17.2 Evidence for the Big Bang
Our Goals for Learning
• How do we observe the radiation left over from the Big Bang?
• How do the abundances of elements support the Big Bang? From Big Bang to Elements
• Universe began with slightly more matter than antimatter; the antimatter annihilates with an equal amount of matter, leaving only matter behind
• That matter (in the form of quarks) forms neutrons and protons as the universe cools
• Neutrons and protons fuse to create heavier elements, but only for a few minutes, because free neutrons decay into protons with a half­life of 8 minutes.
• Density of matter at start of nucleosynthesis era determines how much deuterium, helium, lithium, etc. exists at end of nucleosynthesis era
How do we observe the radiation left over from the Big Bang?
The cosmic microwave background – the radiation left over from the Big Bang – was detected by Penzias & Wilson in 1965, using the radio telescope shown here
Not long after the Big Bang, the universe was very hot. Hydrogen and helium were in the form of nuclei plus electrons, so the universe was opaque.
When the universe cooled to a temperature of about 3000 Kelvin (3300 C), hydrogen and helium formed into atoms, and the universe became transparent.
The infrared light from when the universe was 3000 K hot has traveled through the universe since the universe became transparent: cosmic microwave background.
Background has perfect thermal radiation spectrum at temperature 2.73 K
Expansion of universe has redshifted thermal radiation from 300,000 years after the Big Bang to ~1000 times longer wavelength today: microwaves
COBE (Cosmic Background Explorer)
detected the seeds of future structure formation
WMAP (Wilkinson Microwave Anisotropy Probe) is giving us detailed “baby pictures” of structure in the universe
Background has perfect thermal radiation spectrum at temperature 2.73 K
How do the abundances of elements support the Big Bang?
From Big Bang to Elements
• Universe began with slightly more matter than antimatter; the antimatter annihilates with an equal amount of matter, leaving only matter behind
From Big Bang to Elements
• Universe began with slightly more matter than antimatter; the antimatter annihilates with an equal amount of matter, leaving only matter behind
• That matter (in the form of quarks) forms neutrons and protons as the universe cools
From Big Bang to Elements
• Universe began with slightly more matter than antimatter; the antimatter annihilates with an equal amount of matter, leaving only matter behind
• That matter (in the form of quarks) forms neutrons and protons as the universe cools
• Neutrons and protons fuse to create heavier elements, but only for a few minutes, because free neutrons decay into protons with a half­life of 8 minutes.
Protons and neutrons combined to make long­lasting helium nuclei when universe was ~ 3 minutes old
From Big Bang to Elements
• Universe began with slightly more matter than antimatter; the antimatter annihilates with an equal amount of matter, leaving only matter behind
• That matter (in the form of quarks) forms neutrons and protons as the universe cools
• Neutrons and protons fuse to create heavier elements, but only for a few minutes, because free neutrons decay into protons with a half­life of 8 minutes.
• Densities of neutrons & protons at start of nucleo­
synthesis determines how much deuterium, helium­3, helium, lithium, etc. exists at end of nucleosynthesis
Big Bang theory prediction: 75% H, 25% He (by mass)
Matches observations of nearly primordial gases
Abundances of other light elements agree with Big Bang model having 4.4% normal matter – more evidence for dark matter!
What have we learned?
• How do we observe the radiation left over from the Big Bang?
• Telescopes that can detect microwaves allow us to observe the cosmic microwave background—radiation left over from the Big Bang. Its spectrum matches the characteristics expected of the radiation released at the end of the era of nuclei, spectacularly confirming a key prediction of the Big Bang theory. What have we learned?
• How do the abundances of elements support the Big Bang?
• The Big Bang theory predicts the ratio of protons to neutrons during the era of nucleosynthesis, and from this predicts that the chemical composition of the universe should be about 75% hydrogen and 25% helium (by mass). This matches observations of the cosmic abundances, another spectacular confirmation of the Big Bang theory. Actual observed map of the sky at microwave wavelengths: redder means higher temperature, bluer means lower
1. If you’re moving with respect to the cosmic microwave background, then ­­­ because of the Doppler effect ­­­ that radiation has…
1. A shorter wavelength in the direction on the sky towards which you’re moving
2. A longer wavelength in the direction on the sky towards which you’re moving
3. A longer wavelength in the direction on the sky opposite to your motion
4. A shorter wavelength in the direction on the sky opposite to your motion
5. Both answers 1 and 3 are correct answers
6. Both answers 2 and 4 are correct answers
The Doppler shift changes the peak wavelength lambda of the cosmic background radiation (CBR) across the sky
If you then convert the peak wavelength to a temperature using Wien’s law (lambda * T = constant), you’ll get different temperatures at different places in the sky.
The Doppler shift changes the peak wavelength of the cosmic background radiation (CBR) across the sky
• If you then convert the peak wavelength to a temperature using Wien’s law (lambda * T = constant), you’ll get different temperatures at different places in the sky.
• The amount by which the temperature changes across the sky is larger if you’re moving faster
• So by measuring the temperature change, we can figure out how fast the Milky Way is moving
The Milky Way IS moving at 555 km/sec towards a distant supercluster of galaxies in the constellation of Lyra the Lyre (which contains the bright star Vega).
The Shapley Supercluster is located 600 million light years away towards Lyra. Knowing the distance d to that supercluster, we can use our estimate of the Milky Way’s acceleration a to estimate the supercluster’s mass M, since a=GM/d2=GMgalNgal/d2