In the current issue of Scientific American Adam G. Riess and Michael S. Turner write:
Almost 75 years ago astronomer Edwin Hubble discovered the expansion of the universe by observing that other galaxies are moving away from ours. He noted that the more distant galaxies were receding faster than nearby ones, in accordance with what is now known as Hubble’s law (relative velocity equals distance multiplied by Hubble’s constant). Viewed in the context of Einstein’s general theory of relativity, Hubble’s law arises because of the uniform expansion of space, which is merely a scaling up of the size of the universe.
In Einstein’s theory, the notion of gravity as an attractive force still holds for all known forms of matter and energy, even on the cosmic scale. Therefore, general relativity predicts that the expansion of the universe should slow down at a rate determined by the density of matter and energy within it. But general relativity also allows for the possibility of forms of energy with strange properties that produce repulsive gravity. The discovery of accelerating rather than decelerating expansion has apparently revealed the presence of such an energy form, referred to as dark energy.
Whether or not the expansion is slowing down or speeding up depends on a battle between two titans: the attractive gravitational pull of matter and the repulsive gravitational push of dark energy. . . .
Because telescopes look back in time as they gather light from far-off stars and galaxies, astronomers can explore the expansion history of the universe by focusing on distant objects. That history is encoded in the relation between the distances and recession velocities of galaxies. . . .
Lots of mysteries are waiting for the present and new students of cosmology, physics and mathematics to pursue. Why is the universe still expanding? What are the qualities of dark matter and energy? What are the quantum properties of the black hole? Exciting times. This is another kind of storytelling.