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New class of galaxy has been discovered, made almost entirely of dark matter

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Re: New class of galaxy has been discovered, made almost entirely of dark matter

#41

Post by sandworm101 »

mikeash wrote: post #15
mordocai wrote: post #4

Earlier quoted context omitted.

One thing I don't understand and maybe someone can take the time to explain. Why does it have to be this "Dark Matter" causing the mass differences we've been seeing? How do we know it isn't undetectable-through-normal-means normal matter, or aliens, or mistakes in our calculations/observations, or anything else? Since we haven't been able to detect dark matter, it seems from my layman point of view that any of the a…

"Dark matter" just means "stuff we can't detect except by gravity" so that covers your first case. There's a theory that "dark matter" isn't matter at all, but some modification to the currently accepted laws of physics. Wikipedia lists some of these ideas: https://en.wikipedia.org/wiki/Dark_matter#Alternative_theori... Ultimately, it's generally assumed to be some sort of actual matter because that's what best fits…

I would call 'dark matter' a description rather than an explanation. It seems to effect gravity. So it has mass. So it's 'matter'. But other than that it is unobserved, and so 'dark'. A+B = a very short but accurate description of an observation, rather than an explanation of why that observation happens.

Re: New class of galaxy has been discovered, made almost entirely of dark matter

#42

Post by grahamburger »

mordocai wrote: post #4
Florin_Andrei wrote: post #2

This is significant. Now astronomers can study dark matter with less interference from the regular matter background. So you have a more broad range of objects to study, from galaxies with lots of regular matter, to galaxies with almost none of it. Hopefully this will accelerate DM research. It will certainly provide lots more data.

One thing I don't understand and maybe someone can take the time to explain. Why does it have to be this "Dark Matter" causing the mass differences we've been seeing? How do we know it isn't undetectable-through-normal-means normal matter, or aliens, or mistakes in our calculations/observations, or anything else? Since we haven't been able to detect dark matter, it seems from my layman point of view that any of the a…

> How do we know it isn't undetectable-through-normal-means normal matter

As I understand this is kind of what we think it is, which is why it's called 'dark.'

Re: New class of galaxy has been discovered, made almost entirely of dark matter

#43

Post by mikeash »

orf wrote: post #40
mikeash wrote: post #15

Earlier quoted context omitted.

"Dark matter" just means "stuff we can't detect except by gravity" so that covers your first case. There's a theory that "dark matter" isn't matter at all, but some modification to the currently accepted laws of physics. Wikipedia lists some of these ideas: https://en.wikipedia.org/wiki/Dark_matter#Alternative_theori... Ultimately, it's generally assumed to be some sort of actual matter because that's what best fits…

> some unknown form of batter Mmhhhh... I'm pretty hungry right now and that gave me some wonderful imagery.

Oopsie, stupid hands. Fixed it.

Re: New class of galaxy has been discovered, made almost entirely of dark matter

#44

Post by mikeash »

sandworm101 wrote: post #41
mikeash wrote: post #15

Earlier quoted context omitted.

"Dark matter" just means "stuff we can't detect except by gravity" so that covers your first case. There's a theory that "dark matter" isn't matter at all, but some modification to the currently accepted laws of physics. Wikipedia lists some of these ideas: https://en.wikipedia.org/wiki/Dark_matter#Alternative_theori... Ultimately, it's generally assumed to be some sort of actual matter because that's what best fits…

I would call 'dark matter' a description rather than an explanation. It seems to effect gravity. So it has mass. So it's 'matter'. But other than that it is unobserved, and so 'dark'. A+B = a very short but accurate description of an observation, rather than an explanation of why that observation happens.

Well, it's a bit of both. It's an explanation in that it explains the observational discrepancies in terms of matter that's difficult to observe, rather than, say, different laws of physics or extra dimensions or whatever. But it's also just a description in that it just means some sort of matter with mass that's hard to see.

Re: New class of galaxy has been discovered, made almost entirely of dark matter

#45

Post by okket »

nerfhammer wrote: post #37
Florin_Andrei wrote: post #8

Earlier quoted context omitted.

The only thing we're sure it exists is - there's something out there that acts exactly like normal matter with mass, and it interacts gravitationally with itself and with regular matter, but it doesn't seem to interact in any other way. That's it. The name "dark matter" is a bit optimistic, because we don't even know if it's anything like "matter". It's not a fluke in the equations, it's not a fifth fundamental force…

How can it have mass / interact gravitationally and not make "clumps"?

See http://physics.stackexchange.com/questions/214950/if-dark-ma...

TL;DR: Dark Matter doesn't clump because of missing "friction" from the other forces, esp. electromagnetic/weak.

