Cosmic Event Horizon – Does it have any real effects?

[JimJCW]

Using LightCone8 Cosmological Calculator and PLANCK(2018+BAO) data as input, we can get the following result:

In the figure, Dhor is the event horizon and Dpar is the radius of the observable universe. Currently (t = 13.79 Gyr) Dhor has a value of 16.58 Gly. Does the event horizon have any real effects? How should we view the following writings:

According to the article Event horizon:

In cosmology, the event horizon of the observable universe is the largest comoving distance from which light emitted now can ever reach the observer in the future. This differs from the concept of the particle horizon, which represents the largest comoving distance from which light emitted in the past could reach the observer at a given time.​

Davis and Lineweaver give an example in their paper (see Section II) about galaxies with redshift z ∼ 1.8:

. . . galaxies with redshift z ∼ 1.8 are currently crossing our event horizon. These are the most distant objects from which we will ever be able to receive information about the present day. The particle horizon marks the size of our observable universe. It is the distance to the most distant object we can see at any particular time. The particle horizon can be larger than the event horizon because, although we cannot see events that occur beyond our event horizon, we can still see many galaxies that are beyond our current event horizon by light they emitted long ago.​

Do these writings mean that photons emitted by galaxies outside the event horizon today will never reach our location?

 

[Bandersnatch]

Do these writings mean that photons emitted by galaxies outside the event horizon today will never reach our location?

Yes.

 

[Ibix]

Does the event horizon have any real effects?

Depends what you mean. It's not an invariant feature of spacetime in the sense of a black hole event horizon, and your cosmological horizon and mine differ slightly because we aren't in the same place. But it's a null surface, so nothing can cross it except in one direction.

 

[George Jones]

Do these writings mean that photons emitted by galaxies outside the event horizon today will never reach our location?

Light emitted today by galaxies outside the event horizon today will never reach our location.

It might be the case that there are galaxies outside the event horizon today, but that were inside the event horizon in the past. Light emitted by these galaxies: 1) today will never reach our location; 2) while they were inside the event has reached or will reach our location. For example, in the bottom panel of Figure 1 of Davis and Lineweaver, consider a galaxy that has a comoving distance of 30 Glys.

It might (presumably) be the case that there are galaxies outside the event horizon today, and that have always been outside the event horizon. Light emitted at all times by these galaxies will never reach our location. For example, in the bottom panel of Figure 1 of Davis and Lineweaver, consider a galaxy that has a comoving distance of 70 Glys.

Added edit.

Events outside our particle horizon can never be influenced by us.

Events outside our event horizon can never influence us.

 

[JimJCW]

Davis and Lineweaver give an example in their paper (see Section II) about galaxies with redshift z ∼ 1.8:

. . . galaxies with redshift z ∼ 1.8 are currently crossing our event horizon. These are the most distant objects from which we will ever be able to receive information about the present day. The particle horizon marks the size of our observable universe. It is the distance to the most distant object we can see at any particular time. The particle horizon can be larger than the event horizon because, although we cannot see events that occur beyond our event horizon, we can still see many galaxies that are beyond our current event horizon by light they emitted long ago.​

Let’s label the galaxies in question as G-DL-z1.8 and illustrate the above writing with a diagram. LightCone8 and PLANCK(2018+BAO) data gives the following result:

In the figure, the distances to the event horizon and to the G-DL-z1.8 galaxies are plotted as functions of cosmic time. The results are summarized below:

1. Photons received now from a G-DL-z1.8 galaxy were emitted at t = 3.527 Gyr (z = 1.852). At that time, the galaxy was located at a proper distance of 5.813 Gly from our location.
2. Now (t = 13.79 Gyr), due to space expansion, the G-DL-z1.8 galaxy catches up with the event horizon at a proper distance of 16.58 Gly from our location and at a recession speed of 1.146c.
3. From now on, no photons emitted by G-DL-z1.8 will ever reach our location (i.e., its new images will not be observable).
4. An observer at our location, however, will continue to see the old image of G-DL-z1.8 for a long, long time because the emitted photons when the galaxy was close to, but inside of, the event horizon will have prolonged journeys to reach our location. This is because the recession speeds at those locations are of the order of c. It will be interesting to demonstrate this with calculations using LightCone8.

 

[JimJCW]

An observer at our location, however, will continue to see the old image of G-DL-z1.8 for a long, long time because the emitted photons when the galaxy was close to, but inside of, the event horizon will have prolonged journeys to reach our location. This is because the recession speeds at those locations are of the order of c. It will be interesting to demonstrate this with calculations using LightCone8.

Let’s use Gnedin’s calculator to do the demonstration. If we aim a telescope to a direction in space, we receive photons from galaxies at various distances. Let’s assume that each of these galaxies emits a pulse of photons now at the same time. We want to know when these photon pulses will reach our location, i.e., to get a relation between the distance at emission and the photon arrival time at our location. The result is shown below:

Photons emitted by galaxies close to, but inside of, the current event horizon (16.58 Gly) will have prolonged journeys. For example, while it will take photons emitted now from a distance of 1.479 Gly 1.556 Gyr to reach our location, it will take a photon emitted at 16.52 Gly 99.79 Gyr. Note that this result is consistent with the statement that photons emitted now outside the current event horizon will never reach our location.

 

[JimJCW]

Let’s use Gnedin’s calculator to do the demonstration.

Corresponding result obtained with LightCone8:

 

[Hornbein]

Please allow me to note that for all we know the Universe may someday cease to expand or even contract. I don't believe that it will and the consensus today is against this, but it might. I don't have all that much confidence of extrapolations ahead into billions of years, especially about phenomena discovered during my lifetime.

 

[PeterDonis]

for all we know the Universe may someday cease to expand or even contract. I don't believe that it will and the consensus today is against this, but it might

No, it won't. The "consensus today" does include a range of possibilities, but they have to do with the universe's spatial curvature (our best current model is that the universe is spatially flat, but it could be positively curved with a very large radius of curvature, or even, though AFAIK this is much less likely, negatively curved with a very large radius of curvature), not its ultimate fate. All of the possible models consistent with our current data, which tells us that there is a small positive cosmological constant, have the universe expanding forever. The error bars are not wide enough to include any models where the universe stops expanding and recollapses.

 

[PeterDonis]

The error bars are not wide enough to include any models where the universe stops expanding and recollapses.

Here are two references for this claim:

http://burro.case.edu/Academics/Astr222/Cosmo/Models/unity.html.HOLD

https://ncatlab.org/nlab/show/standard+model+of+cosmology

The first shows the diagram of the two main model parameters, dark energy density and matter density, and shows the regions in the diagram that correspond to accelerating vs. decelerating expansion and to expanding forever vs. recollapsing.

The second shows the current data constraints on where our universe is on the diagram. That locus is well within the "expanding forever" and "accelerating" regions of the diagram; the error bars don't get anywhere close to the boundary of either of those regions. (The error bars do, however, encompass portions of both the "closed" and "open" regions, although the "flat" line goes through the center of the combined data region.)