We finally get organized enough deal with several listener questions: isn’t dark matter just regular stuff we can’t see? how can parts of the Universe be expanding faster than the speed of light? what will Betelgeuse look like when it explodes as a supernova? what’s the speed of gravity? All these and more questions are answered.
(transcript provided by listener David Madison)
Fraser Cain: Hi Pamela
Dr. Pamela Gay: Hey Fraser! How’s it going?
Fraser: Good. Good. Well I’m really excited this week cause we’ve got lots of questions. Actually, we’ve been kinda holding onto telling people we’d be answering questions. We’ve been holding onto them for the better part of three months now, so we have a bunch of good questions to go through. We’ve also got some great audio questions last week which was wonderful. So we’ll be able to play a couple of those. Uhm, and if anyone else has questions from here on out we will be doing these question shows regularly so if you have questions either on a episode that we do or you know you have a more general question about astronomy just send it in and we’re happy to give you a phone call and say you know record your side of the call so that we can actually get your voice on recording to do the show.
So you all ready?
Pamela: I think I’m ready.
Fraser: All right. Ok. So here’s our first audio question which is great. This is from Paul Nelson and he wants a little more information on dark matter which we covered in episode 4.
Paul Nelson: Hello Fraser and Pamela. This is Paul, one of your long time fans. My question has to do with dark mater and it is, why do we think that this matter is a new kind of mass? Why can’t it just be ordinary matter that we just can’t see, is invisible?
Fraser: Ok Pamela. So why is dark matter not just like regular mater? What is it?
Pamela: Well, we’ve been looking for it, and that’s the problem. It’s the more we look for it, the more we don’t find it. And, if it was normal visible matter, we’d be able to see its effects. We’d either be able to see it radiating light, and we don’t see it radiating light. We’d be able to see it blocking light.
For instance, dust. It’s all over the place. And it gravitationally attracts, and dust was for a while a form of dark matter until we figured out how to detect it. And the way we find dust is it’s warm, and so it radiates very long wavelengths of light in the radio and it also blocks light so just like you can see dust motes scattering light as sunbeams go through a dusty room, light from distant galaxies just traveling to Earth gets scattered by dust. So we don’t see dark matter emitting light, we don’t see it blocking light, and we also look for it to magnify light.
There are a couple of different teams that look for gravitational lensing events. This is where the gravitational pull of a high mass object bends light that would otherwise shoot off towards another part of the universe at us here on Earth. So we see more light that we normally would from a distant object. If a lot of these compact objects were out there, we’d see a lot of these magnifying events. And we don’t.
So, we’re not finding dark matter that is magnifying light, we’re not finding it that is emitting or blocking light, and these are all things that normal matter would be doing. So it has to be something else.
Now there are cases where we do see gravitational lensing due to large amounts of dark matter that form halos around large massive systems like galaxies and galaxy clusters, but we’re not finding small balls of it in the numbers that would account for all the dark matter that has to be out there. So when you eliminate all the things you know about, it has to be something you don’t know about. It’s just returning back to Sherlock Holmes.
Fraser: So it’s like we can detect its gravitational effect on the galaxies themselves, but we don’t see it as a compact object.
Pamela: Right. And we don’t see it as small, normal things like dust grains, and we don’t see it as stuff that’s emitting light. So it sort of limits what it can be and what’s left is what we don’t understand.
Fraser: All right. Well, I hope that answers Paul’s question.
Ok. So question number two. Charles London asked a bunch of questions but this is one I really liked; I picked this out of the bunch. So, this is what he had to say. He says, “I’m amazed that scientists can discuss with such certainty about events that happened 10-30 seconds after the big bang. How do people figure out this stuff?” And I think that’s a really good question because there’s no way we can actually go to that moment in time and see it. We can’t see it with our telescopes. So how can we know?
Pamela: Well, and the truth is we actually can’t see anything that happened more than about 400,000 years ag…400,000 years after the big bang occurred when the cosmic microwave background was emitted. So, the whole first almost 400,000 years of the universe is beyond our ability to observe at all, but…
Fraser: That’s because the universe was totally opaque;
Pamela: It was completely opaque. The light couldn’t travel to get out be free and reach us and out telescopes here on Earth. It was sort of trying to look through a fluorescent light bulb. You just can’t do it.
