Just How Sure Are You?

The last 10 or 15 years or so have seen major revisions in the way astronomers have come to understand our universe. One of the biggest surprises (from observations that started to come out in the late ’60s), was that there was a large amount of matter out there, both in the galaxies we see and in clusters of galaxies, that we weren’t seeing – so called dark matter.

Astronomers knew that the matter was there – it had to be, ’cause if it wasn’t, then neither galaxy clusters nor the galaxies themselves could hold themselves together.

But advances in orbiting satellites and in X-Ray astronomy brought about discoveries like this (the bullet cluster), where an unusual event has cleanly separated “normal” matter from the dark.

Well that nails it down, doesn’t it?

Maybe not.

Last August, an astronomer at the University of Arizona at Tucson and his colleagues reported that a collision between two huge clusters of galaxies 3 billion light-years away, known as the Bullet Cluster, had caused clouds of dark matter to separate from normal matter. Many scientists said the observations were proof of dark matter’s existence and a serious blow for alternative explanations aiming to do away with dark matter with modified theories of gravity.
Now John Moffat, an astronomer at the University of Waterloo in Canada, and Joel Brownstein, his graduate student, say those announcements were premature.
In a study detailed in the Nov. 21 issue of the Monthly Notices of the Royal Astronomical Society, the pair says their Modified Gravity (MOG) theory can explain the Bullet Cluster observation. MOG differs from other modified gravity theories in its details, but is similar in that it predict that the force of gravity changes with distance.

Sigh. Just when you thought astronomers actually knew something…

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