The Universe
The Doppler effect occurs when a source of waves (such as sound or light) moves relative to an observer. It applies to any kind of wave, including electromagnetic waves like light.
Blueshift: If a light source moves toward an observer, the wavelength shortens, shifting the light toward the blue end of the spectrum.
Redshift: If a light source moves away from an observer, the wavelength lengthens, shifting the light toward the red end of the spectrum.
Doppler shift is commonly used to measure the motion of stars and galaxies within a gravitational system, such as stars orbiting a galaxy or exoplanets tugging on their host stars.
Redshift Due to the Expansion of the Universe (Cosmological Redshift)
Unlike the Doppler shift, which results from the motion of objects through space, cosmological redshift occurs because space itself is expanding. As light travels through the universe, the space it moves through stretches, increasing the wavelength of the light.
For very distant galaxies, this redshift is not a simple Doppler shift but a result of the entire fabric of space stretching. The farther away a galaxy is, the more its light has been stretched, meaning higher redshifts correspond to earlier times in the universe.
Self-Evaluating Questions
Try answering these questions after completing the reading. If you find any difficult to answer, revisit the textbook to reinforce your understanding.
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a) The star's surface temperature changing
b) The expansion of space between galaxies
c) The motion of the star toward or away from the observer
d) The gravitational pull from nearby planets -
a) A blueshift, as its light shifts to shorter wavelengths
b) No shift at all, since it is moving with space
c) A redshift, as its light stretches to longer wavelengths
d) A doubling of its brightness -
a) It is caused by the star's rotation
b) It only occurs for objects within our galaxy
c) It results from the expansion of space itself
d) It only affects sound waves, not light -
a) The closer a galaxy is, the greater its redshift
b) All redshift is due to the Doppler effect
c) More distant galaxies generally show higher redshifts
d) Redshift indicates a galaxy is moving toward us -
a) It is very hot
b) It is rotating rapidly
c) It is very far away and the universe has expanded significantly since its light was emitted
d) It is part of the Milky Way -
c – The motion of the star toward or away from the observer
c – A redshift, as its light stretches to longer wavelengths
c – It results from the expansion of space itself
c – More distant galaxies generally show higher redshifts
c – It is very far away and the universe has expanded significantly since its light was emitted