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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

#31

Post by tomr_stargazer »

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)?

Additional dust inside a given galaxy would both absorb optical light and emit infrared light (depending on its temperature, which is set by the Galaxy's ambient starlight among other factors); both effects are readily detectable.

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

#32

Post by mikeash »

LeifCarrotson wrote: post #20
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…

I am pretty sure this is satire. But this is the internet, so it is hard to tell. To be clear: to an astrophysicist, "dark" means "does not interact with electromagnetic radiation except through the curvature of space-time by gravity". It only interacts with normal matter in that it affects gravity. It is otherwise completely transparent. It does no mean "is not currently reflecting or producing visible light". Your…

I thought it just meant that it can't (yet?) be detected with electromagnetic radiation, not that it necessarily doesn't interact. For example, MACHOs have fallen out of favor, but were once a decent theory to explain dark matter, and they're just normal matter in a form and location that makes them hard to see.

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

#33

Post by logfromblammo »

LeifCarrotson wrote: post #20
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…

I am pretty sure this is satire. But this is the internet, so it is hard to tell. To be clear: to an astrophysicist, "dark" means "does not interact with electromagnetic radiation except through the curvature of space-time by gravity". It only interacts with normal matter in that it affects gravity. It is otherwise completely transparent. It does no mean "is not currently reflecting or producing visible light". Your…

Well... it might.

Your closet probably has the same concentration of dark matter in it as everywhere else on the surface of Earth. But that quantity is unmeasurable at our current technology level, because any signal we might get is completely obscured by all the noise from bright matter around here.

Dark matter is not matter that does not emit light. A rogue asteroid far from any star is a dark object, but it is not dark matter. If you aimed a radar beam at it, the signal would bounce off, and you could detect the reflected/absorbed/re-emitted signal when it gets back to you. If you somehow found an aggregation of dark matter, your radar beam would not bounce. It would pass right through, like a flashlight beam shining through a crystal ball. Your beam might refract slightly due to the gravity, but it would not reflect. It would be similar to aiming a neutrino beam at the regular asteroid. Most of the beam just passes straight through without interacting.

Not even black holes are dark matter (or if they ever were, they aren't any more), because they absorb light. If one eclipses a known light source, you can see the black spot, along with the lensing around the outside. They interact with light.

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

#34

Post by pc2g4d »

I find myself wondering if the type of inference required to make this claim could be automated. Given the right observations, couldn't dark matter be detected (or at least hypothesized) algorithmically? Couldn't this be used to create a "dark matter scope" by which the dark matter in the universe can be "seen" and visualized?

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

#35

Post by gpderetta »

symmetricsaurus wrote: post #13
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 can explain: the large scale evolution of the universe (together with dark energy but whatever), rotational velocities of galaxies, gravitational lensing and other phenomena. No other theory explains all of these things as well or simply. Also, dark matter is not really so exotic and weird. We already know of one type of dark matter particle, neutrinos. To think that there are other more massive particles…

Exactly, in fact the hypothetical massive sterile neutrinos, which are part of one of the possible explanations for the neutrino masses, are one of the dark matter candidates.

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

#36

Post by fizx »

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…

We're kind of out on the edge of the galaxy. As you might suspect, dark matter tends to be more dense in the middle of the galaxy.

I'll let someone else be more precise:

http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.ht...

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

#37

Post by nerfhammer »

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…

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

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

#38

Post by openasocket »

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…

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 "we don't know," but I'd love to know if we have any guesses.

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

#39

Post by Exras »

"The researchers who found Dragonfly 44 weren't looking for a dark galaxy. Another surprise: They found it using a telescope built of camera parts. The Dragonfly Telephoto Array was built by a group of astronomers at Yale University and the University of Toronto who realized that telephoto lenses — so often used for nature photography and sporting events — were well-suited for spotting the kind of large, dim objects that pose problems for typical telescopes."

I like it, reminds me of the discovery of the Cosmic background radiation by Penzias and Wilson with the Holmdel Horn Antenna. Accept this time nobody had to shovel bird shit :-)

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

#40

Post by orf »

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…

> some unknown form of batter

Mmhhhh... I'm pretty hungry right now and that gave me some wonderful imagery.

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