[extropy-chat] Maybe Dark Matter isn't missing after all?

[Hal Finney]

[This is also in response to Jonathan Despres's message, "Einstein's
Theory 'Improved'?" which is the same result.]

BillK writes:

> <http://www.physorg.com/news10813.html>   February 13, 2006
>
> Chinese astronomer from the University of St Andrews has fine-tuned
> Einstein's groundbreaking theory of gravity, creating a 'simple'
> theory which could solve a dark mystery that has baffled
> astrophysicists for three-quarters of a century.

I guess 'simple' is in the eye of the beholder.  This is a variation of
the theory called MOND, MOdified Newtonian Dynamics, which you can read
about here, <http://www.astro.umd.edu/~ssm/mond/faq.html>.  As they say,
"MOND can be interpreted as either a modification of gravity through a
change to the Poisson equation, or as a modification of inertia through
a breaking of the equivalence of inertial and gravitational mass."

Basically rather than assuming the existence of otherwise unobserved dark
matter, they assume that gravity is slightly stronger at large distances.
So rather than postulating a lot of extra unseen mass to explain extra
gravitational force, they postulate the extra gravitational force itself.

MOND models can work OK to explain various astronomical phenomena,
but you kind of have to tweak them to have just the right strength at
different distances.  I gather that the new Chinese result is called
'simple' because it has a simpler way of smoothing the MOND effects at
different strengths.  It's still pretty complicated, though.

This theory has not been very widely accepted, dark matter fitting much
more comfortably into the dominant paradigm.  However as the years go by
and there continues to be a failure to detect dark matter directly, or
even to figure out what it might be (there was a report recently of what
the temperature of dark matter would have to be, and it was much hotter
than expected), these alternative theories may begin to gain strength.

Hal