Dark Energy Definition & Properties

• Dark energy causes universe expansion to accelerate
• Astrophysicists believe universe expansion is getting faster
• Matter alone cannot cause acceleration - only slows expansion
• Requires negative pressure to accelerate expansion
  • Normal matter & dark matter lack negative pressure
  • Dark energy has negative pressure
• May not be made of actual particles
  • Negative pressure is mathematical construct fitting observations
  • Particle interpretation may be misleading
• "Dark" is misleading term
  • Doesn't swallow light like black holes
  • Light passes through - actually transparent, not dark

Dark Energy vs Dark Matter Differences

• Dark energy: makes universe expand faster
• Dark matter: makes galaxies rotate faster
• Dark matter lacks negative pressure characteristic of dark energy
• Two phenomena have different effects
• Some physicists speculate common origin but unproven

Cosmological Constant Relationship

• Cosmological constant = simplest type of dark energy
• Remains constant over time
• Doesn't change when universe expands
• Weird property: doesn't dilute like matter/radiation
  • Normal matter/radiation density decreases as volume expands
  • Dark energy density stays constant despite expansion

Energy Conservation Question

• Appears to violate energy conservation (constant density + expanding volume = more total energy)
• Actually doesn't violate conservation in general relativity
  • Energy not conserved quantity in general relativity when space volume changes
  • Local conservation of energy densities is fulfilled
  • Mathematics works correctly

Historical Context & Predictions

• Einstein introduced then discarded cosmological constant
• Modern astrophysicists think necessary for observations
• Has small positive value
• Common claim: calculations give result 120 orders of magnitude larger than observed
  • Called "worst prediction ever"
  • Actually not a true prediction
  • Calculated constant is unobservable, doesn't correspond to measurements
  • Measurable cosmological constant is free parameter in Einstein's theory
  • Cannot be calculated by current theories

Dark Energy Generalization

• Dark energy generalizes cosmological constant concept
• Allows energy density & pressure to change with time & space
• Could be field filling entire universe

Observational Evidence

• Part of concordance model of cosmology (Lambda-CDM)
  • Lambda = cosmological constant
  • CDM = cold dark matter
• Not from single observation but combination:
  • Matter distribution in universe
  • Cosmic Microwave Background properties
  • Supernovae redshifts
• Necessary to make concordance model fit data
• Some physicists claim data wrongly analyzed
  • Question whether expansion actually speeds up