But, you have all these theories that as you said go back to 10 to the negative 30th of a second after the big bang and that means it’s zero point 30 zeros and a 1 seconds after the big bang. And the way we’re able to build theories that tell us what we think was happening way at the very beginning of the universe is lots of math. But it’s math that conforms to the observed universe. So, really smart people, smarter than I will ever be, with computers, sit down and they say, “Ok. Here’s what we’ve been able to observe so far. Which, we look at the stars. We look mostly at the structure of the galaxies around us. We look at the large-scale structure of the universe. And, we look at how that’s evolved over time. And we look at the cosmic microwave background and we look at the structure of it.” And using all of these constraints, they build theories that start at time zero and work forward and have to match what we observe in the cosmic microwave background. They have to match what we observe at the universe was a billion years old, the universe was 5 billion years old. And so their models have to grow with the universe. And some of the really good ones are able to predict things that we haven’t been able to observe yet. There was a time before WMap (7:14) when we didn’t have a really good map of the cosmic microwave background. We had Ok maps. Cobe (7:22) did a really good job. But they didn’t get finest of details.
Fraser: So let me see if I understand. The scientists will make these kinds of predictions going back using the math and using their understanding of physics right back to the first moments of the universe. You know, if you push matter this tight together, this is what it should do. If you push it that tight together, this is what it should do. And then, you can then run that process forward again with the expansion with different things happening at different times and you should be able to find things that you can then see in the real universe?
Pamela: And that’s exactly what they’re doing. They made prediction about fine structures, and the numbers of these structures and the cosmic background and what was observed matched with the theories. The predicted how galaxy clusters should form over time. How many there should be at different ages of the universe and they found that. So the theorists are building on current observations and predicting what future observations should hold. And they start their clocks at moment zero and build forward in a completely consistent way such that each step builds on the previous step, builds on the previous step and has to match with what we have observed once they reach the points in time that we’ve been able to see.
Fraser: So it’s not just, I guess inventing numbers, it sounds like it’s a very rigorous process that they have to demonstrate, you know, in the real universe that we see to make those numbers match up. That’s really amazing.
Pamela: It’s a very complicated process. They build three-dimensional models of what the universe should look like in a box in their computer. It’s really quite beautiful work.
Fraser: All right. So, we’ve got another question. Here’s an another audio question about the speed of gravity from John McKee.
John McKee: All right. This is John from Santa Clara. Here’s a question for you. What is the speed of gravity? We’ve heard in recent podcasts about how supernovas eject all that matter out into the universe. Well, if a massive object like a star bends the space around it into a gravity well, an event such as a supernova must certainly have a gravitational consequence. Can we detect that? And if so, how fast does the effect travel? Is it bound by the cosmic light speed limit or does it somehow break that law?
Pamela: Well, in theory, supernova, when they explode, unless they are absolutely perfectly symmetric, which means there is there are the exact same number of blobs of material flying out in exactly all dimensions all the way around the sphere. That usually doesn’t happen. But, in every case, except for those probably impossible symmetrical cases, there are gravity waves emitted when a supernova explodes.
John McKee: Sorry. What’s a gravity wave?
Pamela: A gravity wave is a wrinkly created in the space-time continuum by gravity.
John McKee: I’m sorry I asked [laughter]
Pamela: So gravity manifests itself by bending space. This means that you are sitting in a pit of your own space-time continuum cause you have mass. So you’re curving the space around you towards your body. It’s a very tiny tiny effect and the gravitational pull of the Earth makes a much bigger dent. So you are just basically an itsy bitsy little tiny dimple that no one can really notice in the gravitational hole of the Earth.
John McKee: Right. But I am bending space and time around me.
Pamela: And if you move, and you don’t move at a constant rate in a straight line, the way you move is going to create waves through the space-time continuum the same way you can create waves through water.
John McKee: And would someone with very good equipment, you know, I guess, theoretically be able to detect my movements from afar?
Pamela: I think you’d need equipment that we aren’t capable of creating on the planet Earth.
John McKee: No, no, no. I mean, you know, in theory.
