Dark matter

[CaptainPanic]

I'm always a bit uncomfortable with the explanations offered regarding dark matter (and even more with dark energy, but let's save that for another thread).

 

I know that the popular theory is that dark matter consists of particles that have little or no interactions with other particles, and that these particles are everywhere. This theory is for example explained in

(warning: it's >1 hr!). In fact, it's probably explained much better than I just did .

 

But... what I wanted to discuss: is there any proof that dark matter is not just normal matter, made of atoms?

If dark matter is just normal matter (solid particles of varying size), that is relatively uniformly distributed, then:

1. We would not see it

2. It would block light from stars, which mean we also might get a wrong estimate for the mass of all stars and other objects that give off radiation. (Note that you cannot detect many small objects if they pass in front of a star - there would be no fluctuation in light like when a planet passes in front of it, and no wobbling of the position because particles/dust might be millions of light years closer to us). It would be more like absorption of light.

 

I think that no dust would survive in the vicinity of any large objects (stars, planets) in space, because of gravity, which explains perhaps why our solar system is relatively clean of particles. But perhaps it might survive far away from stars? To double the mass of all stars, the density can still be incredibly low: there is really a lot of "space".

 

I think that this idea is not new (it's quite an obvious "first thought" regarding dark matter), so perhaps somebody can punch a few holes in this idea for me, please?

[insane_alien]

Well, if it was dust the we would be able to see it as some light would be refelcted and then there is the absorbtion spectrum it would show.

 

and not forgetting we mapped a field of dark matter which we could see through perfectly well (appart from the gravitational distortions which allowed us to map it in the first place.) if it was dust then the light from the stars we used would be dim andhave an absorbtion spectrum and the space wouldn't appear like empty space.

[GutZ]

Is the only evidence to dark matter through gravitational effects though. There must be something else i am missing....would it theortically have a structure of an atom without protons and electtrons and still have weak/strong N forces binding it together? It would have no charge either right....

[insane_alien]

all we know is that i has mass and interacts via gravity and possible the strong and/or weak force althoug hthis is unlikely. anything else is currently speculation.

[swansont]

Baryonic dark matter is limited by nucleosynthesis results — the amount of the various elements in the early universe wouldn't match up if there were significant amount of it.

[Airbrush]

...is there any proof that dark matter is not just normal matter, made of atoms?

 

I think that no dust would survive in the vicinity of any large objects (stars, planets) in space, because of gravity, which explains perhaps why our solar system is relatively clean of particles. But perhaps it might survive far away from stars? To double the mass of all stars, the density can still be incredibly low: there is really a lot of "space".

 

Considering the unimaginably vast volume of space, even small densities of nonbaryonic matter will add up to large masses over cosmic scales. Nonbaryonic matter, such as neutrinos, may be denser far away from normal matter. Even though galaxies are flattened shaped, the dark matter exists in a spherical shape centered on the galaxy center.

 

One example of nonbaryonic matter are neutrinos, which move close to light speed, and more than 50 trillion solar electron neutrinos pass through the human body every second. No wonder that kind of stuff is hard to detect. There is far more gas and dust in the universe than all stars, black holes, planets, asteroids, comets, etc combined. The finer the material the more there is of it.

 

http://en.wikipedia.org/wiki/Neutrino

 

They think dark matter is mostly nonbaryonic (not normal) matter because of this:

 

"The total amount of baryonic dark matter can be calculated from big bang nucleosynthesis, and observations of the cosmic microwave background. Both indicate that the amount of baryonic dark matter is much smaller than the total amount of dark matter.

 

"In the case of big bang nucleosynthesis, the problem is that large amounts of ordinary matter means a denser early universe, more efficient conversion of matter to helium-4 and less unburned deuterium that can remain. If one assumes that all of the dark matter in the universe consists of baryons, then there is far too much deuterium in the universe. This could be resolved if there were some means of generating deuterium, but large efforts in the 1970s failed to come up with plausible mechanisms for this to occur. For instance, MACHOs, which include, for example, brown dwarfs (balls of hydrogen and helium with masses ), never begin nuclear fusion of hydrogen [1] but they do burn deuterium. Other possibilities that were examined include "Jupiters", which are similar to brown dwarfs but have smaller masses and do not burn anything, and white dwarfs. Actually, objects with masses around or below the hydrogen-burning limit could be baryonic dark matter."

 

http://en.wikipedia.org/wiki/Baryonic_dark_matter

Edited April 8, 2009 by Airbrush

[CaptainPanic]

I conclude from the above posts that models (which I unfortunately don't know and/or understand) can only explain the observations of the universe if the baryonic matter is roughly what's found in stars.

 

But what I do not understand is:

Both dust/rocks or even planets interact mostly through gravity. Non-baryonic matter (dark matter) would also interact through gravity... so what is the major difference between the two?

 

On which scale do the models require dark matter to hold? If you take the laws of gravity, and calculate the earth's orbit around the sun, do we also require dark matter to make the model fit the observations? Or do we only need dark matter on an intergalactic scale?

[Airbrush]

There are 2 kinds of dark matter, baryonic and nonbaryonic. Baryonic matter is normal matter made up of protons, neutrons, and electrons, but nonbaryonic matter is not, but rather strange particles like neutrinos.

 

Baryonic dark matter is normal matter that we cannot see which can include all kinds of small objects like dust, rocks, even black holes.

 

Dark matter, which they say is mostly nonbaryonic, is noticed only on galactic scales. We would not notice its' effects within our solar system. It seems to cluster all around each galaxy in a spherical shape, not the flattened shape that most visible matter in galaxies seem to take. I don't think you will find much of it between galaxies. That is about all I know about dark matter. Maybe someone can help us out. Martin?

[Klaynos]

Baryonic dark matter is normal matter that we cannot see which can include all kinds of small objects like dust, rocks, even black holes.

 

All of these things you list we can see. We cannot resolve them, but we can see their effects as absorption of light from stars. Dark matter does not absorb any light as it does not interact electromagnetically (only interacts very weakly)...

---

Merged post follows:

Consecutive posts merged

(To note black-holes are visible due to accretion disks)

 

Galaxies (and other more free objects such as planet-forming-disks, accretion disks around black holes) are disk shaped not spheres because of friction, things tend to form a disk as the up and down motion is removed, this takes a LONG time for galaxies hence why they are often closer to spherical than others. This friction is mostly due to EM interactions which dark matters feels only weekly so the friction is enormously reduced, leading to more spherical shapes.

[Airbrush]

Baryonic dark matter CAN be seen, but because it is difficult to see it is called dark matter. In general, I believe, dark matter is any matter that cannot currently be detected other than gravitational effects, but it is possible to detect it. The term "dark matter" generally means nonbaryonic dark matter which is far more elusive, like neutrinos.

 

Some black holes should be detectable because of accretion disks, but probably most have not yet been detected, and therefore the ones not yet detected fall into the category of dark matter. So much for baryonic dark matter. Most dark matter is nonbaryonic.

 

I like your explanation for why galaxies over time will take a flattened shape. Very nice.

[Martin]

Dark matter, which they say is mostly nonbaryonic, is noticed only on galactic scales. We would not notice its' effects within our solar system. It seems to cluster all around each galaxy in a spherical shape, not the flattened shape that most visible matter in galaxies seem to take. I don't think you will find much of it between galaxies. That is about all I know about dark matter. Maybe someone can help us out. Martin?

 

Airbrush, these are good questions! I didn't see your post until just now (was busy with other things most of the day) and I see Klaynos has already given a clear explanation. So I'll just add a comment.

 

You do find concentrations of DM in and around clusters of galaxies. Your basic intuition is right. DM comes in large clouds which tend to coincide with concentrations of ordinary matter----but those don't have to be individual galaxies, they could also be clusters of galaxies and such-like large-scale structures.

 

And there is a chicken-egg problem. Which came first? You might guess that the baryonic concentrations came first and gathered DM in around them. What I've been hearing about, though, are models where the DM (by the action of its own gravity) slowly curdles into large blobs and filamentary structures and that the DM concentrations help the ordinary matter to cluster and clump etc.

