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Chapter 19: The Beginning of Time

Chapter 19: The Beginning of Time. A Hot Early Universe The Story of Creation Evidence for the Big Bang Inflation Did the BB really happen?. A Hot Early Universe. Hubble’s law implies that all things sprang into being at the Big Bang. The universe was hotter and denser in the past.

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Chapter 19: The Beginning of Time

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  1. Chapter 19: The Beginning of Time A Hot Early Universe The Story of Creation Evidence for the Big Bang Inflation Did the BB really happen?

  2. A Hot Early Universe • Hubble’s law implies that all things sprang into being at the Big Bang. • The universe was hotter and denser in the past. • The universe is filled with a faint glow of radiation that allows us to investigate what happened in the first 500,000 years.

  3. The Story of Creation • The Story is divided into eras, or time periods. • Planck era: 10-43 s • GUT era: 10-38 s • Electroweak era: 10-10 s • Particle era: 10-3 s • Nucleosynthesis era: 3 minutes • Nuclei era: 500,000 years • Atoms era: 109 years • Galaxies era: present

  4. Matter and Antimatter • In a very hot plasma photons can transform themselves into matter (electron) and antimatter (positron), and viceversa. • A small asymmetry in the amount of matter-antimatter made our existence possible.

  5. The Plank Era • In the first 10-43 s random mass and energy fluctuations were so extreme that we are unable to describe what happened. • We do not yet have a theory that links quantum mechanics and general relativity.

  6. The GUT Era • Grand Unified Theories predict that at very high temperature the electroweak and strong nuclear forces merge into a GUT force. • When the universe had cooled to 1029K at an age of 10-38 s, GUT predicts that the strong force separated out. • This separation may have produce an inflation.

  7. The Electroweak Era • When the temperature dropped to 1015 K at an age of 10-10 s, the electromagnetic and weak nuclear force separated. • New types of particles emerged, the W and Z bosons. These particles were predicted in the 1970s, and first detected at CERN in 1983. We do not have any experimental evidence of the conditions prior to this era. • Theories concerning eras earlier than 10-10 s are speculative.

  8. The Particle Era • Tiny particles of matter (electrons, neutrinos and quarks) were as numerous as photons. • Near the end of this era quarks combined in groups of three to form protons and neutrons. • When the universe cooled to 1012K at 10-3 s, it was no longer hot enough to produce particle creation from pure energy. • For each 109 protons and antiprotons that annihilated each other, a single proton was left over. This slight excess of matter makes up all the present-day universe.

  9. The Nucleosynthesis Era • Protons and neutrons fused into heavier elements, which were subsequently broken apart due to high temperature. • When the temperature dropped to 109 K, at 3 minutes, nuclear fusion stopped because the density was lower than inside stars. • 75% of the mass remained as protons. 25% fused to He.

  10. The Era of Nuclei • The universe consisted of a very hot plasma of H nuclei, He nuclei, and free electrons for about 500,000 years. • Photons bounced rapidly from one electron to the next, never managing to travel far between collisions. • When the temperature fell below 3000 K, H and He nuclei captured electrons forming neutral atoms. The universe suddenly became transparent. • We still see the released photons as the cosmic microwave background.

  11. The Era of Atoms • 500,000 years after the Big Bang, and during about 1 billion years, the universe consisted of a mixture of neutral atoms and plasma. • Slight density enhancements and the gravitational attraction of dark matter assembled the first protogalactic clouds. • The first stars formed about 1 billion years after the Big Bang.

  12. Steady State Theory • Einstein inserted a cosmological constant in the equation of universal equilibrium to oppose the crunch of gravity. • Burbidge, Hoyle and others proposed the steady-state theory of the universe. • The universe is expanding but it infinitely old. New galaxies continually form in the gaps that open up as the universe expands. • Creation of matter is an ongoing, eternal, process.

