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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

#21

Post by GFK_of_xmaspast »

T0T0R0 wrote: post #19
netcraft wrote: post #16

Earlier quoted context omitted.

im pretty sure this isnt true - if any of those things were massive enough to substitute for the gravitational effects that have been observed they would emit infrared radiation at least.

So go ahead and observe this all-too-obvious infrared, that exoplanets must surely emit. Meanwhile, their gravity is now well known to induce wobble on their parent stars, which are much more luminous, and probably outshines any exoplanet in the infrared.

On the other hand, this has already been done: https://en.wikipedia.org/wiki/List_of_directly_imaged_exopla...

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

#22

Post by semi-extrinsic »

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…

Regarding the "no massive dark bodies" result: does this also cover dark dust? Or is that otherwise ruled out (perhaps by optical density)?

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

#23

Post by zig »

T0T0R0 wrote: post #14
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…

Exoplanets are dark matter. They do not emit light, so they are dark. You can't see an exoplanet, you have to look for stars that wobble. Asteroid belts are also dark matter. If a remote star has an asteroid belt, it wouldn't be easily detectable, since it probably wouldn't elicit the kepler-style wobbles and flickers, or if it did, perhaps less obvious/detectable events. Based on this, the astronomical fad terminolo…

How do you rectify these numbers?

The sun is 99.86% of the mass of our solar system and is quite average. [1]

Dark matter is ~27% of the mass of the observable universe. [2]

Are you claiming that exoplanets and asteroids represent 27% of the mass of the universe when a typical G-type star (not particularly massive, by any means) is almost 100% of the mass of our entire solar system?

I'm confused by your assertion - you either have some data I am lacking, have entirely made up your post, or are, yourself, confused.

[1] https://en.wikipedia.org/wiki/Sun [2] https://en.wikipedia.org/wiki/Dark_matter

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

#24

Post by mikeash »

T0T0R0 wrote: post #14
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…

Exoplanets are dark matter. They do not emit light, so they are dark. You can't see an exoplanet, you have to look for stars that wobble. Asteroid belts are also dark matter. If a remote star has an asteroid belt, it wouldn't be easily detectable, since it probably wouldn't elicit the kepler-style wobbles and flickers, or if it did, perhaps less obvious/detectable events. Based on this, the astronomical fad terminolo…

Exoplanets and asteroids don't explain the discrepancies. There aren't enough of them. Our galaxy appears to have about 20x more dark matter than regular matter, yet our solar system has about 99.9% of its mass concentrated in its star. There's no indication other star systems with planets are radically different in that respect. If there was enough of this stuff to make up the discrepancy, there would be enough of it to see it.

There's also apparently decent evidence, based on things like irregularities in the cosmic microwave background, that most dark matter isn't any form of baryonic matter.

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

#25

Post by mikeash »

zig wrote: post #23
T0T0R0 wrote: post #14

Earlier quoted context omitted.

Exoplanets are dark matter. They do not emit light, so they are dark. You can't see an exoplanet, you have to look for stars that wobble. Asteroid belts are also dark matter. If a remote star has an asteroid belt, it wouldn't be easily detectable, since it probably wouldn't elicit the kepler-style wobbles and flickers, or if it did, perhaps less obvious/detectable events. Based on this, the astronomical fad terminolo…

How do you rectify these numbers? The sun is 99.86% of the mass of our solar system and is quite average. [1] Dark matter is ~27% of the mass of the observable universe. [2] Are you claiming that exoplanets and asteroids represent 27% of the mass of the universe when a typical G-type star (not particularly massive, by any means) is almost 100% of the mass of our entire solar system? I'm confused by your assertion - y…

Note that 27% is of the total mass and energy, where ~68% consists of dark energy. If you ignore dark energy and just look at matter, dark matter is about 85% of the total. Which is pretty crazy! We have no real idea what 5/6ths of the stuff in the universe actually is!

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

#26

Post by auvrw »

Florin_Andrei wrote: post #8
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…

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…

upvoted, and not disagreeing (b/c i know nothing a/b this) -- would like a similarly semisimple explain on how "interaction" thru gravity alone makes any sense at all in the relativity framework: light no longer travels along lines of zero acceleration?

Edit: on second thought, a book reference is probably better

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

#27

Post by maverick_iceman »

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…

Big Bang Nucleosynthesis (BBN) says that the amount of baryonic (i.e. usual) matter cannot be much higher than what we directly observe. However, it accounts for only ~16% of the mass necessary to account for galactic rotation curves, gravitational lensing by galaxies etc. So the leftover must be non-baryonic dark matter.

One can of course say that BBN is wrong but that's very unlikely as it has been tested to high precision from CMB study.

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

#28

Post by maverick_iceman »

semi-extrinsic wrote: post #22
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…

Regarding the "no massive dark bodies" result: does this also cover dark dust? Or is that otherwise ruled out (perhaps by optical density)?

Baryonic dust is ruled out by BBN.

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

#29

Post by mikeash »

auvrw wrote: post #26
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…

upvoted, and not disagreeing (b/c i know nothing a/b this) -- would like a similarly semisimple explain on how "interaction" thru gravity alone makes any sense at all in the relativity framework: light no longer travels along lines of zero acceleration? Edit: on second thought, a book reference is probably better

In relativity, gravity isn't a force that causes acceleration, it's a change in the shape of space which changes the path objects take. Thus, gravity affects the path light takes, but that's still a "straight line," because what gravity is really affecting is what a straight line is.

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

#30

Post by okket »

semi-extrinsic wrote: post #22
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…

Regarding the "no massive dark bodies" result: does this also cover dark dust? Or is that otherwise ruled out (perhaps by optical density)?

If you have four times of visible matter as "dark dust", this dust would clump together, creating stars, becoming visible matter again.
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