goal was actually not to do just the 20 or 30 milliarcseconds, but it actually got down to an arcsecond or less, depending on the star. So it starts to give us a pretty good understanding of exactly where things are in our nearby universe.

Fraser: And so when we see these current maps, these 3-dimensional maps of our surroundings… a lot of the work is done by the Hipparchus spacecraft. It’s amazing to think about that Hipparchus detected all of this motion, that all of the stars that we see in the sky today… it’s just temporary. All the constellations that we see today… the stars are buzzing around… right and left, up and down. Over thousands of years, the constellations will look quite mangled and eventually lose their current shape.

Pamela: And this actually makes setting the zero points on any particular chart rather difficult. You can’t say this particular star is a zero point. In fact even naming the stars gets difficult, because if you look at some of the names of the stars, they’re named Orion number, number, number, number… but that star has moved and is no longer in the constellation Orion. So when we’re creating maps, you have to have some solid reference frame… turn right at the boulder. That’s how we do things when you drive in New Hampshire. But you can’t turn right at the red giant when that red giant is Mira and it’s whipping itself through the galaxy. We actually have to tie all of our coordinate systems to the most distant objects because, yeah, they are moving, but they’re so fare away that we can’t perceive any of that motion. So we tie all of our coordinate systems to quasars, active galaxies off in the most distance regions of the universe… because they don’t move, as near as we can tell.

Fraser: Yeah, in a time frame that astronomers are going to be concerned about. So, in the end, the final mapping tool are these quasars, and so if astronomers want to judge whether things are moving, everything is done against this background of these quasars.

Pamela: And what’s awesome about these quasars, is they actually solve a lot of problems. They don’t just allow us to tie together all the optical data, because luckily there’s quasars in the entire sky and you can see them from satellites. So, if someone in Australia is mapping in the optical this small region of the sky, as long as it has quasars in it… we can tie in their small catalog with all the rest of the catalogs. But you start to get into trouble when you start to look at radio sources. How do I know that this radio sblop on my map corresponds to this optical clear, pretty, shiny galaxy unless I know for certain the two coordinate systems are the same? And again, quasars are nice and polite and occasionally radio-loud… so what we do is we also look for the quasars that give off radio emissions and we use them to ground our radio catalogs to our optical catalogs. We work our way through the entire electromagnetic spectrum this way… looking for things far away, non-moving and shiny, and we make those our zero points.

Fraser: That’s really cool. Well, thanks Hipparchus… and thanks Pamela. We’ll talk to you next week.

Pamela: Sounds good Fraser, I’ll talk to you later.

Transcript: Albedo

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Fraser: Astronomy Cast Episode 180 for Monday March 8, 2010, Albedo. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we 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. Hi Pamela, how’re you doing?

Pamela: I’m doing well. How are you doing, Fraser?

Fraser: I’m doing very well. So, why are some objects in the solar system bright, while others are dim? Much of an object’s brightness is caused by its albedo–or how well it reflects radiation from the sun. If you want to know how big a distant moon, comet, or asteroid is, you gotta know its albedo. Alright Pamela, so mirror, mirror on the wall, which is the brightest thing of them all?

Pamela: Snow! Snow and ice.

Fraser: Snow and ice.

Pamela: Yeah. If you look around our solar system, and you want to find what’s the brightest, shiniest thing in our solar system, all you have to do is find the freshest snow pack out there, and that happens to be the little moon Enceladus and its geysers, which are constantly refreshing its surface.

Fraser: And if you could fly out and take a good look at Enceladus, it would just be like a bright white snowball.

Pamela: Yes, it’s reflecting almost all of its light. It’s up over 90% of the light that’s hitting it is getting reflected off because it has such a fresh, snowy surface. And here on our own planet, we find that the amount that we reflect is directly related to how much snow pack there is. As we look around the solar system, we can find other snowy bodies for looking for shiny things. This is part of how we were able to find Eris, this little dwarf planet out on the edge of the solar system is next to Enceladus the second shiniest object, second highest albedo object in the solar system. So it’s able to reflect almost all of the light that hits it as well.

Fraser: Alright, so let’s sort of transport briefly into your physics classroom, and you’ve written albedo on the board, what is the kind of textbook astronomer understanding of albedo?

Pamela: Well, it in general is how much incident light on a surface of a given wavelength is reflected off of that surface. And albedo numbers that you find in textbooks typically refer to the amount of light reflected in visual wavelengths all averaged together. How much red light is given off, how much blue light is reflected off… average all that together and that gives you the albedo number for an object. Now, if you get into very technical definitions, you also start worrying about how much light is reflected as a function of the angle the light hits the surface. So for instance with water, if the light hits straight down on the surface, the water’s able to absorb almost all of that light and it heats up. But if instead the sunlight hits at a very steep angle just barely grazing and touching the surface, then most of the light is going to be reflected off.

Fraser: Right. And so that’s why when we see pictures of the earth as this ball in space there’s this spot where we can really see the light glinting from the sun.