 

George Smoot gave a neat slide presentation about this to the TED club, in which he played computer animation movies of simulations of structure formation in which the DM was actually playing the driving role and serving as a kind of armature or framework to bring the baryonic structures into being.

Somebody at Chicago did the computer simulations.

 

BTW expansion actually slows things down, drains kinetic energy (not just photon energy by redshift, but massive particle energy). That sounds bizarre but Steven Weinberg has a proof in his Cosmology textbook and other experts seem to accept this. It seems to violate conservation. But it makes it possible for DM to gather in clouds. Analogous to friction drag, but not friction. Weinberg is Nobel---I'm not going to argue with him :-D It is factored into the sims.

Somehow by hook/crook, a self-gravitating DM cloud can eject excess energy and thereby slowly collect and contract.

 

Smoot is another Laureate. Here's his TED presentation

Only 19 minutes, check it out. The structure formation sims are of pure DM.

Edited April 10, 2009 by Martin

[CaptainPanic]

You do find concentrations of DM in and around clusters of galaxies. Your basic intuition is right. DM comes in large clouds which tend to coincide with concentrations of ordinary matter----but those don't have to be individual galaxies, they could also be clusters of galaxies and such-like large-scale structures.

 

So, if Dark Matter is in the same position as ordinary (baryonic) matter... couldn't it be possible that we simply got the calculated mass of stars and galaxies wrong?

 

I believe that the mass of stars is calculated from the luminosity, and distance. (If there is another method, then I'd like to hear it, please). So, if the star is actually more bright than we see, simply because some space dust absorbs and diffuses the rest of the light, then we'll get its mass wrong. And for that you don't need massive amounts of space dust... With very little knowledge of astronomy I dare to predict that there are a few grains of sand between us and a star a million lightyears away... and that therefore it's guaranteed that at least some light is absorbed.

 

Space is pretty empty, but I wonder what the mean free path is for a photon in space. Obviously, we only see those photons which have not collided with anything at all.

[Martin]

I believe that the mass of stars is calculated from the luminosity, and distance. (If there is another method, then I'd like to hear it, please).

 

Several ways to infer mass, some have nothing to do with luminosity.

 

One of the most basic is to use Kepler law on a binary pair of stars.

I assume you know the measure of separation called the semimajor axis.

 

(separation in A.U.)³/(period in years)² = mass in solar masses

 

There are half a dozen different ways to determine the distance to a star or to a group of stars, and there are several ways to determine the masses of stars. The point is that they are consistent. Astronomers have been meticulously calibrating and checking their measures of basic quantities for something like 100 years. New ways of measuring appear in the literature from time to time.

[Airbrush]

Great info above thank you all. That was a fascinating lecture by George Smoot. Sometimes his enthusiasm for his subject brought me to tears. Hahaha.

 

I love the subject of dark matter. Strictly speaking we are baryonic dark matter to someone in another galaxy who can only detect our Sun, but never caught Earth transiting our sun. But the interesting dark matter is nonbaryonic which they believe to be the vast majority of DM. Anyone know what percentage of DM is nonbaryonic dark matter(NBDM)?

 

The universe is like our Earth in that the greatest proportion of animal or plant bio-mass in our oceans is also the smallest, plankton. Among animals isn't it something like the termite or ant, or even a microbe?) that makes up the greatest bio-mass of animals?

 

DM is so rarified. What kind of numbers per cubic meter to account for its' gravitational effects? I believe they estimated the average density of the universe as one atom per cubic meter. Does that include all matter in the universe, including stars, planets, and black holes? Does that include DM? Or is that the average density of interstellar space? Or are they about the same?

---

Merged post follows:

Consecutive posts merged

If DM particles were about the size of neutrinos then how many particles of DM would it take per cubic centimeter? That is at DM average density in and around a galaxy to account for its' gravitational effect.

 

Now I recall an Astronomy magazine discussed the density of atoms in outer space. It ranged from rather dense, many atoms per cubic meter in the region between Earth and the Moon. But in the middle of the great voids between superclusters the density was very low, about one atom per cubic meter. Then from another source I believe they estimated the average density of the universe at about one atom per cubic meter. What is it really?

Edited April 10, 2009 by Airbrush