Lecture 23: The Background Radiation- Static from the Edge

"The more the universe seems comprehensible, the more it also seems pointless."

Steven Weinberg, The First Three Minutes

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    • Date: April 25, 1995
    Reading Assignment: pp. 603-612

    Description : 3K Background Radiation and Tests of Cosmology

    Objectives

  • be able to describe how we can determine the age of the universe using Hubble constant and the average density in the universe
  • be able to define the Cosmological Constant
  • be able to describe observational tests to determine the future fate of the universe and its deceleration
  • be able to describe what the Cosmic Background Radiation is and why it is important in cosmology

    • Lecture Outline

    Slide # 1: The Background Radiation

  • Static From the Edge
    • Slide # 2: Today's Lecture
  • expansion and redshift
  • the Cosmological Constant
  • the Steady State Universe
  • the Big Bang
  • 3K Background Radiation
    • Slide # 3: Cosmological Questions
  • what was it like in the "old days"
  • early universe
  • what will it be like tomorrow
  • fate of the universe
  • why is it like this now?
    • Slide # 4: The Scientific Method
  • testing is ESSENTIAL!!!!
  • something is not scientific unless it is subject to experimental verification
    • Slide # 5: Lost in a Big Universe (GRAPHICS)
  • every direction you look, you see a star's surface
    • Slide # 6: Olber's Paradox
  • if the universe is infinitely old, infinitely large, and uniform
  • why isn't the sky bright?
  • it should be the surface temperature of stars
  • answer - the universe is not infinitely old, infinitely large, and uniform
  • expansion can also solve this problem
    • Slide # 7: The Cosmological Principle
  • the universe is isotropic and homogeneous
  • we are not in a special place
    • Slide # 8: Hubble's Law
  • Hubble's law must obey the Cosmological Principle.
  • Everything must be expanding from everything.
    • Slide # 9: Universal Expansion (GRAPHICS)
  • a loaf of bread with raisins
    • Slide # 10: Universal Expansion (GRAPHICS)
  • as the bread rises, the distance between raisins increases
    • Slide # 11: Universal Expansion
  • the amount of expansion depends on the initial distance between raisins
  • the rate of expansion depends on the distance between raisins
    • Slide # 12: The Universe
  • the rate space between galaxies is expanding depends on the distance between t
  • Hubble's law implies UNIVERSAL EXPANSION
    • Slide # 13: Will the Expansion:
  • continue forever
  • stop
  • reverse
    • Slide # 14: Gravity and the Universe
  • the total mass of the Universe is slowing the expansion
  • the higher the mass, the more expansion will be slowed down
  • the fate of the universe depends on its density
    • Slide # 15: The Universe (GRAPHICS)
  • three possibilities
    • Slide # 16: Critical Density
  • density needed to close the universe
  • 1029 gm/cm3
  • about 6 hydrogen atoms per cubic meter
    • Slide # 17: Vote for Your Universe
  • open- eternal expansion
  • flat - exactly at critical density
  • close - the Big Crunch- the winner!
    • Slide # 18: Fate of the Universe
  • rate of expansion
  • kinetic energy
  • density of the universe
  • total gravitational energy
  • the Cosmological Constant
    • Slide # 19: Cosmological Constant
  • invented by Einstein to offset gravity
  • though the universe was static- no expansion
  • it may have some effect on the fate of the universe
  • we don't know if it is important
    • Slide # 20: The Cosmological Constant
  • invented to preserve an infinitely old, static universe
  • before Hubble expansion was measured
  • the Steady State Universe
    • Slide # 21: The Steady State Universe
  • infinitely old
  • infinitely large
  • constant density
    • Slide # 22: The Steady State Universe
  • infinitely old
  • infinitely large
  • constant density
  • expanding (Hubble's law)
  • CONTINUOUS CREATION OF MATTER
