What Do We Know?

• Evidence from when the universe was young: Distant objects like quasars, seen like they were when the universe was a few billion years old; Old globular clusters, matter and radiation that has been around since then.
• Theory and simulations: With varying amounts of detail, we have developed the hot big bang model, describing the universe back to perhaps 10^-43 s of age! It makes some surprisingly precise predictions.

The Formation of Spacetime and Matter

• The "instant" of the big bang: We don't know if the universe was just a point – probably not – but the it started approximately 13.7 Gyr ago; For the first 10^–43 s ["Planck time"], we don't know how to describe the universe, because there were rapid fluctuations in energy and warping of space and time, and the fabric of spacetime itself had not been "woven together" yet.
• The GUT era: Three of the four forces were unified ("grand unification"), and it lasted until T = 10²⁷ K, at t = 10^–35 s, when the strong nuclear force broke away.

• Inflation: Energy liberated at the end of the GUT era made the universe briefly expand and flatten very fast; Small fluctuations that were present become seeds for galaxies (and their imprints are still seen as fluctuations in the microwave background); Would explain why the universe is so flat, and why it looks almost the same everywhere.

  The Formation of Particles

• The electroweak era: Lasted until all forces became different, like they are today [at T = 10¹⁵ K, 10^–10 s].
• The particle era: In addition to photons, particles like quarks, electrons, and neutrinos were constantly being created and annihilated (quark-gluon plasma); lasted until quarks were combined into protons and neutrons [at T = 10¹² K, 1 ms], just slightly more abundant than antiprotons and antineutrons.

The Formation of Nuclei

• The era of nucleosynthesis: Protons and neutrons form nuclei which then break apart, until at t = 3 min, T = 10⁹ K and nuclei are stable.
• Helium production: He nuclei are produced by a chain of reactions.
• Result/prediction: In terms of mass, 75% of the nuclei are H, 24% He, traces of heavier ones [there were 7 p's for each n available].
• Current observations: We now have 25% He, generations of stars having produced additional He nuclei; numbers agree with prediction.
• The era of nuclei: Matter is a hot plasma of H and He nuclei, mostly; lasts 300,000 yr, until T = 3000 K and protons and electrons stick together.

  The Formation of Atoms

• Decoupling: The era of nuclei ends at about 380,000 yr, when neutral atoms form and most of the matter does not affect photons anymore; matter becomes transparent to visible light.
• Result/prediction: The light that was present at the time is still around, although much redshifted, as microwaves. More energetic photons, like UV, would be absorbed by atoms, but there were not many of those.
• Do we have evidence? The 3 K cosmic microwave background, discovered in 1964 by Penzias and Wilson. The radiation is the same in all directions, except for a blue/redshift in our direction of motion, and very small fluctuations.
• Structure formation: Matter can now form the first gas clouds, that will eventually lead to protogalaxies.

The Formation of Stars and Galaxies

• The era of galaxies: Star formation started after about 200 millions years, at a much higher rate than the present one; Before then, the universe had been dark for some time; After this time, and over a few billion years, stars grouped into galaxies and galaxy clusters around filaments of dark matter under the effect of gravity;* We expect to understand better these processes and their timing with future spacecraft missions.
• Protogalaxies: With lensing, we have started seeing galaxy "building blocks"; Many of the early galaxies are seen as quasars, which later evolve into regular galaxies, or sometimes still active ones.
• First star generations: It seems that the lower abundance of heavy elements favored the formation of large stars, and it definitely prevented stars from having terrestrial planets around them.