What are some of the problems with dark matter? What about modified gravity? Are there any other solutions? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO-GENERATED)

Before I dig into today's topic, I want to share with you a new partnership that I just know you'll really enjoy. I'm excited to announce that I've started an audio book club with chirp. Chirp is an audio book retailer known for great deals without any commitments or subscription, and I've started a book club with them. I know you're always asking me for book recommendations. Well, here it is, folks. At the start of every month, I'll announce the pick here in Chirp. We'll deeply discount the audio book for a limited time. We'll listen to the audio book together, and then at the end of the month, you'll have a chance to share your thoughts and see what other club members thought, too. My first pick is strange New World by Rayyan, which I'm excited to recommend because, as you already know, exoplanets are awesome. We're finding planets bigger than Jupiter and smaller than Mercury. We've got Super Earths and mini Neptunes. We've even got planets orbiting multiple stars at once, and somewhere out there is the ultimate holy Grail, a copy of Earth, potentially another home for life gyre war.

Donna's take on this massive topic is engaging fascinating and fills you in in all the secrets of how astronomers look beyond our solar system and what they hope to find next. To join, go to chirp books dot com slash spaceman and grab my first pick, Strange New Worlds on sale from $19 to 3 99 for a limited time. I wasn't kidding about the great deals. And be sure to press follow to join my club, to stay in the loop on future picks and other exclusive content from yours truly. That's chirp. Books dot com slash spaceman chip CHIRP books dot com slash spaceman I get it. Nobody likes Dark Matter, and that's coming from someone who published their first paper on Dark Matter in 2008. That's basically ancient history and and just a quick recap. For those of you first joining us, there is tons of evidence from all over the universe that tell us that something funky is up.

There's all this missing matter and and observations that conflict with each other, and I'll dig into these observations later in the episode. Uh, but just the sum total when you put this whole picture together of astronomical observations of the universe. We come to the conclusion that up to 80% of all the mass in the universe is of a form completely unknown to the standard model of particle physics. Astronomers, cosmologists, physicists, various and sundry. Other people believe that 80% of the mass of the universe is in the form of a new particle, a particle that we have yet to identify. Or it could be a family of particles we don't know. And the most important property of this particle or family of particles is that it doesn't or they don't interact with light or really each other or really anything else. They just sit there and gravitate and do their thing. That's dark matter. In my undergraduate and graduate training, Dark Matter was a settled thing. By the time I entered graduate school, the the debate was pretty much over.

In the scientific community, Dark matter was just a thing of feature of our universe. It was the best hypothesis that we had. No, we we didn't know what it was and we still don't know what it is, but it was the best explanation we had. For all the available evidence and the dark matter hypothesis was capable of making testable predictions. So we just assumed that Dark Matter was here to stay and we would move on with our work. The more I got involved in science communication, the more surprised I was that many people in the general public, and I don't necessarily mean you. But other people don't think that dark matter is true or accurate or the best hypothesis I've. I've done entire episodes on the nature and evidence of dark matter, and people come out of the woodwork saying the dark matter is wrong. I shouldn't mock people, but when it's basically what they say, and that's how I read it in my head and honestly, I get it. I don't blame anyone for not trusting dark matter. After all, For all its successes and simplicity and cool opportunities for new physics, it does suffer from one tragic flaw.

We haven't seen any yet. It's been decades since some form of dark matter was first proposed in almost a century, depending on how you count dark matter, and there hasn't been a single, direct or indirect detection of it, we only have circumstantial evidence for it This is a fact, and this is coming from someone who thinks that dark matter exists. And so the comments There are a lot of comparisons between dark matter and epi cycles. You know this back in the olden times, uh, we believed that the planets moved in perfect circles around the Earth. But then we refined our observations and perfect circles didn't fit what we observed. So we added circles on top of circles on top of circles in order to fit the observations that there's a a whole discussion to be had out of. That was not a bad idea and and, you know, cut our ancestors some slacker folks because it takes a lot of work and data and a argumentation, and to anyway, that's a whole side discussion.

