Universal Expansion and the Distribution of Light

[Irishwake]

Okay to preface my question, we know the universe is expanding. We know that this is not uniform otherwise the distance between the Earth and Alpha Centauri would be increasing at the same rate as the distance between The Milky Way and M87, for example. I'm over-generalizing here for the sake of simplicity.

Is it at all possible then, that due to the acceleration of the expansion of the universe for a galaxy that we see today to be invisible to us in the future?

I'm thinking the answer is no. I think it is no because the reference frame. If we are traveling away from a far off galaxy at greater than c, then rate of expansion between us in our reference frame is greater than c. However in the reference frame of a photon traveling from the distant galaxy to us this would not be the case. The trip wouldn't take nearly as long and thus the photon would actually reach us.

I could be way off track with this but any guidance would be greatly appreciated. This would answer several questions I have regarding this matter.

[Irishwake]

Okay I was going to edit my post but it won't let me. Regardless, I just read on Wikipedia the following:

"Since the parts of the universe cannot be seen after their speed of expansion away from us exceeds the speed of light, the size of the entire universe could be greater than the size of the observable universe."

So to answer my own question, yes.

[Cosmo Novice]

Yes you have the right answer. The Universe in its entirety is larger than the portion we can see. Inflatuion allows for this without breaking any fundamental laws (rest mass travelling at or > c)

[George Jones]

Okay I was going to edit my post but it won't let me. Regardless, I just read on Wikipedia the following:

"Since the parts of the universe cannot be seen after their speed of expansion away from us exceeds the speed of light, the size of the entire universe could be greater than the size of the observable universe."

So to answer my own question, yes.

In general, I like Wikipedia, but, here, Wikipedia is wrong.
As I said above this isn't true. It is true that recession speeds of galaxies that we now see will eventually exceed c, but it is not true that we loose sight of a galaxy once its recession speed exceeds c. If we see a galaxy now, then we will (in principle) always see the galaxy, even when its recession speed exceeds c. It might seem that moving to a recession speed of c represents a transition from subset 1) to subset 2), but this isn't the case.

Suppose we now see galaxy A. Assume that at time t in the future, A's recession speed is greater than c, and that at this time someone in galaxy A fires a laser pulse directly at us. Even though the pulse is fired directly at us, the proper distance between us and the pulse will initially increase. After a while, however, the pulse will "turn around", and the proper distance between us and the pulse will decrease, and the pulse will reach us, i.e., we still see galaxy A.

In more detail:
I know this is very counter-intuitive, but I really did mean what I wrote in posts #52 and #55. :biggrin:


Thanks for pushing me for further explanation, as this has forced me to think more conceptually about what happens.

This can happen because the Hubble constant decreases with time (more on this near the end of this post) in the standard cosmological model for our universe. Consider the following diagram:


O B A C
* * * *


* * * *
O B A C


The bottom row of asterisks represents the positions in space (proper distances) of us (O) and galaxies B, A, and C, all at the same instant of cosmic time, t_e. The top row of asterisks represents the positions in space of us (O) and galaxies B, A, and C, all at some later instant of cosmic time, t. Notice that space has "expanded" between times t_e and t.

Suppose that at time t_e: 1) galaxy A has recession speed (from us) greater than c; 2) galaxy A fires a laser pulse directed at us. Also suppose that at time t, galaxy B receives this laser pulse. In other words, the pulse was emitted from A in the bottom row and received by B in the top row. Because A's recession speed at time t_e is greater than c, the pulse fired towards us has actually moved away from us between times t_e and t.

Now, suppose that the distance from us to galaxy B at time t is the same as the distance to galaxy C at time t_e. Even though the distances are the same, the recession speed of B at time t is less than than the recession speed of C at time t_e because:

1) recession speed equals the Hubble constant multiplied by distance;

2) the value of the Hubble constant decreases between times t_e and t.

Since A's recession speed at time t_e is greater than c, and galaxy C is farther than A, galaxy C's recession speed at time t_e also is greater than c. If, however, the Hubble constant decreases enough between times t_e and t, then B's recession speed at time t can be less than c. If this is the case, then at time t (and spatial position B), the pulse is moving towards us, i.e., the pulse "turned around" at some time between times t_e and t.

If the value of the Hubble constant changes with time, what does the "constant" part of "Hubble constant" mean? It means constant in space. At time t_e, galaxies O, B, A, and C all perceive the same value for the Hubble constant. At time t, galaxies O, B, A, and C all perceive the same value for the Hubble constant. But these two values are different.

Probably some of my explanation is unclear. If so, please ask more questions.

[Irishwake]

Wall of Text

Thank you so much for posting that, I have been trying to wrap my head around this concept for a few days now and that diagram perfectly illustrates why when I point to a star in the sky and say "That star was THERE X number of years ago", I am wrong.

It was in a different position when the light left, but by the time the light gets to us, it appears to have been in a different position.