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Chapter 1: Problem 7

What does the red shift of the galaxies tell us about their motion with respect to the Earth?

Short Answer

Expert verified
The redshift of galaxies refers to the phenomenon in which light emitted by these celestial objects is shifted towards the red end of the visible spectrum due to the Doppler effect. By observing the redshift of light coming from galaxies, we can deduce their motion relative to the Earth. A positive redshift indicates that a galaxy is moving away from the Earth, while a negative redshift or blueshift suggests that the galaxy is moving towards the Earth. Furthermore, the redshift of galaxies provides crucial evidence for the expansion of the universe, as most galaxies are observed to be moving away from the Earth at increasing rates as their distance increases, supporting the theory of an expanding universe originating from the Big Bang.

Step by step solution

01

Understanding redshift

Redshift is a phenomenon in which the light coming from a celestial object is shifted towards the red end of the visible light spectrum. It occurs when the wavelength of the light emitted by an object is stretched or increased as it travels through space, which causes it to appear more red. This effect can be observed using a spectroscope which separates light into its different wavelength components.
02

The Doppler effect

The redshift can be explained by the Doppler effect - a change in the frequency (and thus the wavelength) of a wave in relation to an observer who is moving relative to the wave source. In the case of light, when an object is moving away from an observer, its light waves stretch, causing wavelengths to increase and the light to appear more red (redshift). Conversely, if the object is moving towards the observer, the light waves get compressed, causing wavelengths to decrease and the light to appear more blue (blueshift).
03

Applying redshift to the motion of galaxies

By observing the redshift of light coming from galaxies, we can deduce their motion relative to the Earth. If the redshift is positive, it indicates that the galaxy is moving away from the Earth, as the observed light's wavelength is stretched towards the red end of the spectrum. If the redshift is negative or if there's blueshift, it suggests that the galaxy is moving towards the Earth, as the observed light's wavelength is compressed towards the blue end of the spectrum.
04

Redshift and the expanding universe

The redshift of galaxies provides crucial evidence for the expansion of the universe. Astronomers have observed that the light coming from most galaxies is redshifted, which means that these galaxies are moving away from the Earth. Additionally, the farther away a galaxy is, the more significant its redshift is, implying that galaxies are moving away from us at faster rates as their distance from the Earth increases. This observation supports the widely-accepted theory of an expanding universe, which originated from the Big Bang.

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Most popular questions from this chapter

How did Eratosthenes calculate the Earth's circumference?

Astronomers discovered that more-distant galaxies move away from the Earth more rapidly than do nearer ones. Why? To answer this question, make a model of the problem by drawing three equally spaced dots along a line; the dot at one end represents the Earth, and the other two represent galaxies. "Stretch" the line by drawing the line and dots again, but this time make the line twice as long. This stretching represents Universe expansion. Notice that the dots are now farther apart. Using the following equation for velocity (Velocity \(=\) Distance/Time) \(-\) if you pretend that it took 1 second to stretch the line (so Time \(=1\) second), measurement of the distance that each galaxy moved relative to the Earth allows you to calculate velocity.

Imagine you hear the main character in a low-budget science-fiction movie say he will "return ten light years from now." What's wrong with his usage of the term?

The horizon is the line separating sky from the Earth's surface. Consider the shape of the Earth. How does the distance from your eyes to the horizon change as your elevation above the ground increases? To answer this question, draw a semicircle to represent part of the Earth's surface; then draw a vertical tower up from the surface. With your ruler, draw a line from various elevations on the tower to where the line is tangent to the surface of the Earth. (A tangent is a line that touches a circle at one point and is perpendicular to a radius.)

Describe how the Doppler effect works.
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