1. 26. Cosmology The significance of a dark night sky
The Universe is expanding
The Big Bang initiated the expanding Universe
Microwave radiation as evidence of the Big Bang
The Universe was initially hot & opaque
The importance of the shape of the Universe
The Universe seems to be filled with dark energy
The Universe’s expansion rate is accelerating
Matter & dark energy determine the future

2. The Significance of a Dark Night Sky
Cosmology
Science of the structure & evolution of the Universe
One fundamental expectation
The Universe is large enough to fill the sky with stars
The night sky should be filled with light
The night sky is not filled with light
This is Olbers’s paradox
The model of an infinite static Universe cannot be correct
One possible solution
Einstein’s special theory of relativity
Distance, mass & time are all relative to different observers
Einstein’s general theory of relativity
Space is warped by massive objects
The cosmological constant L
Introduced to produce the preconceived static Universe

3. The Dark Sky Anomaly

4. The Universe Is Expanding
The Hubble law 1920
Clusters of galaxies are moving away from each other
Observed redshift is conclusive evidence
The relationship is linear
A tenfold distance increase yields a tenfold recessional speed increase
An expansion of space, not of velocity through space
Cosmological redshift rather than Doppler redshift
Space does not expand in objects tightly bound by gravity
Model of solid objects on an inflating balloon
One fundamental conclusion
The center of expansion cannot be identified
Everything is moving away from everything else
Every viewing location observes the same expansion

5. The Expanding Balloon Analogy

6. The Hubble Law & Expansion

7. Fundamental Assumptions
Assumptions in scientific method
Any reasonable assumptions can be made
Assumptions are presumed unproveable for some reason
Future observations may prove some assumptions invalid
Assumptions in cosmology
Observations can verify only one Universe
Cosmologists can imagine but not produce other universes
The cosmological principle
The Universe is assumed to be homogeneous
At the largest scale, all regions of space are identical
The Universe is assumed to be isotropic
At the largest scale, all directions in space look identical

8. The Big Bang & the Expanding Universe
Observational evidence
Reverse the Hubble law
In the distant past, all things in the Universe were close together
Far enough back, all matter & energy were in one point
Reciprocal of the Hubble constant H0 gives age of the Universe
The Universe began 13 Bya presuming H0 = 75 km . sec–1 . Mpc
This assumes that H0 is now & has always been constant
One major problem
Stars cannot be older than the Universe
Observational evidence suggests this may not be the case
Recent evidence may resolve this issue
The Big Bang
“Cosmic singularity” may be a better term
Remarkably similar to the singularity in a black hole

9. The Observable Universe
Expected size of the Universe
This is almost certainly far larger than what we can see
Observational limits
We only see as far back as the Universe is old
This results from the finite speed of EMR in vacuum
Presume the Universe is 13 billion years old
We can only see objects < 13 Bly away
This is our cosmic particle horizon
As the Universe ages, we can see back even farther

10. The Changing Observable Universe
Entire
Universe

11. Microwave Radiation As Evidence
The overabundance of He
Theoretically there is too much He in the Universe
Can be accounted if the early Universe was extremely hot
If so, there should be evidence
This would be severely redshifted to ~ 1.1 mm l
Cosmic background microwave radiation
Penzias & Wilson at Bell Labs Early 1960s
Observed microwave background radiation from space
About 1% of TV noise is this radiation
Earth’s atmosphere is largely opaque at the 1.1 mm l
Satellites are a better option
Cosmic Background Explorer (CoBE) 1989

12. Bell Labs Horn Antenna

13. The Microwave Background Spectrum
Blackbody curve
T = K

14. Temperature of Microwave Background

15. Our Motion Through the Universe

16. Density Dominance in the Universe
Radiation-dominated Universe
This prevailed in the earliest stages of the Universe
This corresponds to a cosmic redshift of z = 25,000
Since then, l’s have been stretched by a factor of 25,000
This occurred when the Universe was ~ 2,500 years old
EMR had l = 40 nm in the UV part of the spectrum
Matter-dominated Universe
This prevailed since the Universe was ~ 2,500 years old
This may prevail for all time in the future

