E6 Galaxies and Expanding Universe

E6.1 A galaxy is a cluster of stars. A galactic cluster is a cluster of galaxies. Galactic clusters grouped together is called a super galactic cluster.

E6.2 As the universe is expanding there spectra are red shifted. Galaxies that are the furthest away are red shifted the most.

E6.3 

Heinneman HL Physics, Chris Hamper

E6.4 Hubble's law is that the distance of celestial bodies is proportional to the recession velocity.

Heinneman HL Physics, Chris Hamper

The gradient of this line is called Hubble's constant. The value of this constant changes depending on the number of data points we obtain and plot.

Heinneman HL Physics, Chris Hamper

E6.6 The recession velocity is calculated by observing the red shift and using the equation above to obtain the recession velocity. The distance is calculated by using the three methods (Stellar parallax, Spectroscopic parallax and Cepheid variable method). Using this, we can plot the data points on the graph to obtain Hubble's Constant. Units: Recession Velocity in Kms^-1 and Distance in Mpc^-1

E6.8

Hubble's equation...

Hubble's constant is about 72kms^-1Mpc^-1

E6.5 The data points are scattered about the line of best fit which means that we have random errors in our measurement. Also the gravitational attraction slows down the recession speed of the galaxies but we are assuming that the recession speed is constant.

 We don't have data points near the origin for the graph because the red shift is so small that the errors in the apparatus become significant.

E6.7 Hubble's constant's units are kms^-1Mpc^-1 which is ditancetime^-1distance^‑1 which is time^-1 Therefore the inverse of Hubble’s constant is the age of the universe. 

Heinneman HL Physics, Chris Hamper

But we are assuming that the velocity is constant but we know that the universe is slowing down. Therefore the speed we measure today is higher than what it actually is which means that the time measured as the age of the universe is an upper limit.