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The trouble with the speed of light


It’s hard to observe the universe when it’s constantly moving away from you, writes Katie Mack.



Naoyuki Noda / getty images

Two of the biggest misconceptions about cosmology are that the universe must have a limit, and that our ability to learn about it doesn’t.

Although we know it had a beginning in time, nothing in our observations suggests that the universe has an edge. The universe seems pretty much the same in all directions. It’s a basic tenet of cosmology that there’s no “special place” in the Universe – it’s called the cosmological principle.

It’s true that things are very different if you’re situated on a planet inside a galaxy rather than floating in empty space. But when you zoom out for a large‑scale view, one region of space looks more or less like another. The universe could just go on forever.

However our knowledge of the universe does have an unsurpassable limit. There are parts of the universe we will never be able to see at all, and for the most distant things we can see, our knowledge about them is stuck in the past.

The culprit here is the speed of light. Light travels at about 300,000 kilometres per second. The distance light travels in a year, about 9.5 trillion kilometres, is called a light-year.

The fact that light has a speed introduces a delay between when light leaves something (say, the Andromeda galaxy) and when it gets to us (2.5 million years later).

The farther away something is, the longer its light has taken to reach us, meaning the snapshot we get is more and more out of date. The most distant galaxy we can see is so far away that its light took 13.4 billion years to reach us.

Because of the light speed delay, it still looks to us like a baby galaxy that existed in a baby universe, only 400 million years after the Big Bang.

Looking even further out, we can see the cosmic microwave background, the afterglow of the Big Bang itself. That light has travelled 13.8 billion years to reach us – almost the entire age of the universe.

This is the most distant light we can see, but since the universe has been expanding since the beginning, the part of the universe it comes from is not, as you’d expect, 13.8 billion light years away, but more like 46 billion light years away.

This distance of about 46 billion light years is called the particle horizon, and it defines the edge of the “observable universe”.

It’s a basic tenet of cosmology that there’s no 'special place' in the universe – it’s called the cosmological principle

It’s not a real edge in space – we have reason to believe that the universe must be exponentially larger than the region we can observe directly – but it is a real edge in our knowledge.

If there is, right now, a galaxy 50 billion light years from Earth, we can’t possibly see it: even starlight that left that galaxy as soon as it came into existence couldn’t possibly have crossed 50 billion light years in the time between the beginning of the universe and today.

What’s worse, we will never see it, or anything else beyond the horizon, no matter how long we wait for the light to arrive.

Nothing can travel faster than light through space, but space itself doesn’t have that constraint, and if it’s expanding in all directions, there will be points far enough away from each other that they’re getting further apart faster than light can travel between them.

The light leaving that galaxy now not only has to travel through the space between us, it has to contend with the fact that the space between us is getting larger all the time. And the expansion is speeding up. If you’ve ever accidentally set a treadmill to a speed that’s faster than you can run, you know how that light beam feels.

As time goes on, we’ll be able to see less and less of the universe around us. Eventually, everything outside our Local Group of galaxies will slowly fade as the conveyer belt of spacetime pulls it away from us faster and faster.

Astrophysicists of the distant future won’t be able to see the rich and beautiful universe we have the privilege to see now.

So we better make the most of it while we still can.

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Katie Mack is an astrophysicist at the University of Melbourne.
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