Measuring dark matter's repulsive force

[taylaron]

I'm trying to find the amount of repulsive force dark matter has on matter.

To my understanding, the presence of dark matter is deduced from the fact that galaxies are being pushed away from one another instead of toward each other. The absence of matter between the galaxies suggests an unknown form of matter is causing this force.

I'm trying to find or calculate the force that dark matter presents between two galaxies based upon the mass of each galaxy, distance apart and 'amount' of dark matter between the galaxies.

Can anyone point me in the right direction?

Thanks,

-Tay

 

[sylas]

I'm trying to find the amount of repulsive force dark matter has on matter.

To my understanding, the presence of dark matter is deduced from the fact that galaxies are being pushed away from one another instead of toward each other. The absence of matter between the galaxies suggests an unknown form of matter is causing this force.

I'm trying to find or calculate the force that dark matter presents between two galaxies based upon the mass of each galaxy, distance apart and 'amount' of dark matter between the galaxies.

Can anyone point me in the right direction?

Thanks,

-Tay

You have the force backwards. Dark matter is considered to have exactly the same attractive force (gravity) as matter. Dark matter is inferred from the need to hold things together; not drive them apart. It works both to hold galaxies together more strongly together than would apply with only the matter we can see; and also to pull galaxies of the whole universe towards one another ... that is, dark matter slows down expansion of the universe in the same way as ordinary matter.

Cheers -- sylas

 

[Arch2008]

Yes, perhaps you meant dark energy?
http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/dark_energy.html

 

[taylaron]

Thank you for the correction Sylas

 

[Ich]

Dark Energy is twice as repulsive as normal matter is attractive. Is that enough for you?

 

[marcus]

As was said earlier, dark matter is attractive to itself and to ordinary matter. It collects in clumps and helps form galaxies and clusters of galaxies. It helps keep galaxies together in clusters. The effect of dark matter is to make each galaxy more massive (because the DM is clumped around the galaxy) and therefore more attractive to its neighbors in the usual Newtonian way.

You were really talking about dark energy instead of DM, so would it be useful to try editing your original question and study a modified version?

I'm trying to find the amount of repulsive force dark [energy] has on matter.

To my understanding, the presence of dark [energy] is deduced from the fact that galaxies are being pushed away from one another instead of toward each other. The absence of matter between the galaxies suggests an unknown form of [energy] is causing this force.

I'm trying to find or calculate the force that dark [energy] presents between two galaxies based upon the mass of each galaxy, distance apart and 'amount' of dark [energy] between the galaxies.

If that is what you want to ask about, then you could start by noting these basics. In the standard cosmology picture, dark energy has a constant uniform density of about 0.6 nanojoules per cubic meter---constant over all space and for all time. That is about 0.6 joules per cubic kilometer.

Dark energy does not clump. It stays uniformly distributed throughout all space. (Even though we cannot see all space, the standard model assumes for simplicity that it all resembles what we can see---it all has the same amounts of dark energy and matter that is in the part we can see.)

Dark energy does not thin out as distances expand. Matter and ordinary energy thins out, but not DE. It stays 0.6 nanojoules per cubic meter.

If you look at anyone object, like a galaxy, there is as much DE on one side as there is on the other. It would be impossible for DE to be "pushing" the galaxy in anyone particular direction.

These are just the basic premises of DE. You may want to think about these basics for a while. For most people they take some getting used to, since they are not especially intuitive. You are encouraged to ask more questions!

 

[Ich]

If you look at anyone object, like a galaxy, there is as much DE on one side as there is on the other. It would be impossible for DE to be "pushing" the galaxy in anyone particular direction.

Well, and in our model uiniverse, there is as much matter on one side than on the other. So it can't pull a galaxy in a perticular direction, but it is still pulling all galaxies together, thus slowing the expansion.
GR is not different from Newtonian gravitation in this respect, with one notable exception: in GR, you can prove that in a homogeneous universe, the dynamics of a spherical region is not affected by the rest of the universe. That renders the old, Newtonian argument for a static universe -...as there are masses everywhere, their net effect cancels out...- invalid.
So there is one source for cosmological acceleration (or deceleration), and that's the trace of the Stress-Energy-Tensor.
As long as there is no shear and no high velocities, this trace is at the base of all "normal" gravitational effects too, see e.g. http://math.ucr.edu/home/baez/einstein/einstein.html" . There is simply no difference between cosmological deceleration and "normal" gravitational attraction - or, for that matter, between cosmological acceleration and not-so-normal gravitational repulsion.
For "fluids", the source of gravitation is $\rho + 3 p$, which is $\rho$ for dust and $-2\rho$ for Dark Energy.

