Dark Energy, Cosmic Acceleration, and Late-Time Dynamics
The discovery of acceleration
Supernovae and beyond
Late-time acceleration was first inferred from Type Ia supernovae and is now supported by combined probes including the CMB, baryon acoustic oscillations, weak lensing, and growth measurements. The simplest description is a cosmological constant with equation of state .
Acceleration condition
Cosmic acceleration requires for the dominant component.
Phenomenological models
From CDM to dynamical dark energy
Common parameterizations include a constant , or the CPL form Alternative explanations include quintessence, k-essence, phantom energy, modified gravity, and inhomogeneity-based ideas, though each faces tight observational and theoretical constraints.
Late-time probes
| Probe | Primary sensitivity |
|---|---|
| Type Ia supernovae | Relative luminosity distances |
| BAO | Standard ruler and expansion history |
| Weak lensing | Growth of structure and geometry |
| Redshift-space distortions | Growth rate |
The cosmological constant problem
Vacuum energy tension
The observed vacuum energy density is extraordinarily small compared with naive quantum field theory estimates. This mismatch is one of the deepest open problems in theoretical physics and strongly motivates searches for symmetry principles, anthropic reasoning, or new dynamical mechanisms.
What is the equation of state parameter of a pure cosmological constant?
A cosmological constant has pressure , giving .
Correct answer:
What does BAO provide in cosmology?
The sound horizon imprinted in the galaxy distribution acts as a standard ruler.
Correct answer: a standard ruler
Growth of structure under acceleration
Background and perturbation interplay
Although acceleration suppresses the growth of linear density perturbations, structure continues to evolve because dark matter overdensities and the late-time dark-energy background coexist. Measuring both expansion and growth helps distinguish dark energy from modified gravity.
Which measurement is especially useful for separating dark energy from modified gravity?
Modified gravity can mimic expansion history but often predicts different growth behavior.
Correct answer: Both the expansion history and the growth rate of structure
Write the CPL parameterization of .
This common two-parameter form captures time variation in the dark-energy equation of state.
Correct answer: w(a)=w0+wa(1-a)