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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

#131

Post by oldmanjay »

Retra wrote: post #102
T0T0R0 wrote: post #98

Earlier quoted context omitted.

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…

Because that would be the kind of 'garbage theory' that would make you look incompetent rather than clever.

If there's one thing I've learned from the internet, it's that people sure prize the appearance of being clever.

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

#133

Post by mmmBacon »

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.

Why is dark matter diffuse? Shouldn't it clump together? I guess if there's no Coulomb force the stuff just moves past itself. However there should be some kind of collision cross-section shouldn't there? Since there's so much of it, what would be the result of even a small probability of collision?

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

#134

Post by evanb »

m_mueller wrote: post #124
evanb wrote: post #107

Earlier quoted context omitted.

While that's the common understanding, general relativity tells us that the spacetime curvature is dictated by the stress-energy tensor, which is accounts for all different types of mass and energy. For example, if you have a crystal at very low temperature it will gravitate less than the same crystal at a high temperature (though for familiar materials it is a VERY slight effect).

Oh wow TIL. So, say a neutron star has stronger gravity than given by its mass alone, due to its energy density? What fraction of gravitational force are we talking roughly for such an object? Edit: Do photons have gravity even though they are massless? Edit2: Is that why gravity can bend photons? GRT suddenly makes a bit more sense to me if the answer to these are 2x yes, so thank you!

The issue is the speed of light is so big. So, you need a LOT of energy in order to source gravity comparably to just a little bit of mass (essentially, because E=mc^2 -- or perhaps phrased more clearly in this case, m = E/c^2).

Even Newtonian Gravity can bend light classically, by Galilean relativity. But, weirdly, it essentially hinges on the fact that the the mass m of the photon cancels from m*a = GmM/r^2. Of course, that is really the equivalence principle---the acceleration of all things under gravity's influence is the same. Whether or not light is a source of Newtonian gravity... I'm not sure. It's a tricky question because m=0. I want to say no because the "equal and opposite" forces should both be 0, even though one of them effects an acceleration on the other. I should emphasize that I'm not sure!

In Einsteinian gravity, the paths of photons (and indeed all things) are bent because spacetime itself is curved. Classical electromagnetic static fields and waves certainly have an energy density that can source gravity, and individual photons do too. But their energy is on the order of hbar. So you're talking a source of gravity like hbar/c^2. THIS IS REALLY TINY unless the photon's frequency is ENORMOUS.

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

#135

Post by evanb »

T0T0R0 wrote: post #122
evanb wrote: post #114

Earlier quoted context omitted.

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).

Yeah, yeah, I get it. And I still say that's a non-explanation with a misleading name. There's no proof of material at all, thus not matter, thus no such thing as dark matter. I'd willingly accept other names such as Dark Question Marks. Or maybe Dark Mathematical Terms Yet To Be Named . Here's a good one: D ark U nobservable N umerically C hallenged E ntities.

As I said above to another commenter, 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 you don't consider all that observational evidence reliable (which maybe you do not, based on your DUNCE name) and will only be satisfied by direct detection experiments on Earth, well, I don't know what to say to you except that lots of things whose existence was deduced from observational astronomy but not from evidence on Earth panned out, including such simple things as Helium.

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

#136

Post by theptip »

mmmBacon wrote: post #133
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.

Why is dark matter diffuse? Shouldn't it clump together? I guess if there's no Coulomb force the stuff just moves past itself. However there should be some kind of collision cross-section shouldn't there? Since there's so much of it, what would be the result of even a small probability of collision?

That's a very good question. The current 'best candidate' model predicts dark matter to be collisionless, i.e. the particles do not (often) collide with each other [1]. In other words, considering a 2-body system, the dark matter particles would accelerate towards each other, and then shoot straight past and decelerate on the other side, continuing to oscillate.

In that paradigm, considering a bulk of dark matter particles, the particles will be attracted to each other, and will fall towards the center of mass of the clump -- but there's nothing to stop them, and so they pass out to the other side of the cloud, where they decelerate. This puts a limit on how dense the cloud can become (I haven't studied the details of the mechanics here, but look into the Virial Theorem if you want the equations that describe these limits). In a normal cloud of gas in space, the particles would collide with something as they fall into the center of the cloud, which would convert their linear motion into random motion, and so they would essentially be trapped.

Note that while dark matter doesn't form dense objects, it does clump to some extent, and this is actually involved in galaxy formation [2]; based on initial small perturbations in the densities of matter before the inflationary period, the Cold Dark Matter forms clumps (halos) which act as the initial seeds of attraction for the baryonic matter (H/He) that formed the first galaxies.

