Podcast: Play in new window | Download
Subscribe: RSS
You’ve probably heard of dark matter and dark energy, but maybe you don’t fully understand what they are. Or maybe the idea itself just rubs you the wrong way and you’d like to know why scientists think they can just make stuff up like this. So you’d like to overturn cosmology? Here’s all you need to do.
Download MP3 | Show Notes | Transcript
Show Notes
What’s a neutrino? (FermiLab)
What is dark matter? (EarthSky)
New study suggests supermassive black holes could form from dark matter (RAS)
The Smallest Galaxies in Our Universe Bring More About Dark Matter to Light (Kavli IPMU)
Factoring in gravitomagnetism could do away with dark matter (Springer)
Dark Energy (Swinburne University)
How Vera Rubin confirmed dark matter (Astronomy Magazine)
How do we know that dark matter exists? (NASA)
The Dark History Of Dark Matter (Forbes Magazine)
Virgo Supercluster (Universe Today)
A Matter of Fact (NASA)
Newtonian Dynamics (University of Texas)
Shining a Light on Dark Matter (NASA)
Hubble finds evidence for widely held ‘cold dark matter’ theory (Astronomy Now Magazine)
Dark Energy’s 10th Anniversary (Berkeley Lab)
The Nobel Prize in Physics 2011 (Nobel Prize)
Planck’s New Cosmic Recipe (ESA)
The Mystery of Cosmic Cold Spots Just Got Even Weirder (Discover Magazine)
How a Dispute over a Single Number Became a Cosmological Crisis (Scientific American)
Will this solve the mystery of the expansion of the universe? (University of Southern Denmark)
Science Made Simple: What is Cosmic Acceleration and Dark Energy? (SciTechDaily)
New 3-D Map of Dark Matter Reveals Cosmic Scaffolding (Phys.org)
Transcript
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast, Episode 598: How You Could Overturn Cosmology. 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. I’m Fraser Cain, publisher of the Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of CosmoQuest. Hey, Pamela. How are you doing?
Dr. Gay: I’m doing well. How are you doing, Fraser?
Fraser: Uh. Did I mention that I’m over the pandemic last time?
Dr. Gay: Yeah.
Fraser: I’m still over it. I’m done.
Dr. Gay: Yeah. My husband had to go get COVID tested today. So, good times!
Fraser: Yeah. Yeah. Yeah, I discovered it at my kid’s school. It’s rampaging on the island. Yet, there’s all of this amazing vaccine news. Then, there’s also all the scary variant news.
Dr. Gay: Yeah.
Fraser: So, it’s – yeah. It’s just like – it’s no longer the action adventure. Now, it’s just…
Dr. Gay: … Reality.
Fraser: Reality. Surviving. Yeah. It’s what it would be like to move to Mars. In the beginning, you’d be like, “Woohoo! I’m on Mars. It’s an adventure.” Then, after a while, you’d be like, “Oh, this is horrible.”
Dr. Gay: Yeah. Mistakes were made.
Fraser: Mistake. Yeah. I’m sad and alone and I want to see an ocean.
Dr. Gay: Yeah. One of the interesting side effects I’m dealing with is because my husband was potentially exposed, we’re trying to quarantine. Which means I can’t really lock the dog somewhere else. So, I’m currently swimming in dogs.
Fraser: Yeah.
Dr. Gay: I keep getting brought toys.
Fraser: Squeaky toys.
Dr. Gay: This is the most recent toy I have been brought and I’m dumping them behind my chair so she can’t squeak them anymore.
Fraser: Right.
Dr. Gay: So, if you see the sudden movement of an object being dropped behind me, this is because I have been quarantined with two dogs and they have ideas.
Fraser: You know, I think, as a podcaster, I think it’s perfectly within your rights to extract the squeaky toys from – the squeaky part from the squeaky toys.
Dr. Gay: Yeah. I’m just also lazy. So.
Fraser: Great. Okay.
Dr. Gay: Dumping them behind the chair works.
