Virgo Cluster and Universe Expansion
Am I right in understanding that everything within the Virgo
Supercluster is gravitationally bound and that everything beyond that is
receding? What is going on in the Virgo Supercluster i.e. is everything
orbiting everything else or is there a centre of gravity that everything
Good question, but no. Everything with the supercluster is gravitationally
based like anywhere else in the universe. But the complex does have a center of
gravity. Recession deals with large scale distances and gravitational geometry.
So local variation does not detract from large scale structure. The Virgo
supercluster hap[pens to include us, as well.
David H. Levy
Think of our own solar system. While all moons orbit around their planets, all planets
orbit around the Sun. In this view, the moons are held (by virtue of the shorter distance
and gravity being a weak, although long-ranged, force) by their local planet, but then
the planets are held by the Sun.
However, if we were to plot the motion of the Moon around the Earth as the Earth orbits
the Sun, we find that the Moon is not actually going around in a circle, nor is it going
backwards as it makes its "circle" around the Earth.
Try it. Draw a circle representing the Earth orbit, and then draw how the Moon would
travel *as the Earth moves about the Sun*. You will see that the Moon kind of forms a
sine wave centered around the circle of the Earth's orbit.
From this perspective then, we see that the moons are actually orbiting the Sun, but
that orbit is perturbed by the presence of the planet.
The conclusion then is that all objects in the solar system (including the Sun, which
can be shown to wobble about as the planets move) orbit around a gravitational center,
but that these orbits are affected or perturbed by the presence of smaller gravitational
Since the dynamics of our solar system are not really any different from galaxies
(gravity is the same everywhere), then we should expect that clusters pretty much
act the same way.
For example, our own Milky Way actually belongs to a local group that includes the
Magellanic Cloud, and this local group affect each other. However, the other nearby
galaxies are much too far in comparison, so the gravitational effect of our local
group does not appreciably affect those galaxies and other factors become more important
(such as the expansion of the universe).
Greg (Roberto Gregorius)
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Update: June 2012