We have learned to pierce the depths of space and see stars and galaxies that lie thousands, and millions, and billions of light-years away.

But our vision is ultimately limited: we live, quite literally, in a bubble. Way beyond the most distant galaxies, the line of sight runs into a backdrop every which way we look. An impassive and impassable wall of fire blocks the view like a hot glowing curtain--a wall beyond which light cannot trespass.

Sure, the sky looks dark to us—the stars shine on a black background—and it looks black to the Hubble telescope as well. But that is just because our eyes, and Hubble’s, can see only a narrow range of wavelengths in the much broader spectrum of light. If we could see wavelengths way beyond the infrared, all the way to the microwave spectrum, the wall of fire’s spectacle would promptly manifest itself.

We know of it (and we have only known of it for less than half a century) because we detect its glow by way of radiation called the cosmic microwave background. Microwave-detecting “cameras” can see it just like ordinary cameras can see the stars.

Once revealed, this boundary appears to us as if we were looking at the inside of a bubble, from the bubble’s center. Everything our telescopes can see lies within the bubble (although in the future we may be able to gather information from earlier epochs with entirely new kinds of observatories, such as the ones that use neutrinos instead of light).

Think of one of those “snow globes” filled with water and specks of white stuff, and imagine holding it in the palm of your hand. Our own galaxy, the Milky Way—including the Earth, the sun, all the planets, and all stars visible to the naked eye—is but a speck of white dust at its middle. All the other visible galaxies are also specks of dust, floating in the water all around it.

The microwave sky, then, is a sphere we look at from the inside. Many of us have seen that sphere—usually portrayed from the outside in—in maps produced by space-based microwave observatories. Those maps represent different “colors” of microwave light as ordinary colors.

To understand the nature of the microwave sky we have to think at almost unimaginable scales of space and time. The snow globe of our universe is so large that the images we glimpse of its outer surface—the microwave light detected by the space probes—took more than 13 billion years to journey from that outer surface to its center.

The boundary of the sphere, the wall of fire, is not an actual physical barrier. Instead it is, in a sense, a trick of our perspective. It is our skewed, partial view of an ancient cosmic epoch called recombination.

For hundreds of thousands of years after the big bang, and long before any stars or planets existed, the universe was filled with a hot broth of subatomic particles. As the universe expanded, this hot broth, called a plasma, slowly cooled. When it was cool enough, particles began to combine into atoms of hydrogen and helium. In a relatively brief event, taking place around 300,000 years after the big bang, all the plasma turned into a gas of hydrogen and helium.

Plasma can be hot enough to glow, but it is not transparent. Thus if anyone had been around at the time, they would have witnessed space go from glowing to transparent. Before recombination, a hypothetical astronaut on a spacewalk might have been unable to see the mother ship. But during recombination, as space became transparent, the ship would have emerged from the glowing haze.

But even assuming that recombination happened everywhere instantaneously and simultaneously—it didn’t—from the vantage point of the astronaut not all of space would have gone suddenly from bright and opaque to dark and transparent. Instead, the astronaut would have seen a bubble of transparent space inflating all around him. Surrounding the bubble, a wall of plasma was receding in all directions, at the speed of light.

To understand why, it helps to think of a more familiar transition than the one from plasma to gas: the transition from ice to water. So imagine you were standing on a sunny day in the middle of a large frozen lake, and that somehow it were possible for the ice to thaw instantaneously, everywhere at once, exactly at 12 noon.

Also, pretend for a minute that light were very slow – one meter per second instead of the usual 300,000 kilometers or so per second. Because light is so slow, you would not in effect see the entire lake thaw at once.

Instead, images arrive at your eyes with an appreciable delay. Thus, a few seconds after noon, even as your immediate surroundings have turned liquid, parts of the lake more than a few meters away from you still look frozen. The rest of the lake is already liquid—you just can’t see that yet. One minute after noon, and the surface of the lake still looks liquid only within a radius of 60 meters—the distance light has traveled in 60 seconds. Ten minutes, 600 meters, and so on.

In other words, what you would see is not the entire surface thawing simultaneously, but rather a disk of liquid surface expanding in all directions like an oil slick. Depending on the size of the lake, it would take many minutes for the spreading circle to cover the entire lake, and so for the lake to look completely thawed.