Pamela: And that’s all about the angle. And when we look out at ocean waves and we see that glint, that’s where the fluctuations in the surface, the waves on the surface of the water are able to catch the light. But most of the time, when it comes to oceans, the water is just absorbing and heating up from all of that light.

Fraser: Now we’re only including the visible light, so we’re not talking about infrared, we’re not talking about radio waves, we’re not talking about x-rays? Is there some calculation that includes that?

Pamela: Well, you can look up albedo across the different wavelengths, but in general albedo refers to visual light. Different objects actually have completely different albedos in different colors, so you might find that something entirely reflects infrared light while lets visual light pass straight through it. This is actually one of the things that we have to deal with with global warming. You end up with clouds that reflect infrared light that’s trying to escape out into space that instead reflect that light straight back down to earth. And so while we’re able to get sunlight in visual colors through those clouds down to the surface, they instead are able to reflect IR trying to escape back out back into the earth and warm up the surface of our planet.

Fraser: Right. So the clouds are highly reflective to the infrared spectrum but transparent into the visible spectrum.

Pamela: And what’s even worse is invisible things like carbon monoxide in the atmosphere is also capable of doing this. Water vapor in the atmosphere is capable of doing this. Many different chemical compounds all like to reflect the infrared light.

Fraser: And it also plays a role a bit in the ice caps where the ocean water is dark while the snowy ice caps are white. So the thinking goes that the more of the ice that melts of the ice caps, then that’s going to cause a bit of an accelerated warming because less of the light is being reflected back away and more is being absorbed by the ocean water.

Pamela: We see this accelerated heating in a lot of different ways. You have as the ice melts and reveals the soil, the soil instead of reflecting the light absorbs the light, heats up, heats up the air around it, drives more melting, and you end up with this runaway melting effect. We also have problems when we build cities, the cement–it just absorbs the heat and you end up with these cities being these extremely hot areas. At the same time though if you chop down a forest, well forests are very good at absorbing the heat and warming up the tropics. Cut them down and instead reveal meadows, instead in some cases let areas become desolate, reveal deserts because you’ve destroyed the land. Those deserts are then going to be reflecting the sunlight back out, and we’re going to have cooling when the forests get cut down. So we have all these strange things going on where in some places by chopping down forests and letting the wrong things happen we end up cooling the areas, but at the same time if you chop down forests and plant farms, a lot of crops that farmers like to plant are even better at absorbing and heating up from the sun than the forests are and you can in some areas raise the year-round average temperature by as much as 3 degrees Celsius just by planting where there used to be a rain forest.

Fraser: Right. So then in terms of astronomy, obviously then it’s only the… you called it the incidented…

Pamela: Incident light.

Fraser: That’s the reflected light? Is that right?

Pamela: Well, the incident light is how much light hits an object. It’s just a fancy way of saying light hits something.

Fraser: Right, so it’s like what percentage…

Pamela: Right. What percentage of the light that hits an object… what percentage of the incident light is reflected off gives us the object’s albedo.

Fraser: And the percentage that isn’t, is absorbed.

Pamela: Is absorbed…yep.

Fraser: It’s on a photon by photon basis, right?

Pamela: Right. And so beyond worrying about global warming, albedo is also something that allows us to start guessing at compositions, guessing at sizes. This is one of the things that recently came up looking out at the dwarf planet Quaoar, and one of these days I’m going to have to find out why we were given such an utterly inpronouncable object. But this little rock out in the Kuiper Belt keeps having its size misestimated because when we look out at it, it’s just a little bit too small to fully resolve. So in an ideal world, if you want to figure out how big something is, you figure how far away it is, then you measure how far across it appears… how many pixels it takes up on your camera. And then you convert from its angular size and distance to its actual physical size. But when something’s one pixel across, that’s not useful because it could be a lot smaller than the one pixel… yeah… you just have to start guessing based on how much light you’re seeing from the object. You assume that a very bright object that’s made out of ice has one size, a very bright object made out of rock is probably a whole lot bigger because rock doesn’t reflect light very well. But you have to know something about the composition. Well, with Quaoar, we have this object that we can see. For a Kuiper Belt object, it’s fairly bright, so we were able to guess it’s fairly big, but it wasn’t big enough to directly detect. So, we’ve been trying to measure its mass, measure its density, by looking at little Weywot, the moon orbiting Quaoar, and Michael Brown, out looking at it, was able to figure out, based on the orbit of this little moon going round and round, that Quaoar is dark, dim, far away and nowhere as big as we thought. It looks like it probably only has a diameter of 1200 kilometers. And that is bigger than we thought, because when we thought it was ice, we guessed smaller. But, it’s still really, really tiny.

Fraser: Definitely not bigger than Pluto.

Pamela: No, no.

Fraser: And I know that the consistency of all of these objects in the Kuiper Belt, have actually fairly vastly different albedo values. So it’s not like you can say, oh, it’s a Kuiper Belt object and they’re all snowy ice of a certain number, and so you can just look at it and know how bright it is, and then use that to know how big it is. It took that second calculation it needed to have a moon, and from the moon you can then calculate the mass and get at the diameter that way.

Pamela: Right.

