Seeing to the Far Limit of the Universe
Let’s go back to the example of the trees. If we have not gone too far into the forest we can see to the edge of it, looking here and there between the tree trunks. But what does the edge look like? The forest grew up uncultivated, perhaps starting with one solitary tree in the middle of a great plain. Left to itself, every year the forest becomes larger, because the trees on the edge scatter seeds some distance out onto the plain. At the edge, last year’s trees are small saplings, and this year’s trees are seedlings. The closer we are to the edge, the younger the trees will be. Looking out from the depths of the forest, we will not be able to see the trunks of the youngest trees, because the seedlings are not tall enough. The saplings will have leafy branches all the way up, because on the edge of the forest all the outer branches are exposed to sunlight. In short, the edge of the forest must be younger and greener, different from the old, dense interior.
With telescopes we can see far away. When we see distant stars and galaxies, we see them not as they are now, but as they were when they were newer, because it takes thousands of millions of years for their light to reach us. The farther away the stars are, the newer they were when their light started on its journey toward Earth.
In the 1950s astronomers recorded their pictures on sensitive photographic plates. Now they record the pictures digitally using a new kind of detector called a charge-coupled device (CCD). These detectors are 100 times more sensitive than film. Light intensity falls off inversely as the square of distance. Therefore an old telescope, newly equipped with a CCD instead of photographic plates, can see ten times farther. The Hale telescope in the 1950s could see out to 2 000 million light years. With a CCD instead of film it can now potentially see as far as 20 000 million light years, but beyond about 13 500 million light years there are no more stars or galaxies to see.
Does this mean that we can now see almost to the edge of the universe? Not really. We can see almost to the beginning of the universe. Stars in their earliest stages look different from fully formed stars. The first stars began as huge clouds of cold gas. Until these clouds contracted and heated up under the action of their own gravity, they emitted no light. That made them as invisible at a distance as the seedlings on the edge of the black forest.
The first stars became visible when their heat reached the temperature of ignition of hydrogen, their nuclear fuel. The first stars had no heavy elements to serve as catalysts for burning hydrogen into helium. They couldn’t start to burn until gravity brought their central temperature to millions of kelvins. When they burn their outer layers reach bluish-white heat. However, we see them as members of dark red galaxies too far away to resolve into individual stars. They look red, not bluish white, because expansion cools their light. The light from the farthest stars is the coldest. Though the most distant stars look red when we photograph them, we must remember that we are seeing the earliest stars. Now they look red, but when their light started out, it was so hot it was bluish white.
With telescopes we can see far away. When we see distant stars and galaxies, we see them not as they are now, but as they were when they were newer, because it takes thousands of millions of years for their light to reach us. The farther away the stars are, the newer they were when their light started on its journey toward Earth.
In the 1950s astronomers recorded their pictures on sensitive photographic plates. Now they record the pictures digitally using a new kind of detector called a charge-coupled device (CCD). These detectors are 100 times more sensitive than film. Light intensity falls off inversely as the square of distance. Therefore an old telescope, newly equipped with a CCD instead of photographic plates, can see ten times farther. The Hale telescope in the 1950s could see out to 2 000 million light years. With a CCD instead of film it can now potentially see as far as 20 000 million light years, but beyond about 13 500 million light years there are no more stars or galaxies to see.
Does this mean that we can now see almost to the edge of the universe? Not really. We can see almost to the beginning of the universe. Stars in their earliest stages look different from fully formed stars. The first stars began as huge clouds of cold gas. Until these clouds contracted and heated up under the action of their own gravity, they emitted no light. That made them as invisible at a distance as the seedlings on the edge of the black forest.
The first stars became visible when their heat reached the temperature of ignition of hydrogen, their nuclear fuel. The first stars had no heavy elements to serve as catalysts for burning hydrogen into helium. They couldn’t start to burn until gravity brought their central temperature to millions of kelvins. When they burn their outer layers reach bluish-white heat. However, we see them as members of dark red galaxies too far away to resolve into individual stars. They look red, not bluish white, because expansion cools their light. The light from the farthest stars is the coldest. Though the most distant stars look red when we photograph them, we must remember that we are seeing the earliest stars. Now they look red, but when their light started out, it was so hot it was bluish white.
