Astronomy

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Video

Materials

• 1 round balloon
• 1 marker pen

Short explanation

The galaxies in the universe are moving apart. But it's not the galaxies themselves that move through space, it's space that expands and the galaxies just follow. This makes an observer in a galaxy see it as if all other galaxies were moving away from him or her, and that his or her galaxy is the center of the universe. But the same phenomenon is seen from every galaxy.

Long explanation

Our Sun is just one of hundreds of billions of stars in our galaxy, the Milky Way. Our galaxy, in turn, is just one of countless galaxies in a giant universe (we don't know how big the universe is).

There are enough astronomical observations today that show that the universe is expanding, and this expansion is widely accepted. The expansion was discovered in the early 1920s by astronomer Edwin Hubble. Through a telescope, he observed that all other galaxies seemed to be moving away from us. In addition, the farther away from the Milky Way a galaxy was, the faster it moved away from us. You can see this phenomenon, called Hubble's law, on the balloon.

But it's not the galaxies that move through space, it's space that expands. Between the galaxies, more distance is created, and the distances between the galaxies increase all the time. This expansion of the universe began with the Big Bang 13.8 billion years ago and has been going on ever since. In "recent years", starting 9.4 billion years after the Big Bang, the rate of expansion has even increased. The driving force for this acceleration is called dark energy, but we don't know what it is. Whether the universe will stop expanding or not isn't known either. But right now there are many observations that indicate that the universe will continue to expand forever, and that the galaxies - actually the galaxy clusters, see below - will eventually be so far apart that they cannot be observed from any other galaxy cluster any more.

In this demonstration, three-dimensional space is simplified as the two-dimensional surface of the balloon. This simplification is common in astronomy in order to be able to "step out" of the three-dimensional space we are in, and observe it from the outside.

There are two errors with the balloon analogy in this demonstration. The first error is that the galaxies appear to be growing in size as the universe expands. This is not true. Galaxies do not stretch out due to the expansion of the universe, but are held together by the gravity between all the stars in them. However, galaxies can grow, but they do so by the merging of several smaller galaxies.

The second error with the balloon analogy in this demonstration is that all galaxies move apart. This is not true either. Not all galaxies do not move apart - galaxies are gravitationally held together in clusters. Two clusters can also attract each other so strongly that they collide. Galaxy clusters are thought to never separate.

Experiment

You can turn this model and demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• Imagine you are in one of the galaxies. How fast does the distance between you and a nearby galaxy increase, compared to between you and a galaxy that is twice as far away as the first?

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