Cosmic Microwave Background Radiation

• Figures borrowed from:  Ned Wright's Cosmology Tutorial
• Smoot Group Education Page
•  Bell Labs on Penzias & Wilson

Outline

• Steady State Theory
• The Prediction of the Background Radiation
• How Can a faint Uniform Source of Radiation be detected
• The CMBR Discovery

Steady State Theory

• Introduced by Hoyle, Bondi & Gold
• philosophical appeal
• Main Ideas
• Cosmological Principle: The Universe is homogeneous and isotropic.
• the same everywhere
• the same in every direction
• Applies to the Big Bang
• The Perfect Cosmological Principle: The Universe is homogeneous and isotropic and unchanging in time.
• new condition: no time dependence
• What about Hubble's Law?
• Since the Universe is expanding, the density was higher in the past!
• But, not if new matter is created between the old galaxies
• violating conservation of energy (or mass)
• but , not by much: 1 Hydrogen atom per  m³ per 10 billion years
• Problem: Bright Radio Galaxies are common at high redshift, but not nearby
• Example: 3C 368 at z = 1.132


• Current View:
• Radio Galaxies are like quasars - they are powered by matter falling in to a central black hole of 100 million solar masses or more.
• Like quasars, radio galaxies were more common (or brighter) in the past because there was more stuff falling into the black holes back then
• The Steady State party line:
• The redshifts of radio galaxies may not be cosmological in origin.

Big Bang Predicts Background Radiation

• Princeton Group (Dicke & Peebles) were working on a Big Bang variation
• The "Oscillating" Universe

• A little like the Steady State Theory
• Universe looks the same as long as we keep our eyes open for 100 billion years!
• We don't understand what happens when the Universe is at 0 size.
• The observable consequences of this theory are the same as the Big Bang
• Since expansion cools the Universe, we know if must have been hotter in the past
• 
• But, how hot is it now?
• Here's what we know about the Universe if the Big Bang model is right (in 1965)
• the Universe is about 10 billion years old
• The Universe is now dominated by matter and not radiation
• The Universe was dominated by the energy of radiation at early times:
• E ~ 1/  ~ 1/(1 + z) for radiation
• E ~ constant for matter
• Also, the Universe was close to the critical density at early times:

• Otherwise it wouldn't have lasted so long.
• We know the density of radiation during nucleosythesis: (when the elements were made)
• But, we don't know the exact density of normal matter.
• However, we can pick the density that gives about 25% Helium production:

• At present, we know (roughly) the density of normal matter
• so using our nucleosythesis prediction of the density at t ~ 100 sec,
• we know how much the Universe expanded
• This gives T_now  from:
• Peebles estimated T_now = 10 K
• Alpher & Hermann estimated T_now = 5 K  many years earlier
• What would the radiation look like?
• Prior to t = 300,000 years, T > 4000 K and Hydrogen is ionized
• photons knock electrons out of atoms
• They efficiently scatter radiation
• Universe is a hot fog
• radiation has "equilibrium" or "blackbody" spectrum at T ~ 4000 K.
• As T drop below 4000 K, neutral atoms form:


• Universe becomes transparent:

• At this point, the radiation still has the blackbody form at a temperature of ~ 4000K

• The radiation is no longer in equilibrium, but the expansion of the Universe cools down the radiation and keeps exactly the blackbody curve!
• Thus, we expect to see blackbody radiation today:

• Maximum is at  ~ 2 mm - this is "microwave" radiation

How to Detect the Background Radiation?

• The predicted radiation temperature was 10K or less
• The surface of the Earth is about 300K (0 C = 273.15 K)
• The apparatus is outdoors at about 300K
• It can radiate itself
• "Background" is much larger than the signal
• Water vapor emits and absorbs very strongly at microwave band
• Observe at longer wavelengths where the atmosphere is transparent
• Penzias & Wilson used 7.35 cm
• Wilkinson & Roll used 3.2 cm
• Shielding:
• The big microwave horn used by Penzias & Wilson provides a shield to help keep out unwanted radiation from the ground:

• Cold Load
• lots of potential sources of noise and a very faint signal
• do a comparison measurement to something at about the same T
• choice: liquid Helium at T = 4.2 K
• both Penzias & Wilson and Wilkinson & Roll did this

Discovery

• Penzias & Wilson looking for faint radiation from the Galactic halo
• Since we are off center, the radiation should be slightly brighter towards the Galactic center

• Long struggle with excess "noise"
• Pigeons deposit "white dielectric" substance

cleaning up after the pigeons?
• back at Princeton, Wilkinson & Roll are almost ready to go
• Peebles spreads the word
• at meeting between Princeton and AT&T Groups
• they conclude that Penzias & Wilson have seen the CMBR
• Penzias & Wilson relieved to have "noise" explained
• announced as proof of Big Bang