 

[Frame Dragger]

I would just make one point, and that is while DE may well be responsible for the ACCELERATION in the recession speed between galaxies, that shouldn't be confused with UNIVERSAL expansion. It's probably also worth noting that DE is a solution to a mathematical problem which is an excellent description of the forces at work, but really not much in the way of the SOURCE of that force. Theories centered on vacuum expectation energy fall far short of the needed energy, and other contenders depend on theories that lie outside of The Standard Model and GR.

I guess the idea of a cosmological constant (although sudden acceleration 3 billion years ago isn't exactly CONSTANT) wasn't as silly as Einstein eventually came to believe. Granted this isn't quite the same thing, but damned if it isn't a bit creepy nonetheless.

 

[sylas]

I would just make one point, and that is while DE may well be responsible for the ACCELERATION in the recession speed between galaxies, that shouldn't be confused with UNIVERSAL expansion.

It should be identified with universal expansion. The recession between widely separated galaxies is a universal phenomenon, and DE applies universally.

I guess the idea of a cosmological constant (although sudden acceleration 3 billion years ago isn't exactly CONSTANT) wasn't as silly as Einstein eventually came to believe. Granted this isn't quite the same thing, but damned if it isn't a bit creepy nonetheless.

There is no proposal of a "sudden acceleration". What is proposed is indeed a cosmological constant term. This refers to a constant energy density in the vacuum, as opposed to the energy density of matter, for example, which reduces as matter disperses. As matter disperses, the mean density of energy throughout the universe approaches in the limit the cosmological constant value. There's a smooth transition from deceleration to acceleration, in the flat ΛCDM model, as the density of matter drops and the dark energy density remains constant.

Cheers -- sylas

 

[Frame Dragger]

It should be identified with universal expansion. The recession between widely separated galaxies is a universal phenomenon, and DE applies universally.



There is no proposal of a "sudden acceleration". What is proposed is indeed a cosmological constant term. This refers to a constant energy density in the vacuum, as opposed to the energy density of matter, for example, which reduces as matter disperses. As matter disperses, the mean density of energy throughout the universe approaches in the limit the cosmological constant value. There's a smooth transition from deceleration to acceleration, in the flat ΛCDM model, as the density of matter drops and the dark energy density remains constant.

Cheers -- sylas

I stand corrected on two points, however, the expected vacuum energy is insufficient to explain the apparent effect of dark energy. I was however, completely wrong about the "sudden" terminology, and I was sorely wrong about the universal nature of the expansion. I stand corrected, and thank you Sylas.

 

[sylas]

I stand corrected on two points, however, the expected vacuum energy is insufficient to explain the apparent effect of dark energy. I was however, completely wrong about the "sudden" terminology, and I was sorely wrong about the universal nature of the expansion. I stand corrected, and thank you Sylas.

No problem. You are delving into really interesting areas, and everyone needs correction as they start to explore the subject. Don't let that stop you! One my own favourite sayings:

They say we learn from our mistakes. That must make me the smartest person in the world.​

With that in mind... here's another. The theoretically expected dark energy from quantum field theory is actually much much larger than what is needed to explain apparent effects in the present. The "expected vacuum energy" according to existing quantum field theory is enormous; about 120 orders of magnitude more than what is used in modern cosmology. This is a major unsolved problem in modern physics.

And if I have that wrong, someone can correct me as well, please!

Cheers -- sylas

 

[Frame Dragger]

I thought it was roughly 100 order of magnitude less... Call me crazy but I'm going to go with your version!

 

[Robert Powell]

I have always had a problem with the Dark Matter theory. In order to explain the unexpected higher velocity of the outer stars in a galaxy, it is theorized that there must be some unknown matter at the edge of those galaxies that causes the outer stars to orbit the center of their galaxy at a speed higher than what would be expected based on gravitational law. That doesn't pass the common sense test. Why would the dark matter always congregate at the outer edge of the galaxies? And why would it congregate in just the right amounts from galaxy to galaxy in order to maintain those higher outer star velocites? Well, the answer to the last question is it doesn't. I think dark matter may be just a property of space.

Recent work by Univ of Maryland astronomer Stacy McGaugh indicates the ratio of normal matter to dark matter decreases as the size of a galaxy decreases and the relationship is systematic with scale. So large galaxies appear to have less dark matter for their size than the small galaxies. How does McGaugh come up with the information to measure dark matter to normal matter ratios between the galaxies? He compares the expected outer star velocities to that based solely on graviational law and the discrepancy is then assumed to be dark matter and from there the amount of dark matter is calculated so as to explain the outer star velocities. McGaugh's results showed a linear variation from 17% normal matter for large galaxies to only .2% normal matter for small galaxies. If one were to believe that dark matter is related to normal matter and is part of the development of a galaxy then those numbers should be fairly constant. But if "dark matter" is nothing more than a property of the continuum of space or a change in the property of gravity with distance then these numbers make more sense. It's almost like an effect that is relational to the amount of space occupied. Almost as if the universe has a built in gravitational constant or acceleration constant across all space. (Maybe those 'zero-point energy' guys are headed in the right direction.)

