1 / 27

The Big Bang

The Big Bang. Koji Mukai. Elements 2002 Workshop. What is the Big Bang? . ‘Big Bang’ Cosmology is The theory that the expansion of the universe began at a finite time in the past, in a state of enormous density and pressure (Weinberg) Tightly constrained by observations

benjamin
Download Presentation

The Big Bang

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. The Big Bang Koji Mukai Elements 2002 Workshop

  2. What is the Big Bang? • ‘Big Bang’ Cosmology is • The theory that the expansion of the universe began at a finite time in the past, in a state of enormous density and pressure (Weinberg) • Tightly constrained by observations • Highly successful family of theories with no obvious competitor Elements 2002: Big Bang

  3. Unanswerable Questions • This subject can generate countless questions: some are based on misconceptions, some are about the unobservable. • Where did the Big Bang occur? • What is the Universe expanding into? • What happened before the Big Bang? • Are there many universes? Elements 2002: Big Bang

  4. Key Concepts • The Universe as a whole is expanding. • Hubble’s Law: recession velocity is proportional to distance • Expansion implies early Universe was dense and hot, sufficient for fusion • Further back in time you go, the more uncertain the theory becomes • Predicts a Microwave Background Elements 2002: Big Bang

  5. A Cosmic Census • What objects are there in the Universe? • A galaxy is made up of billions of stars • Many galaxies are found in groups and clusters Elements 2002: Big Bang

  6. Cosmological Principle • The axiom that the universe is isotropic and homogeneous. • Does not apply to stars within Milky Way Galaxy, or to bright, nearby galaxies • Applies to the average distribution of galaxies on the largest scales • Implies that we are not a privileged observer. Elements 2002: Big Bang

  7. Isotropic Distribution: an Example Elements 2002: Big Bang

  8. Hubble’s Discovery Elements 2002: Big Bang

  9. Hubble’s Law • Hubble’s Law • Recession velocity is proportional to distance • Hubble constant: ~65 km/s per Mpc • I.e., the Universe is expanding! Elements 2002: Big Bang

  10. How Do We Know Their Distances? • Astronomers use a series of steps, or “the cosmic distance ladder” • Parallax for nearby stars • Cepheid and other “standard candles” Elements 2002: Big Bang

  11. How Do We Know Their Velocities? • We use Doppler Shifts. • Redshifts when object is receding Elements 2002: Big Bang

  12. Fingerprints of Atoms • Atoms emit and absorb light at specific wavelengths; these can be used to identify composition and velocity. • Examples (hydrogen, helium, carbon) Elements 2002: Big Bang

  13. Back to Hubble’s Law • Space itself is expanding, not just a few galaxies! • Same expansion in every direction • Same seen from another galaxy Elements 2002: Big Bang

  14. Looking Back in Time • Hubble’s Law describes the current expansion of the Universe. • General Relativity is our current best theory of gravity and motion. • Inputting the current expansion speed and density to GR equations, we can “run the film backward” • Universe began as a “singularity” of infinite density (there is no “before”) Elements 2002: Big Bang

  15. Smaller Means Hotter • The early Universe was not only dense, but was also hot. • You can heat gas by compressing (bicycle pump experiment) or cool it by letting it expand (fridge) • Temperature is the measure of average energy per particle • This applies to photons as well as to protons, electrons, etc. Elements 2002: Big Bang

  16. The History of the Universe Elements 2002: Big Bang

  17. The First Second • We can reconstruct the earliest history of the Universe, based on GR and quantum physics. • More uncertain earlier we go - a family of theories for this stage of Big Bang • T=10,000,000,000 K soup of particles at t=1 s (antiparticles mostly gone) • Electrons and positrons decreasing but numerous, photons, protons, ... Elements 2002: Big Bang

  18. Electrons and Positrons • Electron and its anti-particle, positron, are among the least massive particles. • At high temperatures, they can be created and destroyed ~equally • At T<~10,000,000,000K, annihilation starts to win (not enough energy) Elements 2002: Big Bang

  19. Protons and Neutrons • Isolated neutrons decay into protons with a life time of ~10 min. • When the universe is hot, reactions go both ways so there are equal numbers • Imbalance develops as Universe cools Elements 2002: Big Bang

  20. The First Three Minutes • The first three minutes is the era of Big Bang nucleosynthesis. • Too hot (T~10,000,000,000K), and protons and neutrons won’t stick (left) • Still needs high T and high density for fusion to occur (right) Elements 2002: Big Bang

  21. Deuterium Bottleneck • Deuterium (heavy hydrogen) nuclei are the first step in creating heavier elements, but they don’t stick easily. • Almost all deuterium nuclei will immediately end up as helium • Deuterium formation requires T=1,000,000,000K or less, depending on density of protons+neutrons • Universe cools sufficiently at t=3 min. Elements 2002: Big Bang

  22. Elements Created • The ratio of neutrons to protons when deuterium begins to form determines the ratio of helium to hydrogen. • ~10% He4 • Also He3 etc. • Neutrons disappear Elements 2002: Big Bang

  23. Origin of Hydrogen and Helium • The Big Bang created most of the hydrogen and helium in the Universe. • Protons are stable, and more numerous than neutrons; they survive to become hydrogen nuclei • Most neutrons end up in the very stable alpha particles (2 protons, 2 neutrons), or He4 nuclei • Observed ratios agree with theory Elements 2002: Big Bang

  24. Recombination • There are a lot of photons in the Universe. • Spectrum is determined by the temperature (blackbody) • When atoms form (recombination of nuclei and electrons) at ~3,000K, Universe becomes transparent • The 3,000K blackbody has been redshifting ever since Elements 2002: Big Bang

  25. Cosmic Microwave Background Elements 2002: Big Bang

  26. CMB as Evidence of Big Bang • Cosmic Microwave Background and its approximate temperature was predicted before discovery in 1964: a triumph of Big Bang theory! • Current temperature is 2.725K, and uniform to better than 1 part in 1,000. • No alternative theory can explain the uniformity. • Small “wiggles” are seeds of galaxies. Elements 2002: Big Bang

  27. Elements from the Big Bang Elements 2002: Big Bang

More Related