# Can you view objects that moved beyond the event horizon?

1. ### typical guy

22
Hi, I read a while back that astronomers were able to view an old supernova explosion using a technique that involved looking for light reflected off some hydrogen gas. After thinking about this for a while, I decided to come here with my question. Is it possible to use this technique to view light from an object that was once within the visible universe but has since moved beyond it due to the expansion of space?

2. ### marcus

24,266
T. guy, this is an opportunity to get clear about some standard terminology. The observable universe (aka visible) includes all the matter that we are getting light or other signals from. That includes matter which is today about 46 billion ly from us.

That is distance in the "proper distance" or freeze-frame sense that if you could pause the expansion process to give yourself time to measure a radar beep would take 46 billion years to reach that most distant material.

That 46 Gly (giga for billion) is about how far the matter is that in early times emitted the cosmic microwave background radiation that we are now detecting. So we are in effect LOOKING AT matter that is now 45.5+ Gly from here. It has by now formed galaxies and stars etc. We see it as it was in early days: a hot gas.

That distance is called "particle horizon" to distinguish it from "cosmic event horizon".

The cosmic event horizon (CEH) is only about 16 Gly. It is the proper distance today of the most distant galaxy we can expect to reach with a signal we send TODAY.
If an event happens today in a galaxy that is more than 16 Gly from us, like a supernova explosion, we will never see it no matter how long we wait.
If a supernova explodes today in a galaxy that is LESS than Gly from us (today, freeze-frame i.e. proper distance) then we WILL eventually see it if we wait long enough.

Most of the objects we can see today are well beyond today's event horizon. That is, most of the galaxies we can observe are today more than 16 Gly from us.

So the answer to your stated question is definitely YES. We certainly can continue to observe galaxies which have moved beyond the event horizon. Indeed most of the galaxies we do observe are beyond the event horizon.

Last edited: Feb 1, 2014
3. ### marcus

24,266
This is a different question because you are here not talking about the EVENT HORIZON. Here you are talking about a special technique that INFERS an earlier event from "second-hand light".
That's very interesting.

It seems to me that the answer to what you specifically ask would need to involve some speculation and I hope several people respond and we get some different views.
the current radius of the observable universe is called the "particle horizon" and it is about 46 Gly.
It's roughly (slightly larger than) the distance of the matter that radiated the most ancient light we have so far been able to detect---the CMB. That was around year 380,000 and there was just hot gas. There were no "objects" in the usual sense. Just partially ionized gas. No stars.

The radius of the observable universe is increasing as light comes in from more and more distant matter. And also it is increasing by expansion.

I personally do not see how it would be possible for a material object to move OUT of the observable universe. The recession speed, owing to expansion (not motion thru space) out at 46 Gly is already several times the speed of light and for something to "move out" it would have to move an additional c thru space.

As I see it it is impossible that an object like that exists. If it just moves "due to the expansion of space" well the observable portion of the universe is itself expanding so it keeps up with any object whose distance from us is increasing merely due to expansion of distances.

So I would say the answer to that question of yours is NO, because there couldnt be such an object in the first place. So it would be impossible to detect such an object no matter what technique is used, owing to its non-existence.

4. ### marcus

24,266
T. guy, if you are interested in this kind of thing you might try this link, if you haven't already:

http://www.einsteins-theory-of-relativity-4engineers.com/LightCone7/LightCone.html

See if you can figure out what the columns mean, in some cases.

To get more decimal places displayed, click on "column definition and selection". You will see boxes where you can say how many decimal places you want shown..

It is a calculator made by Jorrie, somebody here at PF, that embodies the standard cosmological model and makes tables of various distances times and expansion speeds.
There is also a "z+1" column that gives the redshift number (plus one) coming from some distance source or (reciprocally) going out to some distant target in the future.

It's good to get some quantitative understanding.

5. ### Mordred

To add to the comments Marcus made lets assume for a second that it is possible for a stellar object to cross the event horizon. Although as Marcus pointed out this is highly unlikely.

The source light in order to reflect to us would need to travel at an angle to the hydrogen cloud then get reflected back to us. So the light path would be larger and take longer to reach us than a direct line of sight. We would have a better chance of seeing the direct path as opposed to the time added to travel to us from a reflection. The greater the reflection angle the longer it would take to see that light.

the second light technique described in the OP is a possible means of seeing a past event that you may have missed with a direct path of light as the reflected path would always take longer.

6. ### marcus

24,266
Objects do cross our event horizon constantly, Mordy. I was trying to explain. the event horizon is at about 16 Gly and it stays approximately at that distance, expanding hardly at all. Objects are always zooming across it.

What I said was I don't think objects pass out of our observable region (because it keeps expanding).
A problem people have with terminology is that they think "event horizon" means current boundary of observable universe.

they are very different. one is around 16 and the other is around 46 Gly.

