The big bang
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The Big Bang. Where do we come from, where are we going?. Goals. What’s the cosmological principle? Where did things begin? Where will things end? What’s the temperature of space?. Hubble’s Law. Recall: All galaxies are moving away from us. The farther away the faster they go.

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The Big Bang

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The big bang

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?

Hubble s law

Hubble’s Law

  • Recall: All galaxies are moving away from us.

  • The farther away the faster they go.

  • V = Ho x D

Large scale structure

You Are Here

Large Scale Structure

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

Cosmological principle

Cosmological Principle

  • 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!

Olber s paradox

Olber’s Paradox

  • 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.

Expanding universe

Expanding Universe

  • If galaxies are all moving away, then at some point they were all much closer.

  • Hubble’s Law implies the Universe is expanding.

Age of the universe

Age of the Universe

  • 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!

Age disagreements

Age Disagreements

  • 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.

The big bang1

The Big Bang

  • 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.

The end of the universe

The End of the Universe

  • 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

Critical density

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.

Critical Density

Density of the universe

Density of the Universe

  • 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!

Are we slowing down



Are we Slowing down?

  • 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

Observational clues

Observational Clues

  • Instant of the Big Bang:

    • Very Dense

    • Very hot

  • Universe one giant blackbody.

    • Temperatures  gamma rays

  • As time goes on the Universe should cool.

3 kelvin

3 Kelvin

  • Recall: lpa 1/T

  • Since longer wavelengths mean cooler blackbodies then space should be cooling.

  • Today, it should be ~3 K

The big bang

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.

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