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View Full Version : Dark Matter vs Alternatives



sanman
2009-Mar-11, 07:03 AM
So I've read that Dark Matter (and Dark Energy) is the leading explanation for why galaxies don't fly apart at their observed rotational speeds, and also for observed gravitational lensing effects.

But what about other phenomenon? For example, could some kind of giant-scale frame-dragging possibly account for the integrity of spinning galaxies?

As for observed gravitational lensing effects, are these occasional discrete events, or just a general diffuse lensing effect?

We've all heard of brown dwarfs, which individually aren't massive enough to produce much light. What about even lower-mass objects spread throughout the galaxy, which would not emit light by would collectively still exert significant gravitational pull? These objects would still be made of conventional matter, of course.

What about neutrinos, which I think were recently found to have more mass than was previously thought.

In a nutshell, what makes Dark Matter the leading candidate over other possible explanations? What particularly phenomena does Dark Matter theory cater to the most, for providing explanations?

Kwalish Kid
2009-Mar-11, 12:45 PM
So I've read that Dark Matter (and Dark Energy) is the leading explanation for why galaxies don't fly apart at their observed rotational speeds, and also for observed gravitational lensing effects.

But what about other phenomenon? For example, could some kind of giant-scale frame-dragging possibly account for the integrity of spinning galaxies?
That's a good question, and it's not one that I have read about, but it probably has been worked out. This might be an interesting area of research. However, it's not likely to pay off, as it would not likely account for the other measurements of dark matter.

As for observed gravitational lensing effects, are these occasional discrete events, or just a general diffuse lensing effect?
The observations in question are, usually, the creation of mulitple images of a distant source. The position of the different images created depend on the mass of the galaxy (and the distribution of that mass), so this serves as a measurement of the mass.

Alternative gravitational theories have traditionally had problems with coming up with the right lensing statistics, but there is some promise. I haven't been able to keep up. (That might be post-thesis work for me.)

We've all heard of brown dwarfs, which individually aren't massive enough to produce much light. What about even lower-mass objects spread throughout the galaxy, which would not emit light by would collectively still exert significant gravitational pull? These objects would still be made of conventional matter, of course.
Sure. And we have cosmological measurements of how much stuff like this there can be out there. This is part of the WMAP project of determining cosmological paramters from teh background radiation. Earlier limits on this material were placed by reasoning about element abundances. The general agreement out there seems to be that there is more of this stuff than we're seeing, but far less than could account for dark matter.

What about neutrinos, which I think were recently found to have more mass than was previously thought.
The only problem with neutrinos is that they don't clump together. Mass or not, they're zipping along pretty fast and aren't likely to slow down and settle around a galaxy.

In a nutshell, what makes Dark Matter the leading candidate over other possible explanations? What particularly phenomena does Dark Matter theory cater to the most, for providing explanations?
I would argue agreeing measurements from independent sources:

Galaxy curves
Gravitational lensing
observations of galaxy clustering history and possible theories of that history
observations of the background radiation and the physics of the early universe
type Ia supernova observations of the change in the Hubble relationship over time

All of these place roughly the same limits on two or more of the parameters of normal matter, dark matter, and neutrinos.

Additionally, there are observations that alternative gravity theories cannot account for, like the observations of colliding galaxy clusters. The normal matter behaves somewhat differently than the dark matter in these situations, and we can observe the normal matter being tugged by the dark matter that is in a different place after a collision. Alternative theories of gravity do not account for a different orientation of gravity.

Amber Robot
2009-Mar-11, 02:09 PM
We've all heard of brown dwarfs, which individually aren't massive enough to produce much light. What about even lower-mass objects spread throughout the galaxy, which would not emit light by would collectively still exert significant gravitational pull? These objects would still be made of conventional matter, of course.

I'm pretty sure the MACHO survey ruled these out as being major contributers to dark matter. But it's a good idea and they were indeed looked for.