
Acceleration found in galaxies challenges current understanding of dark matter - sampo
http://phys.org/news/2016-09-spiral-irregular-galaxies-current-dark.html
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jotato
As someone who can appreciate physics, but doesn't really know much: What are
the implications of this?

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sampo
Galaxies don't seem to obey gravity in their rotation.

Take images of a galaxy, estimate the number and mass of the stars based on
their brightness. Make a computer simulation using this and the known laws of
gravity. In the simulation the galaxy will rotate way differently than in the
observations. This happens with every galaxy, so we just don't know why
galaxies rotate the way they do.

There are two major theories:

1\. Maybe our estimation of the mass distribution in a galaxy is wrong. We
only estimate the mass based on the light of the stars we can see. So people
have invented dark matter. We can add unseen matter in a certain distribution
into the galaxy, until we get it to rotate, under our known laws of gravity,
the way we observe it rotating.

[https://en.wikipedia.org/wiki/Dark_matter](https://en.wikipedia.org/wiki/Dark_matter)

2\. Maybe our laws of gravity are wrong. Maybe the gravitational interaction
works differently over very long distances than it works inside our solar
system. After all, we have directly measured gravity only over distances
inside our solar system.

[https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics](https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics)

Generally physicists feel that the mathematical form of the gravitational
interaction is too precious to meddle with. It is easier to postulate unseen,
unknown, extra mass to all galaxies, than to alter the equations of gravity.
So dark matter is the more generally accepted theory.

This study is purely observational. The authors observe that the deviation in
the rotation of visible mass, compared to out laws of gravity, follows a nice
curve with almost no scatter. This gives slightly more support to the theories
of modified gravitation laws, because the deviations seem to follow a smooth
pattern. But if we postulate some mechanism which distributes the dark matter
just right, to follow the observations, also the dark matter theories would
conform to these observations. So this is a small step in one of the big
unsoveld problem in physics.

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jotato
Thanks!

