PHY111 Our Evolving Universe:
Lecture Summaries

This pie chart shows the make-up of the Universe today, as derived from fits to the "ripples" in the cosmic microwave background and other data. Only about 10% of the "ordinary atoms" is actually contained in the stars and luminous gas that we see: the Universe is actually a very dark place. Image from the WMAP website; credit: NASA/WMAP Science Team.

Summary of Lecture 12 – Echoes of the Big Bang

1. Principal observational evidence for the Big Bang:
   • the universe is expanding according to Hubble's law:
     • suggests (does not prove, cf. Steady State) that the universe was once much smaller and denser (and therefore hotter);
   • there is a nearly isotropic cosmic microwave background with an exact blackbody spectrum:
     • can be explained as photons released approximately 380,000 years after the Big Bang, when electrons combined with nuclei to form neutral atoms, and the universe became transparent;
     • blackbody spectrum shows that the Universe was once hot, dense and opaque (since transparent sources don't produce blackbody spectra);
     • original 3000 K temperature reduced to 2.7 K by expansion of universe (redshifting photons);
     • very strong evidence for Hot Big Bang – other models, such as Steady State, do not have the hot, dense early universe needed to make a blackbody spectrum;
   • galaxies look different at large redshift:
     • evidence that the universe is evolving (evidence against Steady State);
     • can help to determine model parameters (e.g. any model which says that galaxies should form at a time corresponding to redshift=1 is clearly wrong, since galaxies have been observed with redshifts >5);
   • the abundances of the light isotopes deuterium (or hydrogen-2), helium-4 and lithium-7:
     • measured by spectral analysis of old objects;
     • results agree well with predictions of big bang nucleosynthesis;
     • further evidence for hot Big Bang, and strong support for the accuracy of our understanding of the early universe and its evolution;
   • the cosmic microwave background exhibits tiny temperature variations:
     • fitting these to predictions can determine parameters of Big Bang model (see below);
     • results agree well with expectations from Hot Big Bang plus inflation, with derived parameters consistent with other data;
     • further evidence for hot Big Bang, and support for inflation as mechanism for achieving flatness and uniformity.
2. Vital statistics of the universe:
   • Hubble's constant (H₀) – determines expansion rate of universe, and thus its age;
     • main difficulty is distance measurement (redshifts are easy!);
     • HST result: 72 ± 8 km s^-1 Mpc^-1
       WMAP result: 71.0 ± 2.5 km s^-1 Mpc^-1
       recent review: 73 ± 4 km s^-1 Mpc^-1
   • curvature (k) – determines geometry of universe;
   • inflation predicts k = 0 (universe is flat), i.e. Ω_total = 1;
   • best information comes from cosmic microwave background;
   • WMAP result: 0.99 < Ω_total < 1.01 (combining WMAP with other data);
   • density as a fraction of critical density (Ω):
   • Ω is made up of many different components measured in different ways:
   • calculate stellar masses from mass-luminosity relation, and add them up – Ω_stars ≅ 0.004;
   • fit abundances of deuterium and helium-4 – Ω_atoms ≅ 0.04;
   • calculate galaxy masses from their gravitational forces (rotation curves), and add them up – Ω_galaxies ≅ 0.1;
   • calculate masses of galaxy clusters (from motion of galaxies, X-ray emission from cluster gas, or gravitational lensing) – Ω_clusters ≅ 0.3;
   • analyse fluctuations in microwave background – Ω_matter = 0.27 ± 0.02;
   • conclusion: matter accounts for about 30% of the critical density, but (1) only about 1/7 of this is made up of ordinary atoms and (2) even 90% of the ordinary atoms are not in stars (nearly 99% of the matter in the universe is non-luminous);
   • cosmological constant as a fraction of critical density (Ω_Λ):
   • Type Ia supernovae show that expansion is accelerating, not slowing down as expected;
   • combination of supernova data and microwave background gives Ω_Λ = 0.73 ± 0.02;
   • physical interpretation of Λ is that it represents the (quantum mechanical) energy of empty space;
   • nobody currently understands why Λ has its present value, or how it might evolve in future.
3. Our current view of the universe, past, present and future:
   • the universe is currently expanding and cooling from an initial Hot Big Bang which took place about 13.7 billion years ago;
   • a tiny fraction of a second after the big bang, the universe underwent a brief period of rapid (exponential) expansion – inflation – which resulted in an extremely isotropic and flat visible universe, with small variations from quantum fluctuations;
   • the total density of the universe is equal to the critical density (the universe is flat) and has contributions from both matter and the cosmological constant, roughly ¼ matter and ¾ Λ;
   • of the matter component, about 85% is not ordinary atoms, but as yet unidentified non-baryonic dark matter;
   • although the universe is flat, because Λ > 0 the expansion is not decreasing towards zero but instead is accelerating – the unvierse will expand forever at an ever-increasing rate (unless we are wrong about the "cosmological constant" being constant).

Web links

• The PowerPoint file for this lecture.
• Nick Strobel has a chapter on cosmology; Gene Smith has a couple of pages, including a convenient timeline. There is also Ned Wright's Cosmology Tutorial, which is excellent, although the tutorial itself is aimed a bit above our level (the FAQ page is OK).
• Much of our best evidence for the values of cosmological parameters nowadays comes from analysis of the cosmic microwave background. The WMAP mission has been the leading experiment in this field (though it should shortly be overtaken by Planck). WMAP's webpage includes good overviews of WMAP science goals and cosmology in general.
• A short self test for this lecture.

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