Astronomers have created the largest ever map of dark matter | Hacker News Reader

Astronomers have created the largest ever map of dark matter

newscientist.com

77 pointschriskanan33 comments

Maybe someone could help me understand why a simple explanation for dark matter isn't possible.

I get how you can measure the mass of stars, and how you could also maybe measure the mass of interstellar gasses. You add it all up and there is missing mass. But how could you estimate the mass of larger objects? Couldn't the interstellar medium be full of moon sized cold rocks? You'd have no way to observe them on a galactic scale since they're not illuminated, they have effectively no absorption spectrum b/c of their small surface area and yet I'd think they could make up the difference you need for this "dark matter"

The explanation seems too simplistic, so I'm clearly missing something :)

On the mass scales we talk about, even small rocks would add up and lead to an observable dimming.

Nebulae usually are usually even less dense, just a few molecules per cubic metre, but they are still easily observable.

I ran some numbers, for the Milky Way[1], the visible matter is roughly 1e11 solar masses, while the dark matter halo contains[2] about 1-2e12 solar masses (depends on your cut-off), I'll use 1e12.

The moon is about 3.7e-8 solar masses[3], so you' need about 2.7e19 of these "rocks" to make up the Milky Way dark matter halo. For comparison the Milky Way has about 250e9 stars[4].

So we're talking over 100 million "cold rocks" per star in our galaxy.

[1]: https://en.wikipedia.org/wiki/Dark_matter_halo#Milky_Way_dar...

[2]: https://arxiv.org/abs/1510.06409

[3]: https://www.google.com/search?q=mass+of+the+moon+in+solar+ma...

[4]: https://public.nrao.edu/ask/how-many-stars-do-the-andromeda-...

Nebulae are an argument in favor. They are observable b/c they're not dense. They have a very high surface area to mass ratio. Light passes through them and they have absorption spectrum. They also emit a lot of black body radiation.

By contrast a moon size object has a very low surface area to mass ration. It has no absorption spectrum b/c light can't pass through - and any black body radiation would be negligible b/c of relatively small surface area

I am not an expert. My understanding -

(1) The quantity of rocks that would be needed to explain such a gigantic gap in mass is bound to be observed - if they are dark, then at least we should be able to see them when they block light. You have to remember we need dark matter to be around 80% of total mass of the universe. So they either need to be super dense, like tiny black holes [1], or there needs to be so much of it that there is no way we won't see them.

(2) Dark matter is too spread out. If they were rocks or any of the known matter types, they should have coalesced on their own. So they have to be such that dark matter doesn't interact with itself.

Edit: This is a nice intro: https://www.youtube.com/watch?v=fidzLZQyaJE

Edit 2: Also worth pointing out that dark matter is not the only explanation: https://www.youtube.com/watch?v=4_qJptwikRc

[1] https://duckduckgo.com/?q=can+dark+matter+be+tiny+black+hole...

Larger objects, such as the rogue (not members of a solar system) rocks you mention, are classed as MACHOs (massive compact halo objects) in possible dark-matter explanations.

Recent studies [0][1] have pretty much ruled out MACHOs as an explanation, by looking for gravitational microlensing events by MACHOs between us and the Magellanic Clouds and Andromeda Galaxy. They found about 1/1000 of the events predicted by a quantity of MACHOs necessary to explain dark matter's effect on our galaxy's gravity. For very small MACHOs that wouldn't produce enough lensing to be detectable, then you need so many that they'd darken the light, like a gas/dust nebula. This is also not observed.

I don't work in the field, but I studied astrophysics, and I have one or two astronomer friends - it seems these results are regarded as a pretty definitive nail in the MACHO theory's coffin. Current research is mostly looking for WIMPs (weakly interacting massive particles), a novel class of fundamental (atomic/subatomic?) particle, expected to be present throughout all of space, which doesn't interact with the electromagnetic force, so doesn't interact with light, including virtual photons of attraction and repulsion, (such as those holding the particles of your feet together and also repulsing your feet from the ground right now,) that mediate the forces we interpret as "solidity" and "pressure", but has mass.

[0] https://sites.astro.caltech.edu/~george/ay20/eaa-wimps-macho... [1] https://www.nao.ac.jp/en/news/science/2019/20190402-subaru.h...

Sorry to be a pendant, but the forces holding things together is just ordinary EM (well, QED probably better stated) while virtual photons are different (virtual since they only appear in higher order corrections in qft).

I love pedants, don't apologise! Happy to be corrected and keep learning. It's been quite a few years since I was in physics classes, so I went and looked this up, and I think my memory was right, as for example, descriptions of the Coulomb force (electrostatic repulsion and attraction as in my foot example) all seem to be phrased in terms of virtual boson exchange.

I agree the forces holding things together is "ordinary EM", but virtual particles as force carriers seems to be ordinary, not rare.

Thanks for the explanation. It raises other questions, like how they would know small MACHOs haven't darkened the sky (I'm guess either you'd see twinking or it'd shift the stars downward on the Hertzsprung–Russell diagram to some "unnatural" location) - but I get the gist of it. It's an interesting area of research. Really appreciate you illuminating it for me :)

Seems like they observed distortions of space time, but not seeing any (dark)matter?

Just for fun, what if matter actually has coordinates (x,y,z,h1,...,hn). Where hn is some number of hyper dimensions. Visible matter is stuck in our 3 space (x,y,z,0,...,0) and gravity is the only force that permeates across hyperspace. We can’t see the hypermatter except by the gravitational imprint it leaves on 3 space material.

If we had the ability, it’d be neat to see if our visible matter can alter the configuration of dark matter, just as dark matter seems to affect our visible matter.

Then gravity would “leak” and the inverse square law would not perfectly apply.

This has been measured and an extra dimension has been ruled out.

In an infinite universe with random local density of matter, the gravitational constant would vary by location. Anyone know what the consensus is among astonomers if the universe is infinite or not ? Is there even a way to theoretically know ?

Theory can say what shape the universe should have, for example inflation predicts a flat universe[2]. Of course you then have to check if the theory matches what we observe.

As far as I've gathered, the current measurements are consistent with a flat universe. Here's[1] a post about a recent, improved measurement.

[1]: https://astrobites.org/2020/12/03/cosmic-clocks/

[2]: https://en.wikipedia.org/wiki/Flatness_problem#Inflation

Edit: cosmological constant, not gravitational constant

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