The discrepancy between the observed value of the cosmological constant and the value predicted by quantum field theory.

The problem of explaining the origin and evolution of dark energy.

The difficulty in reconciling general relativity with quantum mechanics.

The challenge of understanding the large-scale structure of the universe.

A hypothetical form of energy that is believed to be responsible for the observed acceleration of the universe's expansion.

A type of matter that interacts very weakly with ordinary matter.

A modification of general relativity that allows for the acceleration of the universe's expansion.

A new theory of gravity that explains the observed acceleration of the universe's expansion.

Quantum gravity is a theory that attempts to unify general relativity with quantum mechanics, and it may provide a solution to the cosmological constant problem.

Quantum gravity is a theory that attempts to explain the behavior of matter at very small scales, and it is unrelated to the cosmological constant problem.

Quantum gravity is a theory that attempts to explain the origin and evolution of the universe, and it may provide a solution to the cosmological constant problem.

Quantum gravity is a theory that attempts to explain the large-scale structure of the universe, and it is unrelated to the cosmological constant problem.

Modifying general relativity to allow for a variable cosmological constant.

Introducing a new scalar field that drives the acceleration of the universe's expansion.

Postulating the existence of extra dimensions that can accommodate a larger cosmological constant.

All of the above.

The lack of a complete theory of quantum gravity.

The difficulty in reconciling general relativity with quantum mechanics.

The need for new experimental data to constrain the cosmological constant.

All of the above.