1. The Nature of the Universe
Chapter 18
Cosmology:
The Nature of the Universe 1

2. 18-1 The Search for Centers and Edges
Our basic ideas of the nature and extent of the universe have changed over time. 2

3. Einstein's Universe
Einstein's general theory of relativity substituted curved space for gravitational force.
Closed universe: The state of the universe if its total mass and energy density is greater than a specific value, called the critical density.
A closed universe has positive curvature.
This can be thought of as a two-dimensional plane curved into a sphere. 3

4. An open universe has negative curvature.
Open universe: The state of the universe if its total mass and energy density is less than a specific value, called the critical density.
An open universe has negative curvature.
This can be visualized in two-dimensions as a saddle shape. 4

5. Flat universe: The state of the universe if its total mass and energy density is exactly equal to a specific value, called the critical density.
Critical density: The average mass and energy density of the universe at which space would be flat. It is equal to 3 Ho2 / (8 p G) and is equivalent to about 5.5 hydrogen atoms per cubic meter of space. 5

6. Einstein believed the universe is closed and finite.
He adjusted his theory to include a repulsive force that kept the universe from collapsing.
Cosmological constant: A term (denoted by the Greek capital letter “lambda,” ) in the equations of general relativity that corresponds to a force throughout all space that helps the universe expand.
This hypothetical force is significant only at great distances.
Recent observations suggest an exotic form of energy exists that results in a repulsive force. 6

7. 18-2 The Expanding Universe
Redshifts indicate distant galaxies are moving away from us.
This does not mean we are at the center.
These fleas on a balloon move farther apart as it expands.
More distant fleas are moving away faster than those nearby. 7

8. What Is Expanding and What Is Not? The Cosmological Redshift
It is other clusters that are moving away from our cluster. Anything that is gravitationally bound is not expanding as a result of the expansion of the universe.
Cosmological redshift: The shift towards longer wavelengths that is due to the expansion of the universe.
Clusters do not actually have a velocity, but space itself is expanding. 8

9. Difference in interpretation of redshift as a Doppler effect versus cosmological expansion.9

10. Olber's Paradox
In an infinite, static universe, every line of sight would end on a star and the sky should not be dark.
Olber's Paradox: An argument showing that the sky in a static universe could not be dark.
1. The universe’s finite age limits the distance we see.
2. Light is redshifted and looses energy. 10

11. 18-3 Cosmological Assumptions
Homogeneous: Having uniform properties throughout.
Isotropy (I-SOT-rah-pee): The property of being the same in all directions.
Cosmological principle: The basic assumption of cosmology that holds that on a large scale the universe is the same everywhere.
Universality: The property of obeying the same physical laws throughout the universe. 11

12. 18-4 The Big Bang
Big bang: The theoretical initial explosion that began the expansion of the universe.
All matter and energy was compressed to an unimaginable density.
Afterward the temperature decrease allowed atoms to form, which clustered into stars and galaxies.
The big bang was the universe, not just at one point
Matter is not expanding through space; space is expanding. 12

13. Evidence: Background Radiation
At early times, hot matter was opaque to radiation.
At 376,000 years it cooled to 3000 K, became transparent.
Radiation from then would be redshifted and appear today as radiation from an object at 3 K.
Cosmic microwave background radiation (or CMB radiation): Long-wavelength radiation observed from all directions; thought to be the remnant of radiation from the big bang. 13

14. Additional Evidence for the Big Bang
Time dilation of distant supernovae confirms the cosmological redshift.
The relationship between the number density of distant radio sources and quasars and the energy received from them shows that the universe evolved.
The proportions of light elements (hydrogen, helium, lithium) could not have formed in stars but are consistent with processes in the first few minutes of the hot, early universe. 14

15. Advancing the Model: The Early Universe
At 10-6 s after the big bang, the temperature was low enough for protons and neutrons to form.
For the next 15 minutes, light elements could form.
The predicted abundances depend on the density of the universe and match the observed abundances.
Courtesy of NASA/WMAP. 15

16. The Age of the Universe
An approximate age can be found by assuming the rate of expansion has remained constant.
t = 1 / Ho
Thus if Ho = 20 (km/s) / Mly, then t = 15 billion yrs.
From the most recent data, t = ± 0.11 billion yrs. 16

17. Advancing the Model: The Steady-State Theory
The perfect cosmological principle states the universe is the same throughout time as well as space.
As the expansion is undeniable, matter would be created between galaxies to keep the universe from becoming less dense. The creation rate would be below what is detectable.
The number density of distant radio sources and quasars, the helium abundance in the universe, and the CMB radiation do not agree with this theory. 17

18. 18-5 The Future: Will Expansion Stop?
An open universe would grow indefinitely.
A closed universe would stop growing and contract.
Oscillating universe model: A big bang theory that holds that the universe goes through repeating cycles of explosion, expansion, and contraction. 18

19. Evidence: Distant Galaxies and High-Redshift Type Ia Supernovae
Density parameter (o): The ratio (denoted by the Greek letter “omega” and subscript zero) of the total mass and energy density of the universe to the critical density.
If o = 1, the universe is flat and will expand forever but barely so.
If o > 1, the universe is closed and will collapse.
If o < 1, the universe is open and will expand forever. 19