 

[Vanadium 50]

I have always had a problem with the Dark Matter theory. In order to explain the unexpected higher velocity of the outer stars in a galaxy, it is theorized that there must be some unknown matter at the edge of those galaxies that causes the outer stars to orbit the center of their galaxy at a speed higher than what would be expected based on gravitational law.

No, it doesn't say that this matter is concentrated at the edge.

That makes most of the rest of your post rather moot.

 

[Frame Dragger]

I want to know where on Earth you HEARD or read that dark matter is concentrated at the edges of anything. There is a well known map that purports to show the distribution of dark matter, and it's nothing like you describe. Beyond that... "Dark matter related to normal matter..." wel... in theory everything we percieve came from the same source, so yeah... Otherwise no, dark matter would be very unlike normal matter. Not to mention that dark matter (WIMPs perhaps) are the VAST majority, not "normal" matter... whatever that is.

You know what, Vanadium 50 was right to ignore the rest of your post.

 

[edpell]

Dark energy does not thin out as distances expand. Matter and ordinary energy thins out, but not DE. It stays 0.6 nanojoules per cubic meter.

What about conservation of energy (or energy-momentum four vector)?

This sounds like spontaneous creation of matter (or energy-momentum four vector). Next you will be telling me that maggots form spontaneously inside rotting meat. :)

 

[Dmitry67]

energy is not conserved in cosmology

 

[Dmitry67]

But may be you're right, it is not conserved even locally (in a closed region)

 

[Dmitry67]

No, we are talking about DE

 

[Dmitry67]

Frame Dragger, check page 1.
The title of the thread is incorrect, the poster actually meant DE, not DM

 

[Frame Dragger]

Frame Dragger, check page 1.
The title of the thread is incorrect, the poster actually meant DE, not DM

Yeek, my bad! I completely missed that. Sorry Dimitry.

 

[twofish-quant]

That doesn't pass the common sense test. Why would the dark matter always congregate at the outer edge of the galaxies?

It doesn't. When you are looking at the velocity of something at the edge of a galaxy, that velocity is affected by all of the matter inside the location you are looking at. So you are adding up all of the matter on the inside to get the velocity. One other thing that happens is that as you get further and further out, the area that you are adding to the sphere increases.

Also galaxy rotation curves are independent of the other pieces of evidence that suggests that there is dark matter.

But if "dark matter" is nothing more than a property of the continuum of space or a change in the property of gravity with distance then these numbers make more sense.

This has been considered (see MOND models). The trouble is that it's really hard to make it work once you get down to specifics. If it was possible to add one "fudge factor" to gravity and get both galaxy rotation curves and cosmological observations, then people would be more convinced that it's some sort of gravity effect. The trouble is that people haven't come up with a single "modified gravity fudge factor" that gets everything to work. By contrast, people *have* come up with a single "dark matter fudge factor" that seems to get all of the numbers to work.

Getting into the specific details is something that will take a few pages, and there is stuff on the web about it, but that's a basic sketch as to why people seem to prefer dark matter right now as an explanation over modified gravity models.

Almost as if the universe has a built in gravitational constant or acceleration constant across all space. (Maybe those 'zero-point energy' guys are headed in the right direction.)

One problem with this discussions is that at the end of the day, you have to get down to the nitty-gritty numbers. One thing that people get wrong with cosmology, is to think of it in terms of philosophy. It turns out that that we have tons of data about how the universe works, which is generally a good thing, but it makes it hard to come up with a model that makes all of the data fit.

 

[twofish-quant]

What about conservation of energy (or energy-momentum four vector)?

In general relativity, energy is not conserved and things travel faster than light. Energy conservation results from the a time symmetric system and in GR things aren't time symmetric.

 

[Dmitry67]

How gravitational potential is defined in presence of DE?

When 2 masses (in empty universe) are closer than some distance R they attract to each other.
At distances >R they repel because of DE

 

[edpell]

In general relativity, energy is not conserved

This bothers me. Spontaneous creation not much of an explanation of anything.

and things travel faster than light. Energy conservation results from the a time symmetric system and in GR things aren't time symmetric.

Faster than light? What are you talking about?

 

[edpell]

How gravitational potential is defined in presence of DE?

When 2 masses (in empty universe) are closer than some distance R they attract to each other.
At distances >R they repel because of DE

My limited understanding is that DE is repulsive so I think it is a separate category from gravitation.

