The Limit of the Known Universe
To see farther, beyond the first stars, we must be able to detect light that expansion has reddened even more than the reddening of the first stars. What is redder than red? The answer is: infrared, that is, heat. But the first light has been reddened or stretched or cooled so far it is not now even heat, let alone visible light. The first light has been reddened down through heat and millimeter waves all the way to microwaves. To observe it, we must change our detectors, because charge-coupled devices are not sensitive to microwaves.
The Cosmic Background Explorer Satellite and the Wilkinson Microwave Anisotropy Probe use microwave detectors to photograph the first light. In their pictures we do not see individual stars or even individual galaxies beyond 13 500 million light years. For a distance of 200 million light years beyond the first stars and galaxies there is only darkness, the darkness of the second evening. Beyond that darkness in space, before that darkness in time, we detect the inner surface of what appears to be a luminous spherical wall. The universe looks like it started inside a huge, hollow, bright ball.
What we actually see is a shell of microwaves. We say that it is a shell because theory tells us it is 380 000 light years thick. The thickness of the shell is only one part in 80 thousand of the diameter of the ball. In proportion it is like a balloon 80 inches (2 meters) in diameter, with a plastic skin a thousandth of an inch (25 micrometers) thick.
We know that when the microwaves started out, they were reddish light, but the expansion has stretched and cooled the light to microwaves. This is the light of the first morning. It is like a bright fog, preventing us from seeing through it. The temperature is the same in all directions, to within one part in 50 000. The limit we can see looks like a curtain of clouds in space, but it is really a limit in time.
If we try to see farther than that we will have to look for something that started moving toward us before the first light shone. That would be the cosmic rays at the beginning. However, almost all of the original cosmic rays materialized and broke up into less energetic rays. The shell is the last of them, as they were when reduced to light rays.
If we ask to see something that started moving toward us before the beginning, there is no such thing. It is useless to ask what was before the beginning, because time as well as space began with the first evening.
If we simply wait, we will be able to see very slightly farther, that is, a greater distance, but we won’t see anything different. In the 1950s we thought we were waiting for bigger telescopes to enable us to see farther. In fact, we were waiting for advances in technology to provide better detectors. At present technology is sufficiently far advanced to see all there is to see. Now we are simply waiting to enlarge our field of view. The age of the universe limits how far we can see. Every year we wait, the universe is a year older, and we can see a light year farther. That changes the view very little because one more light year is a tiny fraction of 13 700 million light years.
The amount of past time imposes the present limit. It is not a limit in space. If the universe has a limit in space, then we have not yet seen it. What would a limit in space look like? We don’t expect a brick wall. Who could have made the bricks? Let’s return to the illustration of the forest. If the forest is large enough and the lost people are far enough in the depths, away from the edge, then the forest looks the same to them in every direction. This doesn’t mean that they are necessarily at the center. There are many places far from the edge, but only one center. All places far from the edge look the same.
If they are near the edge, but not close enough to see the edge, then the forest will look different in different directions. The distant trees they can see in the direction of the edge are younger than the trees they can see in the opposite direction. If they are close enough to the edge to see the edge, then in that one direction they can see trees with green leaves all the way to the ground, and bright light beyond them. The edge will look quite different from what they can see in other directions.
In the universe we can detect light reddened to microwaves coming toward us in all directions from beyond the edge of the galaxies. This is analogous to the edge of the dark forest. We can see equally new galaxies at the same great distance in any direction. This doesn’t mean that we are at the center of the universe, however. We would see the same thing from any point in the depths of the known universe. If we were close to the edge of the universe, then the edge galaxies would be closer than the distant galaxies we see in other directions. Instead, we see the cold first light at the same distance in all directions.
The Cosmic Background Explorer Satellite and the Wilkinson Microwave Anisotropy Probe use microwave detectors to photograph the first light. In their pictures we do not see individual stars or even individual galaxies beyond 13 500 million light years. For a distance of 200 million light years beyond the first stars and galaxies there is only darkness, the darkness of the second evening. Beyond that darkness in space, before that darkness in time, we detect the inner surface of what appears to be a luminous spherical wall. The universe looks like it started inside a huge, hollow, bright ball.
