If the universe is 13 billion years old how do we have stars 45 billion light years from earth? Wouldn’t that light take 45 billion years to reach us?



If the universe is 13 billion years old how do we have stars 45 billion light years from earth? Wouldn’t that light take 45 billion years to reach us?

In: Physics

From the way I understand it lightyears is a measure of distance that’s based on time. Even though the universe is only 13 billion years old, that’s the whole universe so something 45 billion light years away could also have been created 13 billion years ago.

If I’m 30 years old and you’re 55 years old, we can be 1000 miles apart.

I think what you might be trying to ask is how we can know that there are stars 45 billion lightyears away from us or if that’s a hypothetical age.

Again, not an expert. I’m basing this answer off of memories of high school science classes.

The thing you have to keep in mind is that space isn’t finite, it’s expanding.

So the light has been traveling for 13 billion years to reach you, but in that time the distance between you and the source of that light has increased and stretched out.

They weren’t 45 billion lightