Dark matter

[Kbramley]

What confuses me with the whole Dark matter thing is why does it not clump together like normal matter ( eg into large structures like stars and dust clouds), surely if it interacts with normal matter via gravity, it also interacts with its self gravitationally too (otherwise it wouldn't be found predominantly around galaxies, but instead evenly distributed throughout all space), thus over time coalescing together and collapsing down, essentially creating dark stars, as no fusion would result. Seems odd how it's so very diffuse

Also it would also mean that large bodies such as the sun would actually have far more gravity than the visible matter would suggest as they would be comprised of mostly dark matter. To me looking at it very simplistically, it just doesn't stack up. It is almost as if gravity on a large scale (100,000s light years) is subtly different than on a solar system scale, thus there isn't any dark matter, just a misunderstanding of gravity, maybe in the same way Newtons laws work fine for everyday situations, but on a cosmic scale at high speeds Einsteins laws give the correct solution.

[sologuitarist61]

Also it would also mean that large bodies such as the sun would actually have far more gravity than the visible matter would suggest as they would be comprised of mostly dark matter. To me looking at it very simplistically, it just doesn't stack up. It is almost as if gravity on a large scale (100,000s light years) is subtly different than on a solar system scale, thus there isn't any dark matter, just a misunderstanding of gravity, maybe in the same way Newtons laws work fine for everyday situations, but on a cosmic scale at high speeds Einsteins laws give the correct solution.

I agreed with you there, I too think it gravity acting differently over greater distances (see my earlier post).

Going back to dark matter (if it exists) not clumping together, perhaps it is in fact quite evenly distributed through space. Perhaps it is like the Oort cloud around the solar system which stretches almost half way to the next nearest star. If the dark matter did the same around the Milky Way that might provide the extra gravity that everyone is looking for. Simplistic I know but it is an idea.

[JulianO]

What confuses me with the whole Dark matter thing is why does it not clump together like normal matter ( eg into large structures like stars and dust clouds), surely if it interacts with normal matter via gravity, it also interacts with its self gravitationally too (otherwise it wouldn't be found predominantly around galaxies, but instead evenly distributed throughout all space), thus over time coalescing together and collapsing down, essentially creating dark stars, as no fusion would result. Seems odd how it's so very diffuse

Thats a really good question - One I asked myself. However, if you think about it, it's a result of its non-interacting. It does clump together, and thats what causes galaxies to form. However it doesn't get really dense.

If you think of it like a load of ping pong balls, all coming together. We would expect them to all drift towards the centre, go in faster and faster, and then go shooting out the other side, or else orbit.

For regular matter, you get cooling effects, such as giving off radiation, and bumping into other particles (electromagnetic interaction) which means there is friction and stuff slows down and falls into the middle as it loses speed. Without them losing energy, the matter wouldn't form stars as it wouldn't get dense enough.

As dark matter doesn't interact like this, it can't emit radiation, it can't bump into regular matter and its unlikely to bump into itself (although it can happen) most of the dark matter just orbits around in a big halo but not contracting. It needs a way to lose energy, and due to it's nature it doesn't really have such a way.

Also it would also mean that large bodies such as the sun would actually have far more gravity than the visible matter would suggest as they would be comprised of mostly dark matter.

There probably is dark matter in the sun, but most of it is shooting through and out the other side, and then back again.

There will be more DM in the sun than outside, because of its gravity well, and astronomers look for DM annihilation signals coming from the sun as one way to detect it.

To me looking at it very simplistically, it just doesn't stack up. It is almost as if gravity on a large scale (100,000s light years) is subtly different than on a solar system scale, thus there isn't any dark matter, just a misunderstanding of gravity, maybe in the same way Newtons laws work fine for everyday situations, but on a cosmic scale at high speeds Einsteins laws give the correct solution.

That's been proposed quite a few times, but its very difficult to make such a theory that works in all situations, and yet works on local scales to a high degree of accuracy. There are plenty of people still playing with such equations though.

[sparkly]

hi

very interesting post and when reading through it i found a question one of those hmm moments, not that i know anything about the subject but (the 'inverse square law'. Basically the effect drops by a factor of four as you double your distance from the object.) so does that mean that gravity loses some of its energy but were does that energy go .

[K Nixon]

Hi Sparkly,

No it doesn't lose energy as it actually doesn't have any energy. It is only a field - essentially a way of modelling where mass has an effect.

