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14.13: Expansion of the Universe

Difficulty Level: Basic Created by: CK-12
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Why does a siren change pitch as it passes you?

The sound of a siren on an emergency vehicle changes as it passes you. It shifts from higher to lower pitch. As the vehicle moves toward you, the sound waves are pushed together. As the vehicle moves past you, the waves are spread apart. This effect with light tells astronomers something about our Universe.

The Universe is Expanding

After discovering that there are galaxies outside our own, Edwin Hubble went on to measure the distance to hundreds of other galaxies. His data would eventually show us how the Universe is changing, and even give us clues as to how the Universe formed.


If you look at a star through a prism, you will see a spectrum. The spectrum is the range of colors seen in a rainbow. The spectrum has specific dark bands where elements in the star have absorbed light of certain energies. An astronomer can use these lines to determine which elements are in a distant star. In fact, the element helium was first discovered in our Sun, not on Earth. This was done by analyzing the absorption lines in the spectrum of the sun.

Astronomers studied the spectra of light from distant galaxies. When they did this they noticed something strange. The dark lines in the spectrum were in the patterns they expected. But they were shifted toward the red end of the spectrum (Figure below). This shift of absorption bands toward the red end of the spectrum is known as redshift.

Stars are redshifted because they are moving away from the earth

Redshift is a shift in absorption bands toward the red end of the spectrum. Redshift occurs when the light source is moving away from you or when the space between you and the source is stretched.

Redshift occurs when the source of light is moving away from the observer. So when astronomers see redshift in the light from a galaxy, they know that the galaxy is moving away from Earth. The strange part is that almost every galaxy in the Universe has a redshift. Of course, this means that almost every galaxy is moving away from us.

The Expanding Universe

Edwin Hubble combined his measurements of the distances to galaxies with other astronomers’ measurements of redshift. He noticed a relationship, which is now called Hubble’s Law: The farther away a galaxy is, the faster it is moving away from us. In other words, the Universe is expanding!

Pictured below is a simplified diagram of the expansion of the Universe (Figure below). Another way to picture this is to imagine a balloon covered with tiny dots. Each dot represents a galaxy. When you inflate the balloon, the dots slowly move away from each other because the rubber stretches in the space between them. If it were a giant balloon and you were standing on one of the dots, you would see the other dots moving away from you. Not only that, but dots farther away from you on the balloon would move away faster than dots nearby.

This simplified diagram of the Universe's expansion illustrates how the universe expanded over time

This is a simplified diagram of the expansion of the Universe over time. Note that the distance between galaxies gets bigger as you go forward in time, but the size of each galaxy stays about the same.

An inflating balloon is not exactly like the expanding Universe. The surface of a balloon has only two dimensions, while space has three dimensions. But it is true that space itself is stretching out between galaxies like the rubber stretches when a balloon is inflated. This stretching of space, which causes the distance between galaxies to increase, is what astronomers mean by the expansion of the Universe.

One other difference between the Universe and our balloon model involves the actual size of the galaxies. On the inflating balloon, the dots you made will become larger in size as you inflate it. In our Universe, however, the galaxies stay the same size; it is just the space between the galaxies that increases as the Universe expands.


  • Almost every galaxy is moving away from us.
  • The spectrum from stars is shifted toward the red; this is known as red-shift. Red-shift of galaxies is evidence that the Universe is expanding.
  • Hubble's Law states that the farther away a galaxy is, the faster it is moving away from us.


  1. How did Hubble determine that the Universe is expanding?
  2. How do astronomers learn what stars are made of?
  3. What is Hubble's Law?

Explore More

Use the resource below to answer the questions that follow.

  1. What is redshift used to determine?
  2. What is the Doppler shift?
  3. How does the Doppler shift relate to light?
  4. What does blueshift tell us?
  5. Why is most of the galaxy redshifted?
  6. What does cosmic redshift tell us?

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redshift Shift of wavelengths of light toward the red end of the spectrum; happens as a light source moves away from us.
spectrum Colors of light that are visible in a continuum.

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Difficulty Level:
6 , 7
Date Created:
Jan 04, 2013
Last Modified:
Aug 29, 2016
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