ASTRONOMY 9: HISTORY OF COSMOLOGY

Handout #23

J. E. Baker
UC Berkeley, Spring 2000

Controversy: Big Bang vs. Steady State

• Genesis of the Big-Bang Model
  • Hubble has discovered the expanding universe
  • Friedmann showed many expanding models have an ``origin'': distances between galaxies goes to zero at a point in the past!
  • Georges Lemaitre (b. 1894)
    • Catholic priest as well as research physicist
    • Works with Eddington (Cambridge), then Shapley (Harvard)
    • First efforts at a scientific understanding of the origin
    • The ``Primeval Atom'' (1931)
      • Wrong in detail, but firmly implants a (correct) evolutionary world-view into cosmology
      • Nuclear fission as an analogy for the creation
      • Universe develops from a giant unstable radioactive atomic nucleus filling all space
      • Cosmic rays (high-energy particles) are the leftover ``fireworks''
      • Nuclear physics is important for the beginning of the universe!
      • Mythical equivalent: the Cosmic Egg
    • Universe has a beginning: endorsed by Pope Pius XIII in 1951 as consonant with biblical act of creation
    • Lemaitre preaches against both literal/scientific interpretation of religion (fundamentalism) and introduction of science into religion
  • Development of Nuclear Physics
    • Atomic nuclei described by two numbers:
      • A = atomic number = number of protons (+ charge)
      • Z = number of protons and neutrons (no charge, discovered 1932)
    • Binding energy of nuclei is maximum for iron A=56
    • So can release energy by combining small nuclei (fusion) or smashing big ones (fission)
    • First atomic bombs worked by fission (U, Pt), later by fusion (H)
    • Nuclei are bound by the ``strong force'' ($100\times$ stronger than electromagnetism, but very short range!)
    • Buildup of elements from smaller ones is called nucleosythensis
  • Richard Tolman and the Hot Big Bang (1930s)
    • Applies thermodynamics to the whole universe
    • Realizes universe cools as it expands
    • Given composition of universe (matter and radiation), can compute change in temperature
    • At early time, universe was very hot--so hot even atomic nucleus could not be bound!
    • Can try to play the movie backwards: figure out the history of the universe!
  • George Gamow (1940s)
    • Escapes Soviet Union, ends up at G. Washington Univ.
    • Nuclear physicist, plants cosmology firmly in realm of physical science
    • With Alpher and Hermann, develops physics of the Big Bang during its first few minutes, basically as we know it today!
    • Universe as a nuclear furnace
    • Adds Hans Bethe (work on formation of elements in stars) to a paper: Alpher, Bethe, Gamow ( $\alpha\beta\gamma$)
    • Universe starts off as ylem: mixture of free, hot protons, neutrons, and electrons, plus hot (high-energy) photons
    • During first few minutes, cools to the point where nuclear bonds can begin to stick ($t\sim 1$ sec, $T\sim 10^{10}$ K)
    • Light elements (deuterium, tritium, helium-3, helium, lithium-7) are formed!
    • Heavier elements are formed only much later, in the stars, which are powered by fusion, and in explosions of stars (supernovae)
    • Today, we know mass of the universe is mostly H, with about 25% He (mostly from early universe, only about 2% added by stars)
    • Crucial predictions of 1948 (largely ignored for 20 years):
      • Can calculate abundance of elements from early universe, add a little bit of processing by stars, and compare to observations
      • The photons from the early universe are still around, but much cooler (Gamow predicts temperature of 5 K)
• Tired Light hypothesis
  • Fritz Zwicky (1929, Caltech) proposes redshifts are not caused by expansion
  • Rather, light gets ``tired'', loses energy, shifts to red, as it travels through space
  • Universe is static
  • Idea resurrected a number of times
  • Idea creates many problems, does not explain many (later) observations which are explained by Big Bang
• The Steady-State Theory (1948-1965)
  • Earlier idea: Macmillan (1918) and Millikan: radiation somehow converted back into matter, individual stars come and go but universe stays the same
  • Perfect Cosmological Principle: universe is same at all points in space and time
  • But, we know the universe is expanding! How can this be?
  • Fred Hoyle, Thomas Gold, and Hermann Bondi (Cambridge) propose steady-state theory in 1948
  • Single creation event (the Beginning) is replaced by continuous creation events!
  • Matter spontaneously appears out of a ``creation field'', at just the right rate so that overall density of universe stays the same
  • Violates conservation of mass-energy, but is not ruled out by experiments: need to create only about one atom per cubic meter every million years!
  • Eternal universe without a creation; mythical equivalent: Jainism
  • Hoyle coins ``Big Bang'' as an insult to his opponents!
  • Controversy now resolved, steady-state conflicts with increasing body of evidence:
    • Universe was clearly different in the past (1955: Martin Ryle shows abundance of radio sources was higher; today we find high-z galaxies are different)
    • Radiation ``echo'' from the Big Bang detected in 1965 (convinced most cosmologists), temperature was hotter in the past
    • Abundance of elements in good agreement with Big Bang nucleosynthesis prediction; no good steady-state explanation

About this document ...

This document was generated using the LaTeX2HTML translator Version 98.1p1 release (March 2nd, 1998)

Copyright © 1993, 1994, 1995, 1996, 1997, Nikos Drakos, Computer Based Learning Unit, University of Leeds.

The command line arguments were:
latex2html -up_url ../../notes.html -up_title Astronomy 9 Lecture Notes -split 0 -t Astronomy 9 (Spring 2000): Handout 22 -dir /coma8/jbaker/public_html/courses/ay9/week12/handout23 handout23.tex.

The translation was initiated by jonathan baker on 2000-04-21