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Dark Matter/Dark Energy


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Let me preface this by saying I know almost nothing about the subject.

I just wanted to ask, when do Dark Matter and Dark Energy come into play?  Are they not a factor at all in our Solar System?  How about our Galaxy?  "Close" Galaxies?  When do we need to throw Dark Matter/Dark Energy in to start explaining things?

I just wish I could be instantly up to speed on where we are today with all this physics so I could take a crack at figuring it out.  If I'm already out of college for many years in another subject, is it too late for me? :)

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If I remember correctly, dark matter comes into play at the galactic level. If you look at the way spiral galaxies rotate, they should be much more massive than is possible with visible matter alone.

Dark energy comes into play at a cosmological level. It is used to explain why the expansion of the universe is accelerating.

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AstroJon has it about right. Broadly Dark Matter is only noticeable on galactic scales. There have been attempts to find it in the solar system, both directly, and by looking for changes to the rotation curve of the planets, which assumes that Dark Matter pools locally around the Sun. However so far nothing significant has been found, but it would probably be a small peak in the galactic trend if it occurred, and any such signal may be washed out depending just how light DM particles are.

Dark Energy is a very weak force that is a property of space, so it only becomes significant where there are large areas of space without other significant forces going on - so intergalactic voids mostly. Otherwise it is drowned out by other forces like the usually weak gravity.

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If you swing water in a bucket, the water will stick in to the bottom.

This is because along the axis the bottom of the bucket is travelling faster through space than the top of the bucket.

This is newtons gravity laws and applies to planets travelling through space around our sun.

Einstein's laws show that spacetime curves around mass, thus the planets are rolling towards the sun.

This is counterbalance for the Newtonian push due to rotation around the sun.

(same as your arm holding the bucket)

This mechanism dictates that the further the object is the faster it needs to rotate the center or the greater mass to create the Einstein spacetime pull.

Within our galaxy the outer part of the spiral arms should rotate significantly faster through spacetime than the inner parts if the mass was uniform across the spiral arms to match our observations.

Counter intuitively this does not occur in our galaxy or other galaxies that have been measured.

This suggests that there is unseen mass in the arms.

I.e. Unseen dark mass.

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We make something up like dark matter, because it is the simplest way to fix many problems. It's not the only way, and people look at tinkering with newtons laws on wide scales and so on. However I would say dark matter is accepted as the most reasonable answer by about 99% of astronomers.

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It just seems like it should come into play everywhere.  In my mind we've got something wrong and that's why we make something up that we can't detect to make things work.

Its not quite a simple as "making something up" . Theoretical and observational evidence shows us that at the macro scale, gravity doesn't work in the way that our theories predict it should. The theories seem accurate as they have been proven time and time again, so something is going on.

Its a bit like tossing a coin in the air- all your life's experiences can lead you to pick a coin, even one that you've never seen before. Then, after a second of holding it in your hand, feeling the weight and size of the coin, you can toss it in the air and catch it again. In other words, your experience allows you to predict how high the coin will go, how long it will take to reach the highest point and start to drop. You can even predict where the coin will be in space and time in the future to allow you to move your arm and hand to the right spot to catch the coin.

Now imagine if you walked outside and tossed the coin in the air. This time, instead of dropping back to Earth, the coin accelerated and disappeared up into the air. Or that it landed 3 metres away from where you expect it to be. You know how heavy it was. You know how hard you tossed it. Yet it behaved in an unpredictable fashion. You pick the coin up and go back inside and try again. The coin behaves as expected. You go back outside and toss the coin in the air and again it lands 3 metres away. Whats happening? Your theory of how a coin moves, the force required to launch it into the air and your predictions of where and when it will land all work inside. You can repeat the experiment over and over inside and all is well. Yet outside the anomalous behaviour occurs.

The same is happening in the Universe. We can predict the motions of stars and bodies to a high degree of accuracy. We can measure the mass in galaxies, again to a high degree of accuracy. Yet, when we look at the motions of stars in the haloes around galaxies, the stars should be flying off into space instead of orbiting the galaxy. There appears to be more mass in the galaxy than we can see, otherwise the galaxy's gravity is not strong enough to hold onto the stars. We cannot, as yet directly detect this extra matter (though we are getting very close to it).

