Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30
Lecture 37: The Whispers of Creation
Readings: Ch 28, sections 28-4 & 28-5; Ch 29, section 29-4
- 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
The "Big Bang" is our name for our physical model of the expanding
Universe. It makes specific predictions that can be tested by
making observations of the Universe.
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
- The Age of the Universe derived from
the expansion rate is consistent with the ages of the oldest stars we see.
- 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
What we see Now:
- The Universe is cold and low density.
- Galaxies (matter) are getting further apart as space expands
- 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?
Pop I Stars (and the Sun):
- 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?
When the Universe was only 1 second old:
- 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
When the Universe was 2 minutes old:
- Temperature dropped to 1 Billion K
Neutrons & Protons fused into Deuterium (2H) Nuclei:
A dense, hot soup of mostly protons (H) and deuterium (2H) in
a 4:1 proportion, and a mix of photons, electrons and other subatomic
- 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
Most of the newly-formed Deuterium fuses to form 4He nuclei
- Other reactions made 3He, Li, Be, and B in
very tiny quantities
By the time the Universe was 4 minutes old:
At this point, the Universe had cooled down so much that fusion stopped
and no further heavy elements were formed.
- Much of the Deuterium had turned into 4He
- Only trace amounts of Deuterium and other light elements leftover
After Primordial Nucleosynthesis stopped when the Universe was about 4
- 4He/H = 20-26%
- D/H = 0.0001-0.1%
- 4He/H = 22-25%
- D/H = 0.001-0.02%
The predictions of Primordial Nucleosynthesis calculations agree very
well with current observations of the primordial abundances of the light
elements relative to Hydrogen.
There are still a few remaining issues to be settled:
- Need refinement of the primordial abundances
- Very difficult observations to make
- Need to know average density of p & n
- light-element reaction rates need refinement
Hot Early Universe
After the Epoch of Primordial Nucleosynthesis, the Universe stays hotter
than 3000 K for a long time:
- Electrons and nuclei cannot combine to form neutral atoms
- The Universe remains fully ionized.
- Free electrons easily scatter photons.
- The Universe is opaque to light during this time.
- Filled with a hot, ionized fog of ions and free electrons
The Early Universe is filled with a hot, opaque ionized gas:
- 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
When the Universe is about 300,000 years old:
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
After Recombination, the Universe is filled with a diffuse,
"relic" blackbody radiation from the initial hot, dense,
As the Universe expands further:
- Blackbody photons redshift with the expansion.
- Spectrum peak shifts to redder wavelengths, hence cooler
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
1965: Arno Penzias & Robert Wilson (Bell Laboratories)
What they had discovered, almost by accident, is the Cosmic Microwave
- 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 Big Bang model makes very specific predictions for the nature of the
Cosmic Background Radiation:
- The spectrum is a perfect blackbody
- Characterized by a single temperature.
- 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.
The current state of observational work has spectacularly confirmed and
greatly strengthened the observational case for the Big Bang Model:
- Perfect blackbody spectrum
- Characterized by a single temperature of T=2.725±0.001 K
- Uniformly fills the Universe
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
Expansion of the Universe:
- Hubble's Law (expansion consistent with Cosmological Principle)
- Age is consistent with ages of the oldest stars
- 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
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Updated: 2006 February 28
Copyright © Richard W. Pogge, All Rights Reserved.