What's Dark Matter, any new theories?

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OgreBattle
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What's Dark Matter, any new theories?

Post by OgreBattle »

Read some stuff about the theory, now I hear the theory is being challenged but I don't fully understand
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Post by Stahlseele »

Dark Matter is the theoretical thing that keeps the universe together.
Basically, the expansion rate of the universe does not fit what it should be, in fact, it is slowing down.
This should not be happening without something creating a huge ammount of gravitational pull. But what is observeable does not have enough mass to actually do so.
So the theory is that there is something we can not detect directly which still has mass and thus creates a gravitational pull of its own.
And seeing how there appears no difference in the expansion rate in different directions, it has to be all around basically . .
So it is a theoretical matter not directly observeable only theoreticized about due to the observed effects.
And this will all go down the drain as soon as somebody finds something more reasonable to explain the slowing down of the expansion of the universe.
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Re: What's Dark Matter, any new theories?

Post by maglag »

OgreBattle wrote:Read some stuff about the theory, now I hear the theory is being challenged but I don't fully understand
As Stahlseele said Dark Matter was theorized to try to explain certain gravity-based observations of the universe's slowing expansion.

But thing is, recently it's been observed that the universe's expansion isn't slowing down that much and may actually be accelerating. Why? Nobody's really sure either, which in turn lead to the new theory of "Dark Energy" to try to explain why the universe's expansion speed is going up.
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Post by Stahlseele »

Which leads to another problem though.
One theory of the big bang is that the universe breathes.
In a cycle of its own existence.
First it goes bang, then it expands.
It gradually slows down and the reverses direction.
Speeding up the collapse gradually untill it goes CRUNCH.
And then when all is compressed into one single spot.
It goes BANG again.
Still does not explain how it all got started, but that is/was a theory at some point.
Now with the universe not actually slowing down its expansion but even increasing pace . . theories about that are slightly newer and probably a good deal more problematic.
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Post by maglag »

Stahlseele wrote:Which leads to another problem though.
One theory of the big bang is that the universe breathes.
In a cycle of its own existence.
First it goes bang, then it expands.
It gradually slows down and the reverses direction.
Speeding up the collapse gradually untill it goes CRUNCH.
And then when all is compressed into one single spot.
It goes BANG again.
Still does not explain how it all got started, but that is/was a theory at some point.
Now with the universe not actually slowing down its expansion but even increasing pace . . theories about that are slightly newer and probably a good deal more problematic.
What's the problem exactly? There's exactly zero evidence of there being anything before the Big Bang so the whole "closed cycle" wasn't a particularly strong theory from the start.

If nothing else, even if all matter even switched direction and collapsed in a new Big Bang, energy in the form of background radiation would not, so each new universe would be colder than the previous one for starters.

Meanwhile Dark Energy may mean that there's actually a way of breaking out of entropy's lockdown. If there's something out there making the universe go faster forever, it means there's something that could be harnessed and we'll never need to worry about everything everywhere freezing to death eventually.
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Post by Username17 »

The core issue of dark matter is that a lot of things spin faster and appear attracted to each other more than can be accounted for by the stuff we can see. We know for an absolute fact that there is some amount of dust and planets and shit that doesn't glow but has gravity - but the back of the envelope calculations make it look like there should be a lot more material out there than seems plausible based on what we know about our immediate vicinity in the galaxy.

It doesn't particularly bother me, since we're talking about the measurements of speeds of things that are very far away, so minor discrepancies in observations could account for very large differences in projected masses. But physicists keep trying to peck away at it, because of course if it turns out that the explanation has something to do with the properties of space having different properties over very large distances we could be looking at the plans for hyper space tubes or something equally out-there.

We ended up being able to make nuclear bombs because someone noticed a hole in the wavelengths of light emitted by heated metal, so when physicists say there's an anomaly in the readings you should always let thing tease at the puzzle.

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Post by deaddmwalking »

The expansion of the universe does seem to be accelerating. This is theorized to be due to Dark Energy, which we don't know what it is, but otherwise has nothing to do with Dark Matter (which we also don't know what it is).

One thing that physicists often ask people to observe is a spinning ice-skater. When the ice skater pulls their arms in, they speed up due to conservation of angular momentum - if they extend their arms as far as they can, they slow down. In very simple terms, galaxies look like they have their arms up, but they're spinning like they have their arms out - so some mass must be part of the galaxy and we can't see it.

There are other observable effects of a large amount of mass that we can't see (and can't explain) so it appears to be something that is effectively invisible, but creating real gravitational effects.
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Post by DrPraetor »

There's a lot to not understand :).

The evidence for dark matter comes from two sources - one of which is angular momentum (things are spinning too fast, for how much they appear to weigh), but the other - which is harder to explain away - is from gravitational lensing. Well, also the cosmic microwave background, and fanatical devotion to the pope:
https://en.wikipedia.org/wiki/Dark_matt ... l_evidence

So gravity bends light (like a lens), and heavier things bend light more. So the amount of light galaxies bend tends to be consistent with more mass than you expect. Thats nice, but if galaxies weigh more for some other reason, this would still be what you see.