17. The Universe Began Hot & Opaque
Basic physical processes
High temperatures tend to ionize atoms & molecules
Hydrogen has only 1 proton in its nucleus
The bond with the 1 electron is therefore relatively weak
Hydrogen ionizes at temperatures > 3,000 K
This was true for ~ 300,000 years after the Big Bang
An ionized gas is called a plasma
Plasma interacts very strongly with EMR
This makes a plasma opaque
Cosmic redshift reduces energy content of EMR
Energy is inversely proportional to wavelength
Recombination
Protons & electrons bind to form neutral hydrogen atoms
Misnomer because they were never combined previously

18. Radiation & Matter In the Universe
Two basic possibilities in the Universe
Something is either energy or matter
Special relativity says they are two forms of the same entity
Important concepts
The Universe’s average mass density of radiation today
rrad = –31 kg . m–3
Equivalent to 550,000,000 photons per cubic meter
Extremely low energy per severely redshifted photon
The Universe’s average density of matter today
rm = 2 to –27 kg . m–3
Equivalent to 1 to 6 hydrogen atoms per cubic meter
Extremely high energy per hydrogen atom

19. Evolution of Density

20. Evolution of Radiation Temperature

21. Before & After Recombination
Free protons Bound protons
& electrons & electrons

22. Microwave Temperature Variations

23. Importance of the Shape of the Universe
Three basic possibilities
Positive curvature of space Spherical
Two parallel beams of EMR will converge
Three-dimensional analog of a sphere
The Universe’s expansion rate will decrease
The Universe will eventually collapse upon itself
Zero curvature of space Flat
Two parallel beams of EMR will remain parallel
Three-dimensional analog of a plane
The Universe’s expansion rate will remain constant
The Universe will continue expanding forever
Negative curvature of space Hyperbolic
Two parallel beams of EMR will diverge
Three-dimensional analog of a saddle
The Universe’s expansion rate will increase

24. Measuring the Shape of the Universe
Hypothetical
Draw a huge triangle
Measure the three angles
Practical
Count the number of extremely distant galaxies
Spherical Concentration is highest nearby
Flat Concentration is uniform
Hyperbolic Concentration is highest far away

25. The Geometry of the Universe

26. Microwave Background & Curved Space

27. A Universe Filled With Dark Energy
The observational evidence
Temperature variations suggest a nearly flat Universe
Galaxy clusters suggest matter density Wm = 0.2 to 0.4
This clearly suggests a non-flat Universe
The tentative conclusion
There must be substantial dark energy in our Universe
Necessarily WL = 0.6 to 0.8
Presumed dark energy must be 60% to 80% of the Universe

28. Proportions of Mass & Energy

29. Universe’s Variable Expansion Rate

30. Universe’s Matter & Energy Distribution

31. The Universe’s Expansion is Accelerating
The observational evidence
Type Ia supernovae in distant galaxies
The tentative conclusion
The expansion rate of the Universe is increasing
Einstein’s cosmological constant may be correct after all ! ! !

32. Varying Rates of Cosmic Expansion

33. Distant Supernovae & Hubble Diagram

34. Limits on the Nature of the Universe

35. The Evolution of Density Revisited

36. The Growth of the Universe

37. Chapter 28: Important Concepts
The expanding Universe
How can the sky be so dark?
Stars should be literally everywhere
The Hubble Law (1920)
Space itself is expanding
Concept of the Big Bang
Matter & energy in a very small space
Explosion created space & time
Space expanded faster than light
Expansion of space has slowed down
Evidence for the Big Bang
The cosmological red shift
The cosmic background radiation
Remnants of heat from the Big Bang
Big Bang produced excess helium
Required extremely high temperatures
Milky Way moves amidst CBR
The Great Attractor
Characteristics of the Big Bang
Extremely hot & opaque plasma
The primordial fireball
Hydrogen atoms eventually formed
Temperature < 3,000 K
Universe became transparent
The shape of the Universe
Three major possibilities
Zero curvature = Flat ***
Positive curvature = Closed
Negative curvature = Open
Two remarkable things
~90% of all matter is “dark”
~80% of all energy is “dark”
The strangest thing yet
Universe’s expansion is accelerating