Now imagine that instead of being a person standing on the ice you had been a fish trapped alive in the depth of the frozen water.

Instants after noon, you are free from the ice. Seconds later, you see a spherical volume of liquid water all around you. The volume looks like a bubble from the inside, expanding inside the ice. As the liquid bubble grows, a spherical surface separating water from ice recedes from you at the speed of one meter per second—the speed of light. Thus at any given time, in every direction you look, you see a white wall of ice.

That wall, mind you, is not real. It is purely an effect of the delay in the spread of information—of the fact that light propagates at a finite speed. It is, if you want, the boundary of the fish’s ignorance.

Now back to our hypothetical astronaut. Just as the fish sees ice all around, surrounding his bubble of water, the astronaut sees a white-hot wall of plasma surrounding his bubble of rarefied hydrogen gas. Or rather, an orange-hot wall: cosmologists estimate that at the time of recombination, the plasma was at about 2,700 kelvins, a temperature that makes matter incandesce with an orange-yellowish hue. In any case, plasma is just as opaque as ice is, so the astronaut can see nothing beyond the wall.

As the eons go by, the astronaut then witnesses his corner of the universe evolve. The hydrogen and helium coalesce into stars, and inside those stars the heavier elements are forged by nuclear fusion. The stars later explode and sprinkle the new kinds of matter into interstellar space, where it again coalesces into stars, and now also into rocky planets. Eight billion years or so after recombination, the astronaut might spot the formation of our solar system, and of our own planet Earth.

Thirteen billion and seven hundred million years after recombination. The present universe looks quite different than it did early on. But that wall of plasma still surrounds us, and it is still receding. It is the boundary of our ignorance.

Because of the finiteness of the speed of light, then, we appear to live in a transparent bubble surrounded by opaque plasma. So why doesn’t the night sky glow orange?

In a certain sense, it does. The only reason that there is a cosmic microwave background rather than a cosmic orange background is redshift. The phenomenon of redshift is the analogue for light waves of the familiar change of pitch we hear in a police siren when the police car zips by us: as the car was approaching, the siren sounded higher in pitch, and once it starts receding, it sounds lower. Lower pitch sound has longer wavelength.

Ever since the big bang, the universe has been expanding. Because of that expansion, the electromagnetic waves that made up the orange light kept getting stretched as they traveled towards us, eventually becoming longer by a factor of about 1,000. If you take the orange light of 2,700-degree plasma and stretch it by a factor of 1,000, you get precisely the microwave wavelengths that astronomers pick up today.

So the sky—one is tempted to say the night sky, but because there was no Earth and no sun, there was no day or night—the sky was once bright-orange. Then the universe expanded and the color of the light changed the same way as the color of an incandescent piece of hot iron changes when the iron cools down, until it stops glowing.

Before it was black, the sky used to be bright red.

By my own back-of-the-envelope calculation, the time when the sky went dark was around 26 million years after recombination—which is essentially the same as saying 26 million years after the big bang. There were still no stars back then, and certainly no planets and no people. But to a hypothetical observer the visible universe would have stretched only for a few tens of millions--not billions--of light years.

Compared to the vastness of the cosmos, this would have been a small neighborhood indeed. If the current galaxies had existed, from the vantage point of our current location the visible universe would have included, apart from our own Milky Way, only Andromeda and a few dozen other major galaxies—not the 100 billion or more that we can see today.

Curiously, a black curtain of a rather different nature might appear again in the remote future: because the universe is now expanding at an accelerating pace, distant galaxies will some day recede from us so fast that the light they emit will never catch up with us.

Further reading:

The Reawakening of X-Ray Delta One

A note to readers: Like the rest of the SciAm Blog Network, Degrees of Freedom is on a hiatus until July 15. Please check back for more posts after that!

While preparing this post, I benefited from conversations with Scott Dodelson and George Musser. The responsibility for any incorrect statements however falls entirely on me.

Fish icon by Bartosz Michalik/Open Clip Art; starry sky by Krystof Jetmar/Open Clip Art; CMB sphere courtesy of NASA