This document discusses the expansion of the universe as evidenced by redshifts in the spectra of distant galaxies. It begins by explaining how Doppler shifts in wavelengths can indicate motion, then shows how measurements of galaxy spectra revealed systematic redshifts that increased with distance. This redshift-distance relationship, discovered by Hubble, provided strong evidence that the universe is expanding uniformly. As space itself expands, the wavelengths of light traveling through this expanding space become stretched or redshifted. More distant galaxies exhibit larger redshifts because the expansion of space between emitted and observed light is greater over larger distances.

1. Red-shift
and the Expansion of the Universe

2. Waves
Both sound and light can behave like a wave.
An important property of a wave is its wavelength.
Wavelength - the distance over which a wave repeats itself
(a) Longer wavelength; (b) shorter wavelength.

3. White Light and Colors
When light passes through a prism, it separates into the
colors that make it up. White light separates into the
colors of the rainbow. This is called a spectrum.

4. Wavelength and Color
Different colors of visible light
correspond to different wavelengths.

5. Visible Light Wavelengths
The wavelength of visible light is very small. Red light
has a wavelength of about 0.0000007 meters and violet
light about 0.0000004 meters.
4
mm
10 000
wavelength
violet
red
7
mm
10 000

6. Visible Light Wavelengths
The wavelength of visible light is very small. Red light
has a wavelength of about 0.0000007 meters and violet
light about 0.0000004 meters.
One billionth of a meter is called a nanometer (nm).
1 nm = 10-9 m
Therefore, Visible light has wavelengths
between about 400 and 700 nm.

7. Spectral Lines
Solar Spectrum
When astronomers separate the light of a star into a spectrum,
the spectrum looks like a regular rainbow of colors—except that
there are dark lines in it. What's going on?
It turns out that elements absorb light of particular wavelengths.
If there are atoms of various elements in the atmosphere of the
star, those atoms will absorb the light at those wavelengths and
produce lines. Each element has a specific “signature”—a
specific set of lines.

9. The Doppler Effect

10. Sound waves moving outward from a stationary car.
Each circle represents a crest of the sound wave moving outward.
The distance between circles is the wavelength of the sound wave.
As the wavelength of the sound is the same in all directions, anyone
that is stationary relative to the car will hear the same pitch.

11. Sound waves moving outward from a moving car.
Because the source of the sound (the car) moves between the times
when two wave crests leave the source, the wave crests are closer
together in the direction of motion and farther apart in the opposite
direction.

12. Sound waves moving outward from a moving car.
So, someone standing at A will hear a higher pitch (shorter
wavelength) and someone standing a B will hear a lower pitch
(longer wavelength).

13. The Doppler Effect also occurs
with light.
Solar Spectrum

14. Motion of a star, galaxy or other object
vt
v
vr
V represents the actual motion of the object. That motion can be
separated into two pieces or components. One component (vt) ,
called the tangential component, is sideways to the observer.
The other component (vr), called the radial component, is along
the line of sight of the observer.
Doppler Effect only measures radial velocity

15. Laboratory Hydrogen Spectrum
(wavelength of Hα (red) emission line = 656 nm)

16. Galaxy

17. Galaxy Spectrum
Modern spectra are usually recorded digitally and represented as plots of
intensity vs. wavelength

18. Galaxy Spectrum
What is the wavelength of this galaxy’s Hα emission line?

19. Galaxy Spectrum
What is the wavelength of this galaxy’s Hα emission line?
676 nm

20. Galaxy Red Shift
Note that the position of the Hα line is no longer where it was in
the laboratory spectrum of hydrogen. Instead, the peak has
been shifted towards the longer wavelength part of the
spectrum, which is the redder end of the spectrum. This
phenomenon is called a “red-shift.”

