Dark energy, modified gravity and backreaction

The expansion of the universe has accelerated within the past few billion years. The reason for the acceleration remains unknown, and it is
considered to be one the biggest enigmas in all of physics. The most studied possibility is that there is a medium with negative pressure and
thus repulsive gravity, called dark energy. Other possibilities include modification of general relativity on cosmological scales or non-linear
effects related to structure formation within general relativity. The cosmology group is probing all three possibilities.

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