The concept of dark energy having a constant density is based on one of the leading theories in cosmology, the cosmological constant, which is often associated with dark energy. According to this theory, dark energy is a form of energy that permeates the entire universe and exerts a negative pressure, causing the accelerated expansion of the universe.
The reason why dark energy with a cosmological constant has a constant density is related to its fundamental properties. In this theory, dark energy is postulated to have a fixed energy density that remains constant throughout space and time. It does not dilute or dissipate as the universe expands.
The constant density of dark energy can be explained by invoking the concept of vacuum energy. In quantum field theory, empty space is not truly empty but is instead filled with fluctuations of fields, often referred to as the vacuum. These fluctuations can give rise to an inherent energy associated with empty space.
According to the cosmological constant theory, dark energy arises from this vacuum energy. The cosmological constant is a term introduced into Einstein's field equations of general relativity to represent this vacuum energy. It is essentially a constant energy density that is uniformly distributed throughout space, providing a repulsive gravitational effect.
Since the energy density of the cosmological constant remains constant, even as the universe expands, it leads to the accelerated expansion of space. As the universe grows, more space is created, but the energy density of dark energy remains the same, resulting in a dominant influence on the expansion dynamics.
However, it's important to note that the nature of dark energy is still not fully understood, and the cosmological constant is just one possible explanation. There are alternative theories that propose time-varying dark energy densities or different mechanisms to explain the accelerated expansion. Ongoing observational and theoretical research aims to shed further light on the true nature of dark energy and its density.