A Galaxy Beyond Gravity and the Hubble Flow

[wolram]

What would happen if the area a galaxy occupied spanned the influence of gravity and the Hubble flow?

 

[jonmtkisco]

Hi Wolram,

I think you need to refine your question further. Normally, a galaxy is considered to "occupy" the volume of its dark matter halo. Therefore by definition, the galaxy itself spans only the area where the force of its gravity dominates over the general Hubble expansion.

The Chernin and Tully papers I talked about in my other threads on the Cosmology forum (e.g., "Hubble expansion within a collapsing supercluster") describe the concept of a "zero-gravity surface", which is simply the spherical boundary surrounding a mass concentration (e.g., galaxy) at which the competing forces of gravity and Hubble expansion are exactly equal and offsetting. Of course the ZGS by definition must be well outside of the galaxy itself. Take the example of our Local Group, which includes the Milky Way and Andromeda galaxies which are .7 Mpc apart. The ZGS of the Local Group is thought be be about 1.1 Mpc in radius. That shows the orders of magnitude.

Wiltshire's papers define another boundary surface called "finite infinity", which is by definition larger than the ZGS. The FI is the (generally spherical) region in which the AVERAGE expansion rate inside is equal to zero. I attached a picture of it to one of my posts.

In my opinion, the best way to think of this subject is that space is intrinsically experiencing an underlying expansion force EVERYWHERE at the cosmic Hubble rate. However, in regions of matter overdensity, the local gravitational force is stronger than the Hubble expansion force and dominates it. In effect, the Hubble expansion force is mathematically subtracted from the gravitational force. This dynamic tension between two competing forces (as well as a third anti-collapse force, virial peculiar motion) can resulting in any particular region undergoing collapse, stability, or expansion, at any rate between the two extremes (black holes; nearly empty voids).

Let me know if I misunderstood your question.

Jon

 

[jonmtkisco]

Hi Wolram,
In my opinion, the best way to think of this subject is that space is intrinsically experiencing an underlying expansion force EVERYWHERE at the cosmic Hubble rate. However, in regions of matter overdensity, the local gravitational force is stronger than the Hubble expansion force and dominates it. In effect, the Hubble expansion force is mathematically subtracted from the gravitational force.
Jon

This reference to the "cosmic Hubble rate" in my last post isn't quite accurate. The cosmic Hubble rate is the AVERAGE expansion rate, so it would be measured as such only in regions which are exactly at average matter density. The expansion rate obviously would be faster than this average inside large voids. Chernin calculates that a theoretically empty void (no void is every fully empty) currently would experience an additional 16 km/s per Mpc of expansion, compared to the cosmic Hubble rate.

Jon

 

[wolram]

Give me some slack guys, i am trying very hard to understand these competing forces.

 

[jonmtkisco]

Hi Wolram,

Did I misinterpret your question? I tried to give you a meaningful answer.

Jon

 

[wolram]

Thanks jonmtkisco , i miss understood, the penny has droped now.

Thanks,

 

[jonmtkisco]

Hi Wolram,

By the way, if the Hubble expansion rate is fast enough at any given point in time, it will cause a bound galaxy to start expanding, at a rate slower than the surrounding Hubble rate. This is what happened in the early universe when the original expansion rate was extremely high. Protogalaxies initially expanded, reached a maximum size, began collapsing, and then pretty much stopped collapsing when their virial (e.g., rotational) motion virialized them.

The same kind of galactic expansion could restart in the far, far distant future if dark energy continues to accelerate the expansion rate. Assuming that the galaxy hasn't collapsed into a black hole by that time.

Jon