Fraser: And then reverse-engineer the albedo.

Pamela: Right.

Fraser: So it’s funny, it’s very similar to whole kind of stellar magnitude problem, right? A star can be… you know in the sky, you can see how bright it is, but that doesn’t tell you how far away it is. It can be really close and be a dim star, or it can be really far and be a super bright star. So how has that caused people problems in the past?

Pamela: Well, it just means that we have these difficulties in figuring out all these little rocky bodies everywhere. There’s lots of asteroids, there’s lots of Kuiper Belt objects that their compositions vary from close to pure ice, where they have densities close to 1, to now we know Quaoar has a density that could be up over 5, which would mean solid rock out there hanging out in the Kuiper Belt with almost no ice involved. And, how dense something is is kinda interesting because we can start guessing what it’s made of. So we have to try to puzzle out all of these pieces. Now we can also learn interesting things about different features on the surface by looking at changes in albedo. For the objects that we can start to resolve, things that are more than one pixel across, which Pluto is. We can start to look at how its amount of reflected light changes as it rotates, as it goes from day to night. And this allows us to start figuring out, oh… this object isn’t just a solid ball of nice shiny ice, or nice dirty ice. It allows us to start sorting out with asteroids in some cases what amount of the asteroid you’re seeing as it’s rotating. So, if we know it has constant albedo, it constantly reflects the same amount of light, then as we see the amount of light change it means we’re going from seeing the broad side of the potato to seeing the skinny end of the potato to seeing the broad side of the potato again. But at the same time it could just be that we have a round object rotating that’s going from low albedo to high albedo. Once we can uncouple any of these problems, we can start figuring out oh, this has shiny ice here… oh, this has dark sooty material here, and we can start answering some rather interesting questions about composition.

Fraser: And does that perhaps tell you about… maybe there’s surface activity… or things are changing, right? Because if you track the albedo over years… and the same spots aren’t dark all the time, then maybe there’s, you know, cryo-volcanism…

Pamela: Outgassing…

Fraser: Outgassing, or changing atmosphere or clouds or things like that.

Pamela: And more than just looking at how albedo changes on a given object, because except with comets, that tends to happen very slowly. What it allows us to get at is how old is the ice in some cases. So if you look at a bunch of nice round moons and you find this one’s really shiny… well really shiny means fresh ice, fresh snow, you have something going on, like cryovolcanism, that is refreshing the surface. At the same time, if you look at something and it’s not highly reflective, that means that you probably have ice that has been dealing with solar wind incident on it, solar radiation incident on it, that’s caused chemical processes that make the surface get dingy that cause the albedo to drop, making it reflect less and less light.

Fraser: And so what is the mechanism that an astronomer uses to determine an object’s albedo?

Pamela: Well we know how much sunlight is getting radiated by the sun… we can go out and measure that conveniently. Then we figure out… ok, we think we know how big this object is. Based on our estimation of its size, we’re able to figure out how much sunlight at its distance hits it. So then you take the amount of light that hits it, the area that it has, and how much light you then receive. And by taking just what is received verses what you know hits it, you can calculate the albedo. The trick is just knowing the size.

Fraser: The only way to know the size is ideally to sort of see how many pixels it covers up in your screen, but if not, to have the mass of a moon that’s orbiting it.

Pamela: And that only gives us the mass… that doesn’t actually give us the physical size.

Fraser: Right, right… so then what do you do, right? How do you get at the size?

Pamela: Well, often we use Spitzer. Spitzer’s very good… it has… yeah… if something’s too small, you’re stuck. With things like these little Kuiper Belt objects we rely on Spitzer taking images and actually it’s more than one pixel across. And when it’s not, then you just have to keep getting better and better data until you get data that shows it’s more than one pixel across.

Fraser: That’s why when an object’s first discovered, astronomers will say it’s between 1000 and 1500 kilometers across because they just don’t know the albedo, and if they knew the albedo, then they would know the answer. There’s no way to know unless you can see a number of pixels.

Pamela: And this is where we guess composition. If you know something is ice… and here we can actually get help because looking at reflected sunlight, different chemicals reflect sunlight differently. So if you look at specific colors, you’re able to go ah, there’s methane ice, ah there’s carbon dioxide ice. Once you start to put together known compositions, it allows you to have error bars, but at least you’re not completely pulling numbers out of a hat.

Fraser: And there was really interesting research… I did an article about it a couple of years ago, about how astronomers might use the albedo of an Earthlike planet to detect how it reflects on its nearby moons, to determine if it’s got clouds, and if so then maybe continents and water and all that kind of thing.

Pamela: Right. There’s been some interesting work just looking at the light that reflects off the dark parts of the moon, where you can use Earthshine to start figuring out… ah, this is what’s going on on the planet Earth. And we don’t think about it because when you look up and don’t see the moon, it tends to be out of sight, out of mind… but if you start using careful instrumentation, the moon’s still there. It’s just not as visible to the naked eye, but all of the light that gets diffused through our atmosphere, all of the light from cities, all of the light that is passing through clouds and getting reflected through our atmosphere from the clouds. All of this is scattering up to the moon. We can measure that and we can start to basically puzzle out what’s going on on the surface of our planet that’s getting reflected.

