Why Is Dark Matter Essential for Understanding Gravity in the Universe?

[VooDooX]

My question is why do we even need dark matter to make things work ? E=mc2 so if ENERGY AND MASS are the same thing if MASS creates gravity why can energy not also create gravity can someone explain this to me? if Under general relativity any form of energy couples with space time to create the geometries that cause gravity why are we even looking for dark matter all the energy in the galaxy should be enough to make up for the lost gravity right? or am i missing something? i mean really microwaves x rays gamma rays LIGHT ITSELF i mean that's ALOT of energy so shouldn't that also be a lot of gravity?

 

[mathman]

Energy does have a gravitational effect. Dark matter is needed since the total mass of ordinary matter plus photons is far too small to account for what is needed to hold galaxies together, etc. under current gravity theory (general relativity). Some people have proposed modifying the theory to avoid dark matter, but their ideas aren't convincing.

 

[VooDooX]

E=mc2 that means a small amount of energy is equivalent to a large amount of mass tho right? and even with all that mass the model still doesn't work? they take into accoutn hawking radiation,electromagnetic and all other forms of energy?

 

[S.Vasojevic]

E=mc2 that means a small amount of energy is equivalent to a large amount of mass tho right? and even with all that mass the model still doesn't work? they take into accoutn hawking radiation,electromagnetic and all other forms of energy?

No, the other way around. 'Small' amount of mass is equivalent to 'large' amount of energy. But anyway, when you add all together, there is simply not enough to hold galaxies together.

 

[Chalnoth]

Mass-energy equivalence leads to a lot of misconceptions. The primary one seems to be that somehow matter can be converted to energy and back again.

This is false.

To try to explain why, first I'll have to talk about using units where c = 1. This is to remove come confusion, and make the equations more clear. Basically, by setting c = 1, we're setting a "standard velocity". In these units, all velocities are compared against the speed of light. And the mass-energy equivalence takes on a form that makes its meaning more clear:

E = m

Now, this isn't entirely correct, though, because it neglects motion. There are a few ways to add motion into the equation, but perhaps the easiest is to invoke momentum. In special relativity, we can then state that:

E^2 = m^2 + p^2

Here we've added the momentum p to the equation. How is momentum related to velocity? Well, in relativistic terms, this is simple:

v = \frac{p}{E}

Remember, the velocity v is a fraction of the speed of light. So we see immediately, for instance, that if the mass is equal to zero, then p = E, and therefore v = 1.

This may seem a bit rambling. But what do the terms in this equation really mean?

Well, momentum is clearly an energy of motion. And mass is an energy that remains even if there is no motion: mass is the energy that is somehow internal to the object in question. An object that has no mass has no internal energy, but can still have lots of energy of motion. Similarly, an object with lots of mass doesn't need to move at all to have lots of energy.

But what it all boils down to, though, is that you can't just split off the energy from matter: there still has to be something there, with or without mass, for there to be energy. It has to either have some energy of motion, or some internal energy. Basically, our observations of dark matter show us that we have to have lots of stuff out there with very, very little momentum, and therefore lots of mass. It also can't be electrically charged (else it wouldn't be dark), and must interact only weakly with itself and normal matter (like neutrinos, but neutrinos would have too much momentum compared to their mass).

 

[Wallace]

E=mc2 that means a small amount of energy is equivalent to a large amount of mass tho right? and even with all that mass the model still doesn't work? they take into accoutn hawking radiation,electromagnetic and all other forms of energy?

Chalnoth has explained the energy-mass misconception, I'll add one small comment. When we model the universe, we do include the gravitational contribution of all forms of energy, including electromagnetic radiation. It turns out that in the early universe, electromagnetic radiation was for a time the dominant component, however at the present time the energy density of radiation is very small, much smaller than 1% of the total energy density.

So radiation is already in the calculations, its just that it has a relatively small contribution at the present time. Of course it is also the means by which we can learn about the universe (because it is what we can see), so of course it is very important, but not in a direct way.

 

[VooDooX]

thanks for clearing all that up for me guys :)

 

[Skolon]

Can you list the basic properties of dark matter? How it is like to be? It is sure that can't be ordinary matter, some undiscovered (yet) elements?

 

[Chalnoth]

Can you list the basic properties of dark matter? How it is like to be? It is sure that can't be ordinary matter, some undiscovered (yet) elements?

The required properties to explain our current observations are:
1. It must not have electric charge.
2. It must interact at most weakly both with itself and normal matter.
3. It must have properties such that it is produced in the right numbers in the early universe to explain the amount of it which we observe.

It can't be ordinary matter, and since elements are made of ordinary matter, it can't be an element either.

 

[Skolon]

Thank you.

