1. The Birth of the Universe

2. Hubble Expansion and the Big Bang The fact that more distant galaxies have higher redshifts indicates that the universe is expanding. This implies that the universe was born in a huge explosion, or Big Bang. 1.53 (billion light years)

4. To detect very distant galaxies, Hubble stared at a small patch of sky for an entire week. This image, known as the Hubble Deep Field, shows that galaxies were smaller and more irregular in the past than they are today. Testing the Big Bang: Has the universe changed? If the Big Bang theory is correct, then the universe was very different in the past. We can test this prediction with images of the faintest and most distant galaxies, and hence looking back in time to when the universe was much younger.

5. Testing the Big Bang: Do we see the expected amount of helium? The universe should have been incredibly hot and dense right after the Big Bang. In fact, during the first three minutes, the entire universe was like the center of a star, and hydrogen fusion should have occurred everywhere. The Big Bang theory predicts how much hydrogen was converted into helium by fusion. The amount of helium relative to hydrogen in the universe today agrees with that prediction (after correcting for the smaller amount of helium made in stars).

7. Testing the Big Bang: Do we see a radio afterglow? In the Big Bang theory, the universe was very hot immediately after it was born, so it would have glowed at short wavelengths (gamma rays). Because of the expansion of the universe, the light produced after the Big Bang should be redshifted over time, and should now appear at radio wavelengths (or more specifically, microwave wavelengths).

8. Prediction vs. Observation The Big Bang afterglow was predicted in 1948, and it was detected with a radio telescope in 1965 (resulting in a Nobel Prize). This afterglow is called the Cosmic Microwave Background (CMB). It appears across the entire sky in every direction, and its spectrum is equivalent to a blackbody with a temperature of 3 degrees above absolute zero.

9. By the time the universe reached an age of 400,000 years, it had expanded enough so that photons of light could travel large distances without scattering. In other words, the universe was now transparent to light. So the microwave background shows us the appearance of the universe 400,000 years after the Big Bang. The Microwave Background

10. By the time the universe reached an age of 400,000 years, it had expanded enough so that photons of light could travel large distances without scattering. In other words, the universe was now transparent to light. So the microwave background shows us the appearance of the universe 400,000 years after the Big Bang. The Microwave Background

11. The Age of the Universe According to the Big Bang theory, the universe has not existed forever, and instead has a finite age. D/V We can estimate the age of the universe from its rate of expansion, which is measured in the Hubble Law. If the universe expanded at a constant rate since the Big Bang, then the age of the universe is D/V for any point along the Hubble Law, which produces an age of about 13 billion years. D V 1.5 (billion light years) 3

12. The Age of the Universe D/V The gravity from the matter in the universe should slow the expansion of the universe over time. If so, the universe would have expanded faster in the past than it does now. As a result, the true age of the universe should be less than D/V, or <13 billion years. In other words, because it expanded faster in the past, the universe reached its current size more quickly than if it had been expanding at a constant rate. gravity Big Bang expansion

13. But stars in some globular clusters are at least 13 billion years old. How can some stars be older than the universe? The solution to this puzzle wasn’t found until the 1990’s, and is explained in the next lecture. The Age of the Universe