Fraser: And one of the interesting things I know is that this Earthshine has been changing over the last few years. So the actual amount of light has been changing from what we’ve been recording.

Pamela: So this is partially, from what I understand, a reflection of pollutions in the atmosphere that cause light that hits the atmosphere on the sunward side of the planet to get bent through the atmosphere and to come out the other side in slightly different ways than without the pollution or with different types of pollution. And at the same time, the cloud cover on our planet has been gradually changing as we throw up different chemicals into the atmosphere. So we are definitely changing the way our own planet behaves, and we can see that reflected in the Earthshine off of the moon. Now trying to detect this around alien worlds is gonna be difficult, but within our own solar system, at least we’re able to say, ah, there’s carbon soot over there. One of the interesting things is the composition is not only reflected in the scattered light, but it’s also reflected in the types of pollutions that get left on things. So when you look out at a glacier, one of the problems they’re having in India is the glacier melt is accelerated by two causes: one… you melt glacier, reveal soil, soil absorbs heat, that heat gets reradiated and melts the glaciers faster. But the other problem you get, is that with all the pollutions we’re letting loose into the atmosphere, that pollution first of all affects how much light our atmosphere scatters, but then when the pollution settles out… when it creates a sooty surface… not just on statues and buildings but on the glaciers, well now those glaciers aren’t shiny fresh snow and ice anymore. Now they’re sooty, nasty, ooky, snow and ice. We’ve all seen this in cities… well, this is happening to glaciers too. And carbon, of all the substances, has the darkest albedo. Carbon, charcoal, coal are the most absorbing substances we know. And so when you get soot coating a glacier, that can accelerate the glacier’s melt by as much as 25%. Now while we produce coal and soot through a lot of industrial processes, you can also get this sent up into the atmosphere through wildfires, you can get this sent up into the atmosphere via volcanism, and so this radical change in albedo, if we were ever able to see it on another world, could reflect some sort of a massive ice age, increasing how much light is reflected, or some sort of horrific wildfire or volcanism causing a planet to suddenly go dark.

Fraser: So, speaking of things that are dark, what is the darkest object in the solar system?

Pamela: Well, exactly what the darkest object is isn’t something that’s easy to find, because we’re finding new rocks everyday. But when you start looking at the big things, Mercury has an albedo of 0.1. This means only about 10% of the light that hits it is getting reflected off. This is one of the darkest objects, and then of course there’s lots of asteroids that are in the exact same category of reflecting only about 10% of the light.

Fraser: And so like the surface composition of Mercury is like lava… cooled lava, and…

Pamela: Right…

Fraser: And meteor-blasted ground, so…

Pamela: It’s basically a lot of basalts… this is the same stuff that the mare on the moon–the dark parts of the moon–are made of.

Fraser: So it’s lava rock everywhere, yeah.

Pamela: Right. So, when you’re looking at these and you’re looking at carbon-based surfaces, all of this stuff is nice and dark. It’s materials like the feldspar in the lunar highlands… these are highly reflective objects. And then like I said… snow and ice… anytime you have snow and ice it just reflects light.

Fraser: And then, brighter than that? Where do we get brighter than that? The moon is actually only a little brighter, though… the moon only has an albedo of 0.14. So it’s pretty dark, too.

Pamela: And that’s the part of the moon that we’re seeing, where again we’re seeing a lot of these basalts, we’re seeing a lot of these lava-based rock. Where we start to get brighter is as we start to get either gassy planets that are able to… off the clouds… reflect a lot of light. That’s another thing that reflects well is clouds. So, when you get cloudy planets, when you have things like Venus… Venus is reflecting all but 35% of its light. It’s reflecting 65% of what hits it back so we can see it bright as our evening star. Our own Earth, though it’s this mix of clouds, mix of water which absorbs most of the light that hits it, it’s a mix of soils and forests and this mixture when we look at the planet Earth, leads to about 40% of the light getting reflected, so it’s not quite as bad as Mercury.

Fraser: And we talked about the brightest object in the solar system…

Pamela: So here we’re looking for the icy objects, we’re looking for things like Eris, like Enceladus, any of these icy moons that are out there. But then again, we also have gassy things. Not up at the 90% level, but Jupiter’s out there reflecting 52%. Venus is out there reflecting 65%, so there’s a whole continuum of ways that you can get bright objects.

Fraser: Right. And could you imagine if the moon was icy like Enceladus, it would be like almost ten times as bright, I mean eight times as bright.

Pamela: Right, and that would make reading by moonlight far far easier. But unfortunately, as close in to the sun as we are, that just wasn’t part of the fate of the moon.

Fraser: Alright, so that wraps up our discussion on albedo… thanks a lot Pamela.

Pamela: My pleasure.

Fraser: Talk to you later.

Pamela: Talk to you later… bye, bye.

Transcript: Mysteries of the Universe, Part 2

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Fraser: Astronomy Cast Episode 179 for Monday March 1, 2010, Mysteries of the Universe, Part 2. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we 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. Hi Pamela, how’s it going?