    • Slide # 23: The Steady State Universe (GRAPHICS)
  • expansion creates hydrogen atoms
    • Slide # 24: Continuous Creation (GRAPHICS)
  • one hydrogen atom appears every year in a one-cubic mile volume
    • Slide # 25: Problems with Steady State
  • doesn't conserve energy
  • no observations of continuous creation
  • evolution of galaxies
    • Slide # 26: The Big Bang
  • all galaxies were closer together in the past
  • Hubble's law in reverse
  • in the distant past, the universe would be a point
  • extension of Hubble's law
  • perhaps everything expanded from a single point in huge explosion
  • the BIG BANG
    • Slide # 27: The Big Bang
  • finite age
  • infinite size (?)
  • expanding
  • universe formed at one time
    • Slide # 28: Constellation Corner (GRAPHICS)
  • Constellation De Jour
    • Slide # 29: Binoculars (GRAPHICS)
  • relatively cheap
  • $40-$500
  • easy to use
  • can be used terrestrial observations
    • Slide # 30: Buying Binoculars (GRAPHICS)
  • objective size
  • determines light gathering power
  • magnification
  • determines size of image
    • Slide # 31: Useful Binoculars (GRAPHICS)
  • too high magnification makes it difficult to hold steady
  • too small an objective makes it difficult to see dim objects
    • Slide # 32: Winter Sky (GRAPHICS)
  • Jan 15 - Midnight - all sky
    • Slide # 33: Winter Sky (GRAPHICS)
  • Jan 15 - midnight - all sky
    • Slide # 34: Spring Sky (GRAPHICS)
  • Mar 15 - midnight - all sky
    • Slide # 35: Spring Sky (GRAPHICS)
  • Mar 15 - midnight - all sky
    • Slide # 36: Summer Sky (GRAPHICS)
  • Jun 15 - midnight - all sky
    • Slide # 37: Summer Sky (GRAPHICS)
  • Jun 15 - midnight - all sky
    • Slide # 38: Star Maps (GRAPHICS)
  • April 18- midnight- 4.0
    • Slide # 39: The Big Bang
  • finite age
  • infinite size (?)
  • expanding
  • universe formed at one time
    • Slide # 40: Finite Age (GRAPHICS)
  • rapid expansion implies a young universe
  • things closer together in the recent past
  • slow expansion implies an older universe
  • things closer together in the more distant past
    • Slide # 41: The Universe
  • three possibilities
    • Slide # 42: Age of the Universe
  • related to 1/H
  • age = 9-20 Billion years
  • depends on Hubble constant
  • density of Universe
  • Cosmological Constant
    • Slide # 43: The Background Radiation
  • the universe was hotter
  • hydrogen was ionized and opaque
  • the gas temperature was about 10,000K
  • it occurred at a high redshift
  • it would appear to be a 3K black body
    • Slide # 44: The Early Universe
  • galaxies used to be closer together
  • same energy, less volume
  • the universe was HOTTER
  • much hotter in the distant past
    • Slide # 45: A Hot, Early Universe
  • at some point in the past, it was hot enough to ionize hydrogen
  • no neutral hydrogen
  • the universe was opaque
  • like a photosphere of a star
  • photons can't pass through it without scattering
  • universe would be 10,000K black body
  • thermal radiation- Planck Curve
    • Slide # 46: Look-Back Time
  • the distant past will be at a high redshift
  • speed of light is finite
  • we can see the distant past in all directions
    • Slide # 47: High Redshift
  • the spectra of the ionized hydrogen will be highly redshifted
  • long ago = big redshift
  • redshift changes the temperature of the observed Planck curve
  • ionized hydrogen in the early universe will appear as a 3K black body
    • Slide # 48: Cosmic Background Radiation
  • a prediction of the Big Bang
  • would appear to be isotropic and a 3K thermal curve
    • Slide # 49: Penzias and Wilson
  • accidently discovered the cosmic background radiation in 1964
  • worked for Bell Labs
  • awarded Nobel Prize in 1978
    • Slide # 50: COBE
  • best ever measurements of cosmic background radiation
  • excellent agreement with big bang model
  • temperature = 2.735 K
    • Slide # 51: Science
  • form hypothesis
  • conduct experiment
  • verify or negate hypothesis