This is Part One of the Spongy Universe Student Text

In this part of the “Spongy Universe” activity, you used the same method that astronomers and cosmologists use as they study the universe. First you described a sponge. Then, you thought about how it might have been formed. Which was more difficult for you? The description or the explanation? Scientists usually find that describing an object is easier than explaining how it was formed. Describing the structures in the universe is even more difficult than describing a sponge, but the development of better instrumentation is making it easier to observe and describe large areas of the universe.

Why is a sponge a good model for the universe?
In nineteen eighty-one, astronomers discovered that there are holes, or voids, in the very large clusters of galaxies called superclusters. A team of astronomers from the University of Michigan discovered the largest void in the Bootes constellation. The Bootes void was considered just a chance event until nineteen eighty-five. That is when another group of astronomers at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, was making observations in a different direction from the Michigan group and found that the universe was full of large bubbles. Cosmologists now refer to these voids as "Hubble bubbles," in honor of Edwin Hubble, an early pioneer in the exploration of the universe.

One of the Cambridge astronomers described the solid matter in the universe as looking like what you would get if you sliced through a sponge. He said that it looked like the solid matter was arranged in an interconnected, stringy network with large bubbles in which little, if any, matter can be seen. So, is there anything in the voids? Are they empty of all matter, or might they contain matter that cannot be seen? After all, we know that although sponges have holes, those holes are filled with the invisible gases in air.

How and when did the “Spongy Universe” form?
Ideas about the origins of "Hubble bubbles" are one of the current space science topics. Explaining how these “bubbles” were formed and why matter is clumped together like a large sponge is even more difficult than explaining how a sponge might have been formed.

Most scientists have agreed that explanations fall into two categories that are based on when the bubbles were formed in the universe's history. One group of explanations is based on the theory that the bubbles were formed late in the history of the universe. The other explanation says that the structures survived from the early times of the universe.

Scientists who think that the voids and stringy structures formed late in the universe's history also think that it is not surprising that the universe resembles "Swiss cheese.” Their thinking includes the idea that if you dig a hole in one place, the dirt is piled in another place. So, if you have clumps of structures, whether those structures are found in a sponge or in a piece of Swiss cheese, or whether they are cosmic structures, you will probably also have holes from which the matter in these structures were dug. Of course we must then ask, what dug those holes? Could an explosion in space have caused the voids? Or could the bubbles have been formed when several smaller bubbles came together?

If, however, the cosmic mass was not uniformly distributed in space at the beginning of the universe, then the solid structures may have formed around the edges of the bubbles, but not in the voids themselves. This would also have given the universe a “Swiss cheese” structure long before the galaxies condensed out of the primordial gas. According to this theory, galaxies may have formed from cosmic strings—long, very dense structures and the spaces between the strands became the voids.

How large are the holes in the Spongy Universe?
Most of us have had no experience with cosmic distances, which are measured in light years. A light year is the distance that light can travel in one year, which is approximately six trillion miles. The "bubble" in the Bootes constellation is about two hundred fifty million light years across. It was found in a supercluster of galaxies, which are typically one hundred million light years in diameter. That is a very large sponge!

Nearly all the visible matter in the universe is contained in galactic superclusters, which are like long strings on which tens of thousands of galaxies are strung like beads in a necklace. The bubbles between these superclusters are called voids, because they are relatively free of visible matter; that is, they contain few, if any, galaxies.

Galaxies themselves are large aggregations of stars that are bound together by gravitational force. There are three major classifications of galaxies—spiral, elliptical, and irregular.The million brightest galaxies are composed of a complex distribution of chains, strings, filaments, and voids, all connected in a lace-like pattern.

Galaxies are not randomly distributed in space but are usually grouped together in what are called galactic clusters. These are structures in which a large number of galaxies are found in close proximity to each other. Over half of all galaxies are associated with clusters of various sizes.

Our sun belongs to a spiral galaxy, the Milky Way, which is part of a cluster called "the local group.” The two largest galaxies in the local group, the Milky Way and the Andromeda galaxy, use the force of gravity to drag at least twenty other smaller galaxies along with them. This cluster is not very large, being only about three million light-years across. Some larger clusters have many thousands of galaxies in them.