21. Galaxy Red Shift
Note that the position of the Hα line is no longer where it was in
the laboratory spectrum of hydrogen. Instead, the peak has
been shifted towards the longer wavelength part of the
spectrum, which is the redder end of the spectrum. This
phenomenon is called a “red-shift.”
It turns out that the amount of the observed red-shift
is proportional to the speed of the source (for speeds that are
not close to the speed of light). For example, for a galaxy
moving away from us at 10% of the speed of light, the lines in
its spectrum will be red-shifted by 10%.

22. Galaxy Red Shift
The Hα line in the galaxy’s spectrum has been
redshifted from 656 to 676 nm.
This is a redshift of ____ nm or _____ %

23. Galaxy Red Shift
The Hα line in the galaxy’s spectrum has been
redshifted from 656 to 676 nm.
20
This is a redshift of ____ nm or _____ %

24. Galaxy Red Shift
The Hα line in the galaxy’s spectrum has been
redshifted from 656 to 676 nm.
20
3
This is a redshift of ____ nm or _____ %

25. Galaxy Red Shift
The Hα line in the galaxy’s spectrum has been
redshifted from 656 to 676 nm.
20
3
This is a redshift of ____ nm or _____ %
The recession velocity of the galaxy is therefore:

26. Galaxy Red Shift
The Hα line in the galaxy’s spectrum has been
redshifted from 656 to 676 nm.
20
3
This is a redshift of ____ nm or _____ %
The recession velocity of the galaxy is therefore:
3 % x 300,000 km/sec = 9000 km/sec

27. Galaxy Red Shift
You will now
have the
opportunity to
analyze the
spectra of these
four galaxies
and calculate
their recession
velocities.

29. Vesto M. Slipher
An astronomer working at the Lowell
Observatory, V.M. Slipher, began in
1912 to measure the radial velocities
of galaxies, which at the time were
called “spiral nebulae”. He noticed
that the lines in the spectra of most
galaxies were shifted toward the red
end of the spectrum.
Photo ca. 1907, courtesy Lowell Observatory

30. Edwin Hubble
Edwin Hubble measured
distances to galaxies and
extended Slipher’s
measurements of their
redshifts.
In 1929 Hubble published
the velocity-distance
relation which, taken as
evidence of an expanding
Universe, is the basis of
modern cosmology.

31. Hubble’s 1931 graph of velocity vs. distance
Image Credit: American Institute of Physics

32. Recent measurements confirm Hubble’s velocity
distance relationship at much greater distances
The Hubble diagram
for type Ia
supernovae.
Kirshner R P PNAS 2004;101:8-13
©2004 by National Academy of Sciences

33. Why would galaxies that are farther
away from us move away from us
with greater velocities?

34. Is there another explanation?

35. Expansion and Red-shift
A red-shift of light can also result if space through which
the light is traveling is expanding. This is called a
cosmological red-shift.

36. Expansion and Red-shift
The wavelength of
light stretches
(increases) with the
expanding space.
Beginning Wavelength
Stretched Wavelength (red-shifted)
Image credit: Wayne Hu,
http://background.uchicago.edu/~whu/beginners/expansion.html

37. Expansion and Red-shift
If the space between galaxies is expanding, light moving
through that expanding space will be red-shifted.
(The collective gravity from stars and matter within a galaxy will
keep space within a galaxy from expanding)
“. . . the observed red-shifts were generally accepted as direct
evidence that the actual universe was expanding at the
present time”. – Edwin Hubble

38. Model the Expanding Universe

39. This model
shows an
imaginary
galaxy field.

40. Expansion
from the
perspective
of galaxy I

41. Expansion
from the
perspective
of galaxy I

42. Expansion
from the
perspective
of galaxy I

43. Expansion
from the
perspective
of galaxy I
Galaxy E
would have
about twice
the red-shift
of galaxy G

44. Expansion
from the
perspective
of galaxy L

45. Expansion
from the
perspective
of galaxy K

46. Conclusion
Expansion of space between the galaxies best
explains the red-shift distance observations.
Therefore, the red-shift distance relationship is
evidence that the universe is expanding.