If you put an object in that gravitational field it has gravitational potential energy. Now if that object moves towards the source of the gravitational field (like moving an asteroid toward a star) the object has loses gravitational potential energy, but like any object falling down, it will get faster and the lost gravitational energy becomes kinetic energy so the total energy stays the same!

I do like physics sometimes.

you can never lose energy. or indeed gain it. except if you're a virtual particle, but we don't talk about those, they mess things up

Kieron

[sparkly]

you can never lose energy. or indeed gain it. except if you're a virtual particle, but we don't talk about those, they mess things up

interesting and thats why i was abit confused (you can never lose energy) gravity is a very interesting subject, does it push or does it pull, repel or repullse.

hmm i am off to search for info on the virtual particle not thats easy on the net these days you seem to get attacked by digital sales reps .

[Grunthos]

does it push or does it pull, repel or repullse.

.

None of the above.

For a true understanding you'll probably want to get to grips with General Relativity.

Don't Panic! This is not as difficult as it sounds...

For starters, try having a look at this (rather simplistic) YouTube video which describes

.

This is the Universe we live in.

I hope you'll become even more interested in "gravity" after that taster.

[SandboxGeneral]

Hello all.

I'm having a difficult time wrapping my brain around the concept of the existence of dark matter. From my extremely limited knowledge of the subject, but based on everything I see in documentaries and read from time to time, I just think that there has to be something more to gravity and mass, rather than an undetected matter out there, where there is more of it than visible matter, yet, doesn't interact with anything.

Does that make any sense or is my question without standing?

Secondly, gravity gets confusing to me on the scale of the vast distances greater than our solar system. If scientists say that the universe is not only expanding, but accelerating faster and faster as it expands, what role does gravity play in these interstellar distances between galaxies? It's affect, if it even has any, must be diminishing right?

[JulianO]

I'm having a difficult time wrapping my brain around the concept of the existence of dark matter. From my extremely limited knowledge of the subject, but based on everything I see in documentaries and read from time to time, I just think that there has to be something more to gravity and mass, rather than an undetected matter out there, where there is more of it than visible matter, yet, doesn't interact with anything.

That's basically what dark matter is. It's a way of balancing things so the laws we have worked out on Earth and the Solar system still work on galactic distances. Either the laws are wrong (and there have been various attempts to modify them) or there is some missing mass, and with it missing gravity that something called dark matter would happily fill in the gap and make everything work. In fact this missing mass would fix a number of issues, so therefore it it acts like matter, works like matter and makes everything fit, then it could well be matter.

Secondly, gravity gets confusing to me on the scale of the vast distances greater than our solar system. If scientists say that the universe is not only expanding, but accelerating faster and faster as it expands, what role does gravity play in these interstellar distances between galaxies? It's affect, if it even has any, must be diminishing right?

Gravity still plays a role in the interstellar distances, but because of the huge distances the small effect of dark energy builds up and has more influence than gravity between galaxy clusters. The dark energy pushes apart the galaxy clusters more than gravity can pull them together.

Its like trying to lift a ball bearing off a table with a magnet, it works fine with the magnet close, move it further away and you may be able to roll the ball along the table - so have some effect, but take it upstairs - and although mathematically it is still having an effect, its dwarfed by other forces like gravity now.

[SandboxGeneral]

That's basically what dark matter is. It's a way of balancing things so the laws we have worked out on Earth and the Solar system still work on galactic distances. Either the laws are wrong (and there have been various attempts to modify them) or there is some missing mass, and with it missing gravity that something called dark matter would happily fill in the gap and make everything work. In fact this missing mass would fix a number of issues, so therefore it it acts like matter, works like matter and makes everything fit, then it could well be matter.

Gravity still plays a role in the interstellar distances, but because of the huge distances the small effect of dark energy builds up and has more influence than gravity between galaxy clusters. The dark energy pushes apart the galaxy clusters more than gravity can pull them together.

Its like trying to lift a ball bearing off a table with a magnet, it works fine with the magnet close, move it further away and you may be able to roll the ball along the table - so have some effect, but take it upstairs - and although mathematically it is still having an effect, its dwarfed by other forces like gravity now.

But wouldn't the effect of the Big Bang exploding everything outwards have what it takes to propel everything to where it is today and beyond? I guess meaning, the momentum or kinetic energy from the explosion is what is causing the continuing the expansion rate not a mysterious matter/energy. Why does there have to be this mysterious dark matter/dark energy?

