Astronomy 162:
Introduction to Stars, Galaxies, & the Universe Prof. Richard Pogge, MTWThF 9:30 |

- Cosmological Principle:
- The Universe is
**Homogeneous**and**Isotropic**on**Large Scales**. - No special places or directions.
- General Relativity predicts an expanding universe.
- Einstein's Greatest Mistake (?)

- Physics of the Universe.
- Distribution of objects on all scales.
- Motions of objects in the Universe.
- Evolution of the Universe.
- Age, Origin, and Fate of the Universe.

**Observational State in 1917**:

- Kapteyn model of the Milky Way was favored by some (but not all) astronomers.
- No agreement on the "spiral nebulae."
- First good calibrations of the P-L relation for Cepheids and RR Lyrae variables.

- "The Universe is
**Homogeneous**and**Isotropic**on the**Largest Scales**."

Critical assumption underlying Cosmology.

**Homogeneous:**

- No special places in the Universe.

**Isotropic:**

- No special directions

**Largest Scales:**

- Large enough to average out the small-scale details of the matter distribution

- The average density of matter is about the same in all places in the Universe.
- The Universe is fairly smooth on large scales.

Does not apply **locally**:

- We see planets, stars, galaxies in regions nearby in space.
- The Universe is locally rather "lumpy".

- The Universe looks the same to all observers.
- The Universe looks the same in all directions as viewed by a particular observer.

Does not apply **locally**:

- We see different numbers of local objects in different directions.

- The spacetime of the Universe could not be static and unchanging.
- The Universe
**must**either expand or contract!

Astronomers assured him that no such general motion was observed.

The Cosmological Constant, L (Lambda):

- "Repulsive" gravitational force term
- Arises from empty space.
- Balances the effects of gravity.
- Halts the motion of spacetime.

Measured the radial velocities of the brightest "spiral nebulae".

Results: 21 out of 25 spirals showed a systematic **redshift**.

- Apparent systematic motion
**away**from us. - Some radial velocities are large: >2000 km/second.

**1920s**:

- de Sitter showed that Einstein had made a mathematical error,
and that his Cosmological Constant made the universe static
but unstable (like a needle balanced on its point).
- Friedmann & Lemaitre showed that without a cosmological
constant, GR predicts that the Universe is
**expanding**.

Modern observations bear out large-scale homogeneity & isotropy on average:

- Large-scale galaxy surveys show that the distribution of galaxies
is essentially the same everywhere on scales larger than 100 Mpc.
(homogeneity)
- Maps of the cosmic background radiation show that it looks generally the same in all directions. (isotropy)

- Quantum ground-state of empty space.
- Acts as an extra pressure on the Universe.

**Distinction**:

- It actually accelerates the expansion of the universe rather than
stopping it.
- There is increasing observational evidence that a Cosmological Constant, or something very like it, may be real!

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Updated: 2006 February 19

Copyright © Richard W. Pogge, All Rights Reserved.