Cosmological Constant (Vacuum Energy)
theoretical prediction · Einstein's 1917 paper 'Cosmological Considerations in the General Theory of Relativity' (1917)
The cosmological constant is a term in Einstein's field equations of general relativity that represents a uniform energy density of the vacuum, capable of driving the accelerated expansion of the universe.
Core Concepts
The Problem
Why is the vacuum energy so much smaller than quantum field theory predicts, and why does it appear to dominate only at the present epoch?
The Claim
The acceleration of the universe's expansion—discovered in 1998 via Type Ia supernovae—is most simply explained by a positive cosmological constant.
Key Evidence
- •Observations of distant Type Ia supernovae (1998) indicating acceleration
- •Cosmic microwave background measurements from WMAP and Planck
- •Large-scale structure surveys showing consistency with a cosmological constant
Practical Implication
A positive cosmological constant implies that the universe will continue to expand at an accelerating rate, ultimately leading to a 'heat death' or 'big freeze' scenario.
Nuance & Limits
While the cosmological constant is the simplest model, it may not be truly constant; alternative theories invoke dynamical scalar fields ('quintessence') or modifications to general relativity. The smallness of its energy scale remains unexplained.
Source Material
Citation Density
vast (tens of thousands of papers)
Related Ideas
Dark energy is the generic term for whatever drives cosmic acceleration; the cosmological constant is the leading candidate.
Gaps
- ⚠ The cosmological constant problem: why is the observed vacuum energy 120 orders of magnitude smaller than naive quantum field theory estimates?
- ⚠ The coincidence problem: why do matter and dark energy densities happen to be comparable today?
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