If the universe originated at the big bang 13 billion years ago, how can there be stars that are 30 billion light years away?
Nick Canning
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Coleraine, County Londonderry, UK
Given that the universe is 13.8 billion years old, you might think the most distant observable object would be 13.8 billion light years away from us, but this isn’t the case.
The light from the most distant object we can see (given the technology we currently have) has indeed been travelling to us for 13.8 billion years, but in that time, the expansion of the universe has increased the distance between us and that object.
The question we need to ask is: how far away is the horizon if the light has been travelling for 13.8 billion years? This is calculated using the best model we have for the history of the expansion of the universe, which shows that the horizon is 46 billion light years away from us.
There may be matter beyond this horizon, but we can’t detect it and it can’t interact with us.
Stephen Johnson
Eugene, Oregon, US
The reason why stars can be 30 billion light years away when the universe itself is less than 14 billion years old is because of two factors.
The first is the inflation that took place immediately after the big bang, when the universe grew by 1026 in less than 10-30 seconds (yes, this is faster than the speed of light, but physics allows this since it was space-time itself that was growing, rather than objects moving away from each other).
The second factor is the continuous expansion of the universe since the end of inflation. Today, the universe is thought to be about 93 billion light years across.
From our position inside the universe, no star is 93 billion light years away from Earth, but most are more than 14 billion light years away.
Danny Kermode
Vernon, British Columbia, Canada
To understand distances that span a significant portion of the known universe, it is helpful to imagine that the cosmos has only two spatial dimensions instead of three.
Think of the universe as a huge, flat sheet of rubber with only a left and right and no up and down. Even with this simplification, visualising the cosmos is still difficult.
For a start, we don’t have enough evidence to conclusively identify the topology of the universe, so we can’t say for certain whether our imagined sheet of rubber extends infinitely in every direction or if it loops back on itself somehow.
The universe is often depicted as having what’s known as a hypersphere topology. To understand this, think of the sheet of rubber as the surface of a balloon that is inflating – this is a good way to think about the ongoing expansion of the cosmos.
Now, consider two points on the balloon’s surface that are 1 billion light years apart. Picture point A sending a pulse of light that travels along the balloon’s surface towards point B.
You can imagine that, depending on how fast the balloon is inflating, the pulse of light might never even reach point B. Let’s say that it takes 10 billion years for the pulse to arrive at point B. When it arrives, how far apart are points A and B?
Hopefully, it now makes sense that points A and B will be more than 10 billion light years apart. What’s more, depending on the rate of inflation, even though the universe may have started inflating from a single point 13 billion years ago, points A and B may now be 30 billion light years apart.
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