I don't Understand Dark Matter

[Observe]

Yet another theory on what dark matter is: 'White Holes' May Be the Secret Ingredient in Mysterious Dark Matter

A black hole is one prediction of Einstein's theory of general relativity. Another is known as a white hole, which is like a black hole in reverse: Whereas nothing can escape from a black hole's event horizon, nothing can enter a white hole's event horizon.

...some white holes in this universe might actually predate the Big Bang. Future research will explore how such white holes from a previous universe might help to explain why time flows only forward in this current universe and not also in reverse

In the meantime, we live and die and...?

[Morkonan]

http://www.bbc.co.uk/news/science-environment-43976977

Measure it.

It's possible. There is a non-zero probability that, given either an infinite amount of time or space that, as long as it is not forbidden, it will happen. That's great. But, it does not prove that it has happened. The maths might work, but that's not a measurement.

There are many different sorts of "multiverse theory." But, aside from a few dealing with some really quircky things, most are simply a response to "we shouldn't be special, therefore we need to explain why we are not."

I'm not saying we are special. I'm not saying that multiverses, however imagined, don't exist. I'm simply saying that until someone measures one in some way, and most maintain that is not possible (Which may be why Hawking''s paper is of momentary interest), the it's nothing more than "theoretical." It's a maybeso or mightbe, not an "is."

[pjknibbs]

Measure it.

It's possible.

Not always. We can't ever measure anything that happens inside the event horizon of a black hole, for instance, and I think the same applies for a multiverse--there's nothing in physics which would allow for travel between universes, so we can't tell what happens there.

[Hank001]

As for dark matter it's somewhat like the British gents who pondered musically on finding how many holes it takes to fill the Albert Hall.

As for multiverses, perhaps, and I hope they are getting along better than we are.

[Morkonan]

Measure it.

It's possible.

Not always. We can't ever measure anything that happens inside the event horizon of a black hole, for instance, and I think the same applies for a multiverse--there's nothing in physics which would allow for travel between universes, so we can't tell what happens there.

Sorry, I should have made the context more clear for "It's possible." I meant that in the context of the next sentence, hence the paragraph change, not in the context of the previous. It is possible in the sense that given infinite space or time, as long as it is not forbidden by natural law, there is a non-zero probability of xx occurring. But, one can not know where or when, relatively speaking, that event will occur. In that situation, "It's possible." But, "probable right now or at any other specific time or place?" Much like my demand for a perfectly cooked steak... no.

There's all sorts of "multiverses." Someone's gonna come up with another one next week, just you watch. I'm going to invent the "Fruit Roll-up Multiverse" where we're all in a spinning tube of delicious not-quite-really-fruit and, due to its velocity, we are not actually experiencing time in any rational sense, just the consciousness excited from what few random fluctuations over bajillions of years induces from the heady aroma of not-quite-really-fruit... I will achieve this for science and in my never-ending battle against the licorice lovers, who are known heretics, cowards, blood-drinkers and butt-sniffers that say mean things to puppies when nobody is around to see them do so. My Nobel is assured....

[Hank001]

Perfect!

[red assassin]

That this question keeps coming up has to be one of the most staggering failures of science communication. It genuinely baffles me that people are still regularly left to assume that since dark matter sounds a bit weird, astrophysics as a field must just have phoned it in without any evidence or attempt to come up with alternative explanations.


There are two things you need to know to get, at a mildly hand-wavy level, our current understanding of dark matter:

1) We can see normal matter by, well, looking at it.
2) We can see where mass is by measuring gravitational lensing. (This is a consequence of general relativity and is very well studied. Mass bends space, this bends light waves, therefore by measuring the effect on light waves you can infer where mass is and how much of it there is.) [1]

Dark matter arises as a theory because the amount of mass we measure in item 2) does not agree with the amount of stuff we see in item 1). The obvious alternative explanation for this, as pointed out, is that maybe we're just a bit wrong about gravity on large scales.