Pamela: It’s going well. How are you doing, Fraser?

Fraser: Good! It’s actually exactly the same as I was for the last show because we’re recording 5 minutes after we finished recording the previous show, so whatever answers I gave you last time–they still stand. All right… so today we tackle more thrilling mysteries of the universe. And by tackle, we mean acknowledge their puzzling existence. Some mysteries will be solved shortly, others will likely trouble astronomers for centuries to come. Join us for part two. Alright, so this time we’re going to focus on some massive problems–galaxies. We talked about the Milky Way, but now we’re going to talk in general about some galaxies and their formation. So here’s the first question–do galaxies form bottom-up or top-down? You threw that question into the mix, so I have no idea what you’re talking about. So what is your question?

Pamela: So there’s two basic ideas on how galaxies originate. One is you take giant clump of gas and dust and other material and let it collapse and you end up forming giant galaxy. The other is you take small spud… form small gas cloud… take another small spud… form small gas cloud… start throwing these things together… they merge… form something slightly bigger. Throw something else in there… it merges… gets slightly bigger. So the idea is either you have galaxies form all at once from the collapse of a giant cloud of gas, dust, and stuff, or you have bunches of little tiny things that collapse out gravitationally, and then together build bigger and bigger objects.

Fraser: Can I take a stab at it?

Pamela: Yeah!

Fraser: I think bottom up… let me tell you why. When you get really big spiral galaxies colliding together, they’re coming at bizarre angles and you get these great big elliptical galaxies. So, if you collide two beautiful spirals together, you get a mush. And so if all the little galaxies were coming together, you would just get mush on top of mush on top of mush. So you would end up with just elliptical galaxies everywhere you looked. A spiral galaxy seems to indicate that one big gas cloud is all just coming together and turning into a spiral. That’s my theory.

Pamela: Now you’d think that. And there’s a lot of great papers out there saying that, but then when we look out there we can actually start to figure out how you make spiral galaxies. So, we can do both… and this is the problem.

Fraser: ‘Course somebody would have thought of that, Fraser… duh.

Pamela: Yeah, yeah…

Fraser: Right. So you’re saying that…

Pamela: You throw things together just right, and you get a disk. Now we’re still trying to figure out where the heck spiral arms come from. These spiral density wave things are kind of crazy. But they work! And they generate spiral arms… we just don’t know where they come from. So, we know how to make disks… you just throw things together like pizza pie and spin them and they flatten nicely.

Fraser: Right, like a solar system… like the way our solar system formed from one gas cloud.

Pamela: Well, you can also do it by throwing things together… small clumps…

Fraser: Right. But it has to be small clumps that all came in on a common center of rotation, right?

Pamela: Well, it depends on the rate at which they come in. Things can get absorbed in. Things can shake themselves out and end up flattening the disk. This is where you have spirals that have warped structure but only for a little while. They ate something that was a little too big and it shook them up. But, over time they flatten themselves back out.

Fraser: Ok, so then the thinking then is you take a galaxy, and as long as it has enough time to spin, it’s going to spin itself back into a nice roughly circular shape. It would be like me spinning a pizza pie–the crust–in the air, then adding a few more globs of dough to it and then giving it enough spins that it flattens itself back out again.

Pamela: As long as you don’t hit it too hard… now you take that nice pretty disk and you hit it at a right angle with something else that’s huge… and it’s just going to get obliterated into nothing.

Fraser: And that’s when you get your big elliptical galaxy.

Pamela: Exactly. So, we think in the modern universe… think… don’t know for certain… think… this is why it’s a mystery… that most galaxies are probably formed by little spuds coming together and building bigger and bigger things. But the problem is, as we look back at the early universe… we still find giant galaxies. And these giant galaxies haven’t had time to form by little things coming together. So we think in the early universe, when you did have giant clumps of stuff floating around in these occasionally anomalously large over-densities, we think that occasionally you were able to have these giant collapsing clouds that formed all at once a giant elliptical galaxy.

Fraser: So is it option C… both?

Pamela: Yeah, that’s what we think. But we don’t know for certain! But this is the type of thing that we should be able to answer in the next few years, and hopefully we’re going to be able to get a good handle on it with the James Webb Space Telescope.

Fraser: Right. And this is the telescope that’s going to be looking at infrared, and so it’s going to be able to see the earliest moments of the universe when visible light is red-shifted out to the infrared, and it should be able to see those either giant galaxies forming all at once or those smaller galaxies coming together. So would you be almost least-surprised to see it be both? To see big galaxies forming and small galaxies coming together?

Pamela: I’m aiming for both.

Fraser: You’re aiming for both… huh…

Pamela: Because “both” gives you this one concept of giant cloud, where giant cloud has varying degrees of giantness, collapses to form something. Sometimes those somethings are really tiny and those tiny things merge to get bigger and bigger. But occasionally, you end up with giant elliptical galaxy all at once. And that’s kinda cool.

