The Race for Dark Energy

November 17, 2015 • by Marc Airhart

Karl Gebhardt explains the two leading ideas for what dark energy might be.

Illustration of galaxies bending space time

Locked in a cosmic tug of war, dark energy (purple) pushes the universe apart, while gravity (green) pulls it together. At the moment, dark energy is winning. Image credit: NASA/JPL-Caltech.


This is the second of a three-part series on general relativity.

Einstein's Theory of General Relativity, which describes how gravity works, turns 100 this month. The theory has successfully explained a lot of what we observe out in the universe; but there are signs that it's incomplete. In the 1990s, astronomers observed that the expansion of the universe is speeding up, as if some mysterious force is pushing everything apart faster and faster. Nearly 20 years later, one of the biggest unanswered questions in science is: what is this dark energy? Not only was dark energy not predicted by general relativity, but its mere existence might mean that the theory needs to be tweaked or even replaced.

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