# Confusion between horizon, particle horizon and event horizon

1. May 26, 2005

### emanaly

Hi All,
I am new here, Iam interested in Cosmology, sometimes I find many questions which I don't find its answers directly in text books or I read it but couldn't understand it, for example I have a confusion between horizon, particle horizon and event horizon, would you please help me in how to differentiate between them.
Eman

2. May 26, 2005

### chronon

General relativity tells us that we can only have received signals from a finite portion of the universe since the big bang. This defines the particle horizon. See http://www.astro.ucla.edu/~wright/cosmo_03.htm for more information.

Black holes have an event horizon, which is the surface beyond which it is impossible for light to escape. In models of the universe where the expansion is accelerating there is a cosmological event horizon, meaning that eventually we will lose contact with distant galaxies.

Note that there is considerable confusion about such horizons. You might want to have a look at http://www.chronon.org/articles/cosmichorzns.html and http://xxx.arxiv.cornell.edu/abs/astro-ph/0310808 [Broken].

Last edited by a moderator: May 2, 2017
3. May 26, 2005

### Chronos

Just one clarification to make. The cosmological event horizon is also receeding, and receeding faster than any object inside of it. Distant galaxies will eventually fade from view as they become too distant to detect, but will not cross an event horizon and suddenly disappear.

4. May 26, 2005

### JesseM

Isn't it true that if the rate of expansion is accelerating, then galaxies can cross the cosmological event horizon and disappear forever? That seems to be what's being said in the last section of the article http://www.sciam.com/article.cfm?articleID=0009F0CA-C523-1213-852383414B7F0147 [Broken]:

Last edited by a moderator: May 2, 2017
5. May 27, 2005

### Chronos

I see no inconsistency there. We will observe galaxies approach the cosmological event horizon, but never see them cross it. The same thing as watching an astronaut fall into a black hole.

6. May 27, 2005

### JesseM

OK, but even if the light from a galaxy gets infinitely redshifted as it crosses the horizon so it appears to be "frozen" at the edge, wouldn't it be true that the galaxy crosses the horizon at some finite cosmological time coordinate, whereas in the Schwarzschild coordinate system of an observer far from the black hole, an infalling observer takes an infinite amount of time to cross the horizon?

7. May 27, 2005

### chronon

In the case of a galaxy crossing our cosmological event horizon at a given proper time T of the galaxy, we will never see any events happening after T. We'll just see events up to time T being asymptotically slowed down and redshifted. In this respect it's the same as watching an astronaut falling into a black hole and crossing the event horizon at proper time T.

I suppose to be consistent with http://www.chronon.org/articles/blackholes.html I ought to deny that the cosmological event horizon exists, but clearly current theory says that it does. I'll have to think about that.

8. May 28, 2005

### emanaly

Thanks a lot Chronon, I am appreciating your help.
Eman

9. May 29, 2005

### robphy

This may be a good reference:
G. F. R. Ellis and T. Rothman, "Lost horizons," Am. J. Phys. 61 (1993) 883.
Here is the abstract:
Cosmological horizons play an essential role in determining the causal structure of spacetime and are of central importance in the inflationary universe scenario. We review the topic of horizons in simple language, pointing out a number of widespread misconceptions. The use of spacetime diagrams plotted in terms of proper time and proper distance coordinates helps sort out some of these difficulties. They complement the widely used conformal diagrams, which show causal relations clearly but severely distort proper distances.

Comments on Lost Horizons'' by G. F. R. Ellis and T. Rothman [Am. J. Phys. 61 (10), 883–893 (1993)] by Michael Rauch, Am. J. Phys. 63 87 (1995)
Past light cone shape and refocusing in cosmology, A Response to Michael Rauch's Comments on `Lost Horizons' '' [Am. J. Phys. 63, 87 (1995)] by G. F. R. Ellis et al., Am. J. Phys. 63 88 (1995)