Event Horizon and Particle Horizon

• I
• Arman777
In summary, the furthest distance that we can see is defined by the Radius of the Particle Horizon which its nearly 46 Gly. However, the cosmic event horizon is nearly 16 Gly. Is this means the galaxies that further than the 16 Gly are just will stay the same in the sky? Since their light can never reach us, in other words, their images on the sky will never change?
Arman777
Gold Member
The furthest distance that we can see is defined by the Radius of the Particle Horizon which its nearly 46 Gly. However, the cosmic event horizon is nearly 16 Gly. Is this means the galaxies that further than the 16 Gly are just will stay the same in the sky? Since their light can never reach us, in other words, their images on the sky will never change?

For example an object at 20 Gly, we will never see its "future" since its light cannot reach us due to the expansion of the universe ?

And after the Event horizon becomes stable at 17.6 Gly, every galaxy that crosses that distance will stay on that horizon and we will see them as getting redshifted to infinity?

How can we calculate the time needed for clusters in our supercluster to pass the cosmic event horizon ?,

More important I don't understand something. If the horizon is the horizon that is furthest distance that can communicate then how it can be growing ?
These horizon things makes me so confused can someone help me, to understand them better, I now just the definitions but I can not grasp the main idea (even I read and watched many things about them)

Can you read a lightcone graph in comoving coordinates? Such as this one:

Those make it so much easier to see and explain.

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Bandersnatch said:
Can you read a lightcone graph in comoving coordinates? Such as this one:
View attachment 236990
Those make it so much easier to see and explain.
I can read it somehow. Hubble Sphere is kind of confusing. Whats the difference between light cone and event horizon ?

For example at comoving distance between 40 Glyr and 20 Glyr at ##a(t_0)##. We cannot receive their light. But we see their light redshifted to infinite?

Is this statement true:
at ##a(t)=1.5## we will be no longer to receive new information from a galaxy, that has a comoving distance ##10Glyr##

Never mind the Hubble sphere, it's going to be confusing in those coordinates.

The event horizon is the past light cone of an observer in infinite future. If you imagine the apex of the light cone going up with time (with its base encompassing more and more comoving distance), it reaches the top of the graph in infinite future, at which point the cone will be coincident with the event horizon.
Arman777 said:
For example at comoving distance between 40 Glyr and 20 Glyr at ##a(t_0)##. We cannot receive their light. But we see their light redshifted to infinite?
Remember, you only ever see what's on your past light cone.
We can't ever receive the light they emit now, but we're seeing the light they emitted in the past (where their comoving coordinate crosses our current light cone) just fine.
The last state we'll ever see is where the coordinate crosses the event horizon. It's a past event, but light emitted at that point will reach the observer only after infinite time.
The currently observed redshift of a galaxy at approx 20 Glyr is z~3. It will gradually increase to infinity, as the future observer's light cone approaches the event horizon.
Arman777 said:
Is this statement true:
at ##a(t)=1.5## we will be no longer to receive new information from a galaxy, that has a comoving distance ##10Glyr##
The galaxy crosses the horizon sometime before that, so we won't be able to receive any information it emits after the crossing. But we'll keep receiving new (in the sense of never before seen) information about the past state of the galaxy, until forever. From the time before it crossed the event horizon. Where, again, the moment of crossing will become observable only after infinite time.Tell you what, check out this post:
https://www.physicsforums.com/threa...increase-bc-of-expansion.912881/#post-5754083
where I give a shoddily-written tutorial on how to read those graphs. See if it clarifies anything, and I'll get back to you tomorrow.

Arman777
Okay thanks, If I still don't understand I ll write you again.

Bandersnatch said:
The galaxy crosses the horizon sometime before that, so we won't be able to receive any information it emits after the crossing. But we'll keep receiving new (in the sense of never before seen) information about the past state of the galaxy, until forever. From the time before it crossed the event horizon. Where, again, the moment of crossing will become observable only after infinite time.
Though eventually the light will be redshifted to the point that wavelengths will be longer than the horizon, making detection impossible. Apparently all galaxies that aren't gravitationally bound to us will become undetectable in about 2 trillion years or so due to this.

kimbyd said:
Though eventually the light will be redshifted to the point that wavelengths will be longer than the horizon, making detection impossible. Apparently all galaxies that aren't gravitationally bound to us will become undetectable in about 2 trillion years or so due to this.
How did you calculated the 2 triliion year period..?

George Jones said:
... and, for this number, Wikipedia references
https://arxiv.org/abs/astro-ph/9902189

See the paragraph that contains equation (4).
Thanks for following the links there! That paper looks like a great read for later. One fun quote: "On the bright side for astronomers, funding priorities for cosmological observations will become exponentially more important as time goes on."

kimbyd said:
funding priorities for cosmological observations will become exponentially more important as time goes on.
As if importance has a linear scale. Or is numeric. Or is a total order. Or is even a partial order. Or is even well defined.

jbriggs444 said:
As if importance has a linear scale. Or is numeric. Or is a total order. Or is even a partial order. Or is even well defined.
I'm sure the statement was meant as a joke rather than anything approaching a realistic appraisal.

1. What is the difference between Event Horizon and Particle Horizon?

The Event Horizon is the boundary around a black hole where the escape velocity is greater than the speed of light, making it impossible for any matter or information to escape. The Particle Horizon, on the other hand, is the maximum distance from which light can have reached us since the beginning of the universe. It is the furthest distance from which we can receive any information or signals.

2. How is the Particle Horizon related to the age of the universe?

The Particle Horizon is directly related to the age of the universe because it represents the maximum distance light has traveled since the beginning of the universe. As the universe expands, the Particle Horizon also expands, allowing us to see more distant objects and events that occurred farther back in time.

3. Can anything escape from the Event Horizon?

No, nothing can escape from the Event Horizon. The escape velocity at this boundary is greater than the speed of light, which is the universal speed limit. This means that even light, the fastest thing in the universe, cannot escape from the Event Horizon.

4. How does the size of the Event Horizon depend on the mass of a black hole?

The size of the Event Horizon is directly proportional to the mass of a black hole. This means that the more massive a black hole is, the larger its Event Horizon will be. For example, a black hole with 10 times the mass of our sun will have an Event Horizon 10 times larger in diameter.

5. Can we observe the Particle Horizon?

No, we cannot observe the Particle Horizon directly. This is because the Particle Horizon represents the maximum distance from which light can have reached us, and any light from beyond this distance has not yet had enough time to reach us. However, we can indirectly observe the effects of the Particle Horizon, such as the cosmic microwave background radiation, which is the oldest light in the universe.

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