But if it really does exist, wouldn't that in effect change the definition of the vacuum of space? Because there would be something filling up that space, thus no longer making it a vacuum by definition, right?

[JulianO]

But wouldn't the effect of the Big Bang exploding everything outwards have what it takes to propel everything to where it is today and beyond? I guess meaning, the momentum or kinetic energy from the explosion is what is causing the continuing the expansion rate not a mysterious matter/energy. Why does there have to be this mysterious dark matter/dark energy?

Well that was what we use to believe, that the big bang started everything racing away from each other. There was then a lot of debate about whether there was enough mass, and therefore gravity in the universe to slow this expansion down, bring it to a halt, and cause a collapse.

However whilst trying to measure and see if there was enough, it turns out that the galaxies are not slowing down which they would be under just gravity, but are in fact moving apart faster, hence needing a new force provided by dark energy.

But if it really does exist, wouldn't that in effect change the definition of the vacuum of space? Because there would be something filling up that space, thus no longer making it a vacuum by definition, right?

Depends how you define a vacuum I guess You sure wouldn't be able to breathe it! This 'vacuum' also contains passing photons, neutrinos, probably dark matter and maybe gravitons and so on. I think we usually define vacuum as a place without atoms in it.

[SandboxGeneral]

However whilst trying to measure and see if there was enough, it turns out that the galaxies are not slowing down which they would be under just gravity, but are in fact moving apart faster, hence needing a new force provided by dark energy.

But why must there be a new force propelling the universe apart? Can't the energy that initially blew out the Big Bang theory be enough to keep it going?

Perhaps the initial blast was so powerful that, it forced all the matter we observe today far enough apart to where gravity doesn't have the restraining effect it does on matter that is still closer together, like the Earth and Moon. Thus providing a means to continue it's expansion unaffected by gravity and without a third-party force called dark energy.

I just think that if dark energy were so powerful as to push the universe apart at the accelerating rate we see, we'd not only be able to observe its effects, but observe it itself. But then again, I just confused myself, because the same thing can be said about gravity, we can see its effects, but not actually "see" gravity. This is why it's hard for me to grasp the concepts, and why I came here asking these questions!

[JulianO]

But why must there be a new force propelling the universe apart? Can't the energy that initially blew out the Big Bang theory be enough to keep it going?

No, or at least not with the physics we know. Once the force stops (the big bang finishes) then things would drift, probably at high speed. Gravity would oppose this, and cause it to slow down, even if only a tiny bit. At best it would continue on at the same speed it ended up with after the big bang. However what we see is it is getting faster and faster. Something is pushing apart galaxies at increasing speed. The further apart they are the stronger the repulsive force.

I just think that if dark energy were so powerful as to push the universe apart at the accelerating rate we see, we'd not only be able to observe its effects, but observe it itself. But then again, I just confused myself, because the same thing can be said about gravity, we can see its effects, but not actually "see" gravity. This is why it's hard for me to grasp the concepts, and why I came here asking these questions!

Dark energy is a very weak force but it grows with space - therefore it only appears where there is lots of space and no matter - such as the voids between galaxy clusters.

Locally, if you took a ruler, then dark energy will make it very very slightly longer than it would be otherwise, but its so weak compared to the electromagnetic force holding the atoms together its not measureable. Its just like the gravity between two atoms is completely swamped by the electromagnetic attraction.

[Stephen]

I'm going to throw this comment in, despite not wanting to get into a conversation I can only just get my head around

The galaxies have some speed yes, but the "apparent speed" of separation is due to space (as a property of the Universe) expanding is it not?

Buttons on a balloon analogy : they all seem to be racing away from each other when you blow it up, but all that's happening is the balloon is getting bigger.

Does this not imply that talking about the kinetic energy and relative velocity of the galaxies might be leading you down an incorrect path?

Is this where dark energy comes in? It's the force that's stretching space apart?

[JulianO]

This might be worth watching - it explains it better than me.

http://video.ted.com/talk/podcast/2012/Prudential/BrianGreene_2012.mp4

[Ponoobersie]

Wow! what an amazing an very informative post!

