New class of galaxy has been discovered, made almost entirely of dark matter • Hacker News Board

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

#111

Post by evanb » Sat, Aug 27, 2016, 1:56 AM UTC

powertower wrote: post #101

sloppycee wrote: post #60

Earlier quoted context omitted.

I've always wondered, if gravity is a wave (has it been confirmed?), then there must be areas in space where different waves superimpose. Would the constructive interference between gravity waves be significant enough to account for 'dark matter'? And what would destructive interference look like (dark energy?)

Gravity, in its normal sense (of matter's gravitational attraction), has no frequency or wavelength. Its only property is that of acceleration.

That's right. Quantum mechanically, potentials are generated by the exchange of off-shell mediators (the clearest explanation I have seen is in Zee's QFT in a Nutshell).

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

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

#112

Post by theptip » Sat, Aug 27, 2016, 1:57 AM UTC

danielmorozoff wrote: post #80

theptip wrote: post #76

Earlier quoted context omitted.

Dark matter wouldn't cause lensing because it's diffuse; only very dense and massive astronomical objects cause light to bend enough for us to detect it.

Could you explain diffuse please? Relativistically, space-time deformation causes both galactic formation and lensing. Collections of stars millions of light years apart(dense locally but not on an average), acting together on light passing near the conglomerate will lens. So what do you mean by diffuse and could you point me to some source I can read up on this?

Sorry, I wasn't specific enough in my explanation. GP was referring to micro-lensing experiments, i.e. lensing around a star or other such object. That experiment is essentially selecting a bright point light source behind a galaxy, and looking for the sort of distortion that would occur if there was an object near the light's path as it passed through the galaxy. If you see lensing but no object, you've found a dark, dense object.

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

You're right that all the dark matter in a galaxy would contribute to gravitational lensing around that galaxy, but that's a different signal; you'd be looking for the lensing at different scale, e.g. Einstein Rings around the edge of the galaxy, not inside it.

I meant 'diffuse' literally, in the physical sense, as in sparse, not dense -- the 'missing mass' that we're trying to explain is, under the dark matter hypothesis, spread out more thinly than if it was accounted for by dense dark objects like planets or stars.

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

#113

Post by asimuvPR » Sat, Aug 27, 2016, 1:58 AM UTC

andrepd wrote: post #109

wallace_f wrote: post #106

I sometimes enjoy to daydream about the possibilities of dark matter, but probably the simplest explanation, and the least spectacular one, is the answer here: dark matter likely is a product of black holes, and probably just an even distribution of black holes. There is already growing a little bit of evidence that this is the case. I would be thrilled to learn about gravity leaking from other dimensions or alternat…

I don't think anything in your post makes the least bit of sense.

Perhaps you could tell why?

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

#114

Post by evanb » Sat, Aug 27, 2016, 2:00 AM UTC

T0T0R0 wrote: post #93

GFK_of_xmaspast wrote: post #92

Earlier quoted context omitted.

You originally claimed that "Exoplanets are dark matter. They do not emit light, so they are dark", which goes against the cited link. Furthermore, are you aware that you yourself are emitting infrared light right at this moment and are presumably not a swirling ball of liquid magma?

are you aware that you yourself are emitting infrared light right at this moment [...] ? WHAAATTT???!!!1one The hell you say! Come on, man. You and I both know that exoplanets are a recent discovery (1988 being the earliest verified potential candidate for the real thing), and thus hard to detect in the visible spectrum. No one is looking at them with an ordinary telescope, tuned into the visible spectrum. Last time…

As long as we're being pedantic nerds: dark matter is not "anything not emitting visible light", although such matter is "dark" in common parlance. Dark matter is called dark because it does not interact electromagnetically at all. No direct interaction with x-rays, radio waves, visible light, UV, IR, etc. etc. etc. It may interact indirectly (eg. by gravitationally distorting spacetime).

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

#115

Post by evanb » Sat, Aug 27, 2016, 2:02 AM UTC

jomamaxx wrote: post #105

Retra wrote: post #103

Earlier quoted context omitted.

>It is, at this stage, a very crude idea. It is an idea. Ideas exist. It does exist. There's nothing here for you to argue with.

A) Yeah, uh, in Science, when we refer to 'things that exist', we usually mean 'materially' and not 'in fantasy'. So no, that it is an 'idea' does not mean 'it exists'. B) Prove to me that dark matter exists. Characterize it. You can't. The only thing we know is that our equations for gravity don't work, and it would be 'nice' if there were this thing called 'dark matter' out there because it would fit nicely with wh…

Well, if you can explain all the observational signatures https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... in another way, you should write a paper! If your idea is simply "that's all bullshit / wrong" then tone it down some, OK?