Anyway, the the key argument is that dark matter is a failed hypothesis that we are holding on to our precious theories too much, and we don't want to let them go. And and we believe in Newton and Einstein and our understanding of gravity. And when they confront reality and fall short of of what we observe, then we we insist that our theories are true and So we start coming up with crazy ideas to to fix our theories, that we have a failed theory that dark matter represents a failure of our imagination or a failure of our understanding of physics. Like saying, Oh, you're saying that dark matter exists? My response is stop adding things to the universe when your theories are so obviously broken. That's the general criticism of the dark matter hypothesis. Sure, dark matter may be wrong. I'm perfectly willing to admit it may be wrong, even though I think it's largely correct, there is a difference there. I believe that dark matter is correct, but I am also willing to admit that it is wrong.

This is not a complicated thought, especially in a scientific mindset. I do believe that dark matter is the correct hypothesis. What could change my mind? Well, a evidence, of course. Evidence. An observation that clearly directly rules out dark matter. Yes, dark matter is a testable hypothesis. We'll get to that. And if you found some clear indication, an observation Yeah, yeah, dark matter doesn't exist. OK, I would stop believing in dark matter or or if we don't find anything that directly rules out dark matter, and we continue to have a lack of evidence for a really long time. We We have a lot of circumstantial evidence, which is a form of evidence, you know, you go to trial. It's not like TV of of murder Cases usually decided on circumstantial evidence. This is reality, but circumstantial evidence is still evidence, and we do have a lot of circumstantial evidence for the existence of Dark matter. But let's say it goes 50 more years, and we still don't have a direct or indirect detection of dark matter, and someone comes up with a very compelling alternative that is able to explain all the data.

If dark matter continues to fail to find verification and somebody cooks up a much better idea, then the scientific community will probably start to lean in that direction. And so will I, because that's where the balance leads me. If dark matter fails to hold up to scrutiny and we just can't find that particle that that hidden particle that doesn't interact with light and we can't find any direct evidence for its existence and somebody is able to say, Hey, here's another alternative explanation that's very compelling and fits all the available data. And it's been a while. Yeah, sure, I'll probably lean in that direction. It's it's no big deal. It's not like I wake up every morning, psyching myself up to fight to the death over dark matter bite. And this is the big butt in response to all those comments that I get in response to all the criticisms. And there are even some scientists who criticize the existence of dark matter. Call it a bad idea and a failed hypothesis. Blah, blah, blah. They, they, they They're real scientists that hold that position.

They are definitely in the minority, but they do exist in response to any criticism of dark matter or says, Hey, dark matter doesn't exist. There is no new particle that doesn't interact with light that makes up the vast majority of the mass of the universe. You're getting it wrong. My response is, you got a better idea. Astronomers, physicists, cosmologists have been working on dark matter for decades. They've written thousands, probably tens of thousands of papers on the subject and performed countless observations and measurements. Most of those papers investigate the nature of dark matter. What could it be made of? What kind of properties does it have? How could we test it or observe it if it had these properties and not those properties? We We went out to make this observation and found this about dark matter and and and not this about dark matter and so on and so forth. And along with those thousands or tens of thousands of papers on dark matter, there have also been thousands of papers on not dark matter alternatives to dark matter, replacements for dark matter, competitors to dark matter.

All those observations that we think are clues to dark matter. Maybe they could be explained by something else. Or maybe we're misinterpreting it. Or maybe there's an alternative that doesn't require dark matter. And these are the observational consequences. How science and I'll give away the punch line. Now there's a reason. A reason that after decades of study and countless hours of debate and arguments and papers and conferences and furious email exchanges, the majority the vast majority of astronomers, physicists and cosmologists believe that dark matter likely exists, and by dark matter, I mean a new particle that does not interact with light that we have not encountered before. The vast majority of scientists who are investigating the nature of the large scale universe and nature of particle physics believes this to be true. And that reason is that even if the dark matter hypothesis is flawed, even if it doesn't neatly explain all the observations and even if we haven't seen it yet, it's better than any other idea we've come up with ever.

