# Size of the visible/observable universe

1. Jun 24, 2015

### DiracPool

I understand the concept of galaxies moving away from us at faster than the speed of light so that they lie outside of a "cosmic horizon" which we cannot see past. This would limit our observations to a "visible" universe which may be far smaller than what the "actual" universe may be. Alan Guth guesses that actual universe may of 25 orders of magnitude larger than the visible universe.

While in principle this makes sense, what does not make sense to me is how there are galaxies that we cannot see because they are outside of some horizon, but for some reason we can see the light from the decoupling/surface of last scattering event. It seems to me that this surface of last scattering would have been receding away from us for far longer than the formation of galaxies which would have formed far after that event. Why doesn't the surface of last scattering lie outside the event horizon?

What am I missing here?

2. Jun 24, 2015

### Chronos

The CMB photons we observe are from the distant past, a time when their source was still within our cosmic horizon. We will never observe light emitted today by sufficiently remote bodies in the universe, we can, however, view photons emitted before they crossed our cosmic horizon. In fact, most currently visible galaxies have left our cosmic horizon since emitting the light we now detect. This is not to say they will someday abruptly disappear, they will merely redshift beyond detectability.

3. Jun 24, 2015

### DiracPool

Ok, that's along the lines of what I was thinking. So does this mean that we pretty much CAN see the entire universe today, only what we see is a "shrunken" version of what exists today as seen, say, equal to or less than 13 billion years ago? All this cosmic horizon stuff means is that some of the more distant galaxies we see today we are no longer getting any recent light from?

Even if that were so, though, I still don't understand where Alan Guth's figure of 25 orders of magnitude come from. Did the surface of last scattering really receed that far since 13 bya? Or am I missing something else?

4. Jun 24, 2015

### DiracPool

Even so, could we in principle detect these galaxies with microwave and radio telescopes? And if so reconnect them to what we knew of their visible versions we knew previously?

5. Jun 24, 2015

### Chronos

Yes, you got the basic idea. The scale factor for CMB photons is about 1100. 25 orders of magnitude is way bigger than that. I dont know the context within which Guth asserted 25 orders of magnitude, but, this wasnt it. A body that crosses our cosmic horizon also crosses our causal horizon. Meaning it loses the ability to detectably interact with us via any means, be it gravity waves, microwaves or anything else.

Last edited: Jun 24, 2015
6. Jun 25, 2015

### Chalnoth

The assertion was probably related to the amount of time inflation lasts. In order to solve the flatness problem, inflation needs to have expanded the universe by at least a factor of $e^{70}$, which is about 30 orders of magnitude. If the energy scale of inflation was on the low side (this would require that the initial inflating patch be larger than if the scale of inflation were at a higher energy) or if inflation lasted a little bit longer than it needed to (say, 2x), then this would cause the entire region that spawned from this initial inflating region to be 25 orders of magnitude larger in each direction than the observable universe.

7. Jun 29, 2015

### RUTA

8. Jun 29, 2015

### DiracPool

9. Jun 30, 2015

### Jorrie

Another way is to look at the Hubble radius (blue curve) versus our past lightcone (the red curve), graphically shown below.

The gold curve is the proper recession rate and one can easily see that where our past lightcone enters the Hubble radius, the recessing rate against scale factor drops through 1, the speed of light. It happens at a ~ 0.38. The units zeit and lzeit are normalized through dividing by the (constant) long term Hubble time, 17.3 Gly.