Fraser: Yeah… I like that. Ok, well then let’s move on to our next question then… now that we’ve solved that one. So which came first… super-massive black holes or their galaxies? We now know that every galaxy pretty much has a super-massive black hole lurking at its heart and that the mass of that super-massive black hole seems to have some relation to the mass of the galaxy. Big galaxies have massive black holes, and small galaxies have less-massive black holes. So the question is do we get a super-massive black hole and then it’s able to attract enough galaxy around it, or when you get a galaxy is it forming a super-massive black hole that’s to scale at the center?

Pamela: And this is one of those things that we’re still sorting out. As we look around, it’s not just the size of the galaxy that the black hole is related to, but very specifically the size of the bulge of the galaxy. So in a spiral galaxy this is that round basketball that seems embedded in the center of the galaxy. In giant ellipticals, it’s just the whole giant elliptical. And consistently, whether the super-massive black hole is millions of times the mass of the sun or billions of times the mass of the sun, consistently it’s about 1/1000 of the size of that bulge in mass. And as we look further and further back in time, we eventually start to hit the point where the galaxies hadn’t quite formed yet. And this is where it gets interesting, because the super-massive black hole had to come from somewhere. It had to eat something to get big, and…

Fraser: It had to accrete, right, it had to form star after star, gas after gas to get bigger and bigger and bigger.

Pamela: Right. The way we know things formed in the early universe is that you started with dark matter, and then the regular matter flowed into the dark matter, and what it’s looking like is maybe… but we don’t know if this was always true because we’re working with an observational sample that you can count on one hand… but it’s looking like maybe the black holes formed first, but what did they form out of… is it simply that you had all the material in one of these dark matter halos collapse down to form a super-massive black hole or is it just the ones we found so far are the naked ones, and as we keep looking we’re going to find ones that are completely surrounded by material. We’re just not sure.

Fraser: So then to give evidence one way or another… what would we be looking for? Would we be looking for a large galaxy that seems to have no super-massive black hole in it?

Pamela: Well, what we need to do is keep looking back and back and back until we find the smallest critters can be defined as a galaxy. And look to see do they still have this ratio of 1/1000 for the black hole to the galaxy mass. At the point that that ratio breaks down we should be able to say “Ah more mass, mass must have come first and mass collapsed into black hole, or ah, more black holes black holes must have formed first.”

Fraser: So, it’s that ratio… that ratio holds true in every galaxy we see around us right now… we just keep looking further and further back in time, further away, until we see it push off that ratio, one way or the other.

Pamela: And we need to consistently see with a sample size bigger than you and I can count on our combined fingers and toes.

Fraser: Right, because right now all we’ve got is gravitational lensing, these…

Pamela: There’s a few examples where we’re looking in the radio… but they’re rare.

Fraser: Right. But once again, James Webb coming to our rescue should help us solve this one.

Pamela: Exactly.

Fraser: So, do you think this is another one that we should nail within… the next decade?

Pamela: I do, I do… I think this another one that James Webb is gonna… I think this one is a combination of James Webb and the Atacama microwave millimeter… ALMA I think it’s going to solve that problem for us… Large Array.

Fraser: Right, and if you had to take a poll… I know this is pointless, but where would you come down?

Pamela: I’m going to…. oh God, I would give you a different answer on a different day of the week. I don’t know. Based on the fact that I’ve been eating gummy bears I’m going to say black holes first.

Fraser: Ok. And is it possible that it’s both? That the black hole formed as the galaxy formed around it in perfect lockstep?

Pamela: I actually wouldn’t be surprised if it’s some combination of the amount of dark matter versus regular matter in a specific over-density affects which happens.

Fraser: Right.

Pamela: And I don’t know how those two play out.

Fraser: Ok. Alright, well let’s move on to the next question. So, your next question, and this is another one that you threw into the mix which is where are the green galaxies? Should there be green galaxies? Because I thought that there really shouldn’t/couldn’t be green stars. Because it’s the way the photons add up… with a star you’re not going to get green. But would you get a green galaxy?

Pamela: This is a matter of green by eye versus green on paper. There aren’t green stars by eye, but if you look at the color of stars on paper, mathematically, what is the wavelength of the peak color of light coming out of the telescope… green stars are out there. We just don’t perceive them as green.

Fraser: Because we’re seeing… even if they’re mostly green… we’re seeing enough on both sides of green that it looks some other color.

Pamela: Yeah, we see them as white, which is kind of annoying.

Fraser: They seem white to us…

Pamela: Yeah it’s boring… Now the problem is on paper, you take galaxies and you do a plot of color versus luminosities and you get this beautiful red cluster… beautiful distribution of red galaxies. All the galaxies with dead stars… mostly ellipticals, not all… there are a very, very tiny rare, rare fraction of ellipticals that are blue… just to be surprising and odd. But, nice, beautiful red branch of galaxies. Then you have this cluster of blue galaxies in the same diagram…

Fraser: Right. With their furious star formation…

Pamela: Right. And some of them have less star formation than others… no big deal. So in this beautiful diagram, you can pretty much draw a line through the valley of green, where there aren’t any.

