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Chapter 26: Problem 9

Explain the difference between a Doppler shift and a cosmological redshift.

Short Answer

Expert verified
In summary, Doppler shifts are caused by the relative motion between the source of a wave and the observer, leading to an apparent change in frequency or wavelength. Cosmological redshifts, however, are caused by the expansion of the Universe causing light waves from distant galaxies to stretch and seem 'redder'. The key difference is that while Doppler shifts involve relative motion of the source and observer, cosmological redshifts involve the expansion of the Universe itself.

Step by step solution

01

Understand Doppler Shift

The Doppler Shift can be observed when the source of wave and the observer are moving relative to each other. It happens with all types of waves including sound, light, and radio waves. When the source is moving towards the observer, the wave appears to have a higher frequency (or a shorter wavelength), this is perceived as a blue shift; conversely, if the source is moving away, the wave appears to have a lower frequency (or a longer wavelength), thus resulting in a red shift.
02

Understand Cosmological Redshift

Cosmological redshift is observed when the Universe itself is expanding, causing distant galaxies to recede from us. This is not due to the motion of the galaxy itself, but due to the expansion of the Universe between the source and the observer. When the light from these galaxies reaches us, it has been stretched or 'redshifted', making it appear to be of longer wavelength and hence 'redder'. This phenomenon is a key piece of evidence for the Big Bang theory, which posits that the Universe is expanding from an initial point of singularity.
03

Highlight Differences

The key difference between Doppler shift and cosmological redshift lies in the cause of the shift. Doppler shifts are caused by relative motion between the source and the observer, often seen in everyday contexts, such as a passing ambulance siren changing pitch. On the other hand, cosmological redshifts are caused by the expansion of the Universe itself, and are not due to any actual movement of the source (e.g., a distant galaxy). Rather, the space between the source and the observer is expanding, causing the wave to 'stretch' and the frequency to appear lower.

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Most popular questions from this chapter

Do you think there can be "other universes," regions of space and time that are not connected to our universe? Should astronomers be concerned with such possibilities? Why or why not?

Use the Starry Night Enthusiast TM program to determine how the solar system moves through the cosmic microwave background. This motion appears to be taking us towards the constellation Leo. First, select Favourites \(>\) Guides \(>\) Atlas to display the entire celestial sphere from the center of a transparent Earth. Open the Find pane and click on the magnifying glass symbol to display the Find categories and click on Constellation. Double-click on Leo to center on this constellation and click again on the Find pane tab to close this pane and display the full screen. Select View \(>\) Constellations \(>\) Astronomical and View \(>\) Constellations \(>\) Labels to display and label the constellations. (a) Draw a sketch showing the Sun, the plane in which the Earth orbits the Sun, and the direction in which the solar system moves through the cosmic microwave background. (b) Use the date controls in the toolbar to step through the months of the year. In which month is the Sun placed most nearly in front of the Earth as the solar system travels through the cosmic background radiation?

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