The cosmological constant and dark energy are central concepts in modern cosmology. The cosmological constant, denoted by Λ, represents a form of energy density that exerts a repulsive gravitational effect, akin to Einstein's original idea. Today, this concept is often referred to as dark energy, which may be a dynamical field that evolves over time. The presence of dark energy is inferred from observations that suggest the universe's expansion is accelerating, and its density is small now due to the universe's age. This alleviates the problem of the dark energy density being too large compared to theoretical predictions. Observational evidence for dark energy comes from cosmological tests, including the thermal cosmic microwave background radiation, light element abundances, and the redshift-magnitude relation. These tests support the Friedmann-Lemaître model, which describes the universe's expansion in terms of dimensionless parameters. The current evidence suggests that the dark energy density is significant, with ΩΛ0 ≈ 0.7, indicating a non-zero Λ. However, the detection of dark energy is not yet conclusive, and further observations are needed to confirm its existence. The cosmological constant and dark energy are related to the vacuum energy of particles and fields. The zero-point energy of the electromagnetic field, for example, contributes to the cosmological constant. However, the value of this energy is much larger than what is allowed by the relativistic cosmological model, suggesting that dark energy may be a dynamical field that evolves over time. This dynamic nature of dark energy could explain why its density is small now and may approach zero in the future. The history of ideas about the cosmological constant and dark energy is rich and complex. Einstein initially introduced the cosmological constant to achieve a static universe, but later abandoned it after the discovery of an expanding universe. The concept of dark energy has since evolved, with various models proposing different forms of dark energy, including scalar fields and vacuum energy. The current understanding of dark energy is still developing, and further observations are needed to determine its nature and properties. The search for a fundamental theory of dark energy remains an active area of research in physics and astronomy.The cosmological constant and dark energy are central concepts in modern cosmology. The cosmological constant, denoted by Λ, represents a form of energy density that exerts a repulsive gravitational effect, akin to Einstein's original idea. Today, this concept is often referred to as dark energy, which may be a dynamical field that evolves over time. The presence of dark energy is inferred from observations that suggest the universe's expansion is accelerating, and its density is small now due to the universe's age. This alleviates the problem of the dark energy density being too large compared to theoretical predictions. Observational evidence for dark energy comes from cosmological tests, including the thermal cosmic microwave background radiation, light element abundances, and the redshift-magnitude relation. These tests support the Friedmann-Lemaître model, which describes the universe's expansion in terms of dimensionless parameters. The current evidence suggests that the dark energy density is significant, with ΩΛ0 ≈ 0.7, indicating a non-zero Λ. However, the detection of dark energy is not yet conclusive, and further observations are needed to confirm its existence. The cosmological constant and dark energy are related to the vacuum energy of particles and fields. The zero-point energy of the electromagnetic field, for example, contributes to the cosmological constant. However, the value of this energy is much larger than what is allowed by the relativistic cosmological model, suggesting that dark energy may be a dynamical field that evolves over time. This dynamic nature of dark energy could explain why its density is small now and may approach zero in the future. The history of ideas about the cosmological constant and dark energy is rich and complex. Einstein initially introduced the cosmological constant to achieve a static universe, but later abandoned it after the discovery of an expanding universe. The concept of dark energy has since evolved, with various models proposing different forms of dark energy, including scalar fields and vacuum energy. The current understanding of dark energy is still developing, and further observations are needed to determine its nature and properties. The search for a fundamental theory of dark energy remains an active area of research in physics and astronomy.