This subject completely fasinates me, I have read before and now on here so many differnt theory's and such about this that as I have seen a few of you say...we can never truely know what it is......and I think this is the best way to look at it....for now. The work being done to identify this ''stuff'' (if I can actually call it that as that would imply it has matter) is amazing and as someone said lets hope in ten years time we are rushing out to buy the new dark matter telescope! haha with the rate at which technology is running at id say this might not be too far from the truth one day.

The mind boggles at what is it? how does it work? why is it there? what are its true effects on the universe?...I think these are all questions that one day will be answered and wont be theory's anymore, hopfully in our life times they are answered.

Cheers

Steve

[callump]

Of course, maybe there is no need for dark matter - 'real' matter was there all along...

Paper just published by the RAS: Do the Milky Way?s companions spell trouble for dark matter?

Callum

[SandboxGeneral]

This might be worth watching - it explains it better than me.

http://video.ted.com/talk/podcast/2012/Prudential/BrianGreene_2012.mp4

Excellent video, thanks.

However, the dark matter/energy thing still bothers me. If the universe is expanding at a greater rate all the time, preventing it from collapsing back in on itself, that implies that there is something at the absolute center of the universe, with almost enough mass and gravity to pull it back in.

Let's say, that at some point, expansion halts and begins to reverse. What is at the center of the universe pulling it all back in? More importantly,where is the center of the universe? Do we know the answer to that? If it is known, I haven't heard about that yet. But if we don't know, why not? Can't scientists measure and calculate the center based upon the direction of expansion and trace it backwards?

I can readily buy into the theory of a multi-verse, based on the possibilities of what black holes could really be. I've seen folks on documentaries talk about black holes sucking in all the matter and light around them, and then perhaps, on the opposite side of the black hole, a new universe is being born. Maybe the black hole is like a jet engine, propelling the matter out the other side which is what is causing the increasing rate of expansion we are measuring today with our own universe. In which case, in my estimation, (wholly limited of course compared to those who study this stuff) that there is no need for the dark stuff to exist. Which in turn could mean there is a super-massive black hole at the absolute center of the universe. If that is true, then there should be no chance of the universe, or any universe, collapsing back in on itself.

[Stephen]

There is no centre of the Universe because space all expands equally (back to the 2D balloon example.)

[SandboxGeneral]

There is no centre of the Universe because space all expands equally (back to the 2D balloon example.)

I admit I don't know what the 2D balloon example is, but how can something expand and not come from a central point?

[Stephen]

I admit I don't know what the 2D balloon example is, but how can something expand and not come from a central point?

Imagine a balloon with buttons stuck all over it.

Now in this example, the universe is the surface of the balloon. Nothing inside the balloon (the air) or outside (you) exist.

As you inflate the balloon, all parts of that surface expand equally. The buttons move away from each other (on the 2 dimensional plane) and the further away they are from each other, the faster they appear to move (apparent speed.)

There is no central point of this expansion, everything is moving away equally from everything else.

This is what is happening to the space inside our universe. You can't picture this mentally, seriously

[SandboxGeneral]

Imagine a balloon with buttons stuck all over it.

Now in this example, the universe is the surface of the balloon. Nothing inside the balloon (the air) or outside (you) exist.

As you inflate the balloon, all parts of that surface expand equally. The buttons move away from each other (on the 2 dimensional plane) and the further away they are from each other, the faster they appear to move (apparent speed.)

There is no central point of this expansion, everything is moving away equally from everything else.

This is what is happening to the space inside our universe. You can't picture this mentally, seriously

But even with a balloon, there is a focal point from where the expansion comes from, right?

[Stephen]

But even with a balloon, there is a focal point from where the expansion comes from, right?

From the perspective of the two dimensional plane of the balloons surface, no there is no point where the expansion comes from. All areas on the surface expand equally.

[SandboxGeneral]

From the perspective of the two dimensional plane of the balloons surface, no there is no point where the expansion comes from. All areas on the surface expand equally.

What happens if say, the expansion reverses itself? There must be a point in which it comes together and stops, right?

It's just that when I read "expansion" that tells me that it is moving in one direction, away from another, and that if that direction were reversed, there would be a point in which it came back together.

Kind of like, "for every action, there is an equal and opposite reaction."

[jflowers]

If you view the expansion of space from the Earth's perspective, it appears that everything is expanding away from us. However, no matter where you are in the universe, that location will appear to be the center of expansion, too. The universe expands from every point!

From

The Universe Adventure - The Expanding Universe