Fraser: Well, sure. All right. You’ve probably heard of dark matter and dark energy, but, maybe, you don’t fully understand what they are. Or, maybe, the idea, itself, just rubs you the wrong way. You’d like to know why scientists think they can just make stuff up like this. So, you’d like to overturn cosmology. Well, here’s all you need to do. And we’ll talk about it in a second, but first, let’s have a break.
And we’re back. We’ve probably mentioned this in past podcasts. This is a constant thorn in our side that – something about dark matter, just don’t like it, man. I think – I feel like last week, we got to the heart of it. Which is that the neutrino is essentially dark matter, just hot dark matter as opposed to cold dark matter.
Dr. Gay: Not.
Fraser: And nobody has a problem with it.
Dr. Gay: Yeah.
Fraser: It’s like the name.
Dr. Gay: Yeah.
Fraser: It’s the name.
Dr. Gay: Yeah. This week, in particular, we’ve seen, in the journals, a number of papers popping up trying to find ways to deal with dark matter, dark energy. I know my inbox, I suspect your inbox, we get this steady stream of emails from people asking us to review their theories.
Fraser: Yes.
Dr. Gay: They always ask us to keep everything in confidence and they mail it all to us before we agree to that, which always confuses me.
Fraser: Yeah. Yeah. That’s not how it works. That’s not how nondisclosure works. I mean, for starters, I will never agree to that. Like ever. If you want to send me a – I’ve mentioned this in the past, at Universe Today, we don’t agree to embargos. Yeah. If you want to send us information, then you’ve already kind of broken it.
Dr. Gay: Yeah.
Fraser: That’s a really valid point. Yeah. Absolutely. I get these one a day. Maybe. Maybe. It’s funny, for me, sending me your theory of Cosmology is worthless. What am I going to do? I’m a journalist. I’m a fancy parrot.
Dr. Gay: I’m an observational astronomer.
Fraser: Right.
Dr. Gay: I do not want to go through your theories.
Fraser: Right.
Dr. Gay: That would require me reviewing far more literature.
Fraser: Yeah.
Dr. Gay: No. No.
Fraser: Yeah.
Dr. Gay: I want to deal with photons and computers.
Fraser: And you’re a specialist.
Dr. Gay: Yeah.
Fraser: Yeah. So, it is funny. I think we know why because…
Dr. Gay: … We’re here.
Fraser: We’re here. Yeah. No. They feel like they can’t get respect from the scientific community. So, they’re going to try to come in on the edges. Try to get some respect through the journalists. The publicity side. I don’t know.
Dr. Gay: I have no idea.
Fraser: Yes. Yeah. We get them. Yes.
Dr. Gay: Yes.
Fraser: Can confirm.
Dr. Gay: The combination of getting these emails and seeing stuff that goes through peer review that you’ve got to wonder who reviewed it. Just got me to thinking, we really need to go over: here are the things, all of the different things you, who are sending us these things, or trying to get them published, need to address in the first few paragraphs of whatever you put together to let us know it’s worth reading further.
Fraser: Right. All right. So, then, let’s pick – do you want to just talk about just overall, general, cosmological ideas? Do you want to just talk about, specifically, the evidence? Sort of, all of the evidence that stands for the individual theories? How do you want to tackle this?
Dr. Gay: Let’s take a look at what are the things you have to meet for dark matter and what are the things you have meet for dark energy.
Fraser: Sure. Sure. Dark matter. I choose dark matter.
Dr. Gay: Okay. So, dark matter. It was first discovered because galaxies don’t rotate the way that you would expect them to, based on luminous matter. A individual object, orbiting around our system, is going to have an orbital velocity that is directly related to how much material is inside of that orbit. You would expect things to go slower and slower the further out you get because the amount of stuff inside is dropping off instead of increasing. But when you look at it, it turns out that that velocity actually flattens out instead of dropping off.
