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

#71

Post by danielmorozoff » Fri, Aug 26, 2016, 9:40 PM UTC

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…

Could you speak a bit more to the idea of gravitational lensing as a proxy to show no regular matter but rather dark matter. If dark matter accounts for the missing 'mass' holding galaxies together, wouldn't you expect it to cause lensing of light?

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

#72

Post by [deleted] » Fri, Aug 26, 2016, 9:42 PM UTC

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…

[deleted]

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

#73

Post by Synaesthesia » Fri, Aug 26, 2016, 9:53 PM UTC

mattmanser wrote: post #69

stcredzero wrote: post #67

Earlier quoted context omitted.

it has always bothered me that such a huge percentage of the matter in the universe would be nearly unobservable. It's the sort of thing that wouldn't pass my normal sanity check in a scientific computing model. How about this? -- A large fraction of bugs in a large, long lived computer system will be difficult to recreate.

It's not the same thing. Imagine being told there's loads of bugs in your system and it's constantly breaking, even though no-one can show you a single one of these bugs and you've not got a single user complaint. That's dark matter. It's obviously bullshit, but it's the 'simplest' explanation.

There's been some pretty specific observations of dark matter like galaxies which have been shown to have varying proportions of DM, there have been measurements where scientists have actually localised the dark matter, i.e. mapped where it should be concentrated based on the perturbation of nearby stars.

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

#74

Post by stcredzero » Fri, Aug 26, 2016, 9:53 PM UTC

mattmanser wrote: post #69

stcredzero wrote: post #67

Earlier quoted context omitted.

it has always bothered me that such a huge percentage of the matter in the universe would be nearly unobservable. It's the sort of thing that wouldn't pass my normal sanity check in a scientific computing model. How about this? -- A large fraction of bugs in a large, long lived computer system will be difficult to recreate.

It's not the same thing. Imagine being told there's loads of bugs in your system and it's constantly breaking, even though no-one can show you a single one of these bugs and you've not got a single user complaint. That's dark matter. It's obviously bullshit, but it's the 'simplest' explanation.

That's not quite right. A better analogy would be:

"Imagine being told there's loads of bugs in your system and it's constantly breaking, and even though no-one can show you a single one of these bugs, tons of users are complaining."

That would fit the situation better: There's only one kind of indication that these things exist.

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

#75

Post by yk » Fri, Aug 26, 2016, 9:53 PM UTC

tokai wrote: post #49

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…

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.cf…

In principle, yes. However, there is not only evidence for dark matter from galaxy rotation curves but also from large scale structure simulations and especially from the CMB. The interesting thing is, the CMB observations, which react to the number of particles in the standard model, agree rather well with rotation curves. (And IIRC there is also a way to disentangle structure formation from simple gravitational interactions, so there are "2 and a half" independent observations which indicate dark matter or something that closely resembles dark matter.

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

#76

Post by theptip » Fri, Aug 26, 2016, 9:53 PM UTC

danielmorozoff wrote: post #71

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…

Could you speak a bit more to the idea of gravitational lensing as a proxy to show no regular matter but rather dark matter. If dark matter accounts for the missing 'mass' holding galaxies together, wouldn't you expect it to cause lensing of light?

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.

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

#77

Post by hinkley » Fri, Aug 26, 2016, 10:03 PM UTC

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?

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

#78

Post by pavlov » Fri, Aug 26, 2016, 10:04 PM UTC

s_q_b wrote: post #65

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…

I'm not even close to a physicist, but it has always bothered me that such a huge percentage of the matter in the universe would be nearly unobservable. It's the sort of thing that wouldn't pass my normal sanity check in a scientific computing model. But I trust that these possibilities are being well considered. Is there any significant mainstream buy-in with regard to alternative theories of gravity?

... it has always bothered me that such a huge percentage of the matter in the universe would be nearly unobservable.

Maybe think of it this way: expecting matter to be easily observable is an anthropocentric point of view, because human intuition defines matter as something that can be observed by the senses.

But the universe was not designed to be observed by us. There's no reason to expect that the majority of matter should be observable.

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

#79

Post by grkvlt » Fri, Aug 26, 2016, 10:11 PM UTC

openasocket wrote: post #57

BurningFrog wrote: post #55

Earlier quoted context omitted.

> I think it would be invisible and fall through your hand (and the earth) Why would't it fall to the center of the earth and stay there, since that is the local gravitational center?

No, it would speed past the center until losing moment on the surface of the other side of the planet, and oscillate back and forth. Because there's no "friction" slowing down the dark matter it will continue like this forever, by conservation of energy. Check out http://physics.stackexchange.com/questions/214950/if-dark-ma...

> speed past the center

What caused these particles to accelerate up to the speeds they are at in the first place, then? Something must be interacting with them quite significantly if they have non-zero velocity?

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

#80

Post by danielmorozoff » Fri, Aug 26, 2016, 10:12 PM UTC

theptip wrote: post #76

danielmorozoff wrote: post #71

Earlier quoted context omitted.

Could you speak a bit more to the idea of gravitational lensing as a proxy to show no regular matter but rather dark matter. If dark matter accounts for the missing 'mass' holding galaxies together, wouldn't you expect it to cause lensing of light?

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?