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4 .1 INTRODUCTION to the universe

4 .1 INTRODUCTION to the universe. Riddles of Cosmology. The vastness of space and time we call the universe still poses many unresolved mysteries. Be prepared to encounter some of the strangest concepts modern science knows (or doesn’t really know). Olber’s Paradox.

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4 .1 INTRODUCTION to the universe

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  1. 4.1INTRODUCTION to the universe

  2. Riddles of Cosmology The vastness of space and time we call the universe still poses many unresolved mysteries. Be prepared to encounter some of the strangest concepts modern science knows (or doesn’t really know)

  3. Olber’s Paradox • Why is the sky dark at night? • Most astronomers believed the universe was infinite. • If that was so, then every line of sight should end up on the surface of a star at some point. • The night sky should be as bright as the surface of stars. • The fact it is dark at night is known as Olber’s Paradox. • Solution: If the universe had a beginning, we can only see light from galaxies having had time to travel to us. • The visible universe is finite!

  4. Cosmic Expansion • In 1929, Edwin Hubble published his discovery the sizes of galaxy red-shifts are proportional to galaxy distances. • Nearby galaxies have small red-shifts and more distant galaxies have large red-shifts. • You may recall the Hubble Law. • Implies the universe is expanding.

  5. Cosmic Expansion 0 On large scales, galaxies are moving apart, with velocity proportional to distance. Space is expanding, carrying the galaxies along! The galaxies themselves are not expanding.

  6. Cosmic Expansion The loaf of bread is representative of the expanding universe and the individual raisins represent galaxies. Analogy to expanding space: • A loaf of raisin bread when the dough is rising and expanding, taking the raisins with it.

  7. Cosmic Expansion 0 You have the same impression from any other galaxy as well: This does not mean we are at the center of the universe!

  8. The Big Bang Theory • If you had a video of the expanding universe and ran it backwards, you would see the matter and energy of the universe compressed into a high-density, high-temperature state. • Cosmologists believe the universe must have begun from this moment of extreme conditions and refer to this idea as the Big Bang Theory.

  9. The Big Bang Theory 0 If galaxies are moving away from each other with a speed proportional to distance, there must have been a beginning, when everything was concentrated in one single point: The Big Bang! Time ?

  10. Age of the Universe • Knowing the current rate of expansion of the universe, we can estimate the time it took for galaxies to move apart as they are today: • Time = distance/velocity • T = 1/H0 Hubble Time τH = 1012/H0 ~ 14 billion years H0= Hubble constant ~ 70 km/s/Mpc

  11. The more distant the objects we observe, the further back into the past of the universe we are looking (“look-back” time)

  12. Cosmic Microwave Background (CMB) Radiation • Discovery of radiation from the time of the Big Bang began in the mid-1960s when two Bell Laboratories physicists, Arno Penzias and Robert Wilson, were measuring the brightness of the sky at radio wavelengths. • With further developments, they recognized the mysterious extra signal they had detected was radiation from the Big Bang known as cosmic microwave background radiation, or CMB. • In 1990, satellite measurements confirmed the background radiation has a temperature of about 2.73 K (1100 times cooler than it actually was at the time of the Big Bang due to the extreme red-shift).

  13. Cosmic MicrowaveBackground (CMB) Radiation • The Wilkinson Microwave Anisotropy Probe (WMAP) satellite mapped the all-sky structure of the Cosmic Microwave Background (CMB) radiation.

  14. The History of the Universe Universe cools down as time passes Universe expands as time passes

  15. The History of the Universe 0 Electron Positron Gamma-ray photon Electrons, positrons, and gamma-rays in equilibrium between pair production and annihilation

  16. The History of the Universe 0 25% of mass in helium 75% in hydrogen Protons and neutrons form a few helium nuclei; the rest of protons remain as hydrogen nuclei No stable nuclei with 5 – 8 protons Almost no elements heavier than helium are produced.

  17. The History of the Universe 0 Photons are incessantly scattered by free electrons; photons are in equilibrium with matter Photons have a blackbody spectrum at the same temperature as matter. Radiation dominated era

  18. History of the Universe 0 Protons and electrons recombine to form atoms universe becomes transparent for photons z ≈ 1000 Transition to matter dominated era

  19. History of the Universe 0 After recombination, photons can travel freely through space. Their wavelength is only stretched (red shifted) by cosmic expansion. Recombination: z = 1000; T = 3000 K This is what we can observe today as the cosmic background radiation

  20. History of the Universe 0 After less than ~ 1 billion years, the first stars form. Ultraviolet radiation from the first stars re-ionizes gas in the early universe. Reionization Formation of the first stars universe becomes opaque again

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