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

#116

Post by vecter » Sat, Aug 27, 2016, 2:05 AM UTC

asimuvPR wrote: post #113

andrepd wrote: post #109

Earlier quoted context omitted.

I don't think anything in your post makes the least bit of sense.

Perhaps you could tell why?

Almost every statement he makes is pure nonsense.

> dark matter is likely a product of black holes

what product, specifically, and through what mechanism?

> and probably just an even distribution of black holes.

That just contradicts the first half of the sentence, which says that dark matter is a product of black holes, but now he's saying that they are black holes themselves.

We understand black hole behavior quite well. They can't be microscopic black holes because they would evaporate almost instantaneously due to Hawking radiation. So they must be bigger. But we know there's dark matter in our own galaxy, and there certainly isn't a uniform ubiquitous spread of black holes in our galaxy. And if they were so uniformly distributed, they would probably merge and become many fewer black holes. But we clearly don't see that either in our own galaxy or elsewhere.

Then some ridiculous vague statement about "gravity leaking from other dimensions"? It's insane, it's like he used a random grammar generator with physics words.

What he said was so crackpot that it would easily qualify as "not even wrong". [0]

[0] https://en.wikipedia.org/wiki/Not_even_wrong

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

#117

Post by Snesker » Sat, Aug 27, 2016, 3:09 AM UTC

New galaxies made of an unproven concept!

Do you seriously believe this shit?

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

#118

Post by dogma1138 » Sat, Aug 27, 2016, 3:54 AM UTC

danielmorozoff wrote: post #80

theptip wrote: post #76

Earlier quoted context omitted.

Dark matter wouldn't cause lensing because it's diffuse; only very dense and massive astronomical objects cause light to bend enough for us to detect it.

Could you explain diffuse please? Relativistically, space-time deformation causes both galactic formation and lensing. Collections of stars millions of light years apart(dense locally but not on an average), acting together on light passing near the conglomerate will lens. So what do you mean by diffuse and could you point me to some source I can read up on this?

For dark matter to explain the missing mass it has to be everywhere and spread evenly - hence defused.

If it was concentrated in only specific spots it would cause different gravitational effects such as lenseing.

The lack of lenseing isn't the only issue it's also the general mass distribution across the galaxy for example the stars in the outer parts of the Milky Way move at nearly the same speed as the stars in the center. Since the center has much more mass the stars should move faster but they don't which means there is a lot of more mass that we do not see and that is distributed evenly across our own galaxy and not clustered in the center like the normal matter.

So to match the observation the dark matter has to be every where think of it like the air around you.

Now it doesn't have to be actual dark matter but it has to gravitationally affect the rest of the matter in the universe one of the version of string theory has dark matter as gravity leaking into our universe from higher dimensions or parallel universes the problem with that is that it still does not explain the diffusion unless the brain it's leaking from for some reason unlike our universe is diffused.

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

#119

Post by gammarator » Sat, Aug 27, 2016, 4:06 AM UTC

hinkley wrote: post #77

From the article it seems to me that it's not the telephoto lenses that were the breakthrough here but the lens coating on the lenses. So is the next step to work with the manufacturer to get those coatings available on purpose-built astronomy equipment? And is it only applicable to refractory telescopes?

Large astronomical telescopes are necessarily reflecting: it's more cost effective to build large mirrors than large lenses. But that necessarily implies that there will be obscuration (secondary mirrors, etc.) in the light path, which complicates the focused image and makes it difficult to look for diffuse, low-surface brightness features.

The other challenge in looking for these sorts of objects is scattered light. What van Dokkum & co realized was that commercial telephoto lenses are exquisitely designed to minimize scattered light, and as refractors have no central obscuration. So a small array of such lenses is actually more sensitive to faint, diffuse features than much larger telescopes. (Plus cheaper!)

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

#120

Post by gammarator » Sat, Aug 27, 2016, 4:09 AM UTC

gus_massa wrote: post #89

pc2g4d wrote: post #34

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?

I'm guessing here, but they probably detected this automatically. There are millions of millions of galaxies and you can't put a even graduate student to look at each one. My guess is that they were preparing a boring paper, like "Analysis of Normal/Dark Matter ratio in WhAtEvEr type galaxies near SoMeWhErE". They put a telescope, some processing and then they transfer the data to Excel to make a nice graphic. Then g…

This was definitely a high-priority, by-hand, single object reduction. They put 33 hours of Keck time into it--you don't do that unless you're certain you have something very interesting.

(For reference, the capitalized value of one night (~8 hours) of 10-m telescope time is roughly $100k.)