What observed feature of the universe motivated scientists to propose the “Big Bang” theory? There is lots of debris in space, as would be expected from an explosion Scientists thought the universe should have a definite beginning, not just be “forever” The universe is expanding and the expansion traces back to a unique time in the past None of the above :10

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?

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 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 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 Background has perfect thermal radiation spectrum at temperature 2.73 K

How do the abundances of elements support the Big Bang?

Protons and neutrons combined to make long-lasting helium nuclei when universe was ~ 3 minutes old

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 Abundances of other light elements agree with Big Bang model having 4.4% normal matter – more evidence for dark matter!

Which of the following abundance patterns is an unrealistic chemical composition for a star? 70% H, 28% He, 2% other 95% H, 5% He, <0.02% other 75% H, 25% He, <0.02% other :10

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… A shorter wavelength in the direction on the sky towards which you’re moving A longer wavelength in the direction on the sky towards which you’re moving A longer wavelength in the direction on the sky opposite to your motion A shorter wavelength in the direction on the sky opposite to your motion Both answers 1 and 3 are correct answers Both answers 2 and 4 are correct answers :10

The Doppler shift changes the peak wavelength λ of the cosmic background radiation (CBR) across the sky

If you then convert the peak wavelength to a temperature using Wien’s law (λT = constant), you’ll get different temperatures at different places in the sky.

The Doppler shift changes the peak wavelength λ of the cosmic background radiation (CBR) across the sky If you then convert the peak wavelength to a temperature using Wien’s law (λT = 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 percentage difference in the temperature of the cosmic background radiation across the sky is the same as the percentage difference in peak wavelength across the sky. If the temperature T is 0.185% higher in your direction of motion, then The peak wavelength of the cosmic background is 0.185% longer in that direction The peak wavelength of the cosmic background is 0.185% shorter in that direction :10

2. The percentage change in peak wavelength of the cosmic background equals the speed of the Milky Way as a percentage of the speed of light c (c=300,000 km/second). If the peak wavelength changes by 0.185%, the Milky Way’s velocity is… 1,621,621 km/second 55,500 km/second 5,550 km/second 555 km/second 55.5 km/second 5.55 km/second 0.555 km/second :10

Speeds up by 400 cm/sec every 1000 years 3. Velocity is acceleration multiplied by time. If the Milky Way is now travelling at 555 km/sec and the universe is 13.875 billion years old, how fast is the Milky Way speeding up (what is its acceleration)? (Hint: 1 km = 1000 meters, 1 meter = 100 cm) Speeds up by 400 cm/sec every 1000 years Speeds up by 40 cm/sec every 1000 years Speeds up by 4 cm/sec every 1000 years Speeds up by 0.4 cm/sec every 1000 years :10

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

4. If a supercluster located d=600 Mly (million light years) away is responsible for accelerating the Milky Way, what number N of galaxies like the Milky Way are in it? Use a=0.003 N/d2 where a=(4 cm/s)/1000 years and d is in units of Mly. 800 1440 1600 144,000 480,000 1,600,000,000 :10

5. The Shapley Concentration consists of about 24 rich galaxy clusters, each containing the equivalent of 2000 galaxies like the Milky Way. What fraction of the Milky Way’s acceleration can be explained by the Shapley Concentration? 0.004 (0.4%) 0.01 (1%) 0.04 (4%) 0.1 (10%) 0.4 (40%) 1.0 (100%) :10

The Shapley Supercluster by itself cannot explain the peculiar velocity of the Milky Way! (It’s likely due to more than one supercluster.)