Pamela: In theory, in a mathematical model, yeah, sure, and more importantly, it’s possible that if two big enough objects are orbiting one another, like to black holes, we’ll be able to detect those gravity waves. And supernovae also create these gravity waves, and these waves propagate though space at the speed of light. This is one of the great things about the theory of relativity is it talks about how it takes time for all changes to communicate that change through space. So when I turn on a light, that light takes time to reach you. When a star explodes, its gravitational field changes because the bits of star flying off in all directions. And it take time for those changes in its gravitational field to communicate themselves to other parts of space.
Fraser: And to just use an analogy like if we, you know, if the Moon, sorry, if the Earth disappears suddenly just phipp, just completely disappears, the Moon would still think the Earth was there for a couple of seconds.
Pamela: For a couple of seconds.
Fraser: It would still go in its orbit and then, you know, whatever the speed of light is you know to reach the Moon, what is it, four seconds?, then the Moon would veer off and go straight into space. But for that intervening time even though the Earth had disappeared.
Pamela: It wouldn’t go straight into space. It would continue going around the sun but it wouldn’t wiggle and wobble the way it does thanks to its orbit around Earth.
Fraser: Right. Right. But it would act if it was still going around the Earth for a few seconds before it then veered off and then went into space.
Pamela: And one of the really neat things is gravity bends light. So we can actually look for this bending light as a potential way to confirm that gravity propagates at the speed of light. And there’ve been some folks who have tried to do it and no one is quite sure if their results are being interpreted correctly, but in theory, it might be possible to look at the light from distant quasars and how that light and its position changes as planets move.
There are some folks back in 2002 who tried to do that looking at the planet Jupiter, and no one’s quite sure that they interpreted the results correctly. Let me rephrase that. There are some people who are quite sure that they did, and people who are quite sure that they didn’t interpret the results correctly. But these results allow us to think of ways that we might in the future get a better handle on exactly what the speed of gravity is which we are quite certain is the speed of light, but it’s always cool to be able to prove you’re right and you know exactly what you’re talking about.
Fraser: And I think, I know we’re going to be planning to do a show on gravity waves. So, that’s a really interesting concept and we’ll cover that in a lot of detail exactly how they plan to detect them and so on and so hang tight for that.
Ok. So next question. Richard Chaplow is following on from Episode 4 where we discussed one of the consequences of dark matter is how it causes a galactic disk to rotate. So here’s his questions. On the dark matter podcast, Pamela explains that the dark matter amount increases with distance from the galaxy’s center, and that explains the radial locking of star velocity. Then how does spiral arms form in galaxies if the dark matter has them locked from forming? Now, we’ve gotten a lot of feedback on this Pamela. Can you clue this up once and for all?
Pamela: Ok. So if we had a nice, happy Keplerian disk where all of the stars, sort of like the planets do in our solar system where the further you are away the slower your velocity is because gravity is pulling on you less, then we’d expect things to be moving apart. So if I dropped my radius by 10%, I hopped off the Earth and moved towards the Sun and moved a tenth of the way from here to the Sun, then I’d expect my velocity to go up by about 5%.
Now, because of dark matter, if I go out in the galaxy and I hop off of our solar system, and move a tenth of the way from our solar system in towards the center of the galaxy, I’d expect my velocity to still go up that 5%, but it doesn’t. My velocity stays exactly the same because of the gravitational pull of dark matter in the outer parts of the galaxy.
Now, let me try and explain this using a reference frame we’re all familiar with. Let’s imagine you and I go down to Antarctica where it’s really cold and we have land. And we start walking in perfect circles around the pole. Now, if I’m closer to the pole than you are and we’re walking at the exact same rate, we’re using marching man steps where we go a certain number of yards in every certain number of steps, well, over short periods of time, as we’re walking along, because we’re walking at the exact same rate, and we’re on really big circles going around the pole, we see ourselves walking side by side.
But, as time progresses, the differences in the circumferences of the two circles we’re walking on will begin to add up. So even though we’re walking at the exact same pace, over time, I, who am on the smaller circle, will begin to move ahead of you. So, if I’m 10% closer to the pole than you are, at the end of the time it takes for you to go all the way around the pole, I will have gone 1.1 times around the pole.
Now, if this was normal Keplerian motion, where by just getting closer to the pole I’m moving faster, we’d start off at time zero and I’d be chugging along faster than you and you would instantly see me move ahead of you. And at the end of the time that it takes you to go one time around all the way around the pole, I’ll have gone 17% further around the pole. I will have gone 1.17 times around the pole.