  13. Evidence for the Big Bang • The BB model has gained wide scientific acceptance for three key reasons: • It predicted that radiation that decoupled from matter at the end of the nuclei era should still be present today. In 1965 the cosmic microwave background was discovered. • It predicted that the abundance of He relative to H should be about 1 to 4. • It explains why galaxies at large distances look younger than present-day galaxies.

  14. Discovery of the Cosmic Microwave Background • Arno Penzias and Robert Wilson found an unexpected noise in every measurement they made with an antenna sensitive to microwave radiation. • The noise was the same all over the sky. • The noise in the Bell labs antenna agreed with calculations made at Princeton for the relic of the Big Bang radiation.

  15. Model of the Cosmic Microwave Background • 500,000 years after the Big Bang, neutral atoms could remain stable for the first time at a temperature of about 3000 K. • Electrons were captured, and photons flew unobstructed. • We are seeing the universe as it was when its age was only 500,000 years.

  16. Temperature of the Cosmic Microwave Background • The heat of an isotropic universe should have an essentially perfect thermal (or blackbody) radiation spectrum. • The universe has stretched by about a factor of 1,000 since the end of the nuclei era, stretching the wavelengths of the photons by the same amount. In 1991 COBE measured T=2.73 K.

  17. Lumps in the CMB • COBE measurements showed that the CMB is not quite perfectly uniform. • Temperature varies from one place to another by a few parts in 105. • Small temperature enhancements may echo much larger density enhancements made up by WIMPs.

  18. Primordial He Synthesis • Everywhere about one quarter of ordinary matter’s mass is He. • At T=1011K protons could convert to neutrons and viceversa. But because n are slightly more massive than p, at T=1010K only p can convert to neutrons. • Calculations of BB conditions give a p/n ratio of 7 to 1, implying a H/He mass ratio of 4.

  19. Inflation • In 1981, Alan Guth realized that the separation of the strong force from the GUT force should have released enormous energy, causing the universe to expand as much as 1030 times in less than 10-36 s. • We call this dramatic expansion INFLATION, and it may have shaped the way the universe looks today. It provides an explanation for some aspects of the universe that are not accounted for by the standard BB model.

  20. Density Fluctuations • According to quantum mechanical principles, energy fields at any point in space are always fluctuating. • Tiny quantum ripples could have been stretched enormously to become the density enhancements that we see imprinted in the CMB.

  21. Smoothness of the CMB • The CMB was created 500,000 years after the BB when the universe had already expanded considerably. • The density of the universe at the end of the era of the nuclei were no more than 0.01%. Distant regions could not have been connected instantaneously because they lied beyond the cosmological horizon. How did they know about each other? • Inflation solves the problem by equalizing the densities in a much smaller volume.

  22. Flatness of the Universe • The matter density of the universe is between 20% and 100% of the critical value, so the universe is remarkably flat. • Inflation flattened any curvature the universe may have had.

  23. Olberg’s Paradox • In an unchanging universe with an infinite number of stars, we would see a star in every direction, making the night sky as bright as the Sun. • The BB solves Olberg’s paradox because we can only see a finite number of stars within the cosmological horizon.

  24. The Big Picture • The early universe was hotter and denser. We do not have experimental evidence for conditions before the electroweak era, and no theory for the Plank era. • The inflationary Big Bang theory is the most successful one for explaining the overall properties of the universe (expansion, CMB uniformity and temperature, large scale structure, cosmic abundances, age of the universe).

  25. Hints for Final Exam • Read chapters 12 through 19. Particularly the following sections: 12.3, 12.4, 12.5, 13.2, 13.3, 13.4, 13.5, 14.2, 14.3, 14.4, 15.2, 15.3, 15.4, 16.1, 16.2, 16.4, 16.5,17.2, 17.3, 17.4, 17.5, 17.6, all of 18 and 19. • Review the most important concepts of each chapter using my ppt presentations. • Practice the problems at the end of each chapter.

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