How Does Quintessence Differ from the Cosmological Constant?

The cosmological constant (Λ) proposed by Einstein is a constant energy density filling space uniformly. Quintessence, on the other hand, is a dynamic scalar field that can vary. This means that while the cosmological constant is a simple fixed value, quintessence involves a field with a potential energy that can change, affecting the rate of the universe's expansion differently at different times.