However, there are situations (primarily galaxy collisions) wherein where the mass is and where the matter is differ - all the gas and dust that makes up the bulk of the regular matter in a galaxy crashes into all the gas and dust from the other galaxy and slows down. Meanwhile, most of the mass keeps on merrily going at the original velocity and separates from the actual matter. This is extraordinarily hard to explain with just "we're a bit wrong about how gravity works" - people have tried very hard, but it just doesn't make sense, while it's trivially explained by "there's some massy but otherwise very weakly interacting matter around".

Hence, dark matter. No, we haven't worked out for sure what it actually is yet, but that doesn't mean we can't tell there must be something there. (I refer you to the search for the Higgs boson.)


[1] There are other ways - dark matter was originally proposed to explain galaxy rotation curves suggesting there was more mass than we could actually see - but gravitational lensing is a lot more powerful as a technique.

[Mightysword]

Don't this whole discussion sound like religion to anyone else? We see something we can't see or explain, so we just hypothesis and make believe that something is there?

[Hank001]

Red Assassin wrote:

Hence, dark matter. No, we haven't worked out for sure what it actually is yet, but that doesn't mean we can't tell there must be something there. (I refer you to the search for the Higgs boson.)

Something is there doing explicable things inexplicably. That much IS observable. Is it one phenomenon or many phenomemon? Time will tell. Though I reserve the right to peer into the depths of starry nights as this and say, "I don't know." When my field of view with naked eye is a small horizon and though a telescope diminishes to mere radians then I say if finding the answers gets us closer to those bodies out there, then yes, it does MATTER.

Edit: Google has this up today as their flash screen offering. As a 3D animator I just had to post this AROUND. Get ready to swipe or scroll it's 360 degrees so follow the action, it too good to miss:

"Back to the Moon"

https://youtu.be/BEePFpC9qG8

Enjoy the trip.

[CBJ]

Don't this whole discussion sound like religion to anyone else? We see something we can't see or explain, so we just hypothesis and make believe that something is there?

No. A scientific theory's predictions can be rigorously tested against measurable facts, even if the thing itself can't be measured, and as soon as any measurable facts contradict its predictions the theory is revised or thrown out. That's not how religion works.

[Bishop149]

However, there are situations (primarily galaxy collisions) wherein where the mass is and where the matter is differ - all the gas and dust that makes up the bulk of the regular matter in a galaxy crashes into all the gas and dust from the other galaxy and slows down. Meanwhile, most of the mass keeps on merrily going at the original velocity and separates from the actual matter. This is extraordinarily hard to explain with just "we're a bit wrong about how gravity works" - people have tried very hard, but it just doesn't make sense, while it's trivially explained by "there's some massy but otherwise very weakly interacting matter around".

Thanks for that, interesting.

As I said I'm totally ok with: "There's something going on, we ddon't understand, our model is flawed." what seems the stretch for me is "The solution is something that is identical to matter in regards to gravitation but is utterly undetectable in any other way . . . . oh and it appears to make up most of the universe"

I mean ok, but that's a extraordinary claim and you know what they say those require?

[Morkonan]

...I mean ok, but that's a extraordinary claim and you know what they say those require?

I think everyone would agree with that.

The thing is, everything keeps pointing to "this one thing." And, "this one thing" does not contradict what we believe is already firmly established understanding. Further - When we take what we know and apply it to what has been proposed, it all "works."

Does that mean it is "true." Well, if you can't measure it directly, yet, but its effects are predictable as you have modeled what it may be, then it doesn't matter if it's a bit of dark matter or a unicorn - It works.

I prefer to think it is as it has been thought of, with possible exceptions if we find something extraordinary. But, it all seems to "work" with what we already depend upon and what ideas we have relied upon to get us this far. And, if that continues I am happy to visualize it however science says we should until something else comes up.

"Dark energy," though... What's the next "Dark" gonna be? I "get it", but are we starting to move into the limits of our ability to comprehend? What next tool is going to help answer these questions and what truths may it reveal that cause us a rethink?

[brucewarren]

Gotta rule out unicorns Mork.

Unicorns, at least the sort they put into kids cartoons appear to emit light. That's no good.