Re: New class of galaxy has been discovered, made almost entirely of dark matter

#46

Post by SolarNet »

Is it possible that an alien race has simply covered that galaxy with dyson spheres? That life is more common than we thought - or perhaps more rapidly advancing once intelligence is hit - and most galaxies are "dark" from their interference?

Re: New class of galaxy has been discovered, made almost entirely of dark matter

#47

Post by dandelany »

openasocket wrote: post #38
tomr_stargazer wrote: post #18

Earlier quoted context omitted.

(PhD student in astrophysics answering here.) This is a great question! You're addressing the idea (roughly along the lines of Occam's razor) that if there's any way to explain the "extra mass" without invoking a new form of matter, it should be preferred. Most of these other possibilities have been largely ruled out via careful observations, which are detailed here: https://en.wikipedia.org/wiki/Dark_matter#Composit…

How dense is the concentration of dark matter? I mean, it's affected by gravity, so could it/does it come together to form "dark stars" of some form? And what would happen if I had a chunk of dark matter in a lab: what would it look like? Would it be invisible? Would I be able to pick it up in my hand, or would the particles just slip through ordinary matter like a ghost? I'm guessing the answer to most of these is "…

Assuming dark matter is a WIMP, I think it would be invisible and fall through your hand (and the earth) when you tried to pick it up. Stars require the particles in question to bang around against each other and interact with each other. WIMP particles wouldn't even be able to interact with each other except gravitationally - ie. they wouldn't "collide" or create pressure in an enclosed space. A cloud of dark matter is likely to stay a cloud for a very very long time rather than collapsing in on itself.

I think of it like an N-body gravity simulation[1] with very very large N and no collision detection.

[1] http://justfound.co/gravity/

Re: New class of galaxy has been discovered, made almost entirely of dark matter

#48

Post by stouset »

mordocai wrote: post #4
Florin_Andrei wrote: post #2

This is significant. Now astronomers can study dark matter with less interference from the regular matter background. So you have a more broad range of objects to study, from galaxies with lots of regular matter, to galaxies with almost none of it. Hopefully this will accelerate DM research. It will certainly provide lots more data.

One thing I don't understand and maybe someone can take the time to explain. Why does it have to be this "Dark Matter" causing the mass differences we've been seeing? How do we know it isn't undetectable-through-normal-means normal matter, or aliens, or mistakes in our calculations/observations, or anything else? Since we haven't been able to detect dark matter, it seems from my layman point of view that any of the a…

> Why does it have to be this "Dark Matter" causing the mass differences we've been seeing? How do we know it isn't undetectable-through-normal-means normal matter, or aliens, or mistakes in our calculations/observations, or anything else?

"Dark matter" is just a name for "a phenomenon that only appears to interact with the rest of the universe through gravitational fields, and not through elecromagnetic fields, the weak, or the strong nuclear force". There are many competing theories for what dark matter actually "is".

The name is independent of any particular theory that describes its true nature, whether that's an exotic form of matter, aliens, or mistakes in our observation.

Re: New class of galaxy has been discovered, made almost entirely of dark matter

#49

Post by tokai »

tomr_stargazer wrote: post #18
mordocai wrote: post #4

Earlier quoted context omitted.

One thing I don't understand and maybe someone can take the time to explain. Why does it have to be this "Dark Matter" causing the mass differences we've been seeing? How do we know it isn't undetectable-through-normal-means normal matter, or aliens, or mistakes in our calculations/observations, or anything else? Since we haven't been able to detect dark matter, it seems from my layman point of view that any of the a…

(PhD student in astrophysics answering here.) This is a great question! You're addressing the idea (roughly along the lines of Occam's razor) that if there's any way to explain the "extra mass" without invoking a new form of matter, it should be preferred. Most of these other possibilities have been largely ruled out via careful observations, which are detailed here: https://en.wikipedia.org/wiki/Dark_matter#Composit…

Wasn't it last year that the presumed count of red dwarf stars in the universe had to be corrected to a much larger number than previously thought?[0]

If we can be wildly wrong about the number of red dwarfs couldn't we just as well be wrong about the number of brown dwarfs, rogue planets and other massive but dim/dark objects in the universe?

edit: Last year? More like 6 years ago. I feel old now. [0]https://www.cfa.harvard.edu/news/2010-26

Re: New class of galaxy has been discovered, made almost entirely of dark matter

#50

Post by the8472 »

Florin_Andrei wrote: post #2

This is significant. Now astronomers can study dark matter with less interference from the regular matter background. So you have a more broad range of objects to study, from galaxies with lots of regular matter, to galaxies with almost none of it. Hopefully this will accelerate DM research. It will certainly provide lots more data.

It's more localized, but we've seen similar things before at a larger scale, e.g. the bullet cluster.

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

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