20. The constant rate of expansion favors a flat universe (o=1) dominated by an exotic energy.
Dark energy: An exotic form of energy whose negative pressure currently accelerates the expansion of the universe.
The magnitudes of distance supernovae are dimmer than they would be if the universe were just slowing due to gravity. 20

21. Radiation density is negligible, so o = m + .
Contributions to the density parameter are matter (m), radiation (rad), and dark energy ().
Radiation density is negligible, so o = m + .
With different models the universe evolves differently.
Dark energy dominates the universe now, with:
m = 0.3
 = 0.7 21

22. 18-6 The Inflationary Universe
The Flatness Problem
The universe seems either flat or very nearly flat.
If the critical density were not 1, it would continue to get further from 1 as time when on.
Because it is close to 1 today, it must have been extremely close to 1 at the beginning.
Flatness problem: The inability of the standard big bang model to account for the apparent flatness of the universe. 22

23. Any great curvature before the inflation became smoothed out.
Inflationary universe model: A modification of the big bang model that holds that the early universe experienced a brief period of extremely fast expansion.
Between and seconds after the big bang, the universe expanded by a factor of 1050.
Any great curvature before the inflation became smoothed out. 23

24. The Horizon Problem
CMB radiation is remarkably uniform in every direction.
A change in one portion can be communicated to another portion only as fast as the speed of light.
The early expansion of the universe was faster than distant portions were able to communicate. Energy could not be exchanged beyond a “horizon.”
Why is the entire universe at the same temperature?
Horizon problem: The inability of the standard big bang model to account for the directional uniformity of the background radiation. 24

25. In the inflationary model, the horizon distance was smaller before inflation occurred.
When inflation occurred, regions originally close enough to communicate became too far apart.
These images show stages of the universe expansion. The CMB radiation we see from 380,000 years has only minute temperature variations.
Courtesy of NASA/WMAP. 25

26. 18-7 The Grand Scale Structure of the Universe
Clusters of galaxies are distributed as if along bubbles.
Inflation smoothed irregularities from the early universe.
For the first 376,000 years the universe behaved as a single fluid of baryons and photons.
Baryons: Subatomic particles, including the proton, the neutron, and a number of unstable, heavier particles.
Courtesy of Seldner, M., Siebers, B., Groth, E.J. and Peebles, P.J.E, 1977. 26

27. Baryons provided inertia, photons provided pressure.
Oscillations in density resulted from the competition between gravity and radiation pressure. The oscillations propagate at the local speed of sound.
At 376,000 years, atoms could form that were transparent to the radiation.
The radiation freely propagated and is now the CMB radiation.
Matter started collecting under the influence of dark matter, which had started to collapse earlier. 27

28. These variations are only about 0.00003 K.
The CMB radiation carries information about temperature variations at the time it decoupled from matter.
These variations are only about K.
Courtesy of NASA Goddard Space Flight Center and the COBE Science Working Group. 28

29. CMB radiation is dominated by hot and cool spots of about 1o.
The geometry of the universe would distort the spots.
The spots are best matched by the simulated flat universe on the bottom.
Courtesy of NASA/NSBF. 29

30. The geometry of space affects the angular size of an object because the curvature determines the path of light beams. 30

31. The sizes of fluctuations in the CMB radiation constrain cosmological parameters.
Courtesy of NASA and the WMAP Science Team. Modified
from M.R. Nolta et al., ApJS 180 (2009): Reproduced by permission of the AAS.
Courtesy of NASA/WMAP. 31

32. Denser regions were hotter, less dense regions cooler.
The shape and peaks depend on the oscillations at the time the CMB radiation decoupled from matter.
Denser regions were hotter, less dense regions cooler.
Big peak corresponds to the horizon size at that time. 32

33. Scattering can also polarize light, and polarization of the CMB radiation has been observed.
Linear polarization: The situation when the electric field in an electromagnetic wave oscillates in a fixed plane.
Courtesy of NASA and the WMAP Science Team.
Modified from M.R. Nolta et al., ApJS 180 (2009): Reproduced by permission of the AAS. 33

34. Courtesy of NASA/WMAP.

35. Sequence of events in the life of the universe.
Courtesy of NASA/WMAP. 35

36. The overlap region gives the latest results:
The combination of data from different projects limits the range of values for the density of matter and dark energy.
The overlap region gives the latest results:
Wm = ± 0.015
WL = 0.728± 0.015
Wo = 1
Courtesy of BOOMERANG Collaboration. 36

37. Cosmic Evolution After about 430 million years the first stars formed.
UV light from these massive stars re-ionized the gas in the universe, slowing down galaxy formation.
Observations of distant quasars show the universe was dominated by fewer, large galaxies.
Heavy elements in their spectra mean supernovae were already occurring in the first 500–800 million yrs.
Evidence of merging supports the hierarchical view of galaxy formation. 37

38. This is a computer model of the universe at the age of 2 billion years.
These maps can match the observed fluctuations with predictions based on the matter and dark energy densities.
Courtesy of ESO. 38

39. Advancing the Model: Dark Energy
There are several ideas about dark energy.
Einstein's cosmological constant.
This could be the vacuum energy, an intrinsic energy of a volume of space.
A time varying energy field called quintessence.
High-energy physics predicts such fields but with energy densities too large for observations.
The fate of the universe depends on the nature of dark energy. 39