PHY306 1 Relativistic models General relativity allows one to calculate the behaviour of space-time in the presence of mass-energy →PHY314 To see basic behaviour  use Newtonian approximation where possible  assume some GR results

PHY306 2 The Friedmann equation Sphere of mass M, radius R S, expanding or contracting where R S = a(t) r S and r S is the radius of the sphere now RSRS

PHY306 3 The Friedmann equation If U > 0  RHS always positive  universe expands forever If U = 0  RHS → 0 as t → ∞  universe expands at ever-decreasing rate If U < 0  ρ(t)  (r S a) −3  U-term  (r S a) −2  at a = −4πGρ 0 /3Ur S expansion reverses  universe headed for a Big Crunch

PHY306 4 The Friedmann equation Replace mass density by energy density  ε = ρc 2  contributions from photons etc. as well as matter Replace U by curvature  k = +1 corresponds to U < 0 (Big Crunch)  R 0 corresponds to present radius of curvature relativistic form

PHY306 5 Ω Energy density for k=0  ε c = 3c 2 H 2 /8πG  define Ω ≡ ε(t)/ε c (t)  then H 2, R 2, a 2, c 2 all positive  Ω = 1 ↔ k = 0  Ω < 1 ↔ k < 0  Ω > 1 ↔ k > 0

PHY306 6 Λ Friedmann equation (like Newton) not static  Einstein believed Universe is static (pre-Hubble)  introduced cosmological constant Λ to allow this  basically an integration constant in Einstein’s equations  can also be expressed as Ω Λ = Λ/3H 2 ; then usually call density parameter Ω m to distinguish the two

PHY306 7 The fluid equation Friedmann equation has two unknowns: ε (or ρ) and a  need another equation  try thermodynamics: dQ = dE + P dV  energy in volume V is E = εV; dE = Vdε + εdV  V  a 3 so dV/V = 3da/a  dQ = 0 for expansion of universe the fluid equation heat supplied change in internal energy work done

PHY306 8 The acceleration equation Multiply by a 2 and differentiate: Simplify: Always deceleration unless pressure is negative!

PHY306 9 The equation of state Unfortunately we have now introduced a third variable, the pressure P!  need to relate P and ε (or ρ)  this relation depends on the substance under consideration  it is called the equation of state of the substance  some useful equations of state:  non-relativistic gas: P = nk B T = ρk B T /μ where μ is particle mass –since 3k B T = μ  v 2 , we have P/ε =  v 2  /3c 2 << 1, i.e. P m ≈ 0  radiation (ultra-relativistic): P/ε = ⅓ so P r = ⅓ ε r  Λ: as its energy density is constant with time, P Λ = −ε Λ –this gives acceleration, since ε + 3P < 0

PHY The fluid equation revisited Radiation  P r = ⅓ ε r →  ε r  a −4 Nonrelativistic matter  P m ≈ 0 →  ε m  a −3   P Λ = − ε Λ →  ε Λ = constant More general form  P = wε →  ε  a −3(w + 1)

PHY Cosmological models Can now substitute in  ε = ε 0 a −3 (matter),  ε = ε 0 a −4 (radiation), or  ε = ε 0 (Λ) to get a differential equation for a(t) First try this with single components… in real life a combi- nation of all three!