Our universe is huge. In every direction in the sky, powerful telescopes are watching galaxies whose light has taken more than 10 billion years to reach Earth.
But wait — if we receive such ancient light from opposite directions, and if the Big Bang happened just 13.8 billion years ago, does that mean the universe is expanding faster than light?
Wouldn’t that violate Albert Einstein’s theory of relativity, which says that the speed of light is an absolute limit?
The answer may surprise you. But first, we need to take a closer look at the expansion of the universe.
We know that the universe is expanding because galaxies seem to be moving away from each other – and the distances between them are constantly increasing. We can observe this through phenomena known as redshift.
But cosmic expansion shouldn’t be depicted as galaxies race through empty space at incredibly high speeds.
Instead, empty space itself is expanding, pushing galaxies further apart.
A helpful analogy to help you imagine this scenario is to think about how a loaf of raisin bread will rise in the oven.
Although the individual raisins don’t travel through the dough, they end up farther apart as the dough expands during the baking process.
You get the picture: Dough is an empty space; Raisins are galaxies.
Even if the galaxies did not move at all, their mutual distances would increase due to the expansion of space.
To represent the infinite universe, the loaf of raisin bread would have to be infinitely large, but even then the rising dough would push the raisins apart.
So what about expansion velocity? Well, if two raisins start 1cm apart, and the bread increases twice as much in one hour, they will end up 2cm apart.
As a result, as indicated by one raisin, the other appears to be moving away at a rate of 1 cm per hour.
But a raisin that was originally farther away, say at a distance of 3 cm, will end up at 6 cm, which corresponds to an apparent stagnation velocity of 3 cm per hour.
The same is true for galaxies in the expanding universe.
Nearby galaxies appear to be receding at a slower rate than distant galaxies. In other words, cosmic expansion does not have a single speed.
If you really want to express it in kilometers per second, you need to know what distance it refers to.
It is useful to define the expansion rate of the universe as a relative growth rate.
It turns out that space is expanding very slowly: at the moment, cosmic distances are increasing by only 0.007% in a million years.
Is the universe expanding faster than light? Well, that depends on what scale you’re looking at.
Suppose two galaxies are in opposite directions in the sky, 20 billion light-years apart.
At the current growth rate of the universe, the distance between them will increase by 0.007% (equivalent to 1.4 million light years) in a million years, which is obviously faster than light.
This rate has changed little over the life of the universe.
So yeah, for really large distances you could say that the universe is expanding faster than light.
But Einstein didn’t mind. Its cosmic velocity limits refer only to the movement of physical bodies through space, from one point in the universe to another.
So in general, the expansion of space has nothing to do with moving objects, and is in no way limited to the speed of light.
This guide originally appeared in the February 2023 issue of BBC Sky at Night Magazine.
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