The Accelerating Universe

The Hubble Diagram

Redshift

Standard Candles

Cepheid Variable Stars

Supernovae

Interpretation

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The Hubble Diagram

CZ(km/s)

D (Mpc)

Δy

Δx

Slope = Δy/Δx = H0

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Redshift

Redshift & Blueshift

when

λe

λo

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Absorption lines can be blue-shifted (galaxy is moving toward us) or redshifted (galaxy is moving away). The velocity can be measured by the shift in the wavelength:

v = cz = (λo - λe)/λo

λo

λe

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Standard Candles

Cepheids & Supernovae

A standard candle is an astrophysical object whose absolute magnitude is well known. By comparing the relative intensity of light observed from the object, with that based on its known absolute magnitude, the inverse square law for light intensity can be used to determine the distance to the object.

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The Inverse Square Law of Light

… relates the absolute luminosity, M of an object to its apparent luminosity m by m = M / r^2 where r is the distance.

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Cepheid Variable Stars

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NGC 4526

Supernova 1994D

Supernovae

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Original Hubble Diagram

CZ(km/s)

D

4

3

2

10

D1 D2

(Mpc)

D3 D4

Z4

Z3

Z2Z1Z0

D0

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InterpretationH4 > H3 Decelerating

Universe

H3 > H2 Decelerating Universe

H2 < H1 Accelerating Universe

H1 < H0 Accelerating Universe

H = kilometres per second per Megaparsec

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Cosmological constant or Quintessence?

What next?

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Only one option!!!

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The Big Rip

At last: proton decay