Friday April 2nd 1999
Announcements:

Lecture notes:

Class 33

Review Cosmic Microwave Background

• from a time when Universe was a hot dense plasma soup.
• T was about 3000K
• age was about 300 000 years
• as photons fly across Universe they are stretched in wavelength by expansion of space so are cooler today.
• good evidence for Big Bang model - predicted to have a precise blackbody shape and COBE confirmed this.

COBE made a sensitive map of the temperature of the cosmic microwave background for all positions on the sky = isotropic.
The microwave background is remarkably smooth to 1 part in 400.  Then you see a dipole anisotropy = systematic differences in the temperature due to the motion of the Earth through space.
Doppler effect - blueshift redshift.

If you take out the dipole anisotropy you can look further for other structure - at a level of 1 in 50 000.  See Galactic
contamination and intrinsic anisotropy in the cosmic microwave background due to the fact that Universe at 300 000 years was not perfectly smooth.

Now we have been moving back to earlier and earlier times in this section of the class.  Scientists think they actually understand the Universe pretty well after about the first 10 seconds of its life.  This may seem surprising since as we talked about in the last class we cannot see this far back.  At times earlier than about 300 000 years the Universe is opaque - matter and radiation are coupled in a dense plasma soup and this prevents image formation - like a fog.

So if scientists cannot "see" any further back how do they know what happened at earlier times?  One of the most reliable ways is primordial nucleosynthesis.

primordial - very early
nucleo - like nuclei of an atom
synthesis - making something.
We learned that stars fusion burn elements in their cores thereby performing "nuclear alchemy" (stellar nucleosynthesis).
Indeed it is thought that stars created most of the elements heavier than Helium in the Universe.

However there is too much He in the Universe to be explained by stellar nuclear reactions and we know something else must have happened to make it.

about 75% of the atomic mass of the Universe is H
about 24% is He
about 1% is all the rest

So how was the Helium made?
During the first 17 minutes or so of its life the Universe was so hot that nuclear fusion reactions could occur in the plasma
"soup" of which the Universe was composed.

At 2 minutes temperature was about 900 million K
At 17 minutes the temperature was about 300 million K
T dropping due to expansion of Universe.

At times before 2 minutes we have.
Proton + Neutron -> Deuterium + energy.
Proton + Proton -> Deuterium + positron + neutrino

From about 2-17 minutes after the Big Bang nuclear reactions such as
proton + neutron - > deuterium + energy
²H + proton -> ³He + energy
³He + neutron -> ⁴He + energy
²H + ²H -> ⁴He + energy
make helium

After about 17 minutes temperatures became too cool for fusion to occur in the plasma.

Scientists can simulate these nuclear reactions in the early Universe with computers to predict how much H He etc. there
should be in the Universe ( Like Chemistry reactions)

The predict about 75% H by mass and about 24% He by mass.
Agrees with observation!

The sun is about 28% He by mass.  Got the extra 4% from stellar nucleosynthesis.

Thus just by studying the composition of objects around us we can learn about conditions just a few minutes after the Big Bang!  We can't "see" it but we can still detect its effects.

Scientists have done a great deal of work to precisely measure He Deuterium and Lithium to test the Big Bang and so far it
has passed the test.

If we find cosmic objects with less than 25% He Big Bang is in trouble.