Dragons. Dragons might work. Ancalagon the Black could blot out the sun. On second thought they breathe fire and fire gives out light. Forget dragons.

Giant spiders? As you know Shelob the spider would consume all light around her. She had the power to suck light from her surroundings and weave darkness itself into her webs, as her mother Ungoliant before her. Yet that's it. Dark matter is created by giant spiders.

[Morkonan]

... Dark matter is created by giant spiders.

Dark Matter is... Australia?

[red assassin]

As I said I'm totally ok with: "There's something going on, we ddon't understand, our model is flawed." what seems the stretch for me is "The solution is something that is identical to matter in regards to gravitation but is utterly undetectable in any other way . . . . oh and it appears to make up most of the universe"

I mean ok, but that's a extraordinary claim and you know what they say those require?

Why is that so bizarre an idea? We've deduced the existence of everything from Neptune to the Higgs particle long before actually being able to observe them directly.

It's not like there haven't been a bunch of candidates for dark matter that don't require any new physics, either - everything from small-ish black holes to neutrinos fit the bill as described in my first post, and the work ruling them out is a bit more subtle. (If there were that many black holes floating around we should be able to see the gravitational lensing from individual black holes making up our own galaxy's dark matter halo, which we can't. Neutrinos are a bit fiddlier - modelling of the large-scale structure of galaxy clusters doesn't produce the observed structure if we use them for dark matter. Obviously it's still possible there's something we've got wrong in the models, but there's quite a lot of work been done at this point and they don't seem to work. There's a few other candidates which have also largely been ruled out, hence a new particle being considered the most likely option at this point. Dark matter isn't the only theory that predicts new types of particle, either.)

Ultimately the options are that astrophysicists have collectively spent a hundred years casually disregarding the scientific method in favour of dark matter because we've all been brainwashed by... big dark matter or something? Or alternatively that we're perfectly well aware of how science works and have in fact expended an enormous amount of effort generating and ruling out alternative explanations and completely failing to rule out dark matter.

[pjknibbs]

everything from small-ish black holes
.
.
.
(If there were that many black holes floating around we should be able to see the gravitational lensing from individual black holes making up our own galaxy's dark matter halo

Wouldn't we also see the Hawking radiation from the black holes, considering that's supposed to get stronger the smaller the hole is?

[Bishop149]

@redassassin

I suppose, I admit the bit I'm most incredulous about is the suggestion that dark matter interacts with gravitation in exactly the same way as regular matter, but in almost every other regard is clearly utterly different. This seems extremely odd, and to my biologists mind would be led to think: "Cool, that should make it pretty easy to find" Followed by "Oh ****, I'm probably barking up the wrong tree here" after I'd spent any significant amount of time failing to find it.

[Hank001]

Is Dark Matter a wave
or in particular does it?
It says it's there, but isn't;
where it's not it's not really;
it's conspicuous by it's
absence.

[Morkonan]

Wouldn't we also see the Hawking radiation from the black holes, considering that's supposed to get stronger the smaller the hole is?

It's a teeny, itty bitty, sort of signature, if at all. It's trying to detect half of a virtual particle pair you're talking about.

I don't know anything about Hawking Radiation getting "stronger" the smaller a black hole is. Relatively more effect on smaller black holes? Sure. But... it's a tiny effect.

[pjknibbs]

It's a teeny, itty bitty, sort of signature, if at all. It's trying to detect half of a virtual particle pair you're talking about.

I don't know anything about Hawking Radiation getting "stronger" the smaller a black hole is. Relatively more effect on smaller black holes? Sure. But... it's a tiny effect.

It's billions of halves of virtual particle pairs, not just one, and the actual power emitted from a black hole by Hawking radiation is proportional to the inverse square of its mass, so small holes emit a *lot* more than big ones do.

However, having looked up the numbers to double-check my thinking here, apparently a black hole would have to have a mass smaller than the Moon in order to emit more Hawking radiation than it currently receives from the microwave background radiation that fills the universe, so consider my question withdrawn.