Now, things may change tomorrow. Oh, yeah, something may come in tomorrow that's gonna totally rock the science world, and we're gonna set down a new path and we'll put down dark matter and pick up this new idea. But tomorrow is not today, which is when I'm recording this podcast. So as of today, dark matter is the best shot we've got. So let's rewind a little bit and go back to the original observations to to look at these alternatives to dark matter and why they don't quite stand up. Uh, back in the 19 thirties, astronomer Fritz Wiki was looking at the coma cluster nearby cluster of Galaxies hosts like 1000 Galaxies or something, and he was looking at the motions of the Galaxies and their mo movements and how quickly they are moving and from their movements. He could calculate how much mass was in the cluster because there's a relationship between the mass of a cluster and the speed of the Galaxies that fly around inside of it. Uh, if if you imagine the the Galaxies are going too fast for the amount of mass, then then then they should just fly away.

Uh, and if they're going too slow, then they should all collapse together. But if the cluster is there and if it's existed for billions of years, which it presumably has, then there should be a state of equilibrium where all the gravity of the Galaxies, their mutual gravitational interactions are able to keep the Galaxies there. Uh, but he found the Galaxies were going too fast. He was able to add up the mass of all the Galaxies and then calculate the maximum speed of the Galaxies that that amount of gravity could support. And then the Galaxies were going like 10 or 20 times faster than that. The coma cluster should have dissolved billions of years ago. It should have flown apart billions of years ago, but it hasn't. So he published that paper. Everyone pretty much ignored it. He's a very famous astronomer, so he had other things to worry about and we didn't know what was going on, but we just kind of put it down and forgot about it. Then in the 19 seventies, Vera Rubin, another fantastic astronomer, was studying rotation curves. She was studying the speed of stars and gas clouds orbiting inside of a galaxy.

And again, there should be a relationship here. This is basic Newtonian physics here, Uh, there should be a relationship between the amount of matter on the inside of an orbit and then how fast an object is orbiting. This is Larian Dynamics. We've known this kind of stuff for hundreds of years, and once again she found that things were going too fast that Galaxies should have spun themselves apart billions of years ago. But they were there. So when you look at the raw data both Z in the 19 thirties and Rubens in the 19 seventies, where stuff is moving too quickly given the amount of mass that we could observe, you have two options at your disposal. I mean, really, three, maybe the data are wrong, but nobody thinks that, so we'll move on from there. That's my usual response, but we've we've checked this. The data are solid. Either there's a new ingredient to the universe that does not interact with light and makes up the majority of the massive Galaxies and galaxy clusters. And it's that mass contribution that is keeping everything together, keeping everything glued together from flying apart or we're getting the laws of physics wrong.

Maybe we don't understand Newtonian dynamics. Maybe we don't understand gravity at galactic scales and cluster scales. And when we plug in these simple equations to estimate Well, uh, if if if this galaxy is moving this quickly inside of the cluster, uh, then this is the mass it should get, Uh, maybe that simple is wrong. The first road where you say there's more stuff to the universe that leads you down to the modern conception of dark matter. The second road leads you down to the the rest of this episode in the first crack at taking a stab at this is mond love that name. It's a very, very catchy name. I really appreciate it. Mond modified Newtonian Dynamics So you take the MO from the first. OK, you get it. And what you get here is exactly what's on the label. Mond replaces the standard relationship between mass and acceleration. You know, F equals MA force equals mass times acceleration. All that your standard Newtonian physics and, uh, replaces it with a you know, a modified version.