This is how science works. We observe the universe. We work on hypotheses to try and explain what we see. We test these hypotheses, by incrementally expanding what we know. we form theories that allow us to predict behaviour. We then observe again to see if the universe is acting in accordance with the theory. So its not quite the same as making something up. The idea of dark matter and energy is built on many observances of what's happening and our widely accepted theories.

We did the same with atoms, sub-atomic particles, the Higgs boson and so on. The Higgs boson wasn't made up. Our theories and observations predicted that it should exist. We built hugely complicated machines to test the theories, And we found it where it was predicted to be. In many ways, it could have been more exciting if we didn't find it, because then we would have had to work on why it wasn't there, and expand our theories.

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However I would say dark matter is accepted as the most reasonable answer by about 99% of astronomers.

The thing about majorities and minorities (percentage of supports) when it comes to scientific ideas is that it's often been the case of the minority idea winning out over the mass follows - eventually.

'Following the crowd' is often the least imaginative and least productive way to go. I'm sure all the great thinkers would agree with that ;)

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The thing about majorities and minorities (percentage of supports) when it comes to scientific ideas is that it's often been the case of the minority idea winning out over the mass follows - eventually.

'Following the crowd' is often the least imaginative and least productive way to go. I'm sure all the great thinkers would agree with that ;)

Well I'd perhaps disagree with if often being the case. It certainly has been the case now and again. Striking out on your own is also a profitable way to make headway too, but everything in moderation. 

In the case of Dark Matter it fits the observed facts incredibly well, enough that most people in the community take it for granted now. As I said people do keep trying out new ideas, but it now has to fit a jigsaw of results, and its not easy to find another piece of the puzzle that is shaped well enough to replace dark matter.

You can fix galactic rotation curves by tinkering with how gravity works on large scales, but then you have to explain why large galaxy cluster bend light by more than the observed mass would tell you. You also have to square it with the planck results, explain the bullet cluster, and why galaxies form so quickly.

Of course it's fun to be a rebel, but you need to have extraordinary evidence to overturn well supported theories, and t doesn't happen that often.

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This February edition of Focus magazine has a piece on dark matter. Not an exhaustive appraisal but an overview of current thinking.

Apparently the mass energy of dark matter hugely outweighs that of normal matter throughout the universe. This dark matter might even form structures and conceivably our own galaxy might even be  entangled with an invisible and very weakly interacting dark galaxy. We can't detect it directly but without it our galaxy (and all the others) couldn't have formed without it's gravitational influence.

None of the dark matter stuff sits easy with me. I always thought it was a bit of a fudge.  But without it there would need to be a re-write of the text books.

I think it's probably a bigger deal than I initially thought!

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The other thing I don't get is how we get the observed mass of galaxies millions or even billions of lightyears away, and how we know it's within a certain accuracy?  So these galaxies are bending light more than they should by the mass we've observed them to be?  And dark matter accounts for the extra bend?  How the heck do we know the mass of a galaxy?  That boggles my mind.

I know the way we determine it is probably very scientific, but still in my mind there's no way we can say we know the approximate mass of something so large and so far away.

Thank you for all the responses so far.  There's some very good reading here.

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There are several ways to weigh a galaxy, or even a cluster of galaxies.

The main way is to assume that the galaxy is not doing anything weird, like just forming or exploding. In this case you measure how fast it is spinning, and assume that the stars will stay in their orbits.

So the force of gravity has to be exactly equal to the circular force trying to make the stars escape. 

So first Newtons law of gravity is

F = Gmgms/r2    - force of gravity (mg is mass of galaxy, ms mass of a particular star, r is the distance between them, G is the gravitational constant).

