The cosmological constant is a term introduced by Einstein in his equations of general relativity and later associated with dark energy. Describe the origin and significance of the cosmological constant in cosmology. Explain how observations of the accelerated expansion of the universe led to the concept of dark energy. Discuss the evidence for dark energy and its impact on our understanding of the universe's fate. Evaluate the theoretical models that attempt to explain dark energy and the challenges they present for modern cosmology.