So in short time scales, if you and I are walking at the same velocity, we stay side by side. But over long periods of time, the person who is closer to he center moves slowly further and further ahead. And this is what happens in galaxies. As the galaxies rotate, the stars that are further from the center they have to go in much bigger circles, so they’re going to slowly lag further and further behind. While the stars that are closer to the center which are moving in smaller circles with the exact same velocity are slowly going to appear to creep ahead, creating in some cases these spiral structures.
Fraser: But if we didn’t have the dark matter it would all be just completely twisted up because it would be like our solar system where the planets are all going around and around and around and Mercury goes around a dozen times when the Earth just goes around once.
Pamela: It would twist itself up faster, so it’s all a matter of time scales.
Fraser: Let’s move on to the next one then. So in episode 15 we discussed supernovae and I had a whole bunch of questions but they were just coming out so fast that I forgot to ask them all. So fortunately, Andy Duncan reminded me of one that I was meaning to ask which was, so here’s his question: “I’d like to know if when one of these massive stars goes into supernova, how visible would it be to us here on Earth?” So how bright would these supernovae be?
Pamela: Well, the two closest stars most likely to explode are Betelgeuse and Eta Carinae. Betelgeuse is the closest of these two. It’s only 430 light-years away, and when it goes, it’s going to appear to be about the same brightness as a half full moon. This is when we look up at the moon, and we call it a quarter moon, but half of the moon, the face of the moon, is lit up. But the thing is all of that light is going to get condensed down to a single point. So take all of that really bright light, and focus it down to a star on the sky. So this is going to be amazingly bright. It’s going to be clearly visible during the day. It’s going to cast shadows. It’s just going to be really really cool.
Fraser: And how long will it last for?
Pamela: It’ll be visible for several months, but it will only be at its brightest for a week or so.
Fraser: And how quickly would it take to get bright? Would it just brighten up with…
Pamela: A couple of days. It is really really cool to look at supernovae light curves. They just brighten straight up, and then slowly decrease back down.
Fraser: So that’s bright…
Pamela: That’s bright [garbled]
Fraser: Ok. Seeing it from the day. No question where it is.
Pamela: Nice big bloody shoulder on Orion the Hunter. It’s kinda poetic to think about.
Fraser: Ok. What about Eta Carinae then? Cause it’s one of the Wolf Rayet stars.
Pamela: Well, it’s not there yet, but that’s where it’s headed. It’s this giant star that’s 100 times the size of the Sun. Huge, doing weird stuff all of the time. We don’t really understand it, and luckily it’s about 17½ times further away than Betelgeuse. It’s located 7,500 light-years away. And since it’s so much further away, when it explodes, if it gets to about the same brightness as Betelgeuse does when it goes supernova, that 17½ times further translates into 300 times fainter. So it’ll be the brightest thing in the sky, it will be giving off more light than the entire Milky Way galaxy, probably visible during the day, but just not quite as cool as Betelgeuse.
Fraser: So the most exciting supernova that we’ve got to look forward to is Betelgeuse.
Pamela: Exactly. So just keep an eye on the Hunter and watch out for him to get wounded by cosmic evolution and stellar evolution in particular.
Fraser: Well I guess we could always you know get lucky as well and have a Type 1a with a white dwarf go off closer by.
Pamela: Uhmm. Depends on your definition of lucky. If it’s too close it could be very bad for the folks who have communication satellites, but quite cool for the astronomers.
Fraser: Right…Right. So if one of those Type 1a supernovas was nearby, say as close as Betelgeuse, how bright would it be compared to Betelgeuse going off as a supernova?
Pamela: So those are about twice as bright, so if a regular supernova goes off, I’m sorry, a Type 1a supernova goes off, it’s about twice as bright, so it would be as bright as the full moon.
Fraser: Compacted down to a single point.
Pamela: Compacted down to a single point. So imagine focusing all of the light from the full moon down to a single point, and you have something pretty painful to look at.
Fraser: Wow! Ok. So let’s go on to our last question then, and I’m going to ask this question because it’s actually come from a lot of people, and this is quite a puzzler. So I think we’ve probably got this question four or five times and this sort of plays off something we mentioned early on with the Big Bang. People want to know how is it possible that the expansion of the universe after the Big Bang with dark energy can go faster than the speed of light. I thought that nothing can go faster than the speed of light.
