Red-shift

and the Expansion of the Universe

Wavelength - the distance over which a wave repeats itself

(a) Longer wavelength; (b) shorter wavelength.(a) Longer wavelength; (b) shorter wavelength.

Both sound and light can behave like a wave.

An important property of a wave is its wavelength.

Waves

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.

Wavelength and Color

Different colors of visible light correspond to different wavelengths.

Visible Light WavelengthsThe wavelength of visible light is very small. Red light has a wavelength of about 0.0000007 meters and violet light about 0.0000004 meters.

410 000

mm

violet

wavelength

red

710 000

mm

Visible Light WavelengthsThe 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.

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.

Spectral LinesSolar Spectrum

The Doppler Effect

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.

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.

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).

Solar Spectrum

The Doppler Effect also occurs with light.

Motion of a star, galaxy or other object

vr

vtv

Doppler Effect only measures radial velocity

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.

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

Galaxy

Galaxy Spectrum

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

Galaxy SpectrumWhat is the wavelength of this galaxy’s Hα emission line?

Galaxy SpectrumWhat is the wavelength of this galaxy’s Hα emission line?

676 nm

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.”

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%.

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 _____ %

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 _____ %20

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 _____ %20 3

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 _____ %

The recession velocity of the galaxy is therefore:

20 3

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 _____ %

The recession velocity of the galaxy is therefore:

3 % x 300,000 km/sec = 9000 km/sec

20 3

Galaxy Red Shift

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

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

Vesto M. Slipher

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.

Image Credit: American Institute of Physics

Hubble’s 1931 graph of velocity vs. distance

The Hubble diagram for type Ia

supernovae.

Kirshner R P PNAS 2004;101:8-13

©2004 by National Academy of Sciences

Recent measurements confirm Hubble’s velocity distance relationship at much greater distances

Why would galaxies that are farther away from us move away from us

with greater velocities?

Is there another explanation?

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.

Expansion and Red-shift

The wavelength of light stretches (increases) with the expanding space.

Image credit: Wayne Hu, http://background.uchicago.edu/~whu/beginners/expansion.html

Beginning Wavelength

Stretched Wavelength (red-shifted)

Expansion and Red-shift

“. . . the observed red-shifts were generally accepted as direct evidence that the actual universe was expanding at the present time”. – Edwin Hubble

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)

Expansion and Red-shift

Model the Expanding Universe

This model shows an imaginary galaxy field.

Expansion from the perspective of galaxy I

Expansion from the perspective of galaxy I

Expansion from the perspective of galaxy I

Expansion from the perspective of galaxy I

Galaxy E would have about twice the red-shift of galaxy G

Expansion from the perspective of galaxy L

Expansion from the perspective of galaxy K

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.

ConclusionConclusion