Quote Originally Posted by oskar View Post
An interesting thought is that there will be a species on a planet a billion years from now when the universe has expanded to the point where there is no cosmic background radiation and everything is so far apart that they will look up into a black night sky and all they know about the universe is within their solar system. What image of the universe will they have?
Way more than a billion. The sun will still be burning in a billion years. It'll have expanded to larger than the size of Earth's orbit, as it will be in its red giant phase, but it'll still be here.

There will always be CMBR, but it is ever more red-shifted over time, such that detection will eventually be impossible.

Assuming that the acceleration due to dark energy that we observe today remains constant or decreases:
They will see the stars in their galaxy. Also, they will see the galaxies in the neighborhood they reside within their galactic supercluster.

Dark Energy is so weak that it only acts over ludicrously long length scales. Galaxies are gravitationally bound together into clusters and those clusters are bound into superclusters. Only on length scales greater than this does the accelerated expansion overpower gravitational acceleration of attraction.

It's not too dissimilar to talking about the electromagnetic forces compared to the gravitational force. On particle scales, gravity is negligible. The dominant forces are orders of magnitude greater, so while gravity's effect isn't truly 0, it's not causing any bad predictions to ignore it. Move out to big enough scales, though and the electromagnetic imbalances average out and you are left with "large" objects which have effectively 0 electric charge, so they don't interact via electromagnetic forces, and the weaker force of gravity expresses dominance.