Issue is, it isn't always consistent with the extra mass being *in* the galaxy - we have cases where we have gravitational lensing from what appears to be empty space next to a galaxy.
https://en.wikipedia.org/wiki/Abell_520

Now, the problem is, we have no good candidates for what this "hot dark matter" (it's called hot dark matter because it's bouncing around at high speed, which is what enables it to not be right on top of the galaxies) is made of. The particles we know about correspond to systems of extremely complicated equations. The standard model of particle physics is so impressive because you solve the same set of equations to get photons (and electric and magnetic fields), and also protons and the forces that hold nuclei together and so on.

https://en.wikipedia.org/wiki/Standard_Model

One thing that the standard model doesn't really deal with is gravity. So, the suspicion is, there are other equations we don't know, that describe quantum gravity, and when you solve those equations you'll get dark matter particles.

You might get dark energy as well? So, dark energy and dark matter might be related in that sense?

Likewise, if dark matter doesn't exist, then whatever modified theory of spacetime might explain the expansion of the universe as well.
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Post by OgreBattle »

This has been an endarkening experience.

So... are there solid or really wild untested theories y'all are fond of to explain dark matter too?

I saw this article the other day that "information" was the mass we couldn't measure https://mindmatters.ai/2020/01/could-in ... rk-matter/

From an RPG building standpoint that sounds like a fun 'science' explanation of mystic powers interacting with mass reality. So this thread is also for mining RPG ideas
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Post by DrPraetor »

I think that's word salad, but I'm not a physicist.
https://sebpearce.com/bullshit/

I usually dislike science-flavoring in my fantasy, but that's extremely subjective. There was another thread about "realistic" mutant superpowers; as a geneticist, that stuff is pure cringe. I suspect if you had a physicist at your table they'd react the same way to magic being something something information content. But, other people might enjoy it, YMMV.

For out-there explanations:
https://www.discovermagazine.com/the-sc ... ark-matter
is a cool one. There are several variants that go all the way up to dark matter being antimatter in a parallel universe.
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Post by Username17 »

The simplest explanation is that the way we measure the angular momentum of distant objects is wrong in some way.

The next simplest explanation is that the universe has a lot of iron in it. Just plain heavy stuff that we can't see because it's not on nuclear fire.

The next simplest explanation is that large areas of empty space have mass or are inherently bent as if there was gravity acting on it.

The next simplest explanation is that the act of creation is ongoing and that as the universe expands more material is coming into being in the newly defined empty space.

The next simplest explanation is that our model of the universe needs an additional force or two (or three) that only matters in really big distances and masses and that gravity starts to stop being the most important force across intergalactic distances the way the strong nuclear force stops being especially relevant across non-microscopic distances.

The next simplest explanation is that there are other universes worth of matter acting on our own from the 'other side.' With the other side being either displaced in a non-spatial dimension or just literally that there are other big bangs moving into our universe from sufficiently far away that we can't see them yet.

We're really quite far down the checklist before we get to 'magic' or functionally similar explanations (such as math itself having weight).
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Post by DrPraetor »

Note: I said "hot dark matter", I meant "warm dark matter" and even then I might be using the terms wrong.

The *simplest* explanation is still that there are WIMPs buzzing around. This has problems - mainly, that we have neither detected nor made any such particles. But "there's some dark matter, it's WIMPs" seems pretty simple to me.
FrankTrollman wrote:The simplest explanation is that the way we measure the angular momentum of distant objects is wrong in some way.
Well, not really, because then we'd have to explain why the gravitational lensing and the angular momentum measurements give the same result. So it's no longer a simple explanation with the other data that's available.
The next simplest explanation is that the universe has a lot of iron in it. Just plain heavy stuff that we can't see because it's not on nuclear fire.
This leads to a slightly different accounting issue:
https://en.wikipedia.org/wiki/Baryonic_dark_matter
but, basically, if you know the hydrogen:deuterium ratio you should be able to figure the iron:deuterium ratio and it doesn't add up to enough heavier elements.
Also, dust clouds and such don't have iron-heavy spectra. So this iron would need to be in exotic stars or something, which isn't simple to explain.
The next simplest explanation is that large areas of empty space have mass or are inherently bent as if there was gravity acting on it.
Whether this is "simpler" than adding particles is subjective.
In the case of the standard model, there's a giant gravity-shaped hole in it that clearly needs to be filled with some hot hard physics, which ought to predict some more particles.
The next simplest explanation is that the act of creation is ongoing and that as the universe expands more material is coming into being in the newly defined empty space.
Again, this doesn't fit the distribution observed from the gravitational lensing, so it's actually a very complicated explanation because it needs additional gimmicks to account for other observations.
The next simplest explanation is that our model of the universe needs an additional force or two (or three) that only matters in really big distances and masses and that gravity starts to stop being the most important force across intergalactic distances the way the strong nuclear force stops being especially relevant across non-microscopic distances.
I think this is actually simpler than the previous one, but I'll agree things at this point and onward require more questions to be resolved than they explain.
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Re: What's Dark Matter, any new theories?

