08 CosmologyAST1002 2017 London LN01Prof. Eric Hsiao

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-Google

Cosmology: the science of the origin and development of the universe.

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Where do we come from?What are we?Where are we going?

-Paul Gauguin

1908: PL relation for Cepheids• Henrietta Swan Leavitt

discovered the period-luminosity (PL) relation for cepheids.

• Longer period, brighter.

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Henrietta Swan Leavitt

1920: The Great Debate

• Sun is not at the center of the Milky Way.

• Spiral nebulae are galaxies like the Milky Way.

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ShapleyCurtis

1924: Distance to Andromeda

• Using Leavitt’s PL relation, Edwin Hubble found the distance to Andromeda Galaxy to be 100x the size of the Milky Way.

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1915: General Relativity

• Gik + Λgik = 8πGTik

• Einstein laid out the mathematical foundation for cosmology.

• Yes, this 1 equation describes the whole universe.

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Geometry ofthe Universe

Stuff inthe Universe

1915: General Relativity

• Gik + Λgik = 8πGTik

• Einstein prefers a static universe. If we don’t occupy a special place, why should we occupy a special time?

• The equation naturally produces a dynamic universe. Einstein added a “cosmological constant” to keep it static.

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cosmologicalconstant

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100-inch Hooker TelescopeMt Wilson Observatory

1929: Hubble’s law

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Hubble diagram:Distance vs. velocity

How did he measure distance?

How did he measure velocity?A positive velocity means…?

1929: Hubble’s law

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Edwin Hubble observed:

1. Practically all galaxies exhibit redshifts, thus moving away from us.

2. The more distant a galaxy, faster it is moving away from us.

What do these observations tell us?

1929: Hubble’s law

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“It is thought premature to discuss in detail the obvious consequences of the present result.”

Edwin HubbleCarnegie, 1929

• Distance between MW and galaxy 2 increases much faster than thedistance between MW and galaxy 1.

• Velocity = distance/time; thus,galaxy 2 is moving away from us with faster speed than galaxy 1.

• Hubble showed that the universe is expanding!

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time

MW

1

2

1929: Hubble’s law

• Hubble shows that the Universe is not static and is expanding.

• What happens when we rewind the tape?

• The expanding universe implies abig bang beginning.

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time

MW

1

2

1929: Hubble’s law

1931: Einstein’s visit to Mt Wilson

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1931: Einstein’s visit to Mt Wilson

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1931: Einstein’s visit to Mt Wilson

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Λ

Einstein reportedly remarked that the inclusion of the cosmological constant Λ is the “biggest blunder of his life.”

Tutorial: Hubble’s Law (p.155)

• Work with a partner!

• Read the instructions and questions carefully.

• Discuss the concepts and your answers with one another.

• Come to a consensus you both agree on.

• If you get really stuck, ask me for help.

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When looking at a Hubble diagram, what is the correct relationship between the slope, the age of the universe, and the universe’s expansion rate?

A steeper slope indicates a younger universe with a slower expansion rate.

A steeper slope indicates an older universe with a slower expansion rate.

A flatter slope indicates a younger universe with a faster expansion rate.

A flatter slope indicates an older universe with a slower expansion rate.

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Which Hubble diagram represents a universe with expansion rate that is increasing with time?

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Which Hubble diagram represents a universe with a constant expansion rate?

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1965: Cosmic microwave background

Arno Penzias and Robert Wilson in a satellite experiment, detected an “excess light.”

After ruling out the Earth, Sun, Milky Way, and bird poop, they concluded the source is outside of our galaxy.

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1965: Cosmic microwave background

This signal is the “cosmic microwave background,”a relic of the big bang emitting a blackbody spectrum of 3K.

Why does such a powerful event emit light at such low energy/long wavelength?(Recall the Hubble Diagram)

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1985: Type Ia supernova

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1985: Type Ia supernova

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1993: Phillips relation

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Fainter supernova fades faster.

Mark Phillips

1993: Phillips relation

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Mark Phillips Henrietta Leavitt

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Type Ia supernovae are bright.

Just like the cepheids, the absolute magnitude of a Type Ia supernova can be determined using by observing how its brightness changes.

Eric Y. Hsiao hsiao@physics.fsu.edu FSU, October 201530

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Climbing the distance ladder

parallax recall the pencil experimentspectroscopic parallax HR diagramcepheids PL relationType Ia supernova Phillips relation

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Armed with this new tool, astronomers set out to use Type Ia supernovae to measure the deceleration of the universal expansion.

Why do astronomer expect that the expansion should slow down?

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Sandage 1972

Hubble 1929

Suzuki+ 2012Type Ia supernova

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Sandage 1972

Hubble 1929

Suzuki et al. 2012Type Ia supernova

1998: Distant supernova race

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Two teams of astronomers independently found the same answer.

The expansion rate is currently accelerating!

What is driving the acceleration?

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Einstein’s original motivation for introducing the cosmological constant was to keep the Universe static.

Hubble showed that the Universe is expanding.

However, the cosmological constant currently provides the best fit to Type Ia supernova Hubble diagrams (and the best description of the acceleration).

Dark energy: the name astronomers have given to the unknown cause of the acceleration.

“On a good day, I can think of 3 or 4 plausible candidates for dark matter. The same cannot be said for dark energy.”

Rocky Kolb, 2004

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today

Universe started acceleratingat ~2/3 of present size velocity

big bang

Type Ia supernova observations show past deceleration and current acceleration.

• Dark matter was discovered using rotation curves of galaxies.

• Dark energy was discovered using Type Ia supernovae.

• But we are only sure of their existence by getting the same answer with many different techniques.

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1998: Distant supernova race

dark

ene

rgy

dark matter

1998: Distant supernova race

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Nobel Prize in Physics 2011

“for the discovery of the accelerating expansion of the Universe through observations of distant supernovae".