ASTR 1102-0022008 Fall Semester

Joel E. Tohline, Alumni ProfessorOffice: 247 Nicholson Hall

[Slides from Lecture21]

Chapter 24: Galaxiesand

Chapter 26: Cosmology

Individual Galaxies• Location in Space

– Coordinate (angular) position on the sky– Distance from Earth

• Motion through Space– Motion across the sky (“proper” motion) – None

observable!– Motion toward/away from us (radial velocity) –

Extremely revealing discovery made by Slipher, Hubble, Humason

Distance Ladder

Hubble used Pop I Cepheid variables to determine distances to nearby galaxies.

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Determining Radial Velocities from Measured “Redshifts”

• Textbook Box 24-2– z = redshift = /0

– If “z” is sufficiently small, then v/c = z

– If “z” not small, use relativistic formula to obtain “v” from “z”.

Plotting measured velocities versus measured distances, Hubble discovered a strong correlation:

Hubble’s Revealing Discovery

• All galaxies are moving away from us! (The exception is our nearest neighbor galaxy, Andromeda.)

• The farther away a galaxy is, the faster it is receding from us!

• Correlation described by simple formula:

v = H0d

H0 is the slope of the line in the “Hubble Law” diagram: H0 = 73 (km/s)/Mpc

Hubble’s Revealing Discovery

• All galaxies are moving away from us! (The exception is our nearest neighbor galaxy, Andromeda.)

• The farther away a galaxy is, the faster it is receding from us!

• Correlation described by simple formula:

v = H0d

H0 is the slope of the line in the “Hubble Law” diagram: H0 = 73 (km/s)/Mpc

Determining Distance from “z”

• If the Hubble Law continues to be relevant at all distances throughout the universe, then we can turn the formula around and use measurements of “z” to give us distances to remote galaxies!

d = v/H0 = (zc)/H0

So … in order to determine distances to all of the galaxies in some region of the sky, all we have to do is measure the (Doppler-shift) redshift of each galaxy.

The “Hubble Constant” H0

• Let’s examine more closely the meaning of the so-called “Hubble Constant,” H0

• H0 = (73 km/s)/Mpc

= (73 km/s)/(3.085 x 1019 km)

= 2.37 x 10-18 /s

• That is,

1/H0 = 4.23 x 1017 s = 13.4 billion yrs

The “Hubble Constant” H0

• Let’s examine more closely the meaning of the so-called “Hubble Constant,” H0

• H0 = (73 km/s)/Mpc

= (73 km/s)/(3.085 x 1019 km)

= 2.37 x 10-18 /s

• That is,

1/H0 = 4.23 x 1017 s = 13.4 billion yrs

The “Hubble Constant” H0

• Let’s examine more closely the meaning of the so-called “Hubble Constant,” H0

• H0 = (73 km/s)/Mpc

= (73 km/s)/(3.085 x 1019 km)

= 2.37 x 10-18 /s

• That is,

1/H0 = 4.23 x 1017 s = 13.4 billion yrs

Interpretation of Hubble’s Law

• Hubble’s Law appears to put us in a special location in the Universe: Everything appears to be expanding away from us!

• Einstein’s general theory of relativity provides a context for interpreting (& understanding) Hubble’s Law that does not put us in a special location.