Post by deaddmwalking »

OgreBattle wrote:Read some stuff about the theory, now I hear the theory is being challenged but I don't fully understand
Setting aside the explanations to this point, the point of every theory is to explain observable phenomenon. Ideally, every theory is testable, but as you get to the edge of known physics a lot of theories may not be testable yet. Importantly, some theories may never be testable, which means even if they're true it might not actually be worth accepting them.

For example, there is a theory that we are more likely to exist as a computer simulation than as a 'real universe'. There are some efforts to confirm this. However, even if it is the objective reality, if we can't prove it to be true then theories that ignore it and focus on what is observable are 'better'.

The areas between and around galaxies have observable effects that are consistent with significant mass. One of the results of Einstein's theories are that we can think of space as being 'bent' by massive object. If the solar system were a blanket stretched tight at the corners and the sun was a bowling ball, the way the blanket 'sinks' around the bowling ball and changes the 'straight line course' of objects rolling across the blanket is an effective model for gravity. When you see other blankets that are bent just as yours is, but you can't see a bowling ball, the question is 'how did space get bent'?

There's really no argument that the space isn't bent - we observe phenomena that are consistent with space that is bent. Is it possible that space can be 'permanently deformed' and if you remove the massive object it'll continue to behave as if the massive object is still there? There's no indication that that's true but there are weird things about 'empty space' For example, empty space creates pairs of particles that wink in and out of existence - this is weird stuff.

In any case, Dark Matter (and Dark Energy) are placeholder names for things that we don't know what they are. Dark Matter may not be matter - but it ACTS like matter in terms of how it produces a gravitational effect.

There are essentially an infinite number of theories but a lot of them are just 'spaghetti monster' style musings on the ultimate nature of reality. The best theories lay out a way we can potentially test them to determine if they can be ruled out immediately (or nearly so). Adopting a particular theory for your game may end up causing problems if the theory is ultimately rejected based on tests that are inconsistent with the theory.

Then you have ether and phlogiston and it only works if you're being intentionally antiquated.
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Post by tussock »

Challenging otherwise stable theories is how they get better.

Quantum theory took decades to get anywhere, and did so because brilliant people kept challenging it until, basically, people ran out of ideas about how to challenge it that were falsifiable and had a mechanism for action.

--

Anyway, it's still reasonable to think of dark matter is a type of particle that reacts only with itself and gravity, and does so with itself more strongly as it moves faster, but still very weakly in any case.

This is the opposite of ordinary matter, of which we are made, that is slightly less likely to react with each other at higher speeds (not it a chemistry sense, but at all), and also interacts with the electric, weak, and strong forces

The current falsifiable challenges with a mechanism for action are basically around the shape of that curve, how much more they react with each other at high speeds (or if they do that at all), which determines the size of dark matter envelopes around galaxies and where the dark matter will tend to concentrate, and the range of possible rest masses for the particle, which likewise constrains the number of reactions that will occur and so the relative density in inner vs outer regions of galaxies.

There's still challenges of "really, this exists at all" but they have consistently been answered that yes, this really exists, with more and more evidence each time, often in unexpected ways. Alternate ways of not having dark matter keep falling apart, the properly falsifiable theories were falsified and thus have died, but people do check to make sure, because that is right and proper.

Like, it's awesome for your theory when something new is found and it agrees with your theory and disagrees with everyone else's, and wimps are gold there in recent years.

--

So, essentially, dark matter can collide with itself only rarely and not at low speeds, so it clusters at galactic scales but cannot cluster at the size of stars or anything smaller. Quite what happens to the energy of the dark matter collisions is unknown, but it's not electromagnetic, and producing proper theories for that is not practical without tighter constraints on the size and mass of the particles and their specific propensity to collide at various speeds.

Which again, is very hard to do, but people are working on it with models about how galaxies would most likely form given different constraints on those various numbers, and trying to see which ranges more often produce the galaxies we actually have when modelling the entire history of the universe. Seeing if anything unexpected pops out and then trying to find that unexpected thing out in the universe somewhere. And of course, those are extremely simplified models at first and then more detailed ones over time, which sometimes find problems only after a lot of work, and new observations keep giving people ideas, and work is ongoing.

And of course, there could be more than one type of dark matter particle, there could be a whole damn family of them, which will make figuring any of it out so much harder.

--

Dark Energy is also a thing, and might be related to the gradual increase in the speed of collapse of superclusters of galaxies over time, that the energy of gravitational collapse on that scale might be able to drive the increasing expansion between the parts of the universe which are not gravitationally connected in the future. But that is also really hard to know without understanding dark matter better, because most all of the collapse energy released would be due to dark matter collisions.
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