Friday March 26th 1999
Announcements:

Lecture notes:
Class 30.
Today will start talking about cosmology.
Cosmology - the study of the structure and evolution of the universe on the largest size scales and over the longest times.

Cosmology is active field of research where many ideas are still charging fairly rapidly - scientists are generally less certain
about the material in this part of the course than about stars galaxies etc.

The rapid changes lead to confusion sometimes.
Newspapers "crisis in cosmology"  e.g. people worked out age of universe but were finding that globular clusters appeared to be older than universe etc.

I will focus on the  things about cosmology that are relatively certain.  In particular we will talk about the Big Bang and the 3
pieces of evidence that support it most strongly.

• Expansion of the Universe.
• Primordial nucleosynthesis
• Cosmic microwave background.

The 3 pillars of the Big Bang cosmology.

The basic idea of the Big Bang is that the Universe was once much smaller and much hotter than it is today.  The Universe has been expanding since the Big Bang.

The Big Bang was not some explosion that occured out in space somehwere since there was no space before the Big Bang.  The fact that there was a Big Bang implies that the Universe is not infinitely old - was born in the past.  Best estimate for age of
the Universe is 10-18 billion years.

Does this mean that the universe has and edge and a center?  - somewhat suprisingly no.

Want to start by talking about 2 simple fundamental observations of cosmology
The sky is dark at night.  The Universe is expanding according to Hubble's Law.

Olbers Paradox.  Heinrich Olbers Vienese physician and astronomer discussed in 1826.  Actually paradox not first from
him.  Thomas  Digges 1756.  Kepler 1610.  Halley 1721.

Suppose the Universe were static eternal and uniformly filled with stars.  If look in any direction then your line of sight
would hit a star.  Thus why isn't every point on the sky as bright as a star?  Distant stars fainter but more of them.

At least one of the assumptions must be wrong.  Uniformly filled with stars assumption - must be wrong since stars are bunched
into galaxies - but doesn't solve problems.

Static and Eternal Assumptions - both of these wrong and contribute to solving problem mainly eternal part.

Static - we learned that galaxies receding - distant galaxies receding faster so very distant ones highly redshifted this makes
the light less energetic and weaker eventually to levels we cannot detect.  Helps to solve about 10% of the problem but
doesn't explain it all.

Main part of the solution is that Universe is not eternal first proposed by Kepler in 1610 and Edgar Allen Poe in 1848.

Universe is not infinitely old.  If we look far enough we look back to a time before stars started to shine.  Powerful idea! -
the Universe must have been born.

The expansion of the Universe.
Edwin Hubble first noted that the spectra of distant galaxies have redshifted spectral lines.
V=H₀D
v=velocity in km/s
d=distance in MPC
H₀=Hubble constant = 50-100 km/s / Mpc

We talked about this before as a distance measuring tool but today we want to use it in a more general way.  This law is
thought to hold everywhere in Universe.

Rough Analogy - loaf of raisin bread expanding.  Expansion of bread pushes raisins away from one another.  Also raisins that
start further apart move apart faster since there is more expanding bread between them.

There would be an analogous "Hubble Law" here.  H₀ is related to how fast the loaf is expanding.  No central point of expansion.

Now I need to discuss further the precise interpretation of the redshift.  Before we spoke of the galaxies as flying away.
Actually space itself expands and stretches the light.  This is the expansion of the Universe.