But what about the constant rotational velocities in galaxies regardless of the distance from galactic center?

Question

"However, all energy and matter creates gravity, so you’d expect that all that extra stuff would affect how gravity works. Specifically, you’d expect the velocity of orbiting objects to all be about the same, regardless of the size of the orbit (still: not obvious). But, to the best of our ability to measure (which is pretty good), no effect has been seen at all in terms of the movement of stars and planets and whatnot." [http://www.askamathematician.com/2011/06/q-what-ex...

Seems to me there is an example of stars movement that is about the same over most of the radius of spiraling galaxies. And that has not been explained as far as I know. So is this not an example of vacuum energy affecting gravity?

Seems to me there is an example of stars movement that is about the same over most of the radius of spiraling galaxies.  And that has not been explained as far as I know.  So is this not an example of vacuum energy affecting gravity?

Anonymous · Accepted Answer

Vacuum energy is as good an explanation as any for what Dark Matter is. The current consensus hypothesis for what Dark Energy is, is centring around Vacuum Energy. So it wouldn't surprise me that Dark Matter might originate from the same source too! Some theories already suggest a common origin for both phenomenon.

So why would virtual particles produce extra gravity, if they just flit in and out of existence so quickly that we can't even measure it? Well, likely at galactic distance scales, billions and billions of virtual particle appearances might create very brief gravitational effects that disappear again. So within small scales, such as the planetary or solar system scales, these things don't last long enough to be a worry. But at galactic+ scales, these things like produce gravitational waves that are traversing at the speed of light and take a long time to disperse. It takes light a 100,000 years to traverse the galaxy, so it should take the same amount of time for a virtual particle's gravitational wave. Obviously each virtual particle will produce negligible gravity, but billions upon billions may produce an accumulative effect, and the effect will last essentially forever, as virtual particles are always appearing and disappearing everywhere.

Brigalow Bloke · Answer

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? · Answer

Little confused about your question, Oldprof. We do not see much gravitational affect from linear and angular momentum since the energy involved in that is comparatively very small compared to the energy density of mass. Most solutions of General Relativity ignore momentum, as well as stress. An exception is the Kerr metric that is basically a Schwarszchild black hole with large angular momentum. With that most of the effects are frame dragging.

Most cosmological solutions ignore momentum and focus on energy and pressure densities.

As you well know, General Relativity is virtually impossible to solve analytically for anything difficult and we need to use numerical approximations. You might be able to find some numerical simulations on the web that include contributions from momentum and vacuum/dark energy.

As for vacuum energy, i would expect that it is too small to be relevant at the galactic level. Cosmological solutions that model this well are extreme idealizations also and consider the universe to be homogenous and isotropic with uniform pressure and energy densities.

If this isn't adressing your question, let me know.

But what about the constant rotational velocities in galaxies regardless of the distance from galactic center?

3 Answers