This is the text of Part Two of the Spongy Universe Student Text.

In this activity, you have been using sponges to model what the universe looks like.

Scientists very often use their observations to construct models, which are scaled-down versions of the natural object or system they are studying.

These models sometimes have some details omitted but they include current and accurate knowledge and observations.

By the early nineteen nineties, many cosmologists had agreed on what is now known as the standard cosmological model of the universe, composed on what are considered to be the known facts about the universe.

This model contains a statement that says that the universe is isotropic and homogenous. If the universe is isotropic, then it should appear to be the same, regardless of which direction we observe it.

If it is homogenous, then we should be able to take equal astronomical-size samples of different large regions of the universe and find a similar mix of galaxies and space no matter where the samples came from.

How can the universe resemble a sponge and, at the same time, be homogeneous?

You have just completed an activity that could explain at least part of the answer to this question. When you examined the real sponge, you could feel the holes and the structure of the solid.

You could also feel the raised sections on the tactile cards that represented the solid structures in the sponge when the sample was large. But what happened as the representation of the sponge got smaller and smaller? The individual raised sections became less well defined until they blended into one larger object.

So, does the distance at which you observe the sample make a difference?

Do individual astronomical structures become less well defined at greater distances and more homogeneous like your sponge sample did?

Cosmologists have found that this is what happens when they observe the universe. You were modeling what happens when even very large objects, like clusters of galaxies are viewed at distances measured in light years.

Cosmologists have also found that the size of the sample observed is important. A piece of space that contains only a star or a planet would probably be a heterogeneous sample.

If this is so, how large a sample of space must be taken before they all start to look alike?

If you observed a portion of the universe that is a billion light years on a side, you would probably get the same mix of galaxies and space, regardless of where you took you sample.

So, on a large scale, is our universe also isotropic?

Isotropic means that a sample looks much the same in every direction. In recent years, structures and voids have been found in samples that measure hundred of millions of light years on a side.

Galaxies, clusters, and superclusters have been found in equal numbers in all parts of the universe, except where clouds of dust and gas in our own galaxy obscure our view of space.