Introduction to Cosmology

1. Introduction to Cosmology

2. Cosmology • Scientific study of the origin and evolution of the universe • “kosmos” – Greek for “order” • Based on laws of physics, incorporates chemistry, mathematics, and astronomy

3. Foundations of modern cosmology 1) General Theory of Relativity (A. Einstein 1915) – describes gravity in terms of the warping of space-time by the presence of mass and energy.

4. Many predictions have already been verified experimentally • Precession of perihelion of Mercury • Bending of starlight around sun • Slowing of time in a gravitational field (gravitational redshift). • Twisting of spacetime by rotation of earth (frame-dragging) • Decay of binary pulsar orbit (through gravity waves, whose presence has not yet been directly observed)

5. 2) The Cosmological Principle Matter in the universe is distributed evenly in all places and in all directions (homogeneous and isotopic), IF YOU LOOK AT LARGE ENOUGH SCALES (about 100 Mpc).

6. Forcing the universe to be finite and static – enter the “cosmological constant”

7. Einstein’s “greatest blunder”

8. Abbé Georges Lemaître, father of the “big bang” • Independently of Hubble, he discovered the expansion of the universe, and proposed the universe began in the decay of a large radioactive atom. • Some religious figures, including Pope Pius XII, embraced it as scientific proof for the Judeo-Christian origin story. • Lemaître himself struggled with the theological implications of his theory -"As far as I can see, such a theory remains entirely outside of any metaphysical or religious question."

9. George Gamow refined the idea Universe began in small, hot, dense state which has expanded and cooled over time. His theory named “Big Bang” as an insult by Fred Hoyle.

10. Initial arguments against the big bang theory included a disbelief that the laws of physics could possibly be pushed back to the origin of the universe. There was also an antireligious backlash against the theory, which reportedly was part of the motivation for the rival steady state model.

11. Despite Lemaître’s caution, some ID proponents still cite the big bang as proof of design • Hugh Ross argues that the big bang renders "the design of the universe irrefutable." • Noted Intelligent Design proponent Michael Behe warns that the big bang, "although friendly to a religious point of view, does not forcibly compel that belief."

12. Tests of the Big Bang 1) Observed expansion of the universe. Slope = H, but is difficult to determine (uncertainties in velocity and distance)  HST Key Project

13. 2) Nucleosynthesis in the first three minutes Predicts percentage of light isotopes relative to hydrogen. Observed composition matches theory with great precision (but restricts amount of normal or baryonic matter that can exist relative to light).

14.  most elements have been created inside stars “We are star-stuff” – Carl Sagan

15. 3) Cosmic Microwave Background Radiation Predicted by George Gamow in 1948, accidentally discovered by Penzias and Wilson in 1965 (1978 Nobel Prize). Matches a blackbody of T = 2.7 K (and is very isotropic!)

16. Misconceptions about Big Bang • It did not occur at a single point in space-time but describes the early evolution of space-time as a whole. • It does not say what happened before the universe, nor does it explain why (or even how) the universe was born, just how it evolved from a hot, dense state.

17. Friedmann-Robertson-Walker models (aka, what shape is the universe?)

18. So which shape is the universe? Ω = ρ / ρcritical = 1 (flat) < 1 (open) >1 (closed) 0.02 < Ω < 2

19. The early universe is described by particle physics

20. Anyone got a “TOE”?

21. Problems with original Big Bang model • Why is CMB so smooth? • Why is universe so close to being flat? • Where does structure come from? • Where are the magnetic monopoles?

22. Enter Inflation! In 1981, particle physicist Alan Guth sought a solution for the magnetic monopole problem and ended up revolutionizing cosmology!

23. Our “universe” is but one small piece of the original Big Bang

24. Solves all four problems: • Our visible universe evolved from a small “smooth patch” • Predicts Ω=1 • End of inflation plants “seeds” for structure formation. • Dilutes magnetic monopoles (hard to find)

25. Another success: age of universe can be determined by: • Hubble constant • Abundances of radioactive isotopes in very old stars • Cooling times of white dwarfs • Models of stellar evolution compared to old stars  All agree with @ 13-14 billion years

26. Another fly in the ointment! The expansion is not slowing down, but is accelerating!  Presence of something like Einstein’s cosmological constant (called “dark energy”)

27. Most precise CMB measurements (WMAP) results agree with “Concordance cosmology” ΛCDM: accelerating, Ω = 1 universe with @ 70% dark energy and @ 25% cold dark matter and @5% “normal” matter So far results are consistent with inflation as well.

28. “The field of cosmology has a grand history of pushing back the boundary between physics and metaphysics. Decades ago, who would have thought that the origin of relative abundances of the different chemical elements would be considered the subject of physical calculation rather than metaphysical speculation! I have no doubt that we will find more opportunities to push the boundary back even further.” - Andreas Albrecht (1994)

29. Big questions we might be able to answer • Will the Big Bang (and general relativity) hold up to improved observations and experiments? • What particle makes up dark matter? • What is dark energy? • Will the acceleration of the universe cease? • Which model of inflation best fits observations? • What is the TOE?

30. Big questions we may never be able to answer • What caused the Big Bang to occur? • Do we live in a multiverse? • Why are the constants of nature what we observe?