Dark matter as micro - MACHOs?

[MTd2]

I was wondering how to rule out the hypothesis that most of dark matter is due micro - MACHOS, that is, if a considerable part of it is due to objects like those of the Oort Cloud and Kuiper belt.

 

[phyzguy]

Both the WMAP data on the cosmic microwave background and the Big Bang Nucleosynthesis (BBN) data pretty much rule out dark matter being composed of ordinary matter ( i.e, it's not made of protons and neutrons). See:

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

 

[MTd2]

I am sorry, but I cannot find the information on the text. Would you help me?

 

[Vanadium 50]

...and the Bullet Cluster, and the microlensing data.

 

[MTd2]

Well, here it says that 10e-8 solar masses was excluded.

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

But, I think Ceres is lighter than that, right?

 

[MTd2]

I came to this idea from rather simple grounds.

Near the galactic core or on denser globular clusters, Newton law is closely followed, so maybe it is the case that most dark matter is comprised of asteroids of frozen hydrogen. So, if there are a lot of stars together, they just might just get near these stars, melt and evaporate these asteroids and become scattered plasma.

I think the total ionized matter observed in galaxies is more or less the missing dark matter in the galaxy core, isn't it?

 

[Chalnoth]

I was wondering how to rule out the hypothesis that most of dark matter is due micro - MACHOS, that is, if a considerable part of it is due to objects like those of the Oort Cloud and Kuiper belt.

The cosmic microwave background is currently the best evidence against this view. At the time the CMB was emitted, no stars, planets, or any such objects could have formed (the universe was too smooth). So this allows us to test whether or not the dark matter is made up of normal matter that we don't see rather easily, because the normal matter and dark matter behave very, very differently within the plasma that existed before the CMB was emitted.

Basically, before the CMB was emitted, our universe was so hot that the electrons and protons were separated, so that our entire universe was made up of this ionized gas, which we call a plasma. The thing about such an ionized gas is that it interacts very strongly with radiation. Contrast this with a normal gas, such as our atmosphere, that is very transparent.

What this means for normal matter is that when it is in this plasma state, and it starts to fall into a region of space that is slightly more dense than the surroundings, it tends to bounce back out. This bouncing behavior is very visible in the CMB.

Dark matter, however, which doesn't interact electromagnetically, doesn't have this property. When it starts to fall into a potential well, it feels no pressure, and so keeps falling inward. This lack of a bounce distorts the bouncing behavior of the normal matter, which means that the CMB itself provides an extremely accurate measurement of the ratio of normal matter to dark matter: normal matter makes up approximately 17% of the total matter in our universe, with an uncertainty of about 1-2%.

 

[MTd2]

But after plasma epoch until reionization there was quite some time for matter to clump. Why should it clump mostly just into stars? Instead why not small hydrogen balls?

 

[Chalnoth]

But after plasma epoch until reionization there was quite some time for matter to clump. Why should it clump mostly just into stars? Instead why not small hydrogen balls?

Most of it didn't clump into stars. Stars make up only a rather small fraction of the normal matter. Most of the normal matter exists as dust and gas.

 

[MTd2]

What I mean is, in the beginning, in the period I mentioned, what if most matter just became invisible clumps, like snowballs, after the CMB went below 14K, and just a small part became stars, dust and gas?

 

[Chalnoth]

What I mean is, in the beginning, in the period I mentioned, what if most matter just became invisible clumps, like snowballs, after the CMB went below 14K, and just a small part became stars, dust and gas?

Well, hydrogen is too light-weight to form objects much smaller than gas giants. But in any event, we know that most of the matter out there isn't normal matter at all.

 

[MTd2]

I don't get it. If they are in thermal equilibrium with the ambient, they should be invisible anyway, right? And just cause gravitational lensening like dark matter...

 

[Chalnoth]

I don't get it. If they are in thermal equilibrium with the ambient, they should be invisible anyway, right? And just cause gravitational lensening like dark matter...

I mean that self-gravitating hydrogen gas has to be above a certain total mass before its gravity is sufficient to hold it into a ball.

