Cosmology

Astronomy 315Professor Lee

CarknerLecture 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