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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

#91

Post by effingwewt »

mikeash wrote: post #29
auvrw wrote: post #26

Earlier quoted context omitted.

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.

This is my favorite explanation of this, mind if I forward this to an old Astronomy Professor? I think he'd love this too.

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

#92

Post by GFK_of_xmaspast »

T0T0R0 wrote: post #52
GFK_of_xmaspast wrote: post #21

Earlier quoted context omitted.

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

This method works best for young planets *that emit infrared light* and are far from the glare of the star. In other words, if they are still swirling balls of liquid magma. So, NOT dark matter.

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?

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

#93

Post by T0T0R0 »

GFK_of_xmaspast wrote: post #92
T0T0R0 wrote: post #52

Earlier quoted context omitted.

This method works best for young planets *that emit infrared light* and are far from the glare of the star. In other words, if they are still swirling balls of liquid magma. So, NOT dark matter.

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 I checked, anything not emitting visible light is commonly referred to as "dark." But wait, let me just check with my specialized visible light emission instrument.

Gee, when I turn off this incandescant light bulb, it goes... dark! Hypothesis verified! Is it still hot? Why yes! Yes, it is still hot. But also dark. Weird!

But hey, while we're being pedantic nerds, I'll just take a moment to correct you, regarding your correction of me.

Most of the examples in the impeccably cited link are measured in multiples of Jupiter's mass, which, you know, pretty much means they're certainly gas giants, and damn near brown dwarf classification, lending to their thermal activity.

So, the heat would likely not be owing to lava or magma.

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

#95

Post by biot »

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…

Given that E=mc^2, how do we know that there isn't simply a form of energy which is overly abundant? If the energy and mass are interchangeable, could it be that a massive amount of energy could be undetectable yet still have the same gravitational effects as what we call dark matter?

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

#96

Post by SmellTheGlove »

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…

> undetectable-through-normal-means normal matter

I'm not a physicist, but my understanding was that dark matter is undetectable-through-normal-means because it doesn't interact with normal matter. If it were undetectable-through-normal-means normal matter, it would be detectable through normal means.

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

#97

Post by m_mueller »

biot wrote: post #95
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…

Given that E=mc^2, how do we know that there isn't simply a form of energy which is overly abundant? If the energy and mass are interchangeable, could it be that a massive amount of energy could be undetectable yet still have the same gravitational effects as what we call dark matter?

as far as I understand, something that invokes gravitational force is by definition 'mass', not energy.

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

#98

Post by T0T0R0 »

logfromblammo wrote: post #33
LeifCarrotson wrote: post #20

Earlier quoted context omitted.

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…

Just going to point out that science is filled with garbage theories, and "Dark Matter" certainly feels like one of them.

Why not just go back to calling it The Æther? Or mayhap a form of non-luminiferous aether, if I may be so bold?

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

  a space-filling substance or field, thought 
  to be necessary as a transmission medium for 
  the propagation of [...] gravitational forces.
You say tomato, I say Tomato. Aether. Why not?

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

#99

Post by mikeash »

effingwewt wrote: post #91
mikeash wrote: post #29

Earlier quoted context omitted.

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.

This is my favorite explanation of this, mind if I forward this to an old Astronomy Professor? I think he'd love this too.

Sure! Just beware that I'm far from an expert and may well have gotten something hilariously wrong. If he rips it to shreds, please let me know what he had to say.

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

#100

Post by zuminator »

logfromblammo wrote: post #33
LeifCarrotson wrote: post #20

Earlier quoted context omitted.

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…

A couple comments here are somewhat conflating dark matter, in general, with non-baryonic matter, which is what we presume to be the primary component of dark matter. Dark matter theories arose because the Milky Way and other galaxies are much heavier than they appear to be based upon the mass of objects in them that we can detect. Therefore we know there's something else out there that has mass, and is spread fairly evenly throughout the galaxy but is otherwise currently undetectable to us. We don't know enough to definitively say what that stuff is or isn't. Part of that mysterious stuff may be MACHOs (massive compact halo objects), which are just made of normal baryonic matter that's aggregated in bodies too small and dark for us to individually detect through existing means, such as small rocky objects which would be perfectly visible if we could get a spotlight on them, and also black holes, neutron stars, dwarf stars, and other dark objects. However, it's currently thought that the majority of dark matter is non-baryonic, which fits the exotic description of invisible and completely non-interacting except through gravitational attraction. MACHOs have the problem that if they're plentiful enough to account for the missing mass, we really ought to be able to detect them through other means. (And furthermore that if baryonic matter were the primary form of dark matter, it would be too abundant for our well-established theories of structure formation and nucleosynthesis to work.) But the bigger problem is that none of the theories of non-baryonic matter are in any way substantiated either, except for garden variety neutrinos, which don't have enough mass and are too energetic to account for the bulk of dark matter. This leaves the door open to even more exotic speculations such as tweaking the theory of gravitation. In a sense, the real darkness to "dark" matter lies in our understanding more than anything else.
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