XIV Cosmology

So far we could test models at many different objects.

Cosmology: test only at our one universe

Why is the night sky dark? Olbers' paradox:

• Is the universe infinitely large?
  • then the infinite number of stars should overlap in the sky
  • very bright sky???

• v_away = H x distance --> The universe expands
• Space itself expands, but Not expansion into something
• Space expands --> photons expand --> "cosmological redshift"
• Expansion seen in the same way from everywhere
• Analogy: raisins in a raisin cake when rising due to yeast

• Steady State: continuous expansion of space
  • but: creation of new matter-> density remains the same
  • see no evolution in distant galaxies
• Big Bang: big explosion in the beginning: density decreases with time
  • clear evolution of distant galaxies
  • beginning with hot --> cooling

  Clues?

Big Bang: Explosion of everything everywhere "together" @ time = 1/H ago (About 15 billion years.)

• Not an explosion into space: space exploded
• Finite age --> "particle horizon" or "observable universe"
• "On a clear day you can't see forever" because photons from far
• away haven't had time to reach us. This is why the sky is dark.
• Resolution of Olbers' Paradox

• Globular clusters:
  • oldest star clusters("e 10 billion years old)
  • checks

• Finite or infinite? Finite: observer on edge would see something different
  • -> infinite universe

How can an infinite universe have expanded from a tiny start?

• Analogy (in 2 dimensions):inflating a balloon
  • -> has curved surface
  • -> curved space in the universe? (from gravity: General Relativity)

• Gravity slows down the expansion of the universe

Consequence:

• Real age = 2/3H due to slowing down of expansion (~ 10¹⁰ yrs.)
• Age Problem: globular clusters seem to be older than the universe!

Enough mass density to bring all flying masses back by gravity... or in other words:

Is the "escape velocity" of the galaxies not high enough to escape forever?

We need to solve "missing mass problem" or measure curvature of space.

Test by measuring:

• area of sphere (does radiation fall of as 1/distance2?)
• is skinny triangle equation correct?
• do parallel lines converge or diverge?
• volume of sphere (how does density of galaxies vary with distance from Earth?)

• Early universe --> hot; dense --> blackbody radiation
• At age of about 3x105 yrs photons decouple from matter (universe 1/1000th present size)
  • (less dense; ions + electrons --> H, He)
• Universe opaque before about 3x105 yrs (many photon - particle collisions)
  • --> we can't see photons from before 3x105 yrs (Curtains of the universe)
• Prediction: universe now filled with photons with T = 2.7K
  • (after cooling due to expansion of universe)
• Detection: 1965: Penzias and Wilson detect 2.7K blackbody radiation (Nobel Prize)

1. Supports big bang
2. Provides a "standard of rest"
   • Our galaxy and other galaxies are 'falling' toward a "Great Attractor"
   • The expansion is not uniform. A riddle
   • 2.7K photons an absolute reference
3. Early universe very uniform (but not perfectly uniform)
   • Riddle - how did galaxies form so quickly?

• Now energy of universe dominated by mass (E = mc2), even though number of photons is about 109 times the number of particles of visible matter

• Before about 3x105 yrs: energy (and gravity) dominated by photons
  • "In the beginning there was light."
• Production of elements:
  • Helium production by fusion in dense, hot early universe:
  • Predict 25% of mass of universe should be Helium, as observed
• Independent support of big bang

a) Matter Problem

• In the radiation-dominated era (universe < about 106 years old) there were
  • 109 photons
  • 109 + 1 particles of matter
  • 109 particles of antimatter
• Why was there one extra particle of matter?

b) Horizon problem.

• 2.7K photons in opposite parts of sky have same temperature
• But those parts were never in contact (particle horizon) before
• How did they "know" to have the same temperature?

c) Smoothness problem.

• Probably solved by detection of non-uniformity of the 2.7K radiation

d) Flatness problem.

• Universe had to be very flat at the beginning for it to be the way it is now
• Problems do not point to wrong physics, but the model does not offer an explanation!

• Contact before inflation solves horizon problem
• Solves flatness problem
  • Analogy: we 'see' a flat earth because we see only a small part of it
• Quantum fluctuations
  • --> non-smoothness which agrees with the observed non-smoothness of the 2.7K radiation

• Why are the world parameters exactly as they are?
• If different by a tiny bit
  • --> duration of universe either too short or no stars possible
  • --> no life --> no man to observe the universe

a) Universe was designed such that man can appear

= Strong anthropic principle

or

b) We live in one of many possible universes where we are able to exist

= Weak anthropic principle

Both offer no explanation, but only a constraint what to look for. These are philosophical and not scientific statements!

Revised E. Möbius 12/96