What else did we know about DM?
It's properties is analog with gas or solid bodies, or maybe exist just as disparate entities? Can it agglomerate in large bodies? Because you said that it interact weakly with itself I suspect that the answer for last question is "no, it can't agglomerate in large bodies".

Is DM uniform distributed in Universe or have some connections or affinities with normal matter, so it have an analog distribution? It is static or can we say that it is affected by expansion?

Because it is so ... enigmatic (we know little about it) how can we say that it is different by Dark Energy (an other very enigmatic thing)?

 

[Dmitry67]

It can't form bodies because of #2 - It must interact at most weakly both with itself
Mathematically it is an ideal gas.

 

[Chalnoth]

Thank you.

What else did we know about DM?
It's properties is analog with gas or solid bodies, or maybe exist just as disparate entities? Can it agglomerate in large bodies?

As Dmitry mentioned, #2 means it can't do this. Basically, galaxies are little bits of normal matter sitting in the center of huge clouds of dark matter. For the matter in the galaxies to become so compact, it had to experience friction. So the fact that dark matter is not compact means it can't experience friction. And for it to not experience friction, it can't interact with itself very strongly.

 

[VooDooX]

this is some new info from a recent article i read

The outer edges of the Milky Way's may be stalked by innumerable invisible galaxies, one of which appears to be crashing into our own. Back in 2005 astronomers discovered the first evidence of mysterious dark galaxies with no starlight -VIRGOHI 21 -a mysterious cloud of hydrogen in the Virgo Cluster 50 million light-years from the Earth found to be colliding with our galaxy - revealed its existence from radio waves from neutral hydrogen coming from a rotating cloud containing enough hydrogen gas to spawn 100 million stars like the sun and fill a small galaxy.

The rotation of VIRGOHI21 is far too fast to be consistent with the gravity of the detected hydrogen. Rather, it implies the presence of a dark matter halo with tens of billions of solar masses. Given the very small number of stars detected, this implies a mass-to-light ratio of about 500, far greater than that of a normal galaxy (which would be around 50). The large gravity of the dark matter halo in this interpretation explains the perturbed nature of the nearby spiral galaxy NGC 4254 and the bridge of neutral hydrogen extending between the two entities.

VIRGOHI21 could prove to be the first discovery of the dark galaxies anticipated by simulations of dark-matter theories. Although other dark-galaxy candidates have previously been observed, follow-up observations indicated that these were either very faint ordinary galaxies or tidal tails.

N2403 Elswhere, Smith's cloud, a high velocity cloud of hydrogen gas located in the constellation Aquila, has a mass of at least one million solar masses. Projecting the cloud's trajectory backwards through time, it is estimated that it had passed through the disk of the Milky Way some 70 million years ago. To have survived this previous encounter, it is thought to be embedded inside a massive dark matter halo. The fact that it survived this previous encounter means that it is likely to be much more massive than previously thought, and may be a candidate for being a dark galaxy.

Many more such dark galaxies may be out there, says Leo Blitz of the University of California, Berkeley. Simulations of galaxy formation suggest a galaxy the size of the Milky Way should feature about 1000 dwarf galaxies, but only a few dozen have been found so far. Some of the missing dwarfs may be dark galaxies that are all but invisible, he says

Blitz's models predict that the universe should contain far more dwarf galaxies than the tiny fraction that astronomers can identify.

If so, Blitz thinks he knows how to find the dark galaxies. "Imagine them plopping through the gas of the outer Milky Way," he says. "They might create some sort of splash or ripple."

These distant reaches are relatively calm, making such disturbances possible to detect. Blitz explains, "It's like throwing darts at a board. As these dark galaxies come at the Milky Way, they're likely going to hit the outer parts because there's more surface area there."

To pinpoint any dark galaxy hot spots, Blitz and his research group are mapping the structure of the Milky Way. In the process, they have been able to characterize the warping of our generally flat galaxy: "It's like hitting cymbals; it's held in the middle and the outer parts are free to vibrate," he says.

Within this structure, Blitz has identified areas of very localized vibrations-an encouraging sign-and is now searching other galaxies for similar characteristics. "That's exactly the kind of signature we look for if the Milky Way were being hit by these dark matter galaxies," he says.

As promising as the mapping looks, Blitz is hedging his bets with a second approach: seeking gassy cores that could be embedded even in dark galaxies. "We're trying to survey regions of the sky to see if there are concentrations of atomic hydrogen that are not associated with known galaxies," he says. "I'm hoping that by making a large enough survey of the sky, we'll be able to find galaxies that contain only hydrogen and no stars. By looking at the motions of the hydrogen, we'll be able to determine the properties of the dark matter that's within it as well."