F = msv2/r         - outward circular force on a star - or anything going in a circle (v is the velocity of the star)

msv2/r = Gmsmg/r2 - set them in equal as the system is in balance - notice the ms term cancels out.

v2= Gmg/r        - cancelled out the star mass, also one of the r's cancel too

mg = v2r/G        - a rearrangement, and we have a formula for the mass of the galaxy (inside distance r)

so if you know the velocity of the stars at any radius, you know how much mass there must be inside that radius to stop them flying apart.

Rotational speeds are easy to measure using spectroscopy as stars are blue and red shifted depending on if they are coming towards us or going away.

You can do the same thing with clusters of galaxies (which is how dark matter was first found in the 1930's). There are several other techniques too - but this is probably the simplest.

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So r is the distance to the center of the galaxy from a certain star in the galaxy?  How do we get r for something so far away?  How do we even single out individual stars at that distance?

Also, is it safe to assume the star isn't gradually escaping the galaxy or plummeting toward the center?  I'm guessing we track the radius over time, but it seems like it would be very hard to get the radius of something so far away.  What if the escape or falling is very very gradual (not sure if that would be possible) and takes years for the radius to change?  Are we accounting for that?

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You can actually locate individual stars in close galaxies, but that isn't necessary.

As you can see the mass of the star vanishes from the equation - so you don't need to know it - and hence don't need a star as such,  all you need to work with is a general velocity at that position ®. This is exactly what spectroscopy does - either IFU or long slit. You basically end up with an image

which has velocities along the axis of the galaxy. So you can work out how fast it is rotating at any position.

As you are looking at the combined light of lots of stars typically, it also averages out their velocities, so you don't get stars with weird proper motion skewing things.

The premise is that galaxies are long lived things, so if you look at a galaxy, and it's not busy crashing into another galaxy, or got some huge outburst of energy from a black hole, then for the most part all the stars in it will be in stable orbits. If they weren't, then galaxies wouldn't live very long!

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There are several ways to weigh a galaxy, or even a cluster of galaxies.

The main way is to assume that the galaxy is not doing anything weird, like just forming or exploding. In this case you measure how fast it is spinning, and assume that the stars will stay in their orbits.

So the force of gravity has to be exactly equal to the circular force trying to make the stars escape. 

So first Newtons law of gravity is

F = Gmgms/r2    - force of gravity (mg is mass of galaxy, ms mass of a particular star, r is the distance between them, G is the gravitational constant).

F = msv2/r         - outward circular force on a star - or anything going in a circle (v is the velocity of the star)

msv2/r = Gmsmg/r2 - set them in equal as the system is in balance - notice the ms term cancels out.

v2= Gmg/r        - cancelled out the star mass, also one of the r's cancel too

mg = v2r/G        - a rearrangement, and we have a formula for the mass of the galaxy (inside distance r)

so if you know the velocity of the stars at any radius, you know how much mass there must be inside that radius to stop them flying apart.

Rotational speeds are easy to measure using spectroscopy as stars are blue and red shifted depending on if they are coming towards us or going away.

You can do the same thing with clusters of galaxies (which is how dark matter was first found in the 1930's). There are several other techniques too - but this is probably the simplest.

So cool!

I love it when somebody "blows my mind"! 

Thanks mate

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I'm uneasy about using large amounts of something undetectable(*) to account for measurements but the alternative - a change to Newton's laws of gravity - is equally unpalatable.

It feels a bit like the attempts to detect the Ether towards the end of the 19thC, and that lead to Relativity and Quantum physics, and no Ether.

Maybe the same thing will happen here, someone will come up with an alternative that's better.

* Undetectable other than by inferring it's presence from gravitational effects.

Chris

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We did the same with the atom. And the neutron

I'm uneasy about using large amounts of something undetectable(*) to account for measurements but the alternative - a change to Newton's laws of gravity - is equally unpalatable.

It feels a bit like the attempts to detect the Ether towards the end of the 19thC, and that lead to Relativity and Quantum physics, and no Ether.

Maybe the same thing will happen here, someone will come up with an alternative that's better.

* Undetectable other than by inferring it's presence from gravitational effects.

Chris

Why?

