PES 106        Spring 2003
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General Astronomy II
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Lecture Notes:

Cosmology

text: Chapter 17 Sections 1 - 3

some references:

Introduction:

cosmology = study of Universe as a whole

Assume:

  • view from Earth is typical
  • our laws of physics are true everywhere

Universe looks the same in every direction

Expanding Universe and the Big Bang

Hubble found
  • (almost) all galaxies are moving away from us
  • speed of recession increases with distance from us

This suggests

  • UCCS is the center of the Universe
  • OR - the Universe is expanding

first choice violates our assumptions

[DEMO: balloon with dots, raisins in raisin bread] Link to animation of balloon analogy

Link to a JAVA applet showing expansion and redshifting

Note: the balloon "universe" has no center and no edge (might be true for ours)

Expansion is NOT matter expanding through space (like a normal explosion)

it is SPACE itself expanding

All galaxies are expanding away from all other galaxies - see Hubble's Law from any point in the Universe

Big Bang and the Age of the Universe:

If everything is expanding - it must have started out together in one place

Universe started with a "Big Bang"

When did this happen ? (How old is the Universe ?)

Use Hubble's Law:

  • age of Universe = 1/H
  • if H - 65 km/second/Mpc, then age = 15 billion years
  • this assumes
    • H is a constant (speed of recession is constant)
    • we know H pretty well (which we might not)
  • oldest stars are around 15 billion years old
    • too close for comfort

We are observing objects which are 10 billion light years away with current telescopes.

Often estimate the radius of the Universe as 15 billion light years (not really accurate)

Olber's Paradox and limits on the Universe

Universe must have a limit - otherwise the night sky would be bright

"dark" sky can be explained by

  • distance beyond which no stars exist
  • time before which no stars exist
  • expansion of the Universe produces big red-shift for distant stars

Cosmic Background Radiation

If Universe was confined in a small space - it must have been very hot
billions or trillions of degrees K => short wavelegth radiation (gamma rays)

As it expanded it must have cooled and radiation was stretched out in wavelength

models predict
  • around 3 degrees K today
  • radiation in the radio (microwave) part of the spectrum

radiation should be everywhere since it comes from the Universe itself

Observe cosmic microwave background radiation

  • very uniform in all directions
  • spectrum is shaped like a "black body" with a temperature of 2.726 K

Composition of early Universe

Probably sub-atomic particles: neutrons, protons and electrons

very close together - so combine into

  • Hydrogen atoms (easy)
  • Helium atoms (harder)
  • Lithium (very hard)

expect formatin of lots of hydrogen, some helium, a little lithium and not much else

current models predict about 76% Hydrogen and 24% Helium which agrees well with what we observe

Future of the Universe

based on properties of the Universe as a whole

Future depends on relative strength of

  • original expansion (pushing out)
  • gravity (pulling in)
  • cosmological constant and dark energy ????

Three possibilities (IF cosmological constant = 0)

  • open universe
    • space is curved
    • IF cosmological constant = 0
      • expands forever
      • slows down but never stops
      • expansion wins
  • closed universe
    • space is curved
    • IF cosmological constant = 0
      • expansion slows down and stops
      • compression begins leading to "Big Crunch"
      • gravity wins
  • flat universe
    • space is flat (no curvature)
    • IF cosmological constant = 0
      • universe stops expanding but does not collapse
      • exact balance between expansion and gravity

This picture is more complicated if the cosmological constant does NOT equal zero.

[Link to a more advanced discussion of the geometry of space: http://www.astro.ucla.edu/~wright/cosmo_03.htm]

How can we tell what will happen?

Measure total density

depends of mass and separation of matter => indicates strength of gravity
  • if density > "critical density" then universe is closed
  • if density < "critical density" then universe is open
  • if density = "critical density" then universe is flat

    critical density = 10-29 grams/cubic centimeter (one hydrogen atom per cubic meter)

    • depends on measurements of Hubble constant (H)

    measured density of universe = 2 x 10-31 grams/cubic centimeter (one hydgrogen atom per ten cubic meters)

    density is < critical density suggests that universe is open and will expand forever

    • unless there is more dark matter that we are not accounting for

Measure the slowing down of the expansion

=> the Hubble constant should change with time if expansion is slowing down
(not really a constant after all !!)
  • large deceleration => universe is closed
  • small deceleration => universe is open
  • intermediate deceleration => universe is flat

    need good measurements of the Hubble constant - difficult to do

    some recent measurements suggest an acceleration !!!

    is something causing the expansion to accelerate ??

    • maybe the Universe is filled with "dark energy" which acts against gravity

      Einstein's theory of relativity has a "cosmological constant" built in which could account for this.

      but what is it ????

    • Link to tutorial on dark energy: http://snap.lbl.gov/brochure/

 Best Guess at this point:

Universe is flat

Cosmological constant is NOT zero

Universe will expand forever

Current composition:

from a Lecture delivered at the National Conference of the American Association for the Advancement of Science (AAAS) February, 2003 by Raymond Orbach, Director, Office of Science, Department of Energy.

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