Fraser: And yet these pretty charts predict them.

Pamela: Well, that’s the thing… they’re not really predicted. There’s just no real reason that they shouldn’t exist. What looks likes is happening is you have galaxies with lots of nice happy star formation…. star formation… star formation… blue galaxy… happy blue galaxy making stars. Then you have galaxies–no star formation. Red stars everywhere. But that intermediate that would give you this nice mix–it leads to green. There’s a few examples in there, but mostly you just have this valley of nothing. So, for whatever reason, across all the different types of galaxies that are out there, star formation has this tendency to just shut off abruptly. And when it shuts off, it’s that abrupt shut off that leads to this valley of green.

Fraser: And it goes red…

Pamela: It goes red.

Fraser: It goes blue to red and it doesn’t have…. and so I guess if we saw green, we would see sort of a slow turn-off of the star formation. We would see a mixture of star-forming and not-star-forming, and then we’d get that in-between stage, but we don’t see that… it’s as you said, it’s a party, and then the party’s over.

Pamela: Yes.

Fraser: Yeah. Hmmm. As opposed to something that’s sort of in-between. Are there any examples at all? Or not?

Pamela: The valley of green isn’t completely empty. But it is still this deep, deep valley in the color-magnitude diagram of galaxies.

Fraser: And so is there any reason? What do you think?

Pamela: Well looking at the things that trigger star formation and end star formation, we have… Quasars have the ability to strangle galaxies. First giving off so much light pressure that they clear out the region around them while at the same time hungrily eating at the beginning. That has some effects on star formation. We have galaxy collisions can cause rapid-fire star formation that eats up all remaining gas and dust in spiral galaxies, and when it’s over, it’s over.

Fraser: So, we have a lot of mechanisms that make star formation start…

Pamela: And, if you have a system that hasn’t had one of these traumas… star formation is just going to keep going par normal. What we don’t have is a mechanism that seems to allow a galaxy to just casually peter itself out… instead they like to die by collision, die by harassment, die by ram pressure stripping, which is just the dirtiest phrase a galactic astronomer ever came up with.

Fraser: So is there some mechanism then that turns off star formation as abruptly and violently as it’s begun?

Pamela: It just… well in all these occasions where you end up with rapid violent star formation, that rapid violent star formation burns through all the gas and dust quickly or blows it out of the system, and it’s usually a combination of the two. What we’re missing is the opportunity for a nice normal galaxy like our own Milky Way galaxy to simply peter itself out. To simply slowly and aging with grace, run out of star formation. And so instead what we end up with is happy blue spiral, spiral that has had a hard life and turned red, violently blue spiral that is in the process of being destroyed, and nothing really in between.

Fraser: Well, let’s move on then. Our next simple question is what is dark matter? And this is a good one because I think we’re getting some pretty tantalizing evidence. Last show we talked about dark energy and you gave it a 50-50 chance that we’d figure it out in our lifetime. But dark matter… dark matter is getting close. Set the background, then, on what dark matter is or what we know… another place-holder name obviously.

Pamela: It started out as a place-holder name… it started out as the way we refer to whatever stuff it was that was causing galaxies to rotate as though they had a lot more mass than we could find with radio and optical and other forms of light telescopes. It was the word we gave for whatever it was that caused the galaxies in clusters to orbit one another too rapidly. All of these places as we look around the universe we see things moving and acting as though there’s substantially more mass than what we can see. That unseen mass we call dark matter.

Fraser: Right, and there were two theories, right? There was that there was a particle that didn’t give off any kind of electromagnetic radiation…

Pamela: Or particles…

Fraser: Or particles, yeah, a collection of particles… a zoo of particles… but yet they could still influence one another and regular matter through gravity. The other theory being a modification of changing gravity as we know it… that over the long distances gravity acts a little funny. But I think now with the evidence that’s piling up, you can sort of get rid of the second theory, right? We can actually see dark matter being separated out of galaxies… stripped away or condensed together… through gravitational interactions, so there’s clearly some great big cloud of particles surrounding galaxies, influencing it through gravity yet invisible to electromagnetic radiation.

Pamela: Right, and not only invisible to electromagnetic radiation, but also just plain refusing to play nice with the electromagnetic force. So whatever this stuff is, and we call it generally… we think it’s some sort of non-baryonic matter, stuff that isn’t like protons and neutrons, whatever it is, it doesn’t interact via the electromagnetic force, it doesn’t interact via light or interact with light or do anything regarding light except gravitationally reach out and change the path of light. And that’s how we find it. We can look through space and see how light from the most distance galaxies gets distorted by the gravitational pull of unseen stuff. We can map out the distribution of this stuff and this is where we’ve learned its distribution around colliding galaxies, this is how we’ve learned its distribution in clusters of galaxies. We’ve been able to come up with phrases that don’t sound pretty… it’s collisionless particles… particles so tiny in cross section that they don’t generally interact with one other directly through collisions.

Fraser: And like we would experience air as particles colliding together… that’s air pressure… particles banging into each other and banging into us, but this would be particles that don’t even do that.

