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Cosmology Lecture 2 Summary from Lecture 1 The cosmological principle Universe is isotropic Universe is homogeneous The universe is expanding, with its current rate given by the Hubble constant Hubble constant is the rate of change of the scale factor at the current time. Today - the impact of relativity Relativity Implications of relativity Special Faster than light expansion General Principle of equivalence Space time curvature More details on relativity later in course and in 4th year Equivalence principle a g=a Inertial Mass = Gravitational Mass Space time curvature Light follows a null geodesic Newton Mass tells gravity how to exert a force Force tells mass how to accelerate Einstein Mass-energy tells space time how to curve Curved space-time tells mass energy how to move Curvature Flat Positive Negative Co-moving co-ordinates Time X=1 2 X=1 2 3 Real distance R = a(t) X 3 Curvature and scale factors Scale factor k>0 k=0 k>0 Time