It's it's in the name. Basically, it says that gravity is different. A galactic scales that the gravity we know and love on the Earth. F equals MA force law Inverse square. All the high school Newtonian gravity and physics that you've learned is just simply different. A galactic scales. OK, it's not a bad idea. No one said that the universal gravitation of Newton had to be that universal all the way up to galactic scales. Maybe just things are different, and we've changed this before. We've had to replace Newtonian physics with general relativity in situations involving strong gravity, and that worked out really well. So maybe when we just blow up to large scales, distances, large distances, gravity starts to be weird. OK, so it's it's it's not a horrible idea. There is a different strength of gravity like a star, and the outer fringes of a galaxy experiences a different strength of gravity than one in the middle by pure virtue of its distance and nothing else.

That's it. It's like so far away that it has a new gravitational law associated with it that it doesn't have close up. Mond was developed in response to the discovery of these weird rotation curves in Galaxies by Vera Rubin. So we're talking about a 19 seventies 19 eighties theory in parallel to a growing understanding and development of dark matter as a new particle, a new component in the universe. So in the seventies and eighties, this was a heated debate, you know, which explains the observations better. And yeah, Bond fits the rotation curves really, really well, which isn't all that impressive because the theory was explicitly designed to match the rotation curves. So that kind of sort of doesn't count as a prediction is more of a what we call a post addiction like the We have this observation and then a theory was generated. In order to explain those observations. Mond does have a big strength here. It's really beyond rotation curves, which it was designed to handle Monde has a big strength.

It's really, really good at capturing the behaviors of galaxy dynamics. What happens to stuff inside of Galaxies when you're just looking at it gravitationally, which, ironically, is something that dark matter has a a rougher time with. That's right, the the dark matter hypothesis. It was generated because the whole motivation for this was the rotation curves in the first place. Don't worry. We're gonna get back to the Galaxy clusters in a little bit. But just focusing on the Galaxy rotation curves. That was the whole impetus that that's what got this whole game started. And right off the bat, people were like, Maybe there's dark matter. And then maybe there's modified Newtonian dynamics and even though that was the star dark matter as a relatively, uh, harder time explaining Galaxy dynamics. Yes, dark matter does explain rotation curves, but it has two sore points when it comes to Galaxies, one is called the core cusp problem, and the other is called the Missing Satellites problem. The core C problem is, uh, if you take a bunch of dark matter according to our simulations and our models and our theory of how dark matter behaves.

It just piles up to super super crazy high densities in the center of Galaxies. It forms what we call a core and more than the core of a galaxy. I'm talking insanely high, like maybe infinitely high, but we're not gonna go there. Just super high densities. Uh, but that's not what we observe. Yes, the cores of Galaxies are more dense than the rest of the Galaxies, but not by that much. And so that's a problem. Another problem is called the missing satellite problem. According to our simulations and our models and our understanding of dark matter, a typical galaxy like the Milky Way should have tons of satellites, small satellites surrounding it. And, yeah, the Milky Way galaxy does have a bunch of satellites, but not that many. So in our vanilla dark matter models, we make cores that are too dense and we overproduce satellites around a galaxy. OK, OK, fair statement, two responses to these issues with dark matter.

Uh, either dark matter is wrong, and we just need to chuck it because it's not fitting. Observations or perhaps Galaxies are more complicated than we thought. And the action of regular matter has a bigger influence than we initially thought. Most cosmologists, physicists, astronomers believe the second explanation. It turns out that the universe is more complicated than we thought, for example, in like the nineties and early two thousands this missing satellite problem. Uh, according to computer simulations of Galaxy evolution, the Milky Way should have had 10 times the number of satellites than we were actually observing in the real universe. But as our simulations became more sophisticated, included the effects of regular matter and how that can influence and interact, uh, gravitationally with the dark matter. In the simulations, the predicted number of satellites started coming down. And then, as we got better and better at making observations and had bigger telescopes, and we could start to pick out some fainter satellites that we weren't seeing before, our count of actual satellites was going up, and then our theoretical predictions of the number of satellites was coming down.

