Visible matter makes up only a tiny amount of the composition of the universe. Dark energy, a mysterious entity that is accelerating the expansion of the universe, dominates, followed by dark matter, invisible material that exerts a gravitational tug.
Credit: NIST

Cosmological data indicate that the expansion of the universe is accelerating. To explain this acceleration, a new paradigm has emerged in which 70% of the mass density of the universe is some kind of “dark energy.” One possibility for this dark energy is the cosmological constant that Einstein called his biggest blunder. Prior to this data, in 1986 I examined the possibility of a time-dependent vacuum energy; a variant known as quintessence is now a favored explanation of the dark energy. In 2002 I also proposed Cardassian expansion, in which a modification of the Friedmann equation (Einstein’s equations as applied to the universe), motivated by the physics of extra dimensions required by string theory, replaces any need for vacuum energy. This model has played an important role in the development of alternative explanations based on modified gravity for accelerated expansion. I have also been working on models for Early Dark Energy, an idea for resolving the Hubble tension (the disagreement between measurements of the expansion rate of the Universe extracted from early vs. late time data). I will continue to search for theoretical understanding of the accelerated expansion of the Universe.