1. The Hubble Constant

2. Henrietta Leavitt had worked out how to measure the distance to the galaxies.
Edwin Hubble was able to calculate how fast they are moving.

3. Hubble’s Findings.
The further away a galaxy is the faster it is moving AWAY from the our galaxy (The Milky Way).

4. Where v is the velocity of recession and d is the distance
The Hubble constant
The relationship between the velocity of recession of the galaxies and distance gives us a measure of how fast the universe is expanding and therefore the time elapsed since the big bang.
The gradient of the graph is called the
HUBBLE CONSTANT (H)
And the graph gives us the relationship
v=Hd
Where v is the velocity of recession and d is the distance

5. The Hubble constant v=Hd
There has been much trouble establishing a value for the Hubble constant because the measure of distance (d) to distant galaxies depends on succesively innacurate approximations.

6. The Hubble constant v=Hd
The most accurate measure of distance is trigonometric parallax which is not applicaple to galaxies because they are so far away.
Hubble relied on Cepheid variable stars as standard candles.
This technique was first used by Henrietta Leavitt to measure the distance to a nearby galaxy

7. The Hubble constant v=Hd
Then the brighter Suprenovas were used as standard candles in Galaxies further away. This technique is less accurate than the more reliable Cepheids.
For very distant galaxies the assumption was that all spiral galaxies have the same dimensions a very uncertain assumption

8. The Hubble constant The most recent observational
determination of the Hubble constant
obtained in 2009 by
using the HST
is H = 74.2 ± 3.6 (km/s)/Mpc.
The SI unit for the hubble constant is s-1
And in this unit H is about about
2.5×10−18 s−1
For the first time the rate of expansion of the universe is pretty much fixed

9. Calculating the age of the Universe
The age of the universe can be calculated from this graph by assuming that the Hubble constant has remained the same over time.
The age of the universe calculated in this way is sometimes called the Hubble time and is given by
Th = 1/H
It gives a very dubious rough value for the age of the universe.

10. Quasars
Quasars (Quasi Stellar Objects) were first discovered in the nineteen sixties. The appear to be point sources like stars but they are very intense radio sources and redshift measurements indicate that they are incredibly distant. They are the most distant objects in the universe that can be identified.
The First Quasar discovered
It is now believed that a quasar is a compact region in the centre
of a massive galaxy surrounding a supermassive black hole.
Its size is 10-10,000 times the Schwarzschild radius of the black hole.
The quasar is powered by the accretion disk around the black hole.