Thermodynamics of The Universe - Dark Energy and Cosmic Inflation

Dark Energy and Cosmic Inflation

Dark energy is a hypothetical form of energy that permeates all of space, and causes an acceleration in the expansion of the universe due to its strong negative pressure: in general relativity, pressure has a gravitational effect similar to that of energy and mass, and while positive pressure causes gravitational attraction and thus decelerates the expansion of the universe, negative pressure causes gravitational repulsion and thus accelerates the expansion of the universe.

According to the equation above,

Thus the more negative the pressure is, the less the energy density reduces as the universe expands. In other words, Dark energy dilutes less than any other form of energy, and will therefore eventually dominate the universe, as all other energy densities gets diluted faster with the expansion of the universe.

In fact, if the dark energy is created by a cosmological constant or a constant scalar field, then its pressure is minus its energy density, and therefore its energy density remains constant (as is expected by definition).

Dark energy is usually assumed to be the Casimir energy of the vacuum, with possible contributions from the energy density of scalar fields which has a non-zero value at the vacuum. It may be that this field can decay at some time in the distant future, leading to a new vacuum state, different than the one we are living in. This is a phase transition, where the dark energy is reduced and huge amounts of energy in conventional forms (i.e. particles) are produced.

Such a series of events is in fact thought to have already occurred in the early universe, where first a cosmological constant much larger than the present one came to dominate the universe, bringing about cosmic inflation. At the end of this epoch, a phase transition occurred where the cosmological constant was reduced to its present value and huge amounts of energy where produced, from which all the radiation and matter of the early universe came about.