TEST Hubble's Law Demonstration


Hubble's Law Demonstration written by Samuel Knutson, Dr. Melissa Hayes-Gehrke, and Dr. Alberto Bolatto in Spring 2011. We acknowledge support from a CAREER award by the National Science Foundation, grant AST-0955836, and from a Cottrell Scholar award by the Research Corporation for Science Advancement, grant 19968. We also thank NASA for the galaxy images, available at nasaimages.org.



Using the Applet

Hover your mouse over an underlined category to view instructions.

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Frequently Asked Questions

  1. What is the Hubble law? What is this demonstration showing?

  2. Hubble's law, named after Edwin Hubble, is the observation that all distant galaxies appear to be racing away from us and from each other. Hubble found that the apparent recessional velocity of a galaxy is proportional to its distance from the observer, which we represent using the equation v = H0d, where H0 is the Hubble constant. This phenomenon is explained by the expansion of the universe, consistent with the Big Bang theory. This simulation shows part of an expanding universe filled with galaxies, and a graph introduced by Hubble to show the relationship between recessional velocity and distance.


  3. What is the graph showing?

  4. The graph in the upper right-hand corner is the same type of graph that Edwin Hubble used when he published his data. Each point on the graph (displayed when the simulation is running or after it has ended) represents a galaxy in the simulation. The graph plots each galaxy's apparent recessional velocity as a function of its distance from the home galaxy. The gray box in the corner of the graph is an information box. Hover your mouse over a galaxy in the simulation or the graph and information about that galaxy will be displayed in this box.


  5. So the expansion of the universe is caused by the Big Bang, which was like a big explosion?

  6. The expansion of the universe is caused by the Big Bang, but an explosion isn't an accurate analogy for this phenomenon. The Big Bang wasn't an explosion of matter into empty space, but rather a rapid expansion of space itself (with all the matter just going along for the ride). A more accurate analogy would be a rising loaf of raisin bread, where the raisins are galaxy groups and the dough is the universe (ignore the edges of the bread). The raisins (galaxy groups) are moving away from each other because the dough (space) in between them is expanding.


  7. Why isn't the expansion of the universe tearing apart galaxies?

  8. Keep in mind that the effect from the expansion of the universe is most significant over very large distances. On local scales, such as galaxies, stellar systems and our bodies, other forces are dominant and “overpower” the relatively small effect from the expansion of the universe. In a galaxy, for example, the gravitational force holding all the stars and other matter together is the most significant force. In our bodies, the electromagnetic force is dominant. Over vast distances, such as those separating distant galaxies, these forces are weak and the effect from the expansion of the universe becomes dominant.


  9. Are all galaxies really moving away from us? I've heard that the Andromeda galaxy is moving towards us!

  10. It is indeed! Again, remember that the expansion of the universe is most significant over very large distances. For nearby galaxies like Andromeda in our local group, gravity is strong enough to "overpower" the small effect from the expansion of the universe. However, all observations of distant galaxies in their respective local groups are consistent with Hubble's law.


  11. Doesn't the fact that all distant galaxies are moving away from us mean that we're at the center of the expanding universe?

  12. Actually, no. Any observer, no matter where s/he was in the universe, would observe all distant galaxies rushing away from him. You can test this in the simulation above; try moving the home galaxy, and see if you ever observe other galaxies getting closer to it over time.


  13. So where is the center of the universe?

  14. In a sense, there is no actual center of the universe. If the Big Bang were an explosion of matter into space, then there would certainly be a center. Instead, the Big Bang was a rapid expansion of space itself (see # 2). Thus every point in the universe was together in a singularity at the beginning. In fact, you could say that every point in the universe is the center of the universe. The expansion in the simulation above is centered around the middle of the screen out of necessity.


  15. I heard nothing can travel faster than the speed of light. Does this mean that there is a limit on the recessional velocity of a galaxy?

  16. No. It is true that a galaxy cannot move faster than the speed of light relative to the space around it. However, the recessional velocity of a galaxy is caused by the expansion of space between the galaxy and the observer, and not by the motion of the galaxy relative to the space around it, so the recessional velocity of a galaxy can exceed the speed of light from our point of view. Imagine dinner plates sitting on a bunched-up tablecloth. If you pull the tablecloth it will move, causing the plates to move away from each other. This motion is not due to the plates moving relative to the tablecloth, but rather the tablecloth between each pair of plates being spread out. Since space is not composed of matter, it can move faster than the speed of light, carrying galaxies and other matter with it.


  17. Is the simulation zooming in on the universe?

  18. No, the simulation is not zooming in. The grid represents space and it is getting larger to simulate the expansion of space itself. The original grid lines are shown in dark gray.


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