Fraser: Right. So, just give a finer point on this, Mercury orbits at something like 45 kilometers a second, while the Earth orbits at 30 kilometers a second. Neptune orbits at 5 kilometers a second. So, the farther away you go from the sun, the slower your orbital velocity is because, as you say, you’re orbiting around this center of gravity. So, you would expect the stars orbiting around the center of the galaxy are going to go super-fast, the ones in the middle are going to go middle speed, and the ones way out at the edges are going to go slow. This is what you expect to find – that you would see if a galaxy was a big solar system.
Dr. Gay: And it’s not. So, we know that you do have to take into account the fact that there’s a whole lot of stuff in the center of the galaxy. There’s bulky arms filled with gas and dust and star formation. You have to account for all of this, but as you get far enough out, you run out of stuff that you can see, other than the occasional cloud of hydrogen gas. So, you’d expect, looking at those clouds of hydrogen gas, that their orbits would slow down in relation to the lack of extra stuff getting thrown into the galaxy that we can see. They don’t, which means there’s a bunch of stuff we can’t see that’s out there.
Fraser: Right. So, pretty much, anywhere you measure the velocity, the orbital velocity of a star going around the center of the Milky Way, it’s the same. It’s – whatever. It’s 250 kilometers a second, I think. So, if you’re close in to the center of the galaxy, it’s that speed. If you’re way out near the outer edges, it’s the same speed. So, that alone should tell you that something weird is going on.
Dr. Gay: Up until the 2000s, all we knew was there was an effect that was either, we didn’t fully understand gravity or there’s a whole bunch of stuff we can’t see. We didn’t know which that was affecting the rotation rate of galaxies. We also saw this in terms of the orbital velocities of galaxies in clusters.
As we scaled up where we were looking, these massive systems, but not our solar system, massive systems like our Milky Way, like our cluster, like the Virgo Supercluster. All of these bigger systems, had this extra affect. So, for a while, you could successfully, more or less, debate: is there this stuff we call dark matter or do we need to modify gravity? Is there a modified Newtonian dynamics out there?
Fraser: We just don’t understand gravity.
Dr. Gay: Right. So, both these things were possible, but then, there were the amazing observations of the Bullet Cluster, which people looking at this system, which is two merging galaxy clusters becoming a bigger galaxy cluster. You could see where the galaxies are in the merging system. You could also look between the galaxies, in this cluster, at the stuff behind it and see how their light was distorted by the gravity of everything in these clusters, including that dark matter. Because we can see that whatever this stuff is that’s causing galaxies and stars to orbit faster, is also having an effect on light.
We knew that it is something that has a gravitational pull instead of an extra term to gravity because an extra term to gravity isn’t going to affect the stuff behind the mass. So, suddenly, we had this – okay. So, it’s not modified Newtonian dynamics. Since then, we’ve been trying to figure out, by looking at the dynamics of how the dark matter appears to be clumped, based on its gravitational effects.
Fraser: All right. Let’s talk about how astronomers actually observe and map out dark matter in a second, but first, let’s have a break.
And we’re back. All right. So, since that observation of the Bullet Cluster was made, astronomers have gotten better and better at seeing dark matter. How do they do this?
Dr. Gay: It’s all about assuming that galaxies, on average, are circles. That if you look at enough galaxies, some are going to be edge on, some are going to be flat, some are going to be mushy messes, but on average, all of these different galaxies can be oriented in every possible way. Because they should be randomly oriented on the sky, they should have an average shape that’s a circle.
So, if you look at a patch of sky, they can contain thousands of galaxies if you look hard enough. You average together all the shapes and if you don’t get a circle, that means there is something out there casually distorting the light like a fun house mirror using gravity.
This is exactly what astronomers do. We look at chunks of galaxies in this volume of space, chunks of galaxies in a further back volume of space. We look at all of the distortions and how they add up layer by layer through the volumes. This allows us to see where the dark matter is by how it warps what we see behind it. There’s projects out there, like Cosmos, that have actually been able to do three dimensional maps of volumes of space showing the clumped up, lumpy mass that is dark matter in our Universe.