What we actually see is a shell of microwaves. We say that it is a shell because theory tells us it is 380 000 light years thick. The thickness of the shell is only one part in 80 thousand of the diameter of the ball. In proportion it is like a balloon 80 inches (2 meters) in diameter, with a plastic skin a thousandth of an inch (25 micrometers) thick.
We know that when the microwaves started out, they were reddish light, but the expansion has stretched and cooled the light to microwaves. This is the light of the first morning. It is like a bright fog, preventing us from seeing through it. The temperature is the same in all directions, to within one part in 50 000. The limit we can see looks like a curtain of clouds in space, but it is really a limit in time.
If we try to see farther than that we will have to look for something that started moving toward us before the first light shone. That would be the cosmic rays at the beginning. However, almost all of the original cosmic rays materialized and broke up into less energetic rays. The shell is the last of them, as they were when reduced to light rays.
If we ask to see something that started moving toward us before the beginning, there is no such thing. It is useless to ask what was before the beginning, because time as well as space began with the first evening.
If we simply wait, we will be able to see very slightly farther, that is, a greater distance, but we won’t see anything different. In the 1950s we thought we were waiting for bigger telescopes to enable us to see farther. In fact, we were waiting for advances in technology to provide better detectors. At present technology is sufficiently far advanced to see all there is to see. Now we are simply waiting to enlarge our field of view. The age of the universe limits how far we can see. Every year we wait, the universe is a year older, and we can see a light year farther. That changes the view very little because one more light year is a tiny fraction of 13 700 million light years.
The amount of past time imposes the present limit. It is not a limit in space. If the universe has a limit in space, then we have not yet seen it. What would a limit in space look like? We don’t expect a brick wall. Who could have made the bricks? Let’s return to the illustration of the forest. If the forest is large enough and the lost people are far enough in the depths, away from the edge, then the forest looks the same to them in every direction. This doesn’t mean that they are necessarily at the center. There are many places far from the edge, but only one center. All places far from the edge look the same.
If they are near the edge, but not close enough to see the edge, then the forest will look different in different directions. The distant trees they can see in the direction of the edge are younger than the trees they can see in the opposite direction. If they are close enough to the edge to see the edge, then in that one direction they can see trees with green leaves all the way to the ground, and bright light beyond them. The edge will look quite different from what they can see in other directions.
In the universe we can detect light reddened to microwaves coming toward us in all directions from beyond the edge of the galaxies. This is analogous to the edge of the dark forest. We can see equally new galaxies at the same great distance in any direction. This doesn’t mean that we are at the center of the universe, however. We would see the same thing from any point in the depths of the known universe. If we were close to the edge of the universe, then the edge galaxies would be closer than the distant galaxies we see in other directions. Instead, we see the cold first light at the same distance in all directions.
Ordinary Darkness
The night sky is filled with microwaves left over from the energetic darkness of the first evening. Our eyes can’t detect microwaves. At present darkness is the absence of light. That is what we mean by “ordinary darkness.”
One can see in the sky a few thousand bright stars, but even if they were gathered all together, they wouldn’t be nearly as bright as the Sun. The Earth receives light and heat mainly from the Sun, the only star close to Earth. The Earth then reradiates the heat it receives into outer space. The average temperature of space is 2.7 kelvins, minus 270º C, or minus 454º F. Exposure to the dark sky at this temperature cools the Earth. The cycle of heating and cooling repeats every 24 hours for any portion of the Earth’s surface that lies between the two arctic circles. This rapid alternation keeps the Earth at a livable temperature. In the next chapter we summarize a partial list of factors that make the Earth suitable for life.
One can see in the sky a few thousand bright stars, but even if they were gathered all together, they wouldn’t be nearly as bright as the Sun. The Earth receives light and heat mainly from the Sun, the only star close to Earth. The Earth then reradiates the heat it receives into outer space. The average temperature of space is 2.7 kelvins, minus 270º C, or minus 454º F. Exposure to the dark sky at this temperature cools the Earth. The cycle of heating and cooling repeats every 24 hours for any portion of the Earth’s surface that lies between the two arctic circles. This rapid alternation keeps the Earth at a livable temperature. In the next chapter we summarize a partial list of factors that make the Earth suitable for life.