From: Allan Wechsler <acwacw@gmail.com> Is the dark matter phenomenon consistent with there being a stack of parallel universes which can gravitationally "feel" each other, but otherwise don't interact?
Note that the universes couldn't be electromagnetically coupled, or we would see the light of parallel stars. Also, electrons would occasionally carom off parallel atoms, and so forth.
--in your scenario, there ought to exist "gravitational lenses" even though there is no visible mass anywhere neat that causes the lensing. Far as I know no such lens has been observed yet. Be way-impressive if you find one! Bring out your computer software, O hackers, and run it on publicly available telescope image databases, to win your nobel prize for discovering parallel universes. Also, dark matter basically is a halo round and about each galaxy of invisible matter, which causes extra gravity which causes the galaxy to rotate faster than it would have if the only stuff gravitating were the visible stuff. And it isn't just a scale factor because the dark matter is distributed in a spherical blob even though the visible matter in that galaxy is distributed like a bar or flat disc or spiral.
From: Eugene Salamin <gene_salamin@yahoo.com> A gram of nucleons is Avagadro's number, 6e23, of particles.? A nucleon has a collision cross section of about 1e-26 cm^2.?
--I find 2*10^(-30) meter^2... ok, about same as you find up to factor 2
So a gram of nucleons has a cross section of 0.006 cm^2.? If dark matter is to be at best weakly interacting, it had better have a cross section way less.
--well, the relevant "cross section" is not for "collision" (whatever that would even mean) it is for "scattering." Two protons interact via EM long range force and will scatter with a cross section far larger than you said if they have low energies compared to their masses. Also, most gas in the universe is "gas" i.e. H atoms, not "plasma" i.e. bare proton and electrons. For an H-atom the radius is the Bohr radius, not the "proton radius" you mentioned. Bohr is about factor 6*10^4 larger, i.e. cross section about 4*10^9 factor larger. But anyhow, I like the calculation you did, if you correct its misleading aspects. The reason they claim dark matter "must" self interact is that there seem to be astronomical observations (they cite) incompatible with the dark matter hypothesis, unless you assume the dark matter particles can scatter off each other with cross section between 0.1 and 1 cm^2 per gram.. This has nothing to do with assumptions about the "weak force" nature of the alleged interaction, it is purely an attempt to agree with observation. (Whether you believe in those attempts and conclusions, is another question... I've got a good deal of skepticism they can compute what they think they can compute, and certainly their so-called deductions far way short of rigorous math standards.) -- Warren D. Smith http://RangeVoting.org <-- add your endorsement (by clicking "endorse" as 1st step)