The Big Bang

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