7. ### marcus

24,266
Typical Guy,
this table shows both the Event Horizon (labeled Dhor) and the Particle Horizon (Dpar) as they evolve in time. You can see that the Particle Horizon, taken to be the radius of the OBSERVABLE UNIVERSE, keeps growing. It actually grows too fast for an object to overtake and pass it and so pass out of the obsrvbl

but the Event Horizon is another matter altogether. It is around 16 now and it stabilizes at 17.3 Gly. It grows in a slow bounded way. It is easy for objects to cross the event horizon. But this does not make them stop being observable!

$${\scriptsize\begin{array}{|c|c|c|c|c|c|}\hline R_{0} (Gly) & R_{\infty} (Gly) & S_{eq} & H_{0} & \Omega_\Lambda & \Omega_m\\ \hline 14.4&17.3&3400&67.9&0.693&0.307\\ \hline \end{array}}$$ $${\scriptsize\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline a=1/S&S&T (Gy)&R (Gly)&D_{now} (Gly)&D_{then}(Gly)&D_{hor}(Gly)&D_{par}(Gly)&V_{now} (c)&V_{then} (c) \\ \hline 0.001&1090.000&0.000373&0.0006&45.332&0.042&0.057&0.001&3.15&66.18\\ \hline 0.003&339.773&0.002496&0.0040&44.184&0.130&0.179&0.006&3.07&32.87\\ \hline 0.009&105.913&0.015309&0.0235&42.012&0.397&0.552&0.040&2.92&16.90\\ \hline 0.030&33.015&0.090158&0.1363&38.052&1.153&1.652&0.249&2.64&8.45\\ \hline 0.097&10.291&0.522342&0.7851&30.918&3.004&4.606&1.491&2.15&3.83\\ \hline 0.312&3.208&2.977691&4.3736&18.248&5.688&10.827&8.733&1.27&1.30\\ \hline 1.000&1.000&13.787206&14.3999&0.000&0.000&16.472&46.279&0.00&0.00\\ \hline 3.208&0.312&32.884943&17.1849&11.118&35.666&17.225&184.083&0.77&2.08\\ \hline 7.580&0.132&47.725063&17.2911&14.219&107.786&17.291&458.476&0.99&6.23\\ \hline 17.911&0.056&62.598053&17.2993&15.536&278.256&17.299&1106.893&1.08&16.08\\ \hline 42.321&0.024&77.473722&17.2998&16.093&681.061&17.300&2639.026&1.12&39.37\\ \hline 100.000&0.010&92.349407&17.2999&16.328&1632.838&17.300&6259.262&1.13&94.38\\ \hline \end{array}}$$

Last edited: Feb 2, 2014
8. ### Mordred

I must have been half asleep lol your correct on that. By the way could you post an up to date link to the calculator, the link in my signature doesn't work correctly for posting on the forum in either large tex or small tex. So I'm assuming Jorrie has a later version. Athough both links look identical in our signatures lol. I tried to post a chart the other day with default values and the tex didn't work properly.

edit: testing

$${\small\begin{array}{|c|c|c|c|c|c|}\hline R_{0} (Gly) & R_{\infty} (Gly) & S_{eq} & H_{0} & \Omega_\Lambda & \Omega_m\\ \hline 14.4&17.3&3400&67.9&0.693&0.307\\ \hline \end{array}}$$ $${\small\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline a=1/S&S&T (Gy)&R (Gly)&D_{now} (Gly)&D_{then}(Gly)&D_{hor}(Gly)&V_{now} (c)&V_{then} (c) \\ \hline 0.001&1090.000&0.0004&0.0006&45.332&0.042&0.057&3.15&66.18\\ \hline 0.003&339.773&0.0025&0.0040&44.184&0.130&0.179&3.07&32.87\\ \hline 0.009&105.913&0.0153&0.0235&42.012&0.397&0.552&2.92&16.90\\ \hline 0.030&33.015&0.0902&0.1363&38.052&1.153&1.652&2.64&8.45\\ \hline 0.097&10.291&0.5223&0.7851&30.918&3.004&4.606&2.15&3.83\\ \hline 0.312&3.208&2.9777&4.3736&18.248&5.688&10.827&1.27&1.30\\ \hline 1.000&1.000&13.7872&14.3999&0.000&0.000&16.472&0.00&0.00\\ \hline 3.208&0.312&32.8849&17.1849&11.118&35.666&17.225&0.77&2.08\\ \hline 7.580&0.132&47.7251&17.2911&14.219&107.786&17.291&0.99&6.23\\ \hline 17.911&0.056&62.5981&17.2993&15.536&278.256&17.299&1.08&16.08\\ \hline 42.321&0.024&77.4737&17.2998&16.093&681.061&17.300&1.12&39.37\\ \hline 100.000&0.010&92.3494&17.2999&16.328&1632.838&17.300&1.13&94.38\\ \hline \end{array}}$$

hrmm working now not sure why it didn't the other night lol

Last edited: Feb 2, 2014