How do Astronomers Measure distance

Cosmology

Chapter 3 and 4 Overheads

Another Big Bang:
The star Eta Carina explodes

How do Astronomers Measure distance?
Often it involves comparing the absolute brightness of an object with it’s apparent brightness
Radar Ranging (near earth)
Stellar Parallax (to 300 light years)
Spectroscopic parallax (to 30,000 light years)
Variable Stars especially Cepheids (to 45 Million lys)
Standard Candles (like supernova’s) (to 600 M lys)
Galactic rotation rates (to 600 M lys)
Hubble’s Law (all the way)

In the 1920’s Edwin Hubble wanted to determine if the spiral nebulae were in our galaxy.

He was able to measure the distance to them using Cepheid variable stars. What did this tell him?

He was also able to measure the velocity of the spirals using the Doppler effect. (Actually it is probably more correct to say that the light got stretch out by the expanding universe--- tired light)

He found that the farther away a galaxy is, the faster it is moving away from us!

This was a HUGE observation. What does it imply?

Not only that the universe as we know it had a beginning
But also that we can estimate the age of that universe!

Why does this imply that the early universe was HOT?

Expanding gasses tend to cool
Things that contract under gravity heat up (like an asteroid burning up in our atmosphere) If the universe is expanding then it must be cooling too.

Scientists could estimate how hot it was!

If it was too hot nuclei would be blasted apart

If it was too cold, they could never overcome their repulsion

If they could estimate how hot it was to begin with, it was just a matter of extrapolating forward until today.

They estimated that the background temperature of the universe would be 5K
(0K is absolute zero, where all motion stops)

This extrapolation also provided a prediction about the primordial abundances of the elements. (75% Hydrogen, 25% Helium)
(All of the larger elements must have formed in stars)

In the 1950’s there were two competing theories of the cosmos. The one that eventually won out was the big bang.

The other one was the steady state theory proposed by Hoyle and Bondi. It proposed that matter is continually forming at the center of the Universe.

It offered a few advantages.

No need to explain origin of the universe
Avoids tragic end to the universe
Earth rocks are 4.5 Billion YO. The universe seemed to be 1.8 billion years. (In fact it is now estimated to be 10 – 20 billion years old.)
Would allow for the formation of the heavy elements

Why was it eventually discarded?

BB correctly predicted …

Temperature of background radiation

Cosmic Background Radiation
Or Blackbody radiation
Glow from the early universe
Has now cooled to about

Relative abundances of Hydrogen and Helium

More Radio Galaxies, and Quasars in the distance (past)