They still don't quite agree in. Instead of like a 10 to 1 mismatch, it's now like 2 to 1, which, honestly, in astronomy is basically nailed it. I'm not joking. That's a whole other episode. Um, but like over time, this problem has eased. And same for the core C problem. Uh, as we get better simulations and better understanding of dark matter, it turns out in in the interaction of dark matter and normal matter through gravity, we see that the densities in the centers are not as high as we initially thought. And and so it could be that these problems will just melt away that these were phantom problems. It's not completely resolved, though, And so there is some wiggle room here for a mod like theory to claim, uh, a slight edge. Mond doesn't have a core C problem. It doesn't have a missing satellites problem. So if you just look at Galaxies, Mond is like, you know, ahead it it it's it's a little bit better at the data. And if all we had were Galaxies, the scientific community would probably lean in a mond like direction.

I do need to make a side note, because I just said a phrase, an interesting phrase, mond like theory. When it comes to both mond and dark matter, there's still one theory. These are more like families of theories or groups of models. There's a general framework of mond. That was the original mond theory. But then, since then, it's been like 50 years. There are different approaches to making the modifications to Newtonian dia dynamics and same with dark matter. There are different approaches to introducing new particles with new properties. Uh, it doesn't mean one thing one way or the other. It's just a general observation that there's no one single theory of dark matter. And there's no one single theory or hypothesis of mond. There have been thousands of papers, and this what I'm providing in this episode is a 10,000 ft view of the debate where I'm not getting into all the little details. Uh, well, this little, uh, hypothesis over here this, uh, developed, uh, some answer and then But there's an alternative dark matter where there's two kinds of dark matter particles that interact with each other.

And there's a we're looking at groups of ideas. Either we add new ingredients to the universe or we modify our theories of physics. And like I said, if you just look at Galaxies, Mond has a slight edge. But where Mon starts to weaken is literally everywhere else. But before I continue, I need to take a quick break for a word from our sponsor. Better help. Mental health is so important. And you know, I'm a firm, firm advocate for mental health. You you take care of your body. You go to the doctor when things are a little off, or just do regular checkups with your doctor. You should also take care of your mind. I know a lot of you tune in to this show to just escape and relax and have your mind blown. Well, maybe you should have your mind helped a little, too. I've gotten a lot out of therapy, and I'm not ashamed to admit it. I think it is a powerful tool for everyday life, and that's where better help comes in. Better help is online therapy.

It's like a podcast where you get to do a lot of the talking. That's pretty cool. And someone's there to listen up real professional over the video, over phone, even live chat only sessions. It's more affordable than in person therapy. You can be matched with the therapist in under 48 hours. This is a powerful tool for your everyday life and I seriously encourage you. Even if you don't think anything is wrong, you will be surprised at how much therapy can help ask a spaceman. Listeners get 10% off of their first month at better. Help dot com slash spaceman. That's better. HE LP dot com slash spaceman Hey, you remember. Remember that coma cluster with Fritz Wiki and the Galaxies moving around inside the cluster way too quickly? Let's bring that back. Turns out that dark matter has a much easier time explaining that, because in the dark matter hypothesis, every observable thing you see at large scales you see a galaxy. It's surrounded or it's embedded inside of a big ball of dark matter, and you just scale that up like, Oh, you got a galaxy cluster.

Well, it has its own giant ball of dark matter, and then that's it. And you can very easily explain the motions of Galaxies inside of clusters at those scales. Uh, has a much more difficult time because the modifications to say, force laws and acceleration relationships all that are tuned to galaxy dynamics. Remember, they're tuned to rotation curves, so they have trouble. This modified these families of modified theories have trouble explaining dynamics at scales anything bigger than a galaxy because they were tuned at the Galaxy scale. Dark matter is much more, uh, universal and much more flexible with one simple idea. Maybe there are giant vaults of invisible stuff. And then, from there you can just explain galaxy clusters. Mond has a much more difficult time. Yes, people have taken some versions of mod and used it to explain some of the motions of Galaxies inside of clusters.

