# Confusion between horizon, particle horizon and event horizon

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

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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].

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Chronos
Gold Member
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.

JesseM
Chronos said:
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.
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]:
What does mark the edge of observable space? Here again there has been confusion. If space were not expanding, the most distant object we could see would now be about 14 billion light-years away from us, the distance light could have traveled in the 14 billion years since the big bang. But because the universe is expanding, the space traversed by a photon expands behind it during the voyage. Consequently, the current distance to the most distant object we can see is about three times farther, or 46 billion light-years.

The recent discovery that the rate of cosmic expansion is accelerating makes things even more interesting. Previously, cosmologists thought that we lived in a decelerating universe and that ever more galaxies would come into view. In an accelerating universe, however, we are surrounded by a boundary beyond which occur events we will never see--a cosmic event horizon. If light from galaxies receding faster than light is to reach us, the Hubble distance has to increase, but in an accelerating universe, it stops increasing. Distant events may send out light beams aimed in our direction, but this light is trapped beyond the Hubble distance by the acceleration of the expansion.

An accelerating universe, then, resembles a black hole in that it has an event horizon, an edge beyond which we cannot see. The current distance to our cosmic event horizon is 16 billion light-years, well within our observable range. Light emitted from galaxies that are now beyond the event horizon will never be able to reach us; the distance that currently corresponds to 16 billion light-years will expand too quickly. We will still be able to see events that took place in those galaxies before they crossed the horizon, but subsequent events will be forever beyond our view.

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Chronos
Gold Member
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.

JesseM
Chronos said:
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.
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?

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.

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

robphy
Homework Helper
Gold Member
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)