As I was preparing to post the first SST3K, my husband suggested that I really ought to do one about dark matter, which I briefly mentioned in that conversation.
MEH: Dark matter should be its own post! Talk about hand-wavy and convenient.
Me: Well, the existence of dark matter is supported by the way light bends in certain places as if there were invisible masses. It also would explain the way the speed of rotation at the edge of galaxies doesn’t slow down the way we would expect it to at the fringe of the galaxy’s mass.
MEH: Yeah, I’m aware of those arguments. I just think it’s a little…unwise…to embrace one explanation so wholeheartedly when there might be other theories that fit the evidence as well.
Me: That’s fair. Why don’t we both do some research? I’ll look into what scientists who support dark matter currently think, and you can look at alternative theories. Then we can reconvene and report to the class on what we learned.
MEH: The class? Isn’t that kind of breaking the fourth wall?
Me: Just call me Deadpool.
(SOME TIME LATER)
MEH: All right, let’s do this. First, you talk about dark matter. I already know that if it exists, it should make up about a quarter of the matter and energy in our universe. I have to say I’m really not impressed with physicists’ naming creativity, since there’s also dark energy, which is thought to take up nearly three-fourths of the matter and energy in the universe and isn’t the same thing as dark matter. Engineers would never have given them such similar names.
Me: Yeah, you have a good point. They’re both called “dark” because we can’t detect them using our normal means, which rely on electromagnetic radiation in some form. (The electromagnetic spectrum covers a huge range of wavelengths, encompassing radio waves, X-rays, and the relatively narrow band of wavelengths that we can see and usually think of as “light”. It’s a pretty big deal to imagine huge amounts of matter that don’t seem to interact with anything on this spectrum.)
MEH: About that “can’t detect them” part, yeah. I know there’s evidence that supports the need for something like dark matter, but as far as I can tell, it seems like kind of a placeholder. Wasn’t it proposed because the stars at the edges of galaxies were moving faster than expected at that distance from the main mass of the galaxy?
Me: That was what started it, yes. Since then cosmologists have found other reasons to expect its existence, like the bending of light in apparently empty areas of space, as well as what we see in the cosmic microwave background. That’s essentially the signal that remains from the very early universe, and we see some odd features in it that can’t be explained by just baryonic matter.
MEH: But no direct evidence, right?
Me: Yeah, nothing direct yet. Dark matter is still best explained in terms of what it would account for, rather than any actual observations of what it is.
MEH: Aha! That’s what I thought. My turn.
It turns out there are a few other theories out there that arguably explain some of these observations as well as dark matter does. Most of these theories fall under the umbrella of “modified Newtonian dynamics” or MOND. Specifically, they modify Newton’s Second Law, which is pretty serious.
Me: Huh. Well, I mean, it’s always been more of an approximation anyway…
MEH: Tell me you’re joking.
Me: (shrugging) Physicist.
MEH: Blecccchh. You people are monsters. Well, anyway, the main idea is that a lot of the effects most cosmologists attribute to dark matter could actually be explained by a change in the gravitational force on objects at very low accelerations, like the stars at the fringe of a spiral galaxy.
One of the latest compelling arguments for some form of this theory is that apparently, the acceleration of galaxies is correlated strongly to their visible matter. That suggests that maybe the acceleration really is determined by the matter we see, rather than being the product of dark matter as well.
Me: Yeah, that is interesting. Dark matter theorists have already responded, though. They claim that models predict the amount of dark matter in a forming galaxy will also correspond to the amount of normal (baryonic) matter. That would explain the correlation between visible matter and acceleration without taking away dark matter’s role.
MEH: Hmm, OK. It sounds like it’s model vs. model on that front, with no evidence on either side. If that was one of the main points in MOND’s favor, I guess it doesn’t really have an edge over dark matter at this point. I can see why dark matter seems like the more likely theory, since it doesn’t change Newton’s laws. It still bothers me that people talk about it like a foregone conclusion, though.
Me: Me too. It’s easy to go the other way too, and depict research right now as a dramatic fight between dark matter and MOND, which isn’t really true. In most ways, it seems like dark matter still offers a better explanation for everything we observe, but a lot of physicists think we shouldn’t throw out other possible explanations. Actually, there’s a fairly recent theory based on extensions of string theory that rejects both dark matter particles and MOND, and it appears to pretty accurately predict the light-bending effects commonly attributed to dark matter, at least in some cases. This one hasn’t been fully worked out, but it’s another example showing that there are people looking at alternatives to dark matter, and they can’t currently be proven wrong.
I agree with you—it’s important to question our assumptions as we’re able to learn more, and it’s even better if we try not to have too many to start with.
MEH: Now you’re speaking my language! I guess engineers and scientists aren’t so different after all.
Me: Oh, I wouldn’t say that…
MEH: Anyway, you’ve convinced me that it isn’t unreasonable to think dark matter is the most likely option. But really, the only way to resolve this is to come up with experimental proof one way or another. There are already ideas floating around on how to test MOND theories here on Earth. Meanwhile, they better get cracking on finding some of the theoretical dark matter particles.
Me: They’re trying! There are a few promising candidates. Those include hypothetical new particles like axions (incredibly low-mass particles) and WIMPs (weakly interacting massive particles, about the mass of protons or bigger). A less likely possibility is that dark matter is made up of regular baryonic matter after all, in the form of MACHOs (massive compact halo objects, like black holes and neutron stars).
MEH: …For real? WIMPs and MACHOs? What did I say about physicists and naming things?
Me: Yeah, yeah, I know. Does that mean you’re naming our children someday?
MEH: Gyroscope and Ratchet sound beautiful, don’t you think?
Me: …let’s talk about this later.