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Aseyhe t1_j45ksax wrote

For example, we cannot rule out that the dark matter might be asteroid-mass black holes (e.g. figure 10 of this article). Why couldn't it just be asteroids?

The main lines of evidence against such a possibility are related to the early universe. This is a time when the the universe was very hot. Asteroids could not exist in such an environment; they would dissociate into diffuse plasma like all the rest of the ordinary matter. In this context, all ordinary matter is equally detectable, in the sense that it has an equal impact on what we observe. But what do we observe?

  1. The relative abundances of light elements throughout the universe. We understand nuclear physics and can predict the ratios of hydrogen, deuterium, helium, etc. that should have emerged from the Big Bang. What we find is largely consistent with ordinary matter comprising only 5% of the total energy density today. If the density of ordinary matter were higher, we should find less deuterium and more helium than we do. The first figure of this paper (page 9) illustrates nicely how the primordial element abundances depend on how much ordinary matter there is.

  2. Temperature variations in the cosmic microwave background. In the early universe, the ordinary matter and photons were tightly coupled, which led to such effects as pressure oscillations and sound waves. Dark matter, on the other hand, only interacted via gravity. This causes them to have very different effects on the evolution of temperature and density variations in the early universe, which manifest themselves to us in the cosmic microwave background. Here's an animation of how changing the density of ordinary matter ("baryons") would alter the "power spectrum" of the cosmic microwave background temperature, which is something we have measured extremely precisely, e.g. the top panel of this figure.

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Interesting-Month-56 t1_j45nggz wrote

I think to restate this simply (with the caveat that simple == less correct) is that we do know it’s not things we can detect easily, like elements (hydrogen, helium, iron, etc). These things tend to clump together and do things like fall into stars where we can detect them by their emission lines. Which how we know things like the hydrogen/deuterium ratios of stars and galaxies. We know the mass is there through observation of galactic rotation. We just can’t see it.

What’s left are largely things that are hard to detect. Like neutrinos. Or theoretical things like subsolar mass black holes or various WIMPs that are also really hard to detect.

As to what dark matter is, there isn’t any definitive proof of one thing over another, though some things are more likely than others.

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Aseyhe t1_j46bqop wrote

> These things tend to clump together and do things like fall into stars where we can detect them by their emission lines.

I think that requires some additional assumptions. Dark matter indeed couldn't be gas, because we can detect gas on its own, and also gas would lose energy and fall into things (whereas dark matter does not). However, high-density low-brightness objects made of ordinary matter (like free-floating asteroids or planets) would behave just like dark matter and would be incredibly difficult to detect. Granted, it would not be easy to explain how they all formed.

But that's why I went to the very early-universe evidence for dark matter. Because irrespective of what form ordinary matter took in the late universe, it was all the same in the hot early universe.

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panzuulor t1_j46p541 wrote

There isn’t enough ordinary matter in the universe to account for it all. All the exoplanets and non-radiating matter is only a tiny fraction of the matter we see. And that only accounts for a small part of the gravity we measure. That’s why dark matter is inferred.

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dastardly740 t1_j47nd3p wrote

To expand a bit, aside from the early universe point in terms of how we get anything smaller than a gas giant for dark matter. One thing that is possible even for hard to detect ordinary matter is that even non-detection sets limits for the size and quantity of those objects. So, surveys watching for occultations and gravitational lensing events on stars in Andromeda or distant galaxies or quasar by these objects if they made up the Milky Way's dark matter halo set limits on the size and quantity of planet sized objects and even planet mass primordial black holes. I believe the current boundaries for size and quantity of objects that have not been eliminated as significant dark matter contribitors is pretty small right now.

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Leemour t1_j484cvk wrote

Couldn't we just be wrong about some aspects of cosmology, something like relativistic gravity?

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Aseyhe t1_j48ddgg wrote

That is possible. However:

The early-universe evidence for dark matter is so strong that even the (non-mainstream) researchers who favor modifications to gravity to explain dark matter in galaxies still have to include dark matter in the early universe. Then they have to get rid of it by the time galaxies start to form to make way for their gravity theory.

Another perspective is that dark matter is a much simpler hypothesis than a modification to gravity. We already know of one dark matter species -- neutrinos -- and it's really not a stretch to suppose that there is another. The only special feature that we know it has to have is that it has to reach the observed abundance.

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forte2718 t1_j49mt2l wrote

I think XKCD said it best:

>Yes, everybody has already had the idea, "maybe there's no dark matter — gravity just works differently on large scales!" It sounds good but doesn't really fit the data.

The bottom line is that, even though we've explored quite a lot of modified-gravity / alternative-cosmology models, none of them have been able to fit all of the data even with some of the wildest and most contrived parameterizations. In basically every case, we can choose parameterizations that work to fit some datasets, but then those same parameterizations then go on to fail miserably at fitting other datasets ... and there just isn't any parameterization that works for them all at the same time, which means none of those models are actually viable as models of cosmology.

On the other hand, not only are dark matter models incredibly simple by comparison, but it is straightforward to parameterize them such that all of the datasets are well-fitted with the same parameterization, and getting to that point doesn't even require any specific model of dark matter — the generic idea of dark matter works so well that there are many different possible models of dark matter that work. It could be sterile neutrinos, or axions, or WIMPs, or light supersymmetric particles, the list goes on and on ...

When an idea works so well and with such simplicity and generality while every other idea explored falls flat on its face no matter how complicated you make it, you just have to take a step back and admit that maybe, just maybe, the simple and general idea is actually the correct one. It's kind of like ... if you're sitting there with pegs of different shapes and you find that the round peg fits the round hole but none of the other pegs fit it, well ...

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Kantrh t1_j477330 wrote

Sterile Neutrinos as dark matter has been ruled out, an experiment running next to a reactor has found no sign of them.

https://phys.org/news/2023-01-results-stereo-sterile-neutrino-hypothesis.amp

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Aseyhe t1_j47fteu wrote

That reads like they are ruling out the possibility that sterile neutrinos are the explanation for a particular anomaly seen in past experiments, and not that they rule out the possibility of sterile neutrinos in general.

Edit: see for example this figure from the article. They rule out parameters to the right of the red/blue curves, in particular ruling out the parameters that would have explained the "reactor antineutrino anomaly (RAA)".

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Kantrh t1_j47hta6 wrote

The article says it rejects the sterile neutrino hypothesis quite strongly, although it doesn't say it rules them out altogether from existing.

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Aseyhe t1_j47qwh0 wrote

Yeah, but in context it's the hypothesis that sterile neutrinos explain the anomaly. I edited the post above after checking the research article.

They also say, > we reject with high CL the hypothesis of a sterile neutrino of mass around 1 eV.

Viable sterile neutrino dark matter models are generally at least 1000 times heavier than that, in the keV range. That's because if the dark matter particle were too light, its thermal motion would eliminate variations in the density of the universe at the scales of dwarf galaxies, preventing those galaxies (which we observe and hence know to exist) from forming.

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daywalkker t1_j4h9nqc wrote

Asteroids are not neay massive enough to cause the gravitational disturbances we see that indicate dark matter.

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Aseyhe t1_j4hbvm8 wrote

An asteroid and an asteroid-mass black hole have the same mass.

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