Pamela: Right, right. So, very small cross-section, doesn’t interact via the electromagnetic force, just generally doesn’t interact with anything. And we have experience with things like this… we just call them neutrinos. And neutrinos may actually be part of what makes up dark matter. There’s a whole lot more out there and it’s possible, if the theories of super-symmetry are right, that the Large Hadron Collider, as it does its experiments, will be able to detect the lightest of these super-symmetric particles that might be dark matter. So, we’re getting there.

Fraser: But there have been more discoveries in the last… even this year, right?

Pamela: Right so we’ve been looking…

Fraser: Closing in on dark matter…

Pamela: Right, so we’ve been using the same types of detectors that we used to detect neutrinos to try to find dark matter. And there’s been some results out there that look like maybe just maybe with more repetition and more crunching and more testing, maybe we’re starting to detect some of these generally refusing-to-interact particles. Because even though they have a small cross-section, that doesn’t mean that they’re zero in size. So occasionally they will cause something to happen, they will cause something to flicker. And it’s those flickers that we’re looking for.

Fraser: And so it’s interesting, even when we were beginning this show three years ago… almost four years ago, we would talk about it, lending a lot of equal credence towards particles modified…

Pamela: Modified Newtonian dynamics…

Fraser: Modified gravity theory… but now I think we’re talking about particles, we’re talking about certain characteristics of particles, what they’re kind of like, the methods we’re going to be using to find them, so it’s interesting to see those theories evolve and so how do you like our odds?

Pamela: I think it’s looking great. For me the turning point for me was when the Bullet Cluster images came out. We’ve also had the COSMOS project which mapped out dark matter.

Fraser: And for those of you that don’t know, the Bullet Cluster–this is this example where you had two huge clusters of galaxies coming together, and the stars were passing right past each other, the dark matter was passing right past each other, but the gas was colliding and mixing in the middle, and so you got this separation like someone had taken a flour sifter to a galaxy, right, and you got the stars and dark matter on one side and you got the gas separated out from it. So clearly there’s some thing that’s there. It’s just amazing.

Pamela: Yeah, it’s so close. We’re getting there… soon.

Fraser: Yeah, so there you go. What is dark matter? We don’t know, but we hope to figure it out soon. Alright, well then as a relation to that question then, where are the dark matter galaxies? So if we do have this process where dark matter is being separated from galaxies, or perhaps they’re just as formed after the Big Bang, could you end up with whole galaxies that are just dark matter?

Pamela: And that’s one of the really, really interesting mysteries that we’re still working to sort out. And the COSMOS project started to get us closer. It did a map of the distribution of luminous matter–the normal stuff we can see–and of dark matter based on gravitational lensing. And what they found was, there are places where we have over-densities… where we have extra amounts of dark matter and there isn’t luminous matter there as well. We’re not seeing the nice dense things we’d identify as a galaxy. Instead we’re seeing these big amorphous halos, but new research is also showing that dark matter interacts weird when you start getting really dense gravitational wells. It doesn’t seem to interact with black holes the way normal matter does. We’re still sorting this out and I have to admit that I need more sleep to reread the paper to better understand the results.

Fraser: But the thinking is that dark matter doesn’t even make its way into black holes in the same way. It just zips past. I guess the point is because regular matter has that larger cross-section, it’s bouncing into itself around the outside of a black hole, and it’s subject to tidal forces, but the cross-section of the…

Pamela: And there’s frictional slowing…

Fraser: Yeah, but because this stuff… you could pile mountains and mountains of dark matter around itself and it’s not going to really be bonking into each other. You’re not going to get that frictional slowing.

Pamela: And it may be that you just can’t get without a black hole in the center, the same sorts of density gradients that we’d recognize as a dark matter galaxy. It may be that we just can’t get that nice super-dense center followed by either a surrounding halo or a surrounding disk. But we do know that there are large amorphous boring-shaped but completely dark density areas of dark matter.

Fraser: So it’s almost like you can get regular matter to do things, but you can’t get the dark matter to do anything, and so you can separate out the regular matter, but you’re still going to end up with a ball of dark matter that isn’t going to collapse, that isn’t going to form, and it isn’t going to black-holify, and it’s just kinda there… doing its own thing, not playing by the rules.

Pamela: So we need to look at more of the universe using gravitational lensing to try to find the distribution. We’ve only looked at a very narrow basically straw through the galaxy.

Fraser: But we don’t see… so far we don’t see dark matter of differing densities, is that what you’re saying?

Pamela: Well, we see it of different densities, but we don’t see any really high densities that look like galaxies, but we need to look more.

Fraser: As you said, we’re looking through a straw and we need to do a better survey. Yeah, are there plans in the works for that?

Pamela: There are lots and lots of different survey teams working to look at these things.

Fraser: Right.

Pamela: It’s just slow.

Fraser: Very cool. Well thanks Pamela. We’ll keep rolling. I can see my list… there are more mysteries.

Pamela: Sounds good Fraser… I’ll be talking to you later.

Fraser: Alright, I’ll talk to you later.

Pamela: Bye-bye.