Astronomy 162: Introduction to Stars, Galaxies, & the Universe Prof. Richard Pogge, MTWThF 9:30

Lecture 37: The Whispers of Creation

Key Ideas

Fundamental Tests of the Big Bang

Primordial Nucleosynthesis

Primordial Deuterium & Helium

Primordial light elements (Li, B, Be)

Cosmic Background Radiation

Relic blackbody radiation from Big Bang

Perfect blackbody spectrum with a Temperature of 2.725±0.001 K

The Three Pillars of the Big Bang

Three fundamental pieces of observational evidence in favor of the Big Bang Model are:

The Expansion of the Universe:

• Explains the observed Hubble Law for Galaxies.
• The Age of the Universe derived from the expansion rate is consistent with the ages of the oldest stars we see.

Primordial Nucleosynthesis:

• The creation of the original complement of light elements during the hot early phases of the Big Bang.
• Specifically: the primordial complement of Deuterium (²H), Helium, & traces of Li, Be, & B

The Cosmic Background Radiation:

• The relic blackbody radiation from the early hot, dense phases of the Big Bang.

The Hot Big Bang

• The Universe is cold and low density.
• Galaxies (matter) are getting further apart as space expands between them.
• As the Universe expands, it cools further.

In the past:

• The Universe was smaller, hotter, and denser

Is there any evidence of this early hot, dense phase?

Where Did all the Helium come from?

• 70% H, 28% He, and ~2% metals
• Pop I metals from Pop II star supernovae.

Metal-poor Pop II Stars:

• 75% H, 25% He, and <0.01% metals

Where did the He in Pop II stars come from?

• If from the first stars, where are all the metals that would also have been produced by those first stars?

Primordial Nucleosynthesis

• Temperature: 10 Billion K
• Too hot for atomic nuclei to exist
• Only had protons, neutrons, electrons, & photons
• About 1 neutron for every 5 protons

General hot, dense soup of subatomic particles and photons.

As it expanded, it cooled off.

Primordial Deuterium Formation

• Temperature dropped to 1 Billion K

Neutrons & Protons fused into Deuterium (²H) Nuclei:

• All of the free neutrons go into making Deuterium
• Leftover protons stay free as Hydrogen "nuclei"
• Proportions: one ²H for every 4 protons (H).

Primordial Helium Formation

• Other reactions made ³He, Li, Be, and B in very tiny quantities

By the time the Universe was 4 minutes old:

• Much of the Deuterium had turned into ⁴He
• Only trace amounts of Deuterium and other light elements leftover

Aftermath

Predictions:

• ⁴He/H = 20-26%
• D/H = 0.0001-0.1%

Observations:

• ⁴He/H = 22-25%
• D/H = 0.001-0.02%

Current Status

There are still a few remaining issues to be settled:

Observational Issues:

• Need refinement of the primordial abundances
• Very difficult observations to make

Theoretical Issues:

• Need to know average density of p & n
• light-element reaction rates need refinement

Hot Early Universe

• Electrons and nuclei cannot combine to form neutral atoms
• The Universe remains fully ionized.
• Free electrons easily scatter photons.

Result:

• The Universe is opaque to light during this time.
• Filled with a hot, ionized fog of ions and free electrons

Blackbody Radiation

• This gas has a perfect blackbody spectrum,
• with a characteristic temperature, T.

As the Universe expands it cools:

• photons redshift
• the peak of the blackbody spectrum shifts redward
• The blackbody temperature drops

Epoch of Recombination

The temperature drops below 3000 K:

• electrons & nuclei combine to form atoms
• not enough free electrons left to scatter photons.

The Universe suddenly becomes transparent:

• Photons stream out through space.
• Photon Spectrum is a 3000 K Blackbody

Cosmic Background Radiation

As the Universe expands further:

• Blackbody photons redshift with the expansion.
• Spectrum peak shifts to redder wavelengths, hence cooler temperatures.

By today, the spectrum will have been redshifted by a factor of 1000 down to a temperature of only about 3 Kelvin.

Wein's Law tells us that a blackbody of this temperature would emit primarily at microwave wavelengths.

Discovery

• Mapping the sky at microwave wavelengths.
• Found a faint microwave background noise.
• First thought it was equipment problems (noisy amplifiers, pigeons nesting in the antenna, etc.).
• Finally determined it was cosmic in origin.

This work won Penzias and Wilson the 1988 Nobel Prize in Physics.

But, is it Blackbody Radiation?

• The spectrum is a perfect blackbody
• Characterized by a single temperature.

Observations:

• Need to work with very cold instruments at the South Pole, high altitude, or in orbit.
• Experiments with balloons, sounding rockets, radio antennas, and orbiting satellites.

You can browse through all of the data from the main space missions devoted to studies of the Cosmic Microwave Backgroun on the Legacy Archive for Microwave Background Data Analysis webpage at the NASA Goddard Space Flight Center.

Spectacular Confirmation

• Perfect blackbody spectrum
• Characterized by a single temperature of T=2.725±0.001 K
• Uniformly fills the Universe

Details:

• The fine structure at the 1 part in 10⁵ level is related to the large-scale structure we see in the galaxies (see the Groups & Clusters Lecture).

Subsequent experiments, like the Wilkinson Microwave Anisotropy Probe (WMAP) and specialized experiments conducted from the South Pole and Northern Chile are studying the details of these temperature fluctuations, which can tell us a great deal about the Universe. We'll discuss these results separately in a later lecture.

Evidence for the Big Bang

• Hubble's Law (expansion consistent with Cosmological Principle)
• Age is consistent with ages of the oldest stars

Primordial Nucleosynthesis:

• Deuterium and Helium in the right proportions.

Cosmic Background Radiation:

• A perfect blackbody with a single temperature.

Together, these constitute strong empirical confirmation of many of the specific predictions of the Big Bang Model for the Expanding Universe.

This is why all astronomers take the Big Bang so seriously, and in fact it has so far provided us with a remarkably robust framework in which to explain a large number of otherwise inexplicable observations about our Universe.