Universe expanding faster than light

[sylas]

Agreed. We already see all that is possible to perceive.

I'm glad you agree, but I don't understand the second sentence. It doesn't make sense; of course there are things in the universe that will be perceived and haven't been perceived yet.

So if you mean that we "already" see everything which can be seen so far, then this appears to be a tautology, and not relevant to the existence or otherwise of event horizons, which is what I was addressing. And if you mean that we "already" see everything that we can hope to perceive in the universe, then that is false.

Note that there is a real substantive difference between, say, a critical density universe and a universe with a cosmological constant. In the first case, there is no event horizon, everything in the universe is in principle able to be seen, if we wait long enough. In the second, there is an event horizon, and there are regions of spacetime that can never be seen. The difference is a real difference, not resolve with philosophical argument but with empirical investigation of whether expansion of the universe admits an event horizon or not.

Cheers -- sylas

 

[Chronos]

We 'see' the CMB, but, will never see beyond it. We will, of course, see more events unfold in the universe as their photons reach us, but, those photons will always be in the CMB foreground. I fail to see how this is relevant to any horizon or density issues. The CMB is our observational horizon [at present] IMO.

 

[sylas]

We 'see' the CMB, but, will never see beyond it. We will, of course, see more events unfold in the universe as their photons reach us, but, those photons will always be in the CMB foreground. I fail to see how this is relevant to any horizon or density issues.

As time passes we see further and further reaches of the surface of last scattering. There is no event horizon here, so its got nothing much to do with the preceding discussion of what is seen when a distant galaxy crosses an event horizon.

 

[Chronos]

For clarity, no galaxy ever crosses our observational 'horizon'. At worst, they 'freeze' against the CMB as they approach infinite time dilation - agreed?

 

[sylas]

For clarity, no galaxy ever crosses our observational 'horizon'. At worst, they 'freeze' against the CMB as they approach infinite time dilation - agreed?

This is analogous to the common problem people have with objects falling into a black hole. We never see an object falling into a black hole... we see it redshifted to infinity. It is, however, incorrect to say that no object crosses the horizon.

Distant galaxies DO cross the event horizon, or our observational horizon, just as we have already crossed the event horizon for those distant galaxies (presuming the FRW model with dark energy and dark matter). You already agreed with this! We merely don't see this occurring for other galaxies.

Cheers -- sylas

 

[Chalnoth]

We 'see' the CMB, but, will never see beyond it. We will, of course, see more events unfold in the universe as their photons reach us, but, those photons will always be in the CMB foreground. I fail to see how this is relevant to any horizon or density issues. The CMB is our observational horizon [at present] IMO.

With photons. With other particles, we might be able to 'see' quite a bit further back in time (and thus further). For example, if we could directly detect the cosmic neutrino background, that would lead a fair amount further back in time. Or, if we could directly detect the cosmic gravity wave background, that would go even further back.

 

[sylas]

With photons. With other particles, we might be able to 'see' quite a bit further back in time (and thus further). For example, if we could directly detect the cosmic neutrino background, that would lead a fair amount further back in time. Or, if we could directly detect the cosmic gravity wave background, that would go even further back.

Yes... this is one of the reasons I really hope LISA gets the nod for NASA projects!

 

[Chalnoth]

Yes... this is one of the reasons I really hope LISA gets the nod for NASA projects!

Yup. But it's worth noting that even without directly detecting these gravity waves, they do leave a polarization signal on the CMB that we can detect indirectly. It's very difficult to measure this polarization signal, but we're working on it.

Planck, by the way, probably won't be sensitive enough to detect the gravity wave signature in the CMB.

 

[Chronos]

A neutrino telescope might allow us to peer beyond the EM veil of ignorance. That would be fascinating. I agree Planck may not be sensitive enough to detect gravity waves, but, should resolve a number of other interesting debates in cosmology.