Fraser: So, just for anyone working on the theory keeping count, you have to explain the rotation of galaxies, how they hold themselves together and how the stars orbit at constant speeds, no matter how far away they are from the galactic core. You have to explain how galaxy clusters can collide and separate themselves out into hot gas and galaxies, themselves. You have to explain the microlensing that’s done – sorry. The gravitational lensing that’s done across the Universe to see all these distortions from some sort of thing with gravity, and many more. We’re just scratching the surface. There are now, I would say probably, 10 separate lines of evidence out there now that show that dark matter is a thing.
Dr. Gay: It also is required to explain how rapidly galaxies and supermassive black holes were able to form at the beginning of the Universe. This is something we went into last week, where if we don’t have cold dark matter as the bulk of the stuff our Universe is made of, then you can’t get the first supermassive black holes as fast as they appear to have formed in the Universe. You don’t have enough stuff to draw in all the luminous matter to form stars and galaxies as early as we’re seeing it in the Universe.
Fraser: Now, you say it has to be cold dark matter. Why is the cold part important?
Dr. Gay: It all comes down to collisions. If you have material that is hot, it’s moving a whole lot faster. So, particles are less able to collapse down and form clumps. A good way to think of this is, if you’ve ever gone ice skating or roller skating with a bunch of friends. If you come up to a friend and your velocities are close to the same and you’re not going too fast, it’s super easy to grab on to each other and to continue skating together. But if you’re whipping around at high velocities, even if you’re both going at similar high velocities, coming together without bouncing off and colliding wildly or just missing each other completely, is much more difficult.
Fraser: I like the idea of, if you want to make ice cubes, you either start with cold water that’s in the fridge, or you start with steam coming out of your kettle. Right? One’s tough.
Dr. Gay: Yeah.
Fraser: One’s harder to make ice cubes with until you cool it down. So, this is way neutrinos – we talked about this last week, neutrinos are a thing has been discovered that is essentially invisible, could pass through a lightyear of lead, 50% of the time can go right through a lightyear of lead, not stop, not interact. You have countless, hundreds of millions of them, going through your body right now. Thanks to the Sun, you don’t even feel it.
Dr. Gay: No big deal.
Fraser: The difference is, those are hot.
Dr. Gay: Yes.
Fraser: So, dark matter is going to be a particle that’s kind of like a neutrino except it’s not hot, it’s going to be cold.
Dr. Gay: Slow.
Fraser: It’s going to be slow moving.
Dr. Gay: Able to clump up nicely.
Fraser: Right. Instead of having neutrinos passing through your body at close to the speed of light, you’ve got dark matter particles moving through your body slowly. That’s what cold means.
Dr. Gay: Yes.
Fraser: Okay. I think that – I’m sort of thinking about the time and that gives us a good list of the top things to try to describe. These observations are, in many cases, rock solid. So, if you think those observations, then that’s where you start. You’ve got to overturn those observations, but if you trust in those observations, then your theory has to explain those things. Plus, like I said, another seven plus lines of evidence.
Dr. Gay: Those are just the big ones.
Fraser: Yeah. Let’s go on and talk about dark energy in a second. But first, let’s have another break.
And we’re back. All right. Dark energy. Badly named, I think. Both are badly named.
Dr. Gay: So, one of the things that is very true is an astronomer does not need to understand a thing to name a thing.
Fraser: Right.
Dr. Gay: Which means that if there’s this nebulous, unknown quantity causing an effect, we’re completely down with naming without knowing what it is or anything else about it other than the effect it is causing.
Fraser: Yes. Well, I think you had one of the greatest analogies and I’ve used this many times since you presented it. Which is that your car is making a funny banging sound as you drive. You don’t know what it is. It could be the wheel. It could be the hub. It could be the axle. It could be the transmission. It could be the steering wheel. It could be all kinds of things. It could be a rock in the tire. It could be something. You don’t know what it is. You know it’s there and anybody who comes and sits in your car for five minutes goes, “Yes, I agree with you. There is a banging sound going on in this car. Let’s call this ‘dark mystery noise’.”
