1. Galaxy motions At the start of the 20 th century, the new technique of spectroscopy, allied with the large telescopes which had begun to become available, enabled observers to use the shifting of spectral lines to measure the velocities of galaxies. This was first done by Vesto Slipher in 1912. He measured the velocity of the Andromeda galaxy (M31), and found it moving towards us at 300 kilometres per second.

2. Hubble’s Law Andromeda turned out to be unusual in coming towards us. Over the next 20 years Slipher measured spectra of 40 more galaxies, and found that the vast majority were receding from us. Edwin Hubble, working with Milton Humason, was able to measure many more spectra. He discovered what we now know as Hubble’s Law. Edwin Powell Hubble

3. Hubble’s Law What Hubble discovered was that the velocity of a galaxy was proportional to its distance from us: v = H d where H is a constant, now known as Hubble’s constant.

4. Hubble’s Law What does it mean? 1) Galaxies are currently moving apart from one another. Our Universe is expanding! 2) Almost certainly the Universe has a finite age, and expanded to its present size from a highly dense initial state, sometimes called the Big Bang. 3) It does not mean that there is a center to the “explosion”.

5. Hubble’s Law Hubble’s Law has just the right form that every observer sees all galaxies receding away such that the Hubble Law is satisfied.

6. Hubble’s Constant Hubble’s Law states that v = H d. The proportionality constant is Hubble’s constant, and its value tells us how fast the Universe is expanding. But what is that value?

7. Hubble’s Constant Attempts over 70 years have been made to measure the constant. H = v/d Typical velocities are measured in kilometres per second. Typical galaxy separations are measured in megaparsecs. So H should be measured in kilometres per second per megaparsec.

8. Hubble’s Constant Hubble’s original measurement gave a Hubble constant of H = 500 km s -1 Mpc -1 We now know that this is far too high, because Hubble drastically underestimated the distances to his galaxies. That is, a galaxy one megaparsec away is expected to be receding at 500 km s -1 Mpc -1.

9. Present attempts The correct answer is now believed to be between 50 and 100 km s -1 Mpc -1, though there is still controversy as to its exact value. In 2001, a group led by Wendy Freedman arrived a a value of H using the Hubble Space Telescope of 72 +/- 8 (km/s)/Mpc. Data from the CHANDRA x-ray satellite gave a value of 77 (km/s)/Mpc +/- 15%. In 2010, data from the WMAP satellite were used to determine a value of 71 +/- 2.5 (km/s)/Mpc