Cosmology Astronomy 315 Professor Lee Carkner Lecture 22 "In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move." -- Douglas Adams, The Restaurant at the End of the Universe

Key Questions  Is the universe infinite?  Does it have spatial boundaries?  Does it have a beginning and an end?  If the universe is not infinite:  How and when did it begin?  How and when will it end?  Where and what are its boundaries?

Caution  The universe is fundamentally unlike everyday objects  We shouldn’t think of the universe as just being a larger ordinary thing  It requires complex math to accurately describe the universe  We will often use analogy to get an idea of what the universe is like, but we shouldn’t take it literally

Types of Universes  Static  Universe does not change or evolve  Infinite in time  Universe has always been here and always will  Static and Infinite  Universe has always been here and has never changed  Infinite in Space  Universe extends endlessly in all directions

Infinite?  The universe is not infinite in space or time  Why?  If the universe was infinite everywhere you look you would see a star  Night sky would not be dark  Called Olber’s Paradox  Universe must be finite in space (run out of stars) or in time (light from some stars hasn’t reached us yet)

Olber’s Paradox

Static?  The universe is not static  Red shifts demonstrate that the universe is expanding  The universe is getting larger  It must have been smaller or more compact in the past  The universe changes and evolves

Expansion  We see all galaxies moving away from us, the most distant ones moving faster  From the Hubble Law: H = v/d  This indicates uniform expansion  Galaxies are not really moving, the space between them is getting “stretched out”  As the universe is stretching, the space between each galaxy increases, making them look as if they were moving

Raisin Bread

Geometry  What is the universe expanding into?  Nothing  Not just matter, but spacetime is expanding  The universe is everything we can observe  Speed of light is absolute limit  Can’t outrun the expansion  The universe may be finite but unbounded  Like the surface of a balloon

The Big Bang  If we run the universe backwards it gets smaller and smaller but has the same amount of matter  The universe was hotter and denser in the past  Called a hot Big Bang  The bang is still going on  Big Bang is a process, not an event

The Cosmic Microwave Background  The early universe was very hot and thus filled with high energy radiation  As the universe expanded the photons have been stretched out by the red shift to about 1 mm  This is detectable with radio telescopes as the cosmic microwave background  The glow from the big bang  CMB is a blackbody with a temperature of about 3 K

Cosmic Blackbody Spectrum

Dark Energy  Recently evidenced has been found for an accelerating universe  Some force is resisting gravity’s attempts to slow the expansion  Called dark energy  Best candidate is energy stored in the fabric of spacetime  Spacetime is “springy” and wants to expand  Dark energy and dark matter may be principle components of the universe

After the Big Bang  The early universe was an extremely strange place  So hot and dense that forces were unified and ordinary matter could not exist  As universe cooled, our present universe took shape  After about 1 million years atoms form  After about a billion years the material started to clump together to form stars and galaxies  Early universe must have been non-uniform for this to happen

Structure of the Universe

The Age of the Universe  We can find the age of the universe by extrapolating backwards  How long has it been since all the galaxies were in the same place?  Since v = d/t, t = d/v, but H = v/d, so:  Age of the universe = 1/H  If we can find H, we can easily get the age of the universe  Known as the Hubble time  For H = 75, age = 13 billion years

History of the Universe

The Fate of the Universe  How will the universe end?  It depends on whether there is enough mass in the universe to gravitationally halt the expansion  Three options:  Open  Universe expands forever  Closed  Universe stops expanding a contracts in a “Big Crunch”  Flat  Universe just barely expands

Curvature of the Universe

End of the Universe  The best observations indicate that the universe is flat  The universe will expand more and more slowly forever  Universe is effectively infinite in time in the forward direction  Beginning but no end

The End  The universe will eventually die a heat death  Stars burn out  Left with white dwarfs, neutron stars and radiation  Can live off of compact objects, but eventually will convert them all to heat  Universe becomes completely random radiation field

Next Time  Quiz #3  Covers everything since Quiz #2  Lectures  Chapters 11,  Same format as other quizzes