Definition
Dark Energy is a hypothetical form of energy that permeates all of space and exerts a negative, repulsive pressure, which is thought to be responsible for the observed accelerated expansion of the universe. It accounts for approximately 68% of the total energy density of the universe.
Why It Matters
Dark energy is the dominant force in the universe, dictating its ultimate fate. Understanding it is the key to solving the greatest mystery in cosmology and discovering whether our universe will expand forever into a “Big Freeze” or end in a “Big Rip.”
Core Concepts
- Accelerated Expansion: Observations of distant supernovae in the late 1990s revealed that the expansion of the universe is not slowing down due to gravity, but is actually speeding up.
- Cosmological Constant (): A term introduced by Einstein in General Relativity, now often associated with the constant energy density of empty space (vacuum energy).
- How to read: “The symbol Lambda.”
- Meaning: The cosmological constant—uniform vacuum energy density driving accelerated expansion in the -CDM model.
- Quintessence: An alternative theory proposing a dynamic, time-evolving field as the source of dark energy, rather than a constant.
- Fate of the Universe: Depending on the nature of dark energy, the universe may end in a “Big Freeze” (eternal expansion) or a “Big Rip” (if the repulsive force eventually tears apart galaxies and atoms).