misterkel said:
Light is redshifted and the greater the z, the further the spectral shift is to the left. Is there a point at which low-frequency EM (radio/micro waves) shifts below the spectra and disappear?
The real question - Is there a z, and thus an implied distance, at which the highest energy electromagnetic emissions - Gamma and x-rays - would shift to this vanishingly low range?
If so, then that would make objects of a certain distance invisible regardless of the age of the universe, wouldn't it?
Misterkel, Phyzguy gave a complete answer, I think. We can still play around with the issues, imagine etc. They are interesting issues to think about, if you want.
Your first paragraph asks about the PRACTICAL limit to observation of EW waves. Is there a longest wavelength beyond which we couldn't build a detector able to detect it. I think so.
How big an antenna can you imagine people ever building.
I picture things fading out to practical invisibility as they get more and more redshifted. Redshift also reduces the energy of an individual photon---so things dim and fade as they redshift too.
Your second paragraph says "is there a z, and therefore a distance..." This assumes a model of the universe, I take that to be the STANDARD MODEL with the usual parameters. You need some definite values of the main parameters in order to calculate distance from redshift. No big deal. Say .27, .73, 71 for the matter fraction, cosmo constant, and current Hubble rate.
In that case z goes to infinity for material that is currently at around 46 billion lightyears. The relationship is not linear.
From a practical standpoint what Phyzguy says is right. Beyond z = 1100 you can't see with light. Maybe neutrinos but not light. There is more to talk about but I have to go.
So beyond a current distance of about 45-some billion LY (the current dist. to the surface of last scattering) you can't see because the universe before that time was opaque. the hot ionized gas would scatter any light.
BUT IF YOU COULD see back thru the fog, you could only see to about 46 because z is blowing up and we already said that z going to infinity imposes another sort of limit.
Have to go...back now.
Your THIRD paragraph raises yet another kind of issue. The issue of horizons which could be not only because the poor photon gets so redshifted but because
it can't even get to us!
There are lots of galaxies which have been photographed and catalogued which are currently more than 16 billion lightyears from us. If you could freeze expansion today and measure with any ordinary instrument (radar for instance) it would be more than 16 billion.
That's a mere redshift of 1.8 or so (I'll check later). In fact I would say that MOST galaxies that we have photographed are currently farther than 16 billion lightyears.
So pick one. If you left today, traveling the speed of light, you could never reach it! Because of expansion.
OK. So that means there is an "event horizon". Somewhere between 15 and 16 I think.
It means if say in some wellknown-to-us galaxy, some event happens TODAY and they send us word of it by flashing a laser flash at us
the news of that event will never reach us! Not in 100 billion years, if we could wait that long, or in 100 trillion years.
And in a certain philosophical sense, what you call "objects" in your post are events. Because gas forms dust forms stars and galaxies and after a while black holes, and even black holes can change--they evaporate. All objects are events, in a certain timeframe.
So are there objects which we cannot see. Yes, objects are events, and we cannot see most of the events in the universe, and will never see those events, even if we know of their likely occurrence in places we have already photographed and catalogued. We will never see those events, or objects, no matter how long we wait.
