Cosmology Astronomy 315 Professor Lee Carkner Lecture 24

Key Questions  Is the universe infinite?    If the universe is not infinite:  How and when did it begin?   Where and what are its boundaries?

Caution   We shouldn’t think of the universe as just being a larger ordinary thing   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   Infinite in time   Static and Infinite   Infinite in Space  Universe extends endlessly in all directions

Infinite?  The universe is not infinite in space or time   If the universe was infinite, everywhere you look you would see a star    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   The universe is getting larger   The universe changes and evolves

Expansion  We see all galaxies moving away from us, the most distant ones moving faster  How are distance and velocity related?   This is the Hubble Law:     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?   Not just matter, but spacetime is expanding   Speed of light is absolute limit   The universe may be finite but unbounded  Like the surface of a balloon

The Big Bang   The universe was hotter and denser in the past  Called a hot Big Bang   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   This is detectable with radio telescopes as the cosmic microwave background (CMB)   CMB is a blackbody with a temperature of about 3 K 

Cosmic Blackbody Spectrum

Nucleosynthesis  If the hot Big Bang model and stellar energy generation models are true, what elements should we see?   Created in first few minutes of universe   Easily destroyed in reactions in stars   Such as O and C

After the Big Bang  The early universe was an extremely strange place   As universe cooled, our present universe took shape   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   Since v = d/t, t = d/v, but H = v/d, so:   If we can find H, we can easily get the age of the universe   For H = 73, age = 13.7 billion years

History of the Universe

The Fate of the Universe  How will the universe end?  Depends on if their enough mass in the universe to gravitationally halt the expansion   Three options:  Open   Closed   Flat  Universe just barely expands

End States of the Universe

Dark Energy   Some force is resisting gravity’s attempts to slow the expansion   Best candidate is energy stored in the fabric of spacetime   Dark energy and dark matter may be principle components of the universe

Possible Expansion Scenarios

End of the Universe  The best observations indicate:   Dark energy is trying to increase the expansion of the universe   Universe is effectively infinite in time in the forward direction  Beginning but no end

The End  The universe may eventually die a heat death   Left with white dwarfs, neutron stars and radiation  Can live off of compact objects, but eventually will convert them all to heat 

Next Time  No reading or homework for Wednesday or Friday  Meet in planetarium lecture hall Friday  Final exam next Monday at 3pm