But if you try to do that, along with fitting, the rotation curves, you end up coming short and you can't explain all the motion of Galaxies inside of clusters. And you still need some form of dark matter to explain everything that doesn't by itself mean that mod or modified Newtonian dynamics is wrong. It just weakens the case for it, because if your alternative theory of dark matter still ends up needing dark matter, then you haven't really advanced the cause of human knowledge very, very far. Dark matter is much simpler. It just says Giant ball of invisible stuff, bigger ball of invisible stuff. What's the big deal? And has a very easy time explaining cluster dynamics where mond struggles. Another big problem for Mond in the original mond theory is that it's not really a theory of physics. Let me explain that because it it sounds like a theory of physics but a theory of physics. It's It needs to jibe with everything else we know about physics.

It's like suggesting a replacement for an engine part like, I don't know, a spark plug. You can't just go in there and take out spark plugs and put whatever you want in place of it, like a live chicken or a piece of cheddar cheese. There, there are rules that spark plugs have to follow in order to do their job. Yes, you can suggest a replacement for the spark plug, but it still has to perform the primary function. It has to connect with all the other parts of the engine to get the machine going. There are rules that laws of physics have to follow to be compatible with all the other knowledge we've acquired about the universe. Like if you want to concoct a new theory of physics, you are more than welcome to, but you needed to obey conservation laws, conservation of energy conservation of momentum you needed to obey known symmetries, et cetera. It needs to be compatible with special relativity, et cetera. You need these baked in because all those other ideas of physics are incredibly well tested and well known.

And so if you want to come up with a new way, you have to be able to connect. Or you can't just take any random piece of the puzzle and take it out and then put in whatever you want and just hammer it in and expect it to work. No, it has to fit. Now. If you want to replace all of physics, go right ahead. I wish you the best of luck. Mond by itself doesn't do that. It's too simple. All it says is that acceleration is a little bit different at extreme distances that doesn't fit with conservation of energy and more momentum known symmetry. Special relativity is too simplistic. So if we want mond to go bigger than galaxy rotation curves or dynamics of Galaxies and clusters, if we want it to use it for anything other than explaining rotation curves, we need to evolve it. We need to grow it. It's It's the same relationship between Newtonian dynamics and general relativity. Newtonian dynamics You can use, uh, looking at star stellar orbits in a galaxy.

You can just use Newtonian physics. General relativity doesn't apply. You're looking at the motions of Galaxies inside of a cluster. General relativity. You know you don't need it. You can just use. You can just use normal Newtonian physics. You go to cosmological scales or gravitational lensing, or any other situation where gravity does become strong and much more interesting. You need to switch to general relativity, so you need the equivalent for mond. If Mond is replacing Newton, then you need to evolve it so that you have a replacement for Einstein. And that's called Tevis. TEVES stands for tensor vector scalar theory. I am not going to get into the nuts and bolts of that, but just know that it's a replacement for general relativity that obeys conservation laws. Known symmetries it it it's a working theory of physics at the scale so it can compete head to head with GR.

It gives you an apples to apples. Comparison to GR to general relativity got a situation that can only be explained by general relativity. Apply Tevis instead and see what happens. You can make testable predictions. Mond is a testable theory of physics, just like dark matter. So the good news is that with this evolution with this new and improved understanding, you can now compete with general relativity head to head. The bad news is that nothing works. For example, there's patreon. Patreon dot com slash PM Sutter, if you wanna. If you have too many epi cycles in your life, then I know how to fix it. Please go to patreon dot com slash PM Sutter to support the show, and I really do appreciate all the contributions. But, for example, the bullet cluster this was observed a few years ago. Uh, this bullet cluster. It's two giant clusters of Galaxies that are crashing into each other. It's about 4 billion light years away. It's amazing. Dark matter gives a very, very simple prediction for what should happen when galaxy clusters collide and they do, and it's awesome.