Its how science works. We postulate and then investigate if real-world data supports the theories.

Think of how the neutron was discovered. It existence was postulated in the 20s but it took until 1932 before Chadwick could prove it's existence.

Or the Higgs boson.

Or the virus.

Personally I think that its a great time to be alive...we are learning at such a rate that huge discoveries are happening all the time. If we do find that fark matter exists then a raft of new science will follow. If we don't, then a raft of new science will follow. Win:Win!

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The classic example is the neutrino - invented to balance the books, virtually undetectable. Proposed in 1930, it was 1956 before it was detected. Up til then it was just a place holder.

Actually the same happened with quarks. There was a time when they were thought to be a useful idea for explaining things, but probably not real. I remember someone working in the field saying soon after the idea came about:

"Its like saying a dollar bill can be made up of 4 quarters, but no amount of poking around the dollar bill will get you the 4 quarters".

Events since have shown they are real!

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There are lots of examples of things that were believed to be a correct explanation but weren't.  Phlogiston, epicycles and so on.

The MOND people claim that their theory is simpler in that it needs fewer adjustments than dark matter. Their point is that Kepler was in the same position when he found that planetary motion in ellipses allowed better predictions than multiple epicycles, it was a good description of what is happening but there is no explanation for it.  As we all know Newton's laws of motion and gravity came up with an explanation that made elliptical movement inevitable.

I don't know what the answer is,  I think it's important to keep an open mind.

Chris

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People do keep an open mind, they try all sorts of different models. For instance here is a paper from just a few months ago.

http://arxiv.org/abs/1310.4009

or here is another

http://mnras.oxfordjournals.org/content/436/1/202.short

there are loads of them. That said, I think its fair to say MOND and TeVes are still considered to be on the fringes. DM just seems to fit the bill easier, and explains things like structure formation and gravitational lensing too.

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The classic example is the neutrino - invented to balance the books, virtually undetectable. Proposed in 1930, it was 1956 before it was detected. Up til then it was just a place holder.

Actually the same happened with quarks. There was a time when they were thought to be a useful idea for explaining things, but probably not real. I remember someone working in the field saying soon after the idea came about:

"Its like saying a dollar bill can be made up of 4 quarters, but no amount of poking around the dollar bill will get you the 4 quarters".

Events since have shown they are real!

To play Devil's Advocate for a sec, Eddington, a neutrino-denier, did predict that our experimentors would be able to create neutrinos.  :grin:  And now we have a Higgs Boson Factory in Geneva!! Only joking. 

Surely 'detecting things by their absence' is a commonplace? The Police and the tax authority routinely consider a lifestyle of swimming pools and Porsches from people with 'no visible means of support'  to be evidence of criminal activity.  They don't know what the activity is but they know it's there.

Olly

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Throughout history we have tried to predict and understand how everything works based on observation and experiment and any theory is formulated to fit the available facts.

There will allways be bits of a theory that dont quite fit but that doesnt matter it gives a focus on what to look for next and dark matter/energy is no different what will probably happen is that some future new observation or unrelated scientific discovery will fill in the gaps and move us forward again with a new set of "fudge factors" to debate over.

One thing that is certain is that we will never know it all.

Alan

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Throughout history we have tried to predict and understand how everything works based on observation and experiment and any theory is formulated to fit the available facts.

There will allways be bits of a theory that dont quite fit but that doesnt matter it gives a focus on what to look for next and dark matter/energy is no different what will probably happen is that some future new observation or unrelated scientific discovery will fill in the gaps and move us forward again with a new set of "fudge factors" to debate over.

One thing that is certain is that we will never know it all.

Alan

A case in point being when Bose gave a lecture in the theory of radiation (early 1920's perhaps) intending to show it's incompatibilities with experimental evidence. He made a 'mistake' in the course of the lecture which showed that the theory was in agreement with experiment. Bose soon realized that his 'mistake' wasn't and some new science was born. The first Bose-Einstein condensate was produced about twenty years ago. (Bosons are of course named in his honour ).

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