Fall is a bittersweet time for stargazers. Nothing beats the view of our own galaxy, the Milky Way as it stretches across the sky like the backbone of the night. In summer, the accumulated glow of billions of stars is hard to beat.
On the other hand, getting the Milky Way out of the way has its advantages. In autumn, with most of our galaxy below the horizon, we can see deeper into space to our larger universe, the realm of the galaxies.
Thousands of them are visible in a moderately sized telescope, and a few of them are visible in binoculars. One of them is even visible to the unaided eye from dark, rural skies unimpeded by urban lighting.
Recently, I spent OWU’s autumn break up in the mountains at Shenandoah National Park. As I looked high in the east, I spotted the constellation Andromeda. There among its stars was a tiny, cigar-shaped patch of light. It was M31, the Andromeda Galaxy, a spinning whirlpool of 300 billion stars. Both galaxies are members of a small cluster of 30 or so galaxies called the Local Group. Andromeda is relatively close to its sister galaxy, our own Milky Way, at only 2.5 million light years away. (One light year is about six trillion miles.)
Astronomers had estimated that the universe contains at least 200 billion observable galaxies. That was until astronomer Christopher Conselice of the University of Nottingham in the UK came along. She and her international team of scientists analyzed an enormous collection of data from the Hubble Space Telescope. They concluded that estimates of the number of galaxies were at least ten times too small. Over time, the universe has been populated by at least two trillion galaxies.
Most of the galaxies they saw were small by comparison to the standard set by the Milky Way and Andromeda galaxies. Also, most of them were very far away, near the “edge” of the observable universe.
The significance of their findings cannot be overstated. As we stare into the universe, we must realize this fundamental fact: Light travels at a fixed velocity. As we look farther and farther out into the universe, the light from distant objects takes more and more time to reach us.
The farther away an object is, the farther back in time we see it. As far as our observation of the world and the universe is concerned, there is no “now.” Now is an illusion caused by our proximity to some of the objects we observe.
Thus, Conselice’s results tell us much about the evolution of the universe. Near the beginning, the universe was much smaller than it is now, and it was populated by an enormous number of small galaxies. As they moved about, they combined together to form larger and larger galaxies. Those larger galaxies continued to cannibalize smaller ones. The process continues to some lesser degree to this very moment even though, on average, the galaxies are much farther apart.
If in fact there are as many galaxies as the team estimates, even the tiniest patch of sky should contain the light from some distant galaxy. The nighttime sky should be filled with light. Why then is it dark at night?
The question is a somewhat modernized version of Heinrich Olber’s famous paradox. If the universe contains an “infinity of stars,” why is the sky dark at night?
The answer is that not all the light gets to us. Some of it is absorbed by dust and gas in our own galaxy and, in this case, the few stray molecules between galaxies. But that doesn’t account for all the lost light.
As the light travels toward us, the space between its source and us is expanding. It loses energy, a process called “reddening.” The energy simply moves out of the visual range into low-energy forms that are beyond our ability to detect visually. In other words, the fact that those myriad galaxies do not light the sky is a sign that the universe is expanding, which provides further verification of the Big Bang theory.
Note that the light from the most distant galaxies is in the range of 12 billion light years away. One might then conclude that the universe is at least 12 billion light years wide. However, that light took 12 billion years to reach our eyes. The universe has continued to expand for 12 billion years. It is as much as 10 times vaster now than when that light began its long journey.
Note also that this is only the light that is getting to us. If, as most astronomers suppose, the universe expanded briefly faster than light speed right after the initial event, we are seeing only that part of the universe for which the light has reached us. As a result, most of the universe is “below the horizon.” The light will never reach us. There are perhaps trillions of more galaxies in a universe far vaster than we will ever know and almost certainly forever beyond our ken.
If we want to know what the distant universe is like “now,” the best we can do is study the closer galaxies like M31 in our Local Group, where the information is only a few million years old, and hope that the same rules apply over the rest of the universe.
Think those matters as you stare at that faint, fuzzy streak, the Andromeda Galaxy, so close to home and yet so unfathomably far away.
Tom Burns is director of the Perkins Observatory in Delaware.