Dr. Gay: Yes.
Fraser: Agreed. Then, now, let’s try to figure out what’s causing it. So, I think we’re at the dark mystery noise point in our story.
Dr. Gay: And if you have a rock inside your tire, kudos.
Fraser: Well, stuck in the tire. Stuck in the tire. You know.
Dr. Gay: Okay. Okay.
Fraser: Yeah. Yeah. So, let’s talk about the observational evidence for dark energy.
Dr. Gay: So, back in 1998, a couple different teams of supernova observers realized that as they looked at Type 1A supernovae, which should all be, more or less, the same brightness – there’s exceptions. Don’t at us. They should be, more or less, the same brightness. When you look at them at a variety of different distances, the rate at which the Universe is expanding at all those different distances is such that our Universe is accelerating. Things were moving slower in the past and they’re moving faster now. What was expected was, either the velocity would be constant over time, the rate of expansion would be constant over time, or it would be slowing down.
Fraser: Right. Slowing down would be what you’d expect.
Dr. Gay: I’m good with constant. But, yeah, slowing down makes the most sense.
Fraser: Well constant’s a little weird.
Dr. Gay: It’s a little weird, but I was good with it.
Fraser: Yeah. Okay.
Dr. Gay: I’m not okay with this acceleration that we actually have.
Fraser: With the acceleration. Yeah. Neither were the astronomers.
Dr. Gay: I am an astronomer.
Fraser: No. No. No. I mean the ones who discovered it.
Dr. Gay: That’s true.
Fraser: The ones who discovered dark energy were not looking for dark energy.
Dr. Gay: No. No.
Fraser: It was a surprise.
Dr. Gay: They got a Nobel Prize for something they weren’t intentionally looking for.
Fraser: Surprise! Here’s your Nobel Prize.
Dr. Gay: Exactly. In realizing this, this caused everyone to look at their data and go, “Huh. Okay.” As people started going through all their data, we were able to get things to, more or less, make sense looking at the WMAP data, later looking at the Planck data to measure the acceleration in the Universe based on the cosmic microwave background. We had multiple teams using supernovaes and gravitational lenses, using a whole myriad of observational properties in the more recent times in the Universe.
For a while, the error bars on all of these results were sufficient that they overlapped with the error bars. But as the error bars have gotten better, they don’t overlap anymore. So, we have a universe that appears to be accelerating apart with a discontinuity, or we don’t understand the physics that goes into interpreting our results, or there’s just something else going on out there.
Fraser: Right. Right. So, again, our job is not to explain what these things are, nobody can. That would get us a Nobel Prize, if we could successfully explain it. What are the big pillars again? If you want to overturn this idea of dark energy, what observations does your theory have to explain?
Dr. Gay: You have to be able to explain the change in brightness or change in velocity of Type 1A supernovae over the age of the Universe. So, either you’re explaining that the supernova actually had different brightnesses over time, or you’re explaining how the acceleration in the Universe changed over time. Both things fit the data, which is a headache.
Fraser: Yeah.
Dr. Gay: You are also explaining the distribution in cold and hot spots in the cosmic microwave background radiation and the other wiggly niggly differences that we find in that tiny splattering of variation that represents the Universe roughly 400,000 years after it formed.
Fraser: Right.
Dr. Gay: You’re, hopefully, explaining why observations of the modern Universe, including with gravitational lenses measuring the physical distances, to galaxies that are changing in brightness in interesting ways, all seem to give one set of numbers and observations of the cosmic microwave background, give a slightly different set of numbers.
Fraser: I think that that is to a person’s advantage. That if you are trying to overturn cosmology, because this is the crisis in cosmology…
Dr. Gay: …Yeah. It’s what they call it is the crisis in cosmology.
Fraser: Yeah. Yeah. So, if anything, this a point where astronomers are now actually very aware that there is a discrepancy between the velocity measurements early on in the Universe and the ones later on in the Universe. Above and beyond what’s predicted by dark energy.
