Wednesday March 31th 1999
Announcements:

Lecture notes:

Class 32

In next 2 classes we will talk about two other main pieces of evidence for the Big Bang - from very early Universe.

Cosmic Microwave Background - 300 000 years after Big Bang.  The very early Universe was
hot and smooth.  A soup of plasma - free electrons and protons. Stars planets galaxies had not yet formed.

We have also talked about how when you look out into space you also look back in time.  About 2 million years for Andromeda Galaxy.  About 10 billion years for most distant quasars.

A natural question then arises.  If we can look back even further past the quasars can we see the very early Universe
when it was hot and smooth?  The answer is Yes!

In the mid 1960s people accidentally discovered radiation that was emitted when the Universe was just 300 000 years old.  At the time the Universe was hot (3000K) almost as hot as the Sun's surface.  Smooth to 1 part in about 100 000.

Two radio astronomers were working with a radio telescope at Bell Laboratories in New Jersey.  They were trying to improve
America's phone system.  They needed to measure the brightness of the sky at microwave/radio frequencies but they kept getting a peculiar source of "noise" in their radio dish.  They couldn't understand this and wanted to get rid of it.  Tried checking system components moving pigeons out of radio telescope cleaning pigeon droppings.  Didn't get rid of noise - they found that the sky was "glowing" in microwaves - uniform in all directions.

Penzias and Wilson didn't understand what they had found - talked to people at Bell Labs and Princeton.  Found out that theorists had predicted such radiation to be left over from Big Bang as early as 1948.  This radiation started out at about 3000K but cooled as Universe expanded.  Wavelength was stretched to cooler temperatures.  Penzias and Wilson won the 1978 Nobel Prize.

Since its discovery the cosmic microwave background has been studied extensively.  Because it can tell us important things
about the early Universe.  Important questions.  What is the spectral shape of the microwave background?

The microwave background has a continuous spectrum.  Blackbody shape with T=3K (2.726K) We talked earlier about blackbodies for star spectra.  It is the spectrum emitted by a hot dense object.

Just like the light from quasars the microwave background is cosmologically redshifted as it travels to us.  CBR photons are
"stretched" to longer wavelength by expansion of the Universe.

Longer wavelength = cooler temperatures.  By the time they reach us they are only about 3K.

Can we look past the microwave background to make pictures of the Universe at even earlier times than 300 000 years?  No.  The Universe at earlier times was opaque.  Light could not travel through it freely.  The early Universe was hot and dense
(smaller) and electrons and protons were free.  The elections and protons scattered photons so they could not travel freely.

Like having a dense fog filling the classroom.  Students in the back couldn't see the front because photons cannot travel
between them.

Eventually the hot plasma thinned out and the electrons and protons coupled into atoms.  Then the photons could fly freely
through the Universe.

Can you see the microwave background?  Yes neat trick.  Turn your TV set to an empty channel where there is static.
About 1% of the blips on the screen are due to microwave background photons that have traveled billions of years through
space.

Modern Microwave Background Studies.

Studied intensively today from ground and by satellite.  Some of the biggest advances came from a satellite launched in 1989 called COBE (COsmic Background Explorer)  It did 2 things really well.

•  Measured the spectrum of the microwave background very precisely.  Showed it had the blackbody spectral shape predicted by the Big Bang model.  When first shown at a meeting of the American Astronomical Society it got a 2 minute  tanding ovation.
•  Made a precise map of the microwave background intensity in different directions.

Microwave background is quite uniform.  On a 0-3.6K scale you see no structures.  Then take out average value -6.6 to 6.6 mk 6.6 mk/2.7k = .2%

Then take out dipole
-.0273 mk to + .273 mk
.273mk/2.7k = 1E-4

See radiation from our Galaxy.  Away from our Galaxy see excess noise.  Universe was not perfectly smooth at early times.  Seeds of Galaxy formation.