The gas inside of a cluster should, uh, tangle up with itself because it's gas and it's gas. You can get shock fronts, it heats up, it gets all tangled up. The star or sorry, The Galaxies, uh, occupy relatively little volume in a cluster. So they just swing by each other like that. That was the sound of Galaxies flying by each other, by the way, uh, like like bees just floating by the dark matter doesn't interact with anybody, including itself. So also the dark matter should swing by each other. So if you look at a a merged galaxy cluster post meger and if you look at where the gas is, it should be in the center. If you look at where the visible light is, it should be scattered at the edges with where all the Galaxies are. And if you use something like gravitational lensing, it should also be at the edges because that's where the mass where the dark matter is, that is exactly what we observed with the bullet cluster. Like I said, testable prediction. And this is what we saw. Tevis has a really tough time explaining the behavior of the bullet cluster really tough time.

Another example. The cosmic microwave background. We look at the statistical properties of the cosmic microwave background and if you include dark matter as a component in the early universe. It matches what we see If you instead take out dark matter and replace it with Tevis. With this modified version of general relativity, you don't get what we see. You do not match those observations. When we look at the growth of structure in our universe and how Galaxies and clusters evolve over time again we have statistical properties of these You plug in dark matter or you say OK, 80% of the mass is invisible and doesn't interact. It just has gravity. You get what we observe at large scales. You take that away and you put in Tevis this replacement for general relativity the evolution of mind You don't get that you look at the 2017 Kila nova. This is when two neutron stars merged and we saw both the flash of light from that collision and the gravitational wave signal from that collision.

Tevis most theories that look like Tevis again Families of theories not one specific theory or model said that the light in the gravitational wave should have arrived at different times wildly different times and they arrived at basically the same time. The ultimate lesson here is that any time you try to extend MOND beyond Galaxies. It has a really, really tough time. We have to be honest. I was honest about dark matter. It had a tough time inside of Galaxies. Totally does. Outside of Galaxies, Mond has a really tough time now. There are some attempts to explain bits of these observations with some successes, but there's no consistent picture from mond or mond like theories or Tevis or Tevis like theories. That explains all of these observations as neatly and cleanly and simply as the dark matter hypothesis. That is just a fact. Yes, there have been theories that have attempted to explain the bullet cluster results.

They also can't give you the cosmic microwave background observations. Yes, there have been attempts to develop a theory of modified gravity that is consistent with the 2017 kan Nova result observation. Those don't get you structure formation or the bullet cluster or rotation curves. There's nothing that fits the bill and all the data. All the observations and that is the challenge with any scientific hypothesis that you want to use to replace dark. Matter is, you have to explain all the data. You can't just cherry pick and say, Well, I explained rotation curves and then walk away. You're not done yet. Like I said, you may not like dark matter, and the dark matter hypothesis certainly isn't perfect. But it's tough to say that you have a better idea. One of the key strengths of the dark matter models is that you can get as crazy as you want with what dark matter is made of. But once you've generated those particles like, OK, I have a new particle and has this mass and and it has these properties and, uh, et cetera, et cetera.

But then after that, you must obey the known laws of physics. And so you can use that to your advantage, to make testable predictions. Whatever you create is automatically consistent with everything else we know. So OK, I think, uh, there's a dark matter particle. And here's its motivation. And here's how I think it interacts with the rest of the particles in the known universe. OK, then you can start calculating because you already have your theories of physics. You're just using GR and electromagnetism and all that, and then you can go out and compare you can plug it in and see how it affects the cosmic microwave background or structure, formation or rotation curves or anything with alternatives. With Mon and Tevis like ideas, you have to create new laws of physics. You're creating new theories, which introduces a lot of ambiguity and opportunities for model fitting, and also since you're creating brand new laws that behave differently in different situations. Uh uh, you know, one way in the solar system, like gravity operates one way in the solar system and another way in Galaxies and then in yet another way at bigger scales.

The theories tend to twist themselves up in knots like Tevis, a GR general relativity is, is clean and consistent, and and it makes testable predictions. Tevis potentially has some serious issues where it may not even be a viable theory of physics, where it may not even be consistent with itself. In certain cases or you know it, it's it's a little bit ambiguous. It's a little unclear because it's such a complicated beast of a theory. Dark matter has its flaws, and it is not able to explain all the available data, which is true for any scientific theory So if anyone ever comes up to you and says, Hey, you dark matter does not explain all the available data. That is true for general relativity itself. That is true for the germ theory of disease. That is true for evolution. That is true for play techno tectonics. No theory, no hypothesis is able to explain all the data because nature is always more complicated than our theories.