 

[Dmitry67]

1 This bothers me. Spontaneous creation not much of an explanation of anything.

2 Faster than light? What are you talking about?

1 Non-conservation of energy <> creation of matter from notheing
2 Yes. Just turn around. You can use ANY frame. Hence Androomeda galaxy moved much faster than light

 

[Frame Dragger]

If you just type my name into wikipedia you'll have a fine example of FTL from an observer's frame of reference (well, frame dragging that is). I'm always surprised that people seem to miss the RELATIVE in Relativity!

Edit: Other good search terms: "Recession Speed, Lens-Thirring Effect, and Heat Death"

 

[twofish-quant]

This bothers me. Spontaneous creation not much of an explanation of anything.

This is one of those "I don't make the rules of the universe, I just figure out what they are."

Also creation of energy really doesn't bother me that much. It turns out that conservation of energy results when you have a system that is time-symmetric. If you have systems that aren't time symmetric, then you don't get conservation of energy.

If you rub your hands together fast enough heat gets created out of nothing. If you have enough energy in one place than matter/anti-matter pairs get created out of nothing.

Faster than light? What are you talking about?

Well just look at hubble's law. V=Hr. If the universe is infinite or even really, really big, then eventually you'll find a galaxy that's moving away from you at faster than light.

 

[Frame Dragger]

This is one of those "I don't make the rules of the universe, I just figure out what they are."

Also creation of energy really doesn't bother me that much. It turns out that conservation of energy results when you have a system that is time-symmetric. If you have systems that aren't time symmetric, then you don't get conservation of energy.

If you rub your hands together fast enough heat gets created out of nothing. If you have enough energy in one place than matter/anti-matter pairs get created out of nothing.



Well just look at hubble's law. V=Hr. If the universe is infinite or even really, really big, then eventually you'll find a galaxy that's moving away from you at faster than light.

Or over time due to a positive cosmological constant, everything geyond the local supercluster could pass beyond the visible horizon at superluminal recession speeds relative to our Frame.

 

[Skolon]

I know that today science are studying matter at fundamental level using particle accelerators. I think that some of that studies were done with intention to discover some parts from dark matter mystery, but I suppose that we need bigger accelerators.

In Universe some "natural" accelerators exist, much bigger that we will can create in near future. I speak about BH accretion disks and other similar phenomena. Aren't these phenomena closely oversee with dark matter discoveries in mind? I suppose they are. But did we found something interesting from this supervises?

 

[Robert Powell]

No, it doesn't say that this matter is concentrated at the edge.

That makes most of the rest of your post rather moot.

My statement that dark matter was at the edge of a galaxy was not meant to be taken literally and is not important as related to the post. Your follow up statement that indicates this makes the rest of my post moot is very closed minded.

 

[Robert Powell]

Twofish-quant, thank you for your responses to my post and your comment on the MOND theory. I appreciate your time and your weighted thoughts.

 

[Frame Dragger]

Vanadium, this type of reply makes you unworthy to hold the title "mentor" near your name. My statement that dark matter was at the edge of a galaxy was not meant to be taken literally and is not important as related to the post. Your follow up statement that indicates this makes the rest of my post moot is very closed minded. When replying to the posts of others, try to do a better mentoring job.

Your post was borderline nonsensical, as is your explanation. You didn't mean the statement that a physical quantity was concentrated in a particular area to be taken literally... how DID you mean it to be taken? Metaphorically?! You just came at this all wrong, so don't vent your spleen at a guy who is very helpful just because he was the first of many to shoot you down. :D

 

[twofish-quant]

I know that today science are studying matter at fundamental level using particle accelerators. I think that some of that studies were done with intention to discover some parts from dark matter mystery, but I suppose that we need bigger accelerators.

The problem is that it's hard to figure out what instrument you need to find something, if you don't know exactly what you are looking for.

There are dozens of experiments that are trying to find dark matter: The basic idea is that you put a detector in some deep dark cave and look for dark matter interacting with normal matter. You might ask what happens if dark matter doesn't interact with normal matter. Well, if you don't find anything, then you know whatever dark matter is, it doesn't interact with normal matter, which narrows down the possibilities for what it might be...

http://lpsc.in2p3.fr/mayet/dm.php

In Universe some "natural" accelerators exist, much bigger that we will can create in near future. I speak about BH accretion disks and other similar phenomena.

They are bigger but they aren't hotter. The energies that you find in a black hole accretion disk really aren't larger than what we can create with things like the large hadron collider.

Aren't these phenomena closely oversee with dark matter discoveries in mind? I suppose they are. But did we found something interesting from this supervises?

Part of the problem is that it's tough to see something if you don't know exactly what you are looking for.