One of the greatest mysteries of astronomy is the problem of
the missing mass: All of the matter scientists can see in the universe accounts
for only a small percent of the observed gravity.
Astronomers often invoke the concept
of dark matter to explain this discrepancy, but some researchers say the
problem is really our understanding of gravity. These scientists tout an idea
called MOND - Modified Theory of Newtonian Dynamics - to explain why the
universe seems to behave as if there's much more matter in it than we think.
Instead of assuming that this missing mass exists in the
form of dark matter, which scientists have yet to detect directly, MOND
advocates say we must alter Einstein's theory of General Relativity.
Under MOND, mass is much more effective at bending
space-time than under General Relativity, so it takes less stuff in the
universe to account for all the gravity we measure.
Fudge factor still needed
Though no one has yet proven or disproven either dark
matter or MOND, supporters of the latter are in the minority. And MOND may
be becoming even more of a long shot, according to cosmologist Pedro Ferreira
of Oxford University in England. Ferreira wrote a review article in the Nov. 6
issue of the journal Science assessing the current state of MOND ideas.
"My personal view at the moment is that dark matter is
a far simpler theory than any of the modified theories that I've seen,"
Ferreira said. Nonetheless, he said MOND shouldn't be discounted out of hand
just because it's the less popular idea, nor because many physicists are loathe
to tamper with Einstein's General
Relativity.
"Very few people have worked on MOND; a very large number
of people have worked on dark matter," said Jacob Bekenstein, a physicist
at Hebrew University in Jerusalem who has researched MOND. "To compare
them is kind of silly because we don't really know enough of whether MOND is
working well or not. Just too little effort has been going into MOND."
Bekenstein admitted that MOND was not yet a fully fleshed-out
theory: It cannot make physical predictions on all scales of the universe.
When applied to just galaxies, MOND can predict very well
the behavior that astronomers observe. But when MOND is applied to larger
structures like clusters
of galaxies, it fails. To make MOND work for clusters, it must include more
complicated concepts, such as entities called dark fields, which are different
from dark matter, but work in a similar way to alter the amount of gravity
present.
"It seems like if you want to build a proper theory of
MOND, you bring in something like dark mater through the back door,"
Ferreira said.
This fudge factor seems to defeat one of the primary
purposes of MOND when it was first proposed, which was to avoid having to
invent a mysterious unseen entity acting in the universe, such as dark matter.
Even Bekenstein admitted that involving dark fields in MOND
is not ideal.
"If you work only on galaxies then MOND doesn't need
any help," he told SPACE.com. "But if you go up to clusters it needs
some help. This is one of the things I hold against MOND."
However ...
Bekenstein pointed out that dark matter isn't perfect
either. Thirty years after it was proposed, scientists have yet
to find the stuff out there in the universe, and the idea isn't yet ideal at
predicting all manner of situations, either.
"In the models of galaxies with dark matter, you have
to carefully adjust the distribution of dark matter," he said. "Since
you don't see the dark matter you're kind of free to adjust what you want, but
it's not very credible in my opinion. It's too free an idea."
Ferreira said some kind of answer may come soon with the
advent of new satellites set to observe the distribution of mass in the
universe more precisely.
"I think things are going to really heat up over the next
10 years," he said.
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