That's a relatively weak argument. Honestly, anyone who claims that they have a better idea than dark matter hasn't done their homework. I've read a lot of papers on dark matter and dark matter alternatives. They almost always focus on one case. There are still papers being published that focus on rotation curves of Galaxies and then are able to make a better fit to rotation curves of Galaxies and dark matter and then stop and then say, OK, that's it. We have a better we have an alternative dark matter. No, you don't. If we never knew about rotation curves, if rotation curves never existed, if we had never made those observations, we would still have a hypothesis for dark matter. Because of all the rest of the universe with the way that Galaxies move inside of clusters with the appearance of the cosmic microwave background with the things like the bullet cluster and gravitational lensing. With the evolution of large scale structures in the cosmos, we still have all of that.

And that would all those things would still lead us to the existence of dark matter. So if someone says they have a better idea and dark matter is a bad idea and they have a better one, they haven't done their homework because they haven't matched what dark matter is able to explain? Will a mond like theory ultimately prove correct? Will some genius pop out an idea that we've never thought of? Maybe, seriously, maybe I'm not going to discount. Like I said, I don't wake up every morning attempting to fight to the death over dark matter. I do believe that dark matter likely exists. I recognize that we have not directly observed it yet any one of the hypothesized particles that it could be. We have not directly seen them. We only have this giant set of circumstantial evidence and that any alternative that people have cooked up, especially alternatives that invoke modifying the laws of physics and our theories of physics and our understanding of gravity come up far short. Yes, dark matter has its weaknesses, but it is miles stronger than any or kilometers.

You your choice than any other alternative. In fact, like I would argue, going back to this epi cycle argument where, uh, you know, we're holding on to our theories and we're just adding modifications. That's the modified theories. Epis. Are we a modification on top of the basic theory of how orbits should work that is mad. That is Tevis, where you're trusting our theories and our theoretical institution rather than the raw observations. And you're trying to, you know, distort our theories in order to fit the observations rather than accepting a simple but radical hypothesis, which is that most of our the mass of the universe is invisible. That's why I'm I believe, that mond like theories are ultimately not going to work out. I my my mind could be changed. And honestly, if anyone says that dark matter isn't creative or we're not responding to the day like isn't a brand new particle that we've never heard of before, and we only see evidence For once we observe the universe, the very largest scales.

Isn't that cool enough for you? Isn't that fun enough? Isn't that awesome enough to imagine that there are new ingredients in the universe that we simply haven't encountered because our horizons have been too small? That's pretty awesome. It could be wrong. We'll see. But there's a lot of reasons to believe it. And there are not a lot of reasons to believe the alternatives. What is dark matter? We don't know. And there's certainly more surprises waiting for us. But we do know what it isn't. And that's still progress. Thank you to Simon S on email at the mayor on Twitter and ENG on email for the questions that led to today's episode. Please please, please go to patreon dot com slash PM Sutter. I really do appreciate everyone who contributes. I do wanna give some shout outs to my top Patreon contributors This month. Justin G, Chris L, Barbara Kay Duncan M Coy D, Justin ZNH and F Nalla, Aaron Scott M Rob H Loyalty. Justin Lewis M, Paul G, John W, Alexis Aaron J, Jennifer M, Gilbert M, Tom B, Joshua Kurt M and Bob H.

It is everyone's contributions that keep the show going, and I truly do appreciate it. That's patreon dot com slash PM. Sutter hit me up with more questions. I really do love all the questions it's ask a spaceman at gmail dot com. Go to the website. Ask us spaceman dot com. Find me on social media hashtag ask a spaceman or at Paul Matts Sutter. You know what to do by now, and I'll see you next time for more complete knowledge of time and space.

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