The supposed need for dark matter is due to the ASSUMPTION/SPECULATION (without observational proof) that Newton’s law of gravity and gravitational constant are also valid outside our solar system. My theory of extended gravity, XTG, is based upon the fact that the equation v*v/r = M*G/(r*r) reduces to M*G=v*v*r and – in the region where the rotation velocities are constant – M*G is a linear function of distance. This led to the false ASSUMPTION/SPECULATION that the linear mass distribution – although not visible – exists as a halo around the visible matter and is dark mattter.

When we take the alternate assution that G has the linear dependence on distance, r, the need for dark matter is removed – and researchers can spend their time and funding on more productive matters.

Details will be on my web site (undergoing major updates). Included will be interesting clarification of dark energy, expanding universe, inflation, CMB, information in black holes, and more. Do a Google search on my name.

Comments will be appreciated.

Further on, Julianne, you invoke a cliché for the distant past: “… back when I was a grad student (you know, when dinosaurs roamed the earth and the iPhone did not yet exist) …”

I don’t know how any self-respecting person in your field neglects to add “… and when Pluto was a planet.”

Just sayin’.

There, I talk about the importance of bookkeeping in Science, which is also a response to Sean’s cheeky comment:
“If you know that something exists, what’s the point in thinking about it?”
As I mention there, and contrary to what you say, baryonic accounting doesn’t seem to exactly work out, even in galaxy clusters. The discrepancy is not quite as big as what you are working on, but it is (in my opinion) statistically significant.

If it gets hot enough, like in clusters of galaxies, then you can detected it in the X-ray with current instruments. But, there’s a decade or so of astrophysically interesting temperatures where it doesn’t emit enough x-rays to be readily detected. You can get it in absorption, but that’s tougher to interpret except in a broad statistical sense.

Thanks! Also, is the universe invisible below certain radio frequencies? I mean, the ionized gas must have some plasma frequency below which radio-waves do not propagate….

On a related topic, I was wondering(/hoping) if you could take a few minutes to talk about the (possible) connections between galaxy formation and super-massive black holes.

It’s been a while since I knew anything about the subject, but I recall a time when people were surprised and annoyed to learn that, in simulations, relaxing dark matter clouds tended to develop singularities of infinite density at their cores. One can try to constrain halo models to be non-singluar (“softened isothermal sphere”); but is it entirely a coincidence that when you look into the cores of real galaxies you find … singularities of infinite density? ie super-massive black holes (SMBH).

In your understanding — orders of magnitude better than mine, I trust — what are the best ideas for how SMBH’s form in galaxies? Can they grow by accreting appreciable amounts of dark matter? Should we expect SMBH’s to exist in the cores of galactic cluster halos? or just in galaxies? (this may be strongly connected to the baryon question.)

Also, here’s my favorite question, if you’re up for it: within whatever mechanism you might favor, why would we expect a given galaxy to have just _one_ SMBH and not several?

Thanks for whatever wisdom you can share, plus references.

Regards,

Paul

Why wouldn’t we able to see such large quantities of hot gas?

If it gets hot enough, like in clusters of galaxies, then you can detected it in the X-ray with current instruments. But, there’s a decade or so of astrophysically interesting temperatures where it doesn’t emit enough x-rays to be readily detected. You can get it in absorption, but that’s tougher to interpret except in a broad statistical sense.

From your description, you have most of the story correct. Both baryons and dark matter do tend to concentrate in the same places, but because baryons can lose energy (usually by emitting photons), while dark matter can’t (since it’s dark), the baryons can concentrate even further within the dark matter halo. Thus, we have a solar system that is very dense in baryons, but find little dark matter within the radius of the outer planets.

Well, I’m amused by the fact that there’s currently an ad for “Montana Dude Ranches” over in the sidebar, evidently because Julianne used “Dude” in the title…

Theorist have only been rewarded for accounting for nontrivial and far reaching experimental facts and for correct nontrivial predictions of far reaching consequence.

So you are in very good company. Maybe you’ll even run into a Nobel prize

If so, perhaps a decent paper for Sean or Mark, etc.

However, even if so, probably not enough photons for that anyway with present-day telescopes.

If you know that something exists, what’s the point in thinking about it?

Because it may not exist in the way you think it does.