But in any event, as I mentioned, the total amount of dark matter out there is set by CMB observations which are completely independent of this sort of consideration.

 

[MTd2]

What is the pattern in CMB that makes people conclude it is not normal matter?

 

[Chalnoth]

What is the pattern in CMB that makes people conclude it is not normal matter?

I tried to explain it before. At the temperatures the matter was before the CMB was emitted, the normal matter is a plasma. But only about 17% of the matter in that early universe behaved in that fashion, with the rest behaving as if it couldn't interact with radiation at all.

 

[MTd2]

Alright, alright. But how do you know that what you see in the CMB is not matter? But the speed of clumpiness that is greater than expected?

 

[Chalnoth]

Alright, alright. But how do you know that what you see in the CMB is not matter? But the speed of clumpiness that is greater than expected?

Well, it's a bit difficult to explain, but I tried in that previous post. It has to do with the fact that normal matter in a plasma state feels pressure, and tends to bounce, while dark matter does not.

The way we measure this is that we look at how much the temperature varies from place to place on the CMB sky as a function of scale. At about the degree scale, we have matter that is at what is known as the "sound horizon", which is matter which has had enough time to just fall into a potential well since our universe began. Both normal matter and dark matter bunch up here. At about half a degree, you have a distance where normal matter has had enough time to fall into a potential well, then bounce back. The normal matter also bunches up at this scale. But the dark matter does not. So the ratio of the one degree peak to the half-degree peak gives us an estimate of how much normal matter there is compared to dark matter.

 

[phyzguy]

And don't forget that it's not just the CMB measurements. At least three completely different measurements (CMB, BBN, and galaxy densities from SDSS) all agree on the values of dark matter and ordinary matter densities. This is a good paper describing the measurements:

http://arxiv.org/abs/astro-ph/0310723v2

Look carefully at figures 3 and 4, and note how the different measurements all overlap. This is one of the areas where attempts to explain the universe without dark matter fail. They can explain one of these measurements, but not all three.

 

[MTd2]

The way we measure this is that we look at how much the temperature varies from place to place on the CMB sky as a function of scale. At about the degree scale, we have matter that is at what is known as the "sound horizon", which is matter which has had enough time to just fall into a potential well since our universe began. Both normal matter and dark matter bunch up here. At about half a degree, you have a distance where normal matter has had enough time to fall into a potential well, then bounce back. The normal matter also bunches up at this scale. But the dark matter does not. So the ratio of the one degree peak to the half-degree peak gives us an estimate of how much normal matter there is compared to dark matter.

Is there any paper on arxiv that teaches all this?

 

[Chalnoth]

Is there any paper on arxiv that teaches all this?

I'm not sure of any that are accessible. These days this is mostly textbook stuff. This website, however, looks like it offers a pretty good popular description of the whole thing:
http://abyss.uoregon.edu/~js/ast123/lectures/lec23.html

 

[MTd2]

I don't want a popular exposition. I want a technical exposition. A textbook one, as you say.

 

[Chalnoth]

Well, if you really want to get into the physics of the CMB, see here:
http://arxiv.org/abs/astro-ph/0110414

 

[phyzguy]

Well, if you really want to get into the physics of the CMB, see here:
http://arxiv.org/abs/astro-ph/0110414

Chalnoth,

This is a nice paper - thanks for posting it. I especially like Plate 4, where the impact of the various parameters is shown graphically.

 

[Tanelorn]

Why can't we find even one single particle of dark matter on earth? Is it because it would be completely undetectable? Is dark matter made of particles? Could it be made of any combination of any known particles?

 

[Chalnoth]

Why can't we find even one single particle of dark matter on earth? Is it because it would be completely undetectable? Is dark matter made of particles? Could it be made of any combination of any known particles?

Well, dark matter wouldn't be "on" Earth. Rather, the particles travel right through it. It's just that the densities are rather low and so are the interaction cross sections.

But we're definitely trying very hard to find dark matter colliding with normal matter here on Earth. There have been some tentative detections, but as yet there's nothing definitive. Many think that these tentative detections are probably just noise or instrumental effects. We'll see.