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The Big Bang The Big Bang EvidenceEvidence

The Science of Creation

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Cosmological Principle

• Isotropy – The view from here is the same in all directions. (observation)

• Homogeneity – We live in an average place the same as any other. (statistics)

• This is the cosmological principle.

• Implies: – Universe has no edge!– Universe has no center!

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Olbers’ 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.

• Either:– Universe has edge.– Universe has a

beginning.

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Concept Test

• Olbers’ paradox asks why the night sky is dark, when every line of sight must eventually fall on a star. Which of the following reasons would best explain the darkness at night? It is because the universe is:a. Infinite and mostly empty.b. Clumpy, so not every sightline intercepts a star.c. Expanding, so distant stars are red-shifted.d. Young, so there are only stars to a finite distance. e. None of the above.

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Scientific Evidence

• A scientific model must do two things:– Explain what is seen.– Predict what will happen accurately.– Or, predict what can be seen before one

thinks to look.

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Cosmic Microwave Background

• The Big Bang:– Massive amount of energy.– Very high temperature thermal radiation

• As Universe expands:– Temperature cools thermal radiation gets

redder.or– Cosmological redshift same thing.

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Origin of the CMB

Once cool enough for atoms, radiation could move freely through space.

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The Temperature of Space

• After 500,000 yrs: T=3000Kpeak 1/T

peak = near infrared.

• The Universe was red.• After 16 billion yrs:

Universe 1000x bigger.peak = 1000x longer.

peak = microwaves (radio).

T = 3K!

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Where do you look?

• Recall Lookback time.• Universe is a time machine.• Look beyond the youngest galaxies.• Look everywhere.

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What We See

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Cosmic Microwaves

• Big Bang theory predicted cosmic microwave background.

• In 1965: discovery of radio waves (microwaves) from sky.

• Prediction confirmed.

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COBE

• Early 1990s:NASA Cosmic Background Explorer

• Measure the intensity of radiation at many wavelengths.

• 3K thermal emission!

peak = (3 x 10-3 m/K) / 3 K = 0.001m = 1mm

T

constpeak

.

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COBE cont.

• Also measure the (temperature) intensity and wavelength in various directions.

• Look for fluctuations out of which galaxies came.

V = 600 km/s

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Large Scale Structure

WMAP

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Concept Test

• The cosmic background radiation is visible in every direction because:a. We are at the center of the universe.b. It is just now passing by us at the speed of

light.c. It pervades all space.d. It has reflected in every direction over the

age of the universe. e. All of the above.

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Evidence of Eras

• Can’t see what happened before origin of CMB.

• Can estimate given change of temperature and density with time.

• Look for Proof.– CMB (Age of Nuclei)– Ratio of H/He and Li (Age of

Nucleosynthesis)– Weak Bosons (Electroweak Era)

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A Good Theory?

• The Big Bang is a very successful theory.

• Makes predictions:– CMB.– Hydrogen, Helium, Lithium abundances.– Formation of structure.– Expansion of Universe.

• Are other Creation theories:– Scientific?– Successful?

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Homework #19

• For Monday, read NY Times article plus WMAP website: http://map.gsfc.nasa.gov/m_mm.html

• Answer the following:1. What is the inflation theory?2. What new observations support the

theory?3. How does this theory

change/help/augment the Big Bang theory?