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The Big Bang

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The Big Bang

Where do we come from, where are we going?

- What’s the cosmological principle?
- Where did things begin?
- Where will things end?
- What’s the temperature of space?

- Recall: All galaxies are moving away from us.
- The farther away the faster they go.
- V = Ho x D

You Are Here

Farther away we look, further back in time we see!

- Isotropy – The view from here is the same in all directions.
- Homogeneity – We live in an average place the same as any other.
- This is the cosmological principle.
- Implies:
- Universe has no edge!
- Universe has no center!

- Why is it dark at night?
- In an infinite and unchanging universe:
Every line of sight should end at the surface of a star.

- Universe is neither.

- If galaxies are all moving away, then at some point they were all much closer.
- Hubble’s Law implies the Universe is expanding.

- Since all galaxies are moving away from us, how long has it been since all galaxies were together?
time = distance / velocity

velocity = Ho x distance

time = distance / (Ho x distance)

time = 1/Ho

- MeasuredHo ~ 65 km/s/Mpc
T ~ 15 billion years

- Recall: Recent HST result says the oldest white dwarfs are 13 billion years old!

- Until recently, much disagreement on the value of Ho and therefore, the age of the Universe.
- Need to know the distance to some galaxies in order to know the slope of velocity versus distance.
- Different methods yielded different distances.
- Some values of Ho yielded an Universe younger than some of its stars.
- HST has helped solve the problem: Cepheids.

- Big Bang: the explosion out of which the Universe began expanding.
- Into what did the Universe expand? Nothing.
- Where was the Big Bang? Everywhere.
- Where is the center of the Universe? Nowhere.

- Will the universe expand forever?
- Depends on the density of the Universe.
- Too big: Big Crunch
- Closed Universe
- Bound Universe

- Too small: Big Freeze
- Open Universe
- Unbound Universe

Dividing line is the critical density.

Wo is the ratio of measured density to the critical density.

If Wo > 1then closed, Wo < 1 then open.

- Add up all the mass we see and Wo = 0.01
- But we know there is some dark matter in galaxies and clusters.
- How much?
- Think ~10 x more dark matter than “light” matter.
- Cosmologists think Wo < 0.3
- Result: Open Universe Big Freeze!

Slowing

Accelerating

- In our experience, things slow down over time.
- Is the Universe slowing down at all?
- Use supernovae as “standard candles.”
- Get distance to distant supernovae.

- Plot distance versus velocity.

Dark Energy

- Instant of the Big Bang:
- Very Dense
- Very hot

- Universe one giant blackbody.
- Temperatures gamma rays

- As time goes on the Universe should cool.

- Recall: lpa 1/T
- Since longer wavelengths mean cooler blackbodies then space should be cooling.
- Today, it should be ~3 K

Satellite measured the intensity of energy from space at a range of frequencies.

2.7 K

- Amazingly uniform temperature in all directions.
- Deviations < 0.0002 Kelvin

10 x more energy in background than everywhere else.