Let’s pretend for a moment that in the far, far future there is an advanced civilization living on a planet in a solar system similar to ours, in a galaxy similar to or even our own. They have technology equal to or better than what we have today. However, due to the expansion of the universe, they can no longer see any galaxies except their own…all other galaxies have either merged with their own (there are several that seem to be going to do this in our own galaxy several billion years from now) or have moved away so far that they can no longer see light from them. The astronomers in this theoretical future civilization…would they conclude that the universe is only the size of the galaxy they can see? Is there anything that would indicate to them that, in fact, the universe is vastly larger, with uncounted other galaxies out there that they simply can’t see? Would we be able to determine that, if it was the case today? Or would we conclude that the entire universe was merely what’s in the (expanded) Milky Way?
(this is all my understanding of the expansionist theory of the universe taken to what I assume are the eventual logical conclusions. If I have it fundamentally wrong, feel free to explain)
It wouldn’t be impossible to detect other galaxies in the far future, but it would be difficult. It’s not that light from other galaxies wouldn’t reach us, it’s that it’s going to be red-shifted so much that you’d need bigger and bigger detectors to see any of it.
But if we go far enough in the future then all other sources of light will eventually become truly undetectable. By then I imagine all stars will have burned out anyway, so there won’t be much to look at inside or outside our galaxy.
I don’t think that’s true. If the universe is expanding basically everywhere in every direction at roughly 72km per second per megaparsec, there comes a point where you’re so distant that the light will never reach you because effectively the universe is expanding at a speed greater than light. Or to say it another way, more space is created between 2 points in a given period of time than light could travel in that period of time. This is independent of red shifting.
Yes, Lawrence Krauss discusses this in his talk, ‘A Universe From Nothing’ and also in his book by the same name:
We naturally think that, if there are cosmologists in the year 2 trillion AD, their vision of the universe will be expanded over ours. Not so—and this is one of the many shattering conclusions I take away on closing this book. Give or take a few billion years, ours is a very propitious time to be a cosmologist. Two trillion years hence, the universe will have expanded so far that all galaxies but the cosmologist’s own (whichever one it happens to be) will have receded behind an Einsteinian horizon so absolute, so inviolable, that they are not only invisible but beyond all possibility of leaving a trace, however indirect. They might as well never have existed. Every trace of the Big Bang will most likely have gone, forever and beyond recovery. The cosmologists of the future will be cut off from their past, and from their situation, in a way that we are not.
This is not quite true! I’ve pointed this out before, but in the standard cosmological model we can see objects that have always been receding from us FTL since the big bang! The Hubble sphere (the sphere at which beyond objects are receding from us faster than the speed of light) is not the same as the cosmological horizon (the sphere beyond which we cannot see objects) and neither does the existence of a Hubble sphere imply the existence of a cosmological horizon and vice versa.
My question, from watching Krauss’ talk, is could there be astronomical information that has been forever lost to us—and is likewise impairing our cosmological understanding of the universe—due to our living in this epoch? That’s the question I wanted to ask Krauss after seeing that talk.
I have to admit, I made an error because I didn’t read SenorBeef’s post properly. He was talking about the event horizon of the Universe and not the cosmological horizon. Just to make this clear:
Cosmological/particle horizon = the sphere which is the boundary beyond which we cannot currently see and hence is the boundary of the observable universe. It is how far light away light from the past could’ve traveled from for us to be able observe it now.
Event horizon = the sphere which is the boundary beyond events happening now cannot affect us in the future. It is how far away light from events happening now can travel and be able to reach us at some point in the future.
Hubble sphere/horizon = the sphere which is receding from us faster than the speed of light.
My last post was referring to the independence of the existence Hubble horizon and the particle horizon. However it is also true that the event horizon (of the Universe) and the Hubble horizon are not the same thing and there existences are also independent (i.e. the fact that a cosmological has an event horizon does not absolutely imply that it always has a Hubble horizon and the existence of a Hubble horizon most definitely does not imply the existence of an event horizon). However to be fair I believe in any half-way sensible model that has an event horizon there will also be Hubble horizon and over time the two spheres will converge with each other.
To answer your question: I’m not sure exactly what point Krauss is trying to make as in the future, in principle at least, we will be able to see galaxies that we cannot currently see because they are outside of our current particle horizion. This is because within our current event horizon there is light that has traveled from galaxies outside of our current particle horizon that is yet to reach us (edited to add: on re-reading I think he is referring to the fact that all light from other galaxies will be so faint and redshifted that for all practical purpose it will be impossible to see).
One way, even if you can only see your own galaxy, to still detect evidence of cosmic expansion might be hypervelocity stars that, due to some violent event within the galaxy (near-miss with the central black hole, for example) are ejected from it. Studying their behaviour will yield evidence of accelerated expansion.