The Dawn of Time - II. A Cosmos is Born

Transcription

1 The Dawn of Time - II. A Cosmos is Born

2 Learning Objectives! Why does Olbers paradox show the Universe began?! How does Hubble s Law tell us the age of the Universe? If Hubble s Constant is large, is the Universe old or young?! What is the Big Bang? Where in the Universe did it occur?! What is the CMB? Why are the expansion of the Universe (Hubble s Law), and the CMB, convincing evidence that, at its birth, the Universe was very hot and very small?! What is the temperature of the CMB? If the Big Bang was very hot, why is the CMB very cold? It may help to think about both cosmological redshift and Wien s Law! Is the CMB perfectly uniform in temperature? How do we see the seeds of large clusters of galaxies in the CMB?

3 Olbers Paradox! If the Universe extends infinitely in space, as it might, then the accumulated light from an infinite number of distant stars and galaxies should seemingly cause the sky to be bright at all times, night and day If space is infinite, every line of sight from Earth out into space should eventually terminate on the surface of a star or galaxy

4 The necessity of a beginning! The more distant galaxies are so far away that light from them has not yet reached the Earth! If you look out far enough, you look back to a time that is greater than the age of the Universe! The night sky is dark because the Universe had a beginning

5 Looking Back! Hubble s observations tell us the Universe is expanding! In its past, the Universe must have been more dense! Looking backwards in time, the entire Universe was crammed into a singularity (similar, mathematically, to the center of a black hole)! There must have been a point at which the expansion of the Universe began

6 Hubble s Law and The Age of the Universe! Imagine watching a movie of the expansion of the Universe! Now, reverse the movie! Expansion becomes contraction! If the Universe expanded at a constant rate! time = distance/velocity! Recall, v=h o d! Time = 1/H o ~1/70 km/s/mpc ~ 14 billion years

7 The early Universe must have been hot and dense! Earlier in time, the Universe must have been more compact! As light from the Universe shifted to the red (so, had less energy) the Universe cooled! The expanding Universe must have begun from conditions of extremely high density and temperature! Astronomers call those conditions the Big Bang

8 Not An Explosion! The Big Bang occurred everywhere at once! The Universe was suddenly filled with energy hot and dense! The beginning of space-time, matter, and energy! As space-time expanded, the Universe redshifted and became less dense and cooler (redder = cooler)! Eventually forming the galaxies we see today

9 Major Evidence for the Big Bang! 1) Expansion of the Universe! There must have been a point at which this expansion began! 2) Cosmic Microwave Background! Predicted remnant radiation from when the Universe was hot and dense long, long ago! 3) Light Element Formation! Light elements (Hydrogen, Helium, Lithium) are seen in the exact amounts predicted by the Big Bang, if the Universe was once hot and dense

10 Evidence for the Big Bang! Stars are hot and dense, so If the early Universe was hot and dense, shouldn t it, too, have emitted blackbody radiation?! Could we detect the spectrum of that radiation?! Yes! It s redshifted to microwaves! Called the Cosmic Microwave Background Other Guy shot pigeons! Detected by Robert Wilson and Arno Penzias Removed white dielectric material

11 CMB: A Very Uniform Blackbody! All over the sky, we see blackbody radiation! Its temperature = 2.73 K (about -270 o C)! Compelling evidence for the Big Bang Theory! Almost perfectly isotropic! i.e. the same in every direction! Indicates that, over large scales, the Universe is uniformly spread out

12 How to Understand Sky Right: Sphere of the Earth wrapped around in projection Maps Left: Sphere of the sky wrapped around in projection (showing our view towards the center of our Galaxy)

13 Small Scale Variations - Cosmological! Cosmological variations are less than 1 part in 100,000 around the 2.73 K background temperature of the Universe

14 Small Scale Variations - Cosmological! Originally quantum fluctuations! instabilities on scales comparable to the size of single atoms! These small differences in temperature represent tiny differences in density of stuff in the early Universe! These overdense regions ultimately collect the most mass together through gravity to become superclusters, clusters, galaxies

15 Structure Formation! Regions of higher density became the seeds of galaxies, clusters, and superclusters! Collapsed under their own gravity! Well-fed supermassive black holes at galaxy centers became quasars, etc.

16 Review: Main 3 Reasons the Big Bang Must Have Happened! The Hubble Law: v=h 0 d + Einstein s General Relativity = Expanding Universe with an age of ~14 billion years! Cosmic Microwave Background Remnant of the Big Bang when the Universe is ~500,000 years old Tiny fluctuations become galaxies! Light Element Formation H and (almost all) He come from the Big Bang, which precisely predicts how much H and He we should see

17 Next Time Extrasolar Planets