The discussion revolves around the hypothesis that dark matter may be composed of micro-MACHOs, such as objects similar to those found in the Oort Cloud and Kuiper Belt. Participants explore various aspects of this hypothesis, including evidence from cosmic microwave background (CMB) data, Big Bang Nucleosynthesis (BBN), and microlensing data, as well as the implications of these observations on the nature of dark matter.
The discussion features multiple competing views regarding the composition of dark matter, with no consensus reached on the role of micro-MACHOs versus other forms of dark matter. Participants express differing interpretations of the evidence from CMB and other measurements.
Participants note limitations in understanding the formation of matter clumps and the dependence on definitions of dark matter. There are unresolved questions about the implications of CMB observations and the nature of matter in the early universe.
This discussion may be of interest to those exploring theories of dark matter, the implications of cosmic microwave background data, and the nature of matter in the early universe.
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.MTd2 said: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.
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 said: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?
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 said: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?
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.MTd2 said: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 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 said:What is the pattern in CMB that makes people conclude it is not normal matter?
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.MTd2 said: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 said: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.
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:MTd2 said:Is there any paper on arxiv that teaches all this?
Chalnoth said:Well, if you really want to get into the physics of the CMB, see here:
http://arxiv.org/abs/astro-ph/0110414
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.Tanelorn said: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?