Cosmic Microwave Background Radiation II

• Figures borrowed from: Smoot Group CMB Anisotropy Page
•  Princeton CMB Group

Why Study the Cosmic Microwave Background Radiation (CMBR)?

• Test the Big Bang Theory
• The Big Bang Predicts a black body spectrum
• if the Universe is homogeneous and isotropic, then the CMBR should be isotropic
• i.e. the same temperature in every direction
• Study the Universe when it was simple
• CMBR probes the Universe at t = 300,000 years
• current Universe is "messy"
• visible Universe is dominated by stars and galaxies
• stars and galaxies are complicated
• formation of stars and galaxies cannot be calculated in detail
• stars and galaxies are very much denser than the average density of the Universe
• galaxies > 100 times denser than average
• stars > 10²⁰ times denser than average
• At  t = 300,000 years, the Universe was simpler
• density constant to better than 0.1%
• allows simple approximations
• perfectly uniform Universe with small corrections
• Learn the absolute velocity of the Sun through space
• Learn about the primordial density fluctuations which grew to form galaxies
• Observations are difficult
• atmosphere (especially water vapor) is not transparent to full spectrum of CMBR
• temperature variations are very small

CMBR Spectrum

• Test the Blackbody prediction
• deviations at short wavelengths can be caused by energetic processes shortly after the Universe becomes transparent
• Example: stars of a million solar masses might have formed and released lots of radiation
• mostly visible and ultra-violet radiation
• emitted at t = 1-10 million years
• generates some excess radiation at wavelengths shorter than 1 mm
• high T, smaller z -> shorter wavelength
• modified Big Bang might be able to live with small deviations
• Early tests focused on long wavelengths
•  Here  is a plot of many spectrum measurements
• Short wavelength measurements were done from space
• Here's the current observed spectrum from the COBE-FIRAS space mission:

Measurement Techniques and Problems

• At wavelengths of a few mm and longer
• use radio receivers
• radio wave radiation shakes electrons up and down in antenna
• creates electronic signal
• H₂O vapor not a huge problem
• until very high sensitivity is required
• At wavelengths shorter than a few mm
• needed to confirm blackbody curve
• H₂O vapor a big problem
• no atomic states at mm and sub-mm wavelengths
• molecular excitations
• rotational states
• vibrational states
• quantized just like atomic states
• How to get away from H₂O vapor
• hot and dry location: say Yuma, AZ?
• NO! hot air holds a lot of moisture
• cold and dry location:
• YES!
• Saskatoon
• South Pole: 
• problems - cold
• high altitude:
• White Mt., California (14,000 ft)
• Chile's high desert (17,000 ft.): 
• problems: altitude sickness - altitude stupidity
• balloons (> 100,000 ft): 
• requires automation
• U2 spy plane (~100,000 ft):
• requires automation
• sub-orbital rockets
• flights last a few minutes
• rocket exhaust may travel with instrument
• detectors for  < a few mm
• radio receivers don't work
• Bolometers do:
• a bolometer's temperature will increase when it absorbs radiation
• must be cold
• cooled to T = 4K with liquid Helium at first
• now, state of the art is T = 0.1K with "adiabatic demagnetization" refrigerator
• problem:
• bolometers can absorb all types of radiation
• non CMBR radiation (visible, infrared) must be excluded

CMBR Anisotropy

• Big Bang predicts approximate isotropy
• Universe is homogeneous and isotropic on large scales
• First deviation from Isotropy
• recall the following formulae:

   if 

• The first equation is valid when z << 1
• and the 2nd equation is valid for all z
• we can combine them to get  for 
• So the doppler effect gives a temperature shift
• This was first measured by the U2 flight led by Smoot:
• Here's their temperature map:
• This is a full sky map in Earth based coordinates
• Note the missing part of the map in the Southern Hemisphere.
• The velocity of the Sun with respect to the CMBR is about 0.001c or 300 km/sec.
• This is the motion of the Sun with respect to the "cosmic rest frame."
• This is thought to be caused by the gravitational attraction of nearby galaxies, clusters of galaxies and even superclusters of galaxies
• Here's a graphical representation of this result:

• Here's a more recent, and more accurate map from the COBE mission: