How big is the observable universe? It seems a reasonable question to ask. But the answer is not so simple. Let’s have a go at this question anyway because, well, as intelligent beings who live in the universe, it is something we should know. And arriving at the size of the universe is more interesting than the answer itself…
Why is it so hard to determine the size of the observable universe? For one thing, the universe is expanding, as Edwin Hubble discovered in the 1920’s and Einstein’s theory predicted a decade before. The universe is bigger today than it was yesterday, it’s been expanding since the Big Bang some 13.7 billion years ago, and it will continue to grow as space itself expands. As with an inflating balloon, the size of the universe depends on when you measure it.
But here’s a helpful concept. By measuring the speed at which distant galaxies recede from us, astronomers get an idea of how space expands over time. They invented a useful idea called the “scale factor”, sometimes written as “a” or “S”. The scale factor tells us the size of the universe in relative terms. According to the scale factor, the universe today has size “1”. About 10 billion years ago, the scale factor was ⅓, so the universe was one-third its current size. About 10 billion years from now, it will double in size compared to today, and so on. The scale factor is a very powerful tool, although it only tells us the relative size of the universe. But at least we are making progress.
The farthest thing we can observe is the cosmic microwave background (CMB), the first light to escape from the universe when it was just 380,000 years old. This light has been traveling from the visible edge of the universe for 13.7 billion years. So a little thought might suggest the observable universe is twice this size, or 27.4 billion years across.
This isn’t quite right, of course, since space has been expanding continuously as the light has been traveling towards us. The surface on which we see the CMB (call it the “sky”) must now be much farther away from us than 13.7 billion light years. And when the light first started out, the surface must have been much closer. So there are actually three distances we can talk about… the light travel distance, which is 13.7 billion years, the lesser distance when the light began its journey, and the much larger current distance of the surface, which is the edge of the observable universe.
Here’s an example to make it clear. Say you were flying from Sydney to Perth, Australia, a distance of 3300 km. If your flying speed was 330 km/hr, it would take you 10 hours to make the trip. Easy. Now imagine instead that Australia grew in size by 30% during your trip. As you set out, your destination is 3300 km away. When you land, your departure point is 4920 km away. And how far did you travel? Somewhere in between these two numbers, depending on how the size of Australia scaled during your trip.
So it is with light from the cosmic microwave background. This light, the most distant light we can see, has traveled 13.7 billion years. Using the scale factor of the universe, astronomers can calculate the current distance to the surface from which this light appears to come is now much farther away… about 46.5 billion light years. So from our point of view, the observable universe appears to be 93 billion light years across right now.
And the universe was much smaller when the light from the CMB first started on its way. Since this light has been stretched by a factor of 1000 to longer wavelengths, astronomers deduced the CMB was 1000 times smaller, or only 46.5 million light years away when it began its journey some 13.7 billion years ago, and the universe was far, far smaller than it is today.
Of course, the universe may be much larger than the observable universe. We also know the universe exploded in size for a tiny fraction of a second after the Big Bang. The event, known as “inflation” caused space to expand by a factor of 1078 almost instantaneously until the entire universe was almost perfectly flat and smooth. By some calculations, because of inflation, the observable universe may be an very tiny fraction of the total universe. But since we can’t see beyond the observable universe, it’s impossible to measure the size of the entire universe, or even know if it makes sense to ask how big the universe really is.
But the observable universe is now about 93 billion light years across.