The resulting map of interstellar hydrogen could help answer another paradox in astronomy: why today's galaxies haven't yet run out of gas. According to observations, most galaxies have just enough fuel left to make stars for another billion years or so. Yet galaxies have endured for most of the age of the universe, making it unlikely that so many should blink out at once.

Blitz thinks they could be topping up their tanks with interstellar gases. As galaxies interact gravitationally, gases from their edges will get torn loose. These gases may eventually fall onto other galaxies, just as water vapor gets recycled back into rain. "There should be enough material between galaxies to be able to make up for the stars that are currently being formed," he says. "That's measurable with the Allen Telescope."

Casey Kazan.

Image top: The 76-m Lovell Telescope at Jodrell Bank Observatory where the dark galaxy was first detected. The graph shows the signal that was picked up by the telescope showing the peak at the 21 cm Hydrogen-Line emitted by the Hydrogen gas in the dark galaxy. (Copyright University of Manchester.)

Highly Recommended:

 

[Wallace]

Just to be clear, when we say 'DM doesn't interact with itself' that doesn't preclude gravitational interaction. Dark matter most certainly does form 'bodies' which we call 'dark matter haloes' due to gravitational interaction. These bodies are very big, very massive and don't have very dense cores, but none the less they are bodies, and they are their existence is what drives the formation of galaxies.

 

[Dmitry67]

I would rather call them 'clouds', not 'bodies' as they don't have any rigidity.

 

[Wallace]

You say potatos I say potartoes. The point is that Skolon in post #10 had taken 'DM doesn't interact' to mean that it is distributed uniformly. In fact dark matter is distributed in a very non-uniform way, and much of it exists in gravitationally bound, roughly spherical objects reffered to as dark matter haloes. Neither 'clouds' or 'bodies' are words used to normally describe them.

By the way, in post #11 you suggest DM is mathematically an ideal gas. In fact an ideal gas in general has pressure, whereas DM has no pressure. You could say that DM is an 'ideal pressureless fluid'; that is the most minimal description you can make.

 

[Dmitry67]

By the way, in post #11 you suggest DM is mathematically an ideal gas. In fact an ideal gas in general has pressure, whereas DM has no pressure. You could say that DM is an 'ideal pressureless fluid'; that is the most minimal description you can make.

Sorry to disagree with you.
But it has pressure.

DM can not interact with normal matter, so that gas can not apply force to any surfaces, so the pressure can not be measured. yet you can calculate it based on the velocity and mass of DM particles. And that pressure creates additional gravity based on GR.

 

[Chalnoth]

Sorry to disagree with you.
But it has pressure.

DM can not interact with normal matter, so that gas can not apply force to any surfaces, so the pressure can not be measured. yet you can calculate it based on the velocity and mass of DM particles. And that pressure creates additional gravity based on GR.

That sort of pressure is only non-negligible if they have relativistic velocities, which they can't have to form the haloes we observe. Dark matter is thus effectively zero-pressure.

 

[Wallace]

Dmitry, there is a technical way when you do calculations in GR that let's you define whether a fluid is pressureless of not. I am using that definition as will any textbook on the subject.

In all GR calculation done in the presence of dark matter it is taken as a pressureless fluid. At a microphysical level this can be related to assuming a dispersion velocity of zero. All calculations to date that include a non-zero dispersion velocity (equivalent to assuming a non-zero pressure) have been found to be inconsistant with observations. Dark matter really does have to be pressureless to fit the data and a non-zero pressure is measureable.

At some level the dispersion velocity is probably not zero, but it is very small.

 

[Dmitry67]

That sort of pressure is only non-negligible if they have relativistic velocities, which they can't have to form the haloes we observe. Dark matter is thus effectively zero-pressure.

Effectively zero pressure = very low
My disagreement was mathematical.
It is very low, but it is not =0. The fact if particles are interacting or not is irrelevant for the calculation of a pressure.

 

[Dmitry67]

Dmitry, there is a technical way when you do calculations in GR that let's you define whether a fluid is pressureless of not. I am using that definition as will any textbook on the subject.

In all GR calculation done in the presence of dark matter it is taken as a pressureless fluid. At a microphysical level this can be related to assuming a dispersion velocity of zero. All calculations to date that include a non-zero dispersion velocity (equivalent to assuming a non-zero pressure) have been found to be inconsistant with observations. Dark matter really does have to be pressureless to fit the data and a non-zero pressure is measureable.

At some level the dispersion velocity is probably not zero, but it is very small.

Ah, I see.
So what you are saying is that the trajectories of the DM particles (if we could track them) are highly correlated. There are 'flows' of DM, and inside the flow the 'temperature' of DM particles is very cold, like that are flying together in parralel with the same speed, correct?