[1]: http://www.ncbi.nlm.nih.gov/pubmed/10828999 [2]: https://en.wikipedia.org/wiki/Dark_matter_halo

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

#137

Post by wallace_f »

JumpCrisscross wrote: post #125
vecter wrote: post #116

Earlier quoted context omitted.

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…

I read it as meaning "the gravitational effects we attribute to dark matter may be a product of even more massive black holes at galactic cores". I once wondered this. Then I learned our studies of how star revolution rates taper off as a function of distance from the core were not explicable with a giant mass at the core; there needed to be a diffuse mass.

What I intended as the meaning was close, it was basically this (hopefully it makes more sense): the gravitational effects we attribute to an unknown source, dark matter, is likely the product of matter inside of black holes.

In other words, there may be weird quirks of physics that are not yet discovered. what they are I don't know. Maybe mass density or gravitational density has an upper limit, and the universe conserves mass by redistributing it in some strange way.

The point is also just this: fantastic explanations that explain what dark matter is (alternate universe's, leaking gravity) are probably the least likely to to be true. Instead, the most likely explanation is the one which uncovers the least fantastic possibilities -- it likely is explained using the least modifications to our current observations/understanding of the universe.

I don't know how it didn't make any sense, but I'm sorry that it didn't - Can someone please enlighten me?

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

#138

Post by wallace_f »

vecter wrote: post #116
asimuvPR wrote: post #113

Earlier quoted context omitted.

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…

You are right on many points, and I should have qualified my statement that it is more philosophy than science itself. I had little intention to have very much explanatory utility about the mechanisms dark matter. Also, I have no advanced degree in physics, I just have an interest in it.

All I was trying to say, I explained below, in a response to @JumpCrissCross. I understand it's different to mainstream physics, that was the intention. Perhaps you can see the point I am trying to make.

Edit: Here is an article that appears to discuss black holes as a possible candidate to explain what we call dark matter: http://hub.jhu.edu/2016/06/16/dark-matter-in-primordial-blac...

I realize another hypothesis for dark matter is WIMPs but as I understand we have found no evidence for their existence. On the other hand, you're saying what I wrote is entirely crackpot despite academic publications and evidence that may support that alternate theory? I think you are being a bit harsh -- but you tell me, you're the apparent expert, and I'm a hobbyist, but there's a source, go read it yourself.

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

#139

Post by m_mueller »

evanb wrote: post #134
m_mueller wrote: post #124

Earlier quoted context omitted.

Oh wow TIL. So, say a neutron star has stronger gravity than given by its mass alone, due to its energy density? What fraction of gravitational force are we talking roughly for such an object? Edit: Do photons have gravity even though they are massless? Edit2: Is that why gravity can bend photons? GRT suddenly makes a bit more sense to me if the answer to these are 2x yes, so thank you!

The issue is the speed of light is so big. So, you need a LOT of energy in order to source gravity comparably to just a little bit of mass (essentially, because E=mc^2 -- or perhaps phrased more clearly in this case, m = E/c^2). Even Newtonian Gravity can bend light classically, by Galilean relativity. But, weirdly, it essentially hinges on the fact that the the mass m of the photon cancels from m*a = GmM/r^2. Of cou…

To me, the speed of light is actually really slow - meaning, if you think classically you assume c infinite (at which point, as far as I understand, relativity theory essentially behaves like classical physics).

Learning that information travels way slower than the universe expands is quite unnerving. Similarly, learning that earth's fate could be determined already since hundreds of thousands of years through a hypernova directed at it - that we'll only know about when it hits us and wipes out our atmosphere. Well... light speed is far too slow for my taste ;).

Edit: It still doesn't answer my first question though: If you have something really energy dense like a neutron star - what fraction of gravity does energy make out then? 1E-3? 1E-10? 1/2? I'd find that interesting to know. According to wiki, neutron stars fall in temperature within years of creation from up to 1E12 K to 1E6 K. Six orders of magnitude. Depending on how much this decreases gravity I could imagine this effect alone influencing stellar orbits (I assume that a supernova would still allow other stars in a multi star system to continue existing). Has such a thing ever been measured?

Sorry.. there's just a whole can of worms opened about this in my head right now. Need to find an astro physicist to shake down :D.

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

#140

Post by jomamaxx »

evanb wrote: post #115
jomamaxx wrote: post #105

Earlier quoted context omitted.

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?

I didn't say that 'Dark Matter' was Bullt or that it was 'wrong'.

I said that Scientists should not speak of it as in any way an established theory, or that it even exists.

Dark Matter should always be written "Dark Matter" and communicated not that it is 'something' but that it is an idea.

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