Dark Energy and Cosmic Acceleration
The expansion of the universe is speeding up, driven by a mysterious component called dark energy that dominates the cosmic energy budget and acts as a repulsive form of gravity.
Definition
Dark energy is the component of the universe with negative pressure that drives the observed acceleration of cosmic expansion; in the standard model it is described by a cosmological constant, equivalent to a constant vacuum energy density, and it makes up roughly two-thirds of the total energy content.
Scope
This area covers the discovery that the cosmic expansion is accelerating, the cosmological constant and vacuum energy as the leading explanation, alternative dynamical models such as quintessence and modified gravity, and the equation of state and observational probes used to characterize dark energy.
Sub-topics
Core questions
- How was the acceleration of the universe discovered?
- What is dark energy, and is it simply a cosmological constant?
- How is the nature of dark energy being tested observationally?
Key concepts
- Dark energy
- Cosmic acceleration
- Cosmological constant
- Vacuum energy
- Equation of state
- Quintessence
- Negative pressure
Key theories
- Accelerating expansion
- Observations of distant Type Ia supernovae show they are fainter than expected, indicating that the expansion of the universe has been accelerating, which requires a component with negative pressure.
- Cosmological constant
- A constant vacuum energy density, represented by Einstein's cosmological constant, provides the simplest description of dark energy and fits the data, though its tiny value is theoretically puzzling.
Clinical relevance
Dark energy reshaped cosmology: its discovery established the accelerating, flat, Lambda-CDM universe, it accounts for about seventy percent of the cosmic energy budget, and its nature, whether a true constant or something dynamical, is among the deepest open questions linking cosmology to fundamental physics.
History
Two teams using Type Ia supernovae announced cosmic acceleration in 1998 and 1999, an unexpected result that revived the cosmological constant and earned the 2011 Nobel Prize; subsequent surveys of the cosmic microwave background, baryon acoustic oscillations, and large-scale structure confirmed a dark-energy-dominated universe.
Debates
- The nature of dark energy
- Whether dark energy is a true cosmological constant, a dynamical field such as quintessence, or a sign that general relativity needs modification on cosmic scales remains unresolved, with the cosmological-constant problem of its tiny value a central puzzle.
Key figures
- Saul Perlmutter
- Brian Schmidt
- Adam Riess
- Steven Weinberg
- Albert Einstein
Related topics
Seminal works
- riess1998
- perlmutter1999
Frequently asked questions
- What is the difference between dark matter and dark energy?
- Dark matter is unseen mass that clumps under gravity and helps form structure, while dark energy is a smooth component with negative pressure that pushes the expansion to accelerate; they have opposite effects and are believed to be entirely different things.
- Will the accelerating expansion continue forever?
- If dark energy is a true cosmological constant, the acceleration continues indefinitely, leading to an ever-emptier, colder universe; if dark energy evolves, the future could differ, which is one reason measuring its equation of state is so important.