SO it resebles flows of very cold gas or presureless fluid, not a hot gas?

 

[Wallace]

Yes, exactly right. In the past it was hoped that maybe Neutrinos could be dark matter, then we'd just have to explain why there was a lot more of an already known particle in the Universe than we'd previously thought. The problem is that a big bath of Neutrinos behaves like a hot gas, not a cold pressureless fluid and it turns out that these gives different observational predictions. The observations didn't agree with the predictions from Neutrinos (and more generically, any other light relativistic particle).

At a micro-physical level, this means that if DM is some new particle, the particle itself must be quite heavy, hence the acronym WIMP for Weakly Interacting Massive Particle. The M is an important element in that description.

 

[Dmitry67]

Any wild guesses about the mass?
As I understand, velocities of DM particles must be in range of 100km/s (if less, they won't form a halo, locked a the center, if much more, they would escape from galaxy)?

 

[Wallace]

The mass limits that we have depend on the model you assume for the particle. For instance if you google scholar for the mass limits of neutralino dark matter you'll find a number of articles. A lot of these limits actually come from direct detection experiments, rather than astrophysics. I think the astrophysical constraints are not very tight, due to the model dependance.

I'm still not sure what you mean by 'if less they won't form a halo'. In the standard CDM model, we assume that particle have zero dispersion velocity, and they do form halos?

 

[Dmitry67]

I assume that DM particles are locked in the gravitational well of the galaxy. They go from one side to another, crossing the center. Other orbits are also possible.

If velocity of DM particle is low (when it corssescore regions of the galaxy), say, 10km/s, then gravity will stop it and return back to the core long before it reaches the visible outer border of the galaxy

 

[Wallace]

Ah okay, yes there will be bulk motions of dark matter that have those kinds of velocities with respect to the centre of the halo.

 

[Dmitry67]

Hm... after thinking for a while... if at some moment, say, 10By ago, movement of DM particles was correlated, then 10By after it there will be total chaos! Orbits are not stable... minor differences in trajectories 10By ago lead to dramatic difference now.

The flow of particles falling to center will pass without any friction thru the flow of particles going from the center and decelerating.

I don't see, however, how any observation data can tell such chaotic movement from correlated movement if distiribution of Density(distance from center) is the same

 

[Wallace]

Have a look at http://arxiv.org/abs/0906.1730" paper on neutralinos. It goes into this kind of thing in quite a bit of detail.

 

[VooDooX]

Strongest evedience yet of dark matter infact its a dark matter halo

http://www.dailygalaxy.com/my_weblog/2009/12/image-of-the-day-a-dark-matter-halo-.html"

 

[Skolon]

Very interesting discussion.

But it is something from my previous post that you don't discuss yet: "can we say that it (DM) is different by Dark Energy"?
After my post I found some strange theories based on this idea: "Dark fluid" models.

Are these models accepted by "official science"?

 

[Chalnoth]

Very interesting discussion.

But it is something from my previous post that you don't discuss yet: "can we say that it (DM) is different by Dark Energy"?
After my post I found some strange theories based on this idea: "Dark fluid" models.

Are these models accepted by "official science"?

They're considered to be extremely speculative. Nearly all such speculative models turn out to be wrong. But I don't think they're considered ruled out just yet.

 

[Wallace]

Yes, in the LCDM model, DM and DE behave very differently. DM clusters gravitationally, and indeed this clustering is responsible for the structure we see in the Universe, but DM is also pressure-less.

DE on the other hand does not cluster (it is very close to, or exactly homogeneously distributed) and has significantly negative pressure. This negative pressure is what cause the late time acceleration of the expansion of the Universe.

Now, thinking of alternatives to LCDM there are some attempts to construct single field models in which a single energy term has strange properties that allow it to both cluster on small scales but drive acceleration on large scales. These theories are not yet fully developed (at least as I understand it) in terms of having worked out all the details and implications that such a theory would have on the things we can observe.

There is no such thing as 'official science' but these aren't crackpot theories if that's what you mean. More that they are as yet under-developed possibilities that are being investigated. I don't know a lot about this to be honest, most of what I know comes from a conversation I had for several hours with one of the proponents of this idea. It seemed there were still a lot of details to be worked out before definite model predictions could be made in order to test this against data.

Probably a case of watch this space, though my personal view is that there doesn't seem to be a great motivator for this kind of approach, apart from a desire to simplify the theory by only needing one, rather than two, new unseen forms of energy. To me it seems that the single fluid would need to in fact act in a way that's much more complex and fine tuned that the two fluids we currently put in the model. In any case it's difficult to judge this yet in the absence of more details.