Physical Cosmology - Energy of The Cosmos

Energy of The Cosmos

Light elements, primarily hydrogen and helium, were created in the Big Bang. These light elements keep being spread too fast and too tightly in the Big Bang process (see nucleosynthesis) due to which they formed stable medium-sized atomic nuclei, like iron and nickel. This fact allowed for later energy release which leads to intermediate-sized element formation in our time. The formation of such atomic power and the steady energy-releasing reactions in stars, also contributes to sudden energy releases, such as in novae. Gravitational collapse of matter into black holes is also thought to power the most energetic processes, generally seen at the centers of galaxies (see quasars and active galaxies).

Cosmologists are still unable to explain all cosmological phenomena exactly on the basis of known conventional forms of energy, such as those related to the accelerating expansion of the universe. Cosmologists therefore invoke a yet unexplored form of energy called dark energy(dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe) which account for certain cosmological observations mostly related to "accelerating expansion of Universe". One hypothesis is that dark energy is the energy of virtual particles (which mathematically must exist in a vacuum due to the uncertainty principle).

There is no definite or clear way to define the total energy of the universe due to the most widely accepted theory of gravity, general relativity. As a result it remains controversial whether one can meaningfully say that total energy is conserved in an expanding universe. For instance, each photon that travels through intergalactic space loses energy due to the redshift effect. This energy is not obviously transferred to any other system, so seems to be permanently lost. On the other hand, some cosmologists insist that energy is conserved in some sense; this follows the law of conservation of energy.

Thermodynamics of the universe is a field of study that explores which form of energy dominates the cosmos - relativistic particles which are referred to as radiation, or non-relativistic particles which are referred to as matter. The former are particles whose rest mass is zero or negligible compared to their energy, and therefore move at the speed of light or very close to it; the latter are particles whose kinetic energy is much lower than their rest mass and therefore move much slower than the speed of light.

As the universe expands, both matter and radiation in it become diluted. However, the universe is also cooling down, meaning that the average energy per particle is getting smaller with time. Therefore the radiation becomes weaker, and dilutes faster than matter. Thus with the expansion of the universe, radiation becomes less dominant than matter. The very early universe is said to have been 'radiation dominated' and as such radiation controlled the deceleration of expansion. Later, as the average energy per photon becomes roughly 10 eV and lower, matter dictates the rate of deceleration and the universe is said to be 'matter dominated'. The intermediate case is not treated well analytically. As the expansion of the universe continues, matter dilutes even further and the cosmological constant becomes dominant, leading to an acceleration in the universe's expansion.