1. The Accelerating Universe The Hubble DiagramRedshiftStandard CandlesCepheid Variable StarsSupernovaeInterpretation

2. The Hubble Diagram 2 CZ (km/s) D (Mpc) ΔyΔy ΔxΔx Slope = Δy/Δx = H0

3. Redshift Redshift & Blueshift 3 when λeλe λoλo

4. 4 Absorption lines can be blue-shifted (galaxy is moving toward us) or redshifted (galaxy is moving away). The velocity can be measured by the shift in the wavelength: v = cz = (λo - λe)/λo λoλo λeλe

5. Standard Candles Cepheids & Supernovae 5 A standard candle is an astrophysical object whose absolute magnitude is well known. By comparing the relative intensity of light observed from the object, with that based on its known absolute magnitude, the inverse square law for light intensity can be used to determine the distance to the object.

6. The Inverse Square Law of Light 6 … relates the absolute luminosity, M of an object to its apparent luminosity m by m = M / r^2 where r is the distance.

7. Cepheid Variable Stars 7

8. NGC 4526 Supernova 1994D Supernovae 8

9. Original Hubble Diagram 9 CZ (km/s) D 4 3 2 1 0 D1D2 (Mpc) D3D4 Z4 Z3 Z2 Z1 Z0 D0

10. Interpretation H 4 > H 3 Decelerating Universe H 3 > H 2 Decelerating Universe H 2 < H 1 Accelerating Universe H 1 < H 0 Accelerating Universe H = kilometres per second per Megaparsec 10

11. Cosmological constant or Quintessence? 11 What next?

12. Only one option!!! 12

13. The Big Rip At last: proton decay 13