By Robert Hazen, Ph.D., George Mason University
A major effort is underway to map the distribution of galaxies in three dimensions. This effort, of course, is difficult because if one looks at the heavens, what is seen is a two-dimensional surface, and one has to measure the third dimension: the distance to every galaxy that is seen in the telescope.
Hubble’s Discovery and Universal Expansion
Edwin Hubble’s discovery was based on measuring the redshift of a galaxy to get the approximate distance to it. This means that if you have a relationship between distance and red shift, then all you have to do to measure the distance to a galaxy is to determine its redshift.
You don’t have to worry about Cepheid variables, or standard candles, or exploding supernovae, and things like that. You just measure the galaxy’s distance by seeing how much the hydrogen lines are shifted; and that’s very easy because every star, every galaxy, is sending us photons from hydrogen, so we can see those hydrogen lines even from the most distant galaxies.
Also, more distant objects are moving away from us—that’s called universal expansion. The universe is expanding, and that has very great consequences about the origin and the ultimate fate of the universe.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Mystery of Empty Regions
As telescopes have become more powerful, the number of known galaxies that can be measured has grown tremendously. The Hubble Space Telescope did a really interesting experiment recently. It focused on a small area of the sky that you might call an empty region. This is a region of the sky where there were no stars whatsoever from the Milky Way galaxy. It was just a piece of black space.
Any normal telescope, looking at that region of space, would see nothing. But the Hubble Space Telescope is very powerful; it has such high resolution that it was able to focus on that small area of space in the Northern sky for several days and actually get a very high-resolution image of that distant slice of space.
What it found is absolutely remarkable. It found, not just a few, but dozens of galaxies: galaxies that were billions, or tens of billions, of light-years away, in what’s called a dark-field image of the Northern sky. To make sure that there was no bias in the result by randomly picking an area that might have had extra galaxies, the same thing was done in the Southern Hemisphere. There it found another piece of the dark sky.
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Distribution of Galaxies
If you take that small area, and you figure out how many galaxies are in it, and then you multiply over the whole vast area of the entire sky, the result is absolutely staggering. The minimum estimate for the number of galaxies is on the order of 50 billion galaxies. And each of those galaxies has tens or hundreds of billions of stars.
Now, with the redshift of each galaxy, astronomers can get the approximate distance to each of those galaxies, so they can start mapping, in three dimensions, the distribution of galaxies throughout the universe. This has been done now: every year, there are thousands of more galaxies that are added to the list.
Computer programs generate three-dimensional models of the universe, and what they’ve found is fascinating. Galaxies are not just randomly distributed. Galaxies are distributed in clusters, local clusters, and more large-scale clusters, and then superclusters of galaxies. Separating those clusters, there are vast voids, in which there are very few galaxies indeed.
Milky Way Galaxy and the Virgo Cluster
The Milky Way galaxy is part of the Local Group of galaxies; it’s about 30 galaxies. There are two big ones: the Milky Way, at one end of this cluster, and Andromeda, about two million light-years away, at the other end; and this local cluster is all gravitationally bound together. The Milky Way and Andromeda are actually very slowly rotating around a common center and herding along with the smaller galaxies.
Then there are several other clusters of galaxies that are relatively nearby. By far, the largest of these is called the Virgo Cluster; it’s about ten million light-years across, and it has hundreds of galaxies. The Virgo Cluster is about 50 million light-years away from us, but it’s so massive that this Virgo Cluster actually has a gravitational attraction with our local cluster, and our local cluster is being pulled towards the Virgo Cluster.
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Local Supercluster of Galaxies
So here’s one small group of galaxies where instead of seeing a redshift, we see a blueshift. Because we’re moving towards them, wavelengths are being piled up; and so there are actually some galaxies in the heavens that are reasonably far away, a few million or tens of millions of light-years, that have the blueshift.
The Local Group, the Virgo Cluster, and several other clusters seem to be part of an even larger gravitationally bound group called the Local Supercluster of galaxies. That’s arranged in a disk like arrangement about 100 million light-years across. The total mass of this collection of galaxies is estimated to be a million billion stars.
The Local Supercluster is part of various surfaces and voids in a much vaster dimension that goes outwards to ten billion light-years and more. This is a vast scale; it’s hard to imagine how vast our universe is.
Common Questions about Mapping the Distribution of Galaxies in the Universe
Edwin Hubble’s discovery was based on measuring the red shift of a galaxy to get the approximate distance to it. This means that if you have a relationship between distance and red shift, then all you have to do to measure the distance to a galaxy is to determine its redshift.
The Hubble Space Telescope is so powerful and has such high resolution that it was able to find dozens of galaxies that were billions, or tens of billions, of light-years away, in what’s called a dark-field image of the Northern sky.
Astronomers have come up with amazing results by three-dimensionally illustrating the distribution of galaxies. They found that not only are galaxies distributed randomly, but they are distributed as clusters, enormous clusters, and superclusters.