Pamela: Well, it’s not a matter of an object is moving at the speed of light, it’s a matter that the stuff between things is growing at a normal reasonable rate but there’s so much stuff between you and a distant object that it seems that distant object is moving at the speed of light. So let me see if I can make sense of this.
Astronomers have measured the expansion rate locally. Right now, at this moment, as we sit here recording, uhm, the one megaparsec of space around us is expanding at rate of about 70 kilometers per second. Now, one megaparsec is a kind of weird number and it works out to be 3,261,636.26 light years, so that lots of light-years is expanding at 70 kilometers per second. So every second that megaparsec of space gets 70 kilometers bigger.
Now that means that if something is two megaparsecs away the space between us and it is growing at 140 kilometers per second. If something is 25 megaparsecs away, the space between us and it is increasing at 1,750 kilometers per second, ignoring all sorts of second order relativistic effects and stuff. So the further something is away, the faster it appears to be moving because the space between us and it is expanding, and for every one megaparsec of distance, we add 70 kilometers of space every second.
Now, as you get to greater and greater distances, this gets to mean something. So, if you decide to fly off and sit 1,000 megaparsecs away from me, I will see you appear to be moving at .23 the speed of light. Uh, that’s pretty fast. That’s about a quarter of the speed of light. And it’s not that you are moving at a quarter of the speed of light. It’s that we’re shoving 70,000 kilometers of space between you and I every second. So it’s that 70,000 extra kilometers between you and I that gives the appearance of you moving.
Now it works out that someone has to be about 4,285, 86 megaparsecs away to be moving at faster than the speed of light. So if I stick you at exactly 4,285.7 megaparsecs with a speed of light of 300,000 kilometers per second, which there is rounding going on in there, uhm, I’ll see you as moving at the speed of light.
Fraser: How far across the universe is that?
Pamela: That’s actually further than the universe has been around for me to actually get to see. That’s 13.9 billion light-years away.
Fraser: And the universe is 13.7 billion years old.
Pamela: Things that are beyond the horizon of light’s even had time to reach us. Uhm, they’re moving so fast that we never actually get to see them. Now this means that there’s stuff relative to all of us that appears to be moving faster than the speed of light. And it also means that relative to other points in the universe we appear to be moving faster than the speed of light. But it is all an illusion brought on by the fact that the universe is growing.
Fraser: So what’s going to happen to us trying to see those things? If they start to move they’ll be moving at a certain speed and then suddenly they’ll be moving faster than the speed of light away from us, what will we see?
Pamela: They’ll disappear.
Fraser: So we’ll just see like the last photons there were able to make it and then everything else is just gone.
Pamela: Well there’s lots of weird relativistic effects. It’s sort of like as something falls into a black hole, it appears to stop moving, it just hangs out there. So as things disappear over the edge of the horizon, they’ll just slowly appear to stop and fade away.
Fraser: So I guess, you know. When you say that nothing can move faster than the speed of light, it’s kinda a misnomer. In fact, relative to people, everything in the universe is moving faster than the speed of light. It just depends on where you look.
Pamela: But we can’t go faster than light relative to space. So as I sit here and you sit here, the space between us can increase at faster than the speed of light, but we can’t move through that space at faster than the speed of light.
Fraser: You know, I think we’re going to get more questions on that. So
Pamela: It’s one of those neat mind bending concepts.
Fraser: No kidding! Well, that was great Pamela. Thanks a lot. I think we got through our first question show. We got some great questions. Uhm, and I hope that people will send us in some more.
So let’s just close up, give more people information about the show. Uh, you can find more information about AstronomyCast at our website, at astronomycast.com. You can send us comments, questions, or feedback to firstname.lastname@example.org and you can participate in the forums, there’s a link every episode on our website. You can subscribe to our show by pointing your podcatching software at astronomycast.com slash podcasts.xml or subscribe to it from within iTunes, just search for AstronomyCast.
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All right Pamela, we’ll see you next week.
Pamela: See you next week.
This has been AstronomyCast, a weekly facts based journey through the cosmos. Music provided by Travis Searle. Thanks for listening.