Thank you J S Princeton, your post actually helped me alot…
And you’re right, I should read more. I was actually only thinking logically, not scientifically.
Yes, this is exactly what I was talking about. Of course, things that are gravitationally bound don’t get farther apart. Nevertheless, the space in between them is still expanding. To use layman’s terms, if two particles were not bound and were “stationary” with respect to each other, they would actually be getting farther apart. Because of inertia, the particles “want” to stay affixed to their point in space even though the space between them is getting bigger. In other words, you must exert a force on them to get them to stay in the same relative position, i.e. intertial frames of reference are actually “accelerating” with respect to everything else in the universe. The farther apart these particles are, the greater the force exerted on them must be.
It would seem to follow that, in effect, the expansion of space effectively weakens the force of gravity. The net acceleration outward caused by the expansion of the universe is counteracted by the force of gravity. The larger the scale, the smaller the net force until, at some scale, the force of gravity, which is weakening with the square of the distance, is overcome by the “force” of the expansion of the universe, which, though small, is increasing linearly with the distance.
The thing is, this ought to have measurable effects at reasonable scales. Has this been measured? Is this effect corrected for? I’ve actually asked a few astronmers about this and was surprised when none of them knew the answer. One opined that GR somehow prevents the expansion of space on a small scale. Unfortunately, neither one of us actually knew enough about GR to offer an informed opinion.
I responded to this earlier, but apparently the board ate it.
Truth Seeker, if you want tto look at Hubble Flow as another “force” (really it’s not, it’s simply an initial condition), then you need to compare its magnitude to that of the other forces we are considering. When you do that, you find that ]i]on the scales where the forces act* the other attractive forces are far stronger than the Hubble Flow expansion. This means that the space expanding between these objects is not enough to expand the objects at all. They just compensate by falling in ever-so-slightly into each other to maintain their size throughout the evolution of the universe. The ONLY scales at which Hubble Flow becomes a concern is at large structure scales (larger than galactic clusters). You can convince yourself of this simply by working the out the math for, say, how much expansion you’d expect over time for a sub-cluster object. You’ll find that the time-scales for destruction of these features is far greater than the timescales needed for virialization (that’s basically a fancy word describing a type of gravititational equilibrium) to occur. In short, the expansion is just too tiny for us to notice when we are as small as we are.
That’s not to say that in the future the effect won’t become more noticeable. As the universe looks right now, we are entering into an exponential acceleration phase. The universe is speeding up its expansion. As this happens, more and more stuff will begin flying faster and faster away from us. Given enough time, we may find our universe expanding so much that all the other galaxies cannot be seen as they’ll be pushed beyond the observational edge. Then the Milky Way will truly be an “Island Universe” as was postulated to be the model of the universe before Hubble made his discovery that many of the nebula were actually galaxies outside our own.
Well, I’d think the effect would be noticeable now, or if not noticeable due to other uncertainties, at least ought to be corrected for. For example, our solar system is moving in orbit around the galactic center at about 220 km/s. I calculate that the space between the galactic center and us is expanding at about 400 m/sec. Pretty insignificant, considering that the galactic center is 8.5 kiloparsecs away. However, it’s not so insignificant if you treat that 400 m/sec as a velocity vector pointing out along the radius of the orbit.
Truth, you cannot treat the Hubble flow as a straight velocity vector since
- It acts in all directions
- there are OTHER FORCES that are acting in between (most significantly, gravity) causing peculiar velocities.
It isn’t true that we don’t have a radial component to our velocity. We happen to live in a Spiral galaxy which shows more of a tangential component, but that’s basically because of the equilibrium we sit in. If we consider the galaxy to be virialized, then we need to consider the 220 km/sec velocity as the representative velocity for our infall (the fact that we’re orbiting tangentially does not matter because if we fall in slightly, our orbit increases in velocity ala water going down a drain). This means the Hubble Flow on the scale of our galaxy is a correction of 2 parts in 1000 for our velocity wrt to the center. The Hubble Flow is a perturbation that pushes us ever slightly outwards and gravity whallops it by pulling us back. This effect, together with the so-called “peculiar velocities” which occur do to the fact that there are random interactions between our area of the universe and other (relatively) dense areas of the universe demands that Hubble Flow be unnoticeable on small scales. For example, the Andromeda Galaxy is blueshifted because we are on a collision course with it. Hubble Flow doesn’t get a chance on these scales. ONLY when you get beyond the local group, therefore, will you see the effect. The Virgo cluster is arguably the closest object that is redshifted from us (that is, where the peculiar velocities are clearly not dominating the picture). This does’t happen until a redshift of 0.0036, or about 1000 km/sec recessional velocity. You can see why this is, since our peculiar velocity in the galaxy is 220 km/sec.