This could be the "big unanswerable question" for future generations as they look to the heavens.
The universe was a pretty simple and static place a century ago when Einstein was first heard of. Common wisdom at the time had it that all creation consisted of an island of stars and nebulae known as the Milky Way, and we were surrounded by infinite nothingness.
We now know space contains countless other galaxies rushing away from one another as a result of, what could be, the big bang. We like to think we're pretty smart, and that many of life's hard questions are being answered, but our ancestors may have no way of finding out about the Big Bang or the expanding universe.
According to a paper by Laurence Krauss and Robert J. Scherrer, in another hundred billion years the only galaxies left visible in the sky will be the half-dozen bound together gravitationally in what's known as the Local Group. What's more, is that this cluster of galaxies (which we're part) will likely not expand any further and collapse into a large starry ball.
Without being able to see galaxies flying away, future astronomers will not know if the universe is expanding and will instead think the universe is much as we imagined it 100 years ago. According to the paper written by the authors, "obervers in our 'island universe' will be fundamentally incapable of determining the true nature of the universe."
Sad news, indeed. Anyone who enjoys science fiction may have grown up with the idea that as we learn more about the universe, we learn more about ourselves. Instead, what we see here is that no matter how advanced a civilization might become, they my never know the true scale of the universe.
Worse than that, it makes you wonder what we should think about what we've already learned. If future generations (albeit several billion generations from now) may create incorrect theories of the universe, whose to say that our is correct?
“There may be fundamentally important things that determine the universe that we can’t see,” Dr. Krauss said in an interview. “You can have right physics, but the evidence at hand could lead to the wrong conclusion. The same thing could be happening today.”
As with everything in physics recently, this is the direct result of dark energy's effect on the universe. This is the theoretical force that is accelerating the cosmic expansion which seperates the galaxies at an ever increasing rate of speed. The cosmological constant is the fundamental reason for this expansion.
So, according to this constant, as the universe expands there is more space. The more space, the more force is being exerted on the galaxies, pushing them faster and faster away from us. As the approach the speed of light (which would be an interesting thought experiment), the galaxies will disappear from our skies. Thier ever lengthening wavelengths dimmed to the point where normal background radiation from our own galaxy blocks them out completely.
If the models play out, in the very distant future, this ever-expanding dark energy will absorb all the life from our universe. Though luckily, and I use the term loosely, future generations of astronomers will never learn this bit of information. Instead, they'll wonder why the universe consists of only six galaxies. Like Einstein, they may worry instead that their galaxy could collapse into a ultra-massive black hole and propose some kind of cosmic repulsion to prevent it, but they will have no idea to know if this is correct.
Naturally, we don't really need to know about dark energy to see that the universe is expanding. Hubble proved the universe was expanding long before we ever knew about this elusive property. Eventually, the galaxies will just be too far away to be seen under current means, setting the stage for cosmic ignorance.
For the moment, we have some 100,000,000 years or so to record our observations of the universe, and to develop the technologies required to help us learn more about the wonderful gift of existence. For the moment, we don't know what we don't know. But we'll learn and, hopefully, record it for future generations.