Dr. Gay: What’s going to be fascinating in watching theorists try and deal with this – the observers are out there beating on their error measurements, replicating what they’re doing.
Fraser: Yeah. Yeah.
Dr. Gay: The observers, they’re having a good time with this one while making their technology work at the edge of its bestness. The theorists, on the other hand, are out there writing papers, that have no more than two authors, with potential explanations. This is what we’re going to see, is papers with no more than two authors because that’s how many people can get the Nobel Prize. There was an interesting one that came out this week that we discussed in The Daily Space, on how, maybe, there was a phase transition in dark energy where it changed from one state to another and that with it changed the evolution of our Universe.
Fraser: Yeah. I saw that.
Dr. Gay: I’ve seen stuff like pop up before where people point at, “Well, there is this age of inflation in the first few minutes of the Universe, where crazy stuff happened and then it stopped. What if there was, yet another, change over time?” So, those are intriguing, but they’re not definitive.
Fraser: Yeah.
Dr. Gay: So, we need something that is able to explain all of this and convince all of us that it explains all of it.
Fraser: Well, I think that’s a good place to wrap this. So, if you have a theory of everything, if you think that you have figured out why Einstein was wrong, it’s more than just Einstein.
Dr. Gay: Yeah.
Fraser: I think the first, best, step that anyone should take is to fully educate themselves at the state of the science as it stands today. Be able to replicate the mathematics that the people who you would like to be peer reviewing your paper, are going to be doing if they actually are willing to peer review your paper. To not do that, is, I think, a sign of, I don’t know, almost disrespect to the amount of work that’s gone into the discoveries that have been made so far.
Dr. Gay: Yeah.
Fraser: People have not just made this up.
Dr. Gay: Never write to somebody and say, “I have this idea, but I’m unable to do the maths. Can you do the maths to prove my idea for me?”
Fraser: Yeah.
Dr. Gay: I don’t even like to do my own math, people.
Fraser: Right.
Dr. Gay: Don’t ask me to do yours.
Fraser: Yeah. A guy coming up with an idea for a novel and talking to an author. All right. Thank you so much, Pamela.
Dr. Gay: Bye, bye.
Fraser: Now, do you have some names for us?
Dr. Gay: I do. I do. I just need to move things around on my screen. So, as always, we’re brought to you by you. Your contributions on Patreon, your donations. You are what allow us to keep bringing you content week after week, year after year. This week, I’d like to thank some of our Patreon community members, specifically I’d like to thank: Antony Burgess, Donald E Mundis, Andrew Stephenson, Anitusar, Scott Beiber, Jen Greenwald, Rachel Fry, Bart Flaherty, Kenneth Ryan, Sean Freeman (Blixa the cat), The Air Major, Cemanski, Dean, Tim McMackin, Glenn McDavid, Benjamin Davies, Naila, Kseniya Panfilenko, Shannon Humber, Kimberly Rieck, Gabriel Gauffin, Frode Tennebø, Nial Bruce, Daniel Loosli, Alex Raine, Corinne Dmitruk, Neuterdude, David Gates, Justin Proctor, Abraham Cottrill, Joe Wilkinson, Claudia Mastroianni, Jean-François Rajotte, Eran Segev. Thank you so much, all of you, for being here for us and allowing us to do what we do.
Fraser: Thanks, everybody. We’ll see you all next week.
Dr. Gay: Bye, bye.
Astronomy Cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy Cast is released under a Creative Commons Attribution license. So, love it, share it, and remix it, but please credit it to our hosts, Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website: astronomycast.com. This episode was brought to you thanks to our generous patrons on Patreon. If you want to help keep this show going, please consider joining our community at patreon.com/astronomycast. Not only do you help us pay our producers a fair wage, you will also get special access to content right in your inbox and invites to online events. We’re so grateful to all of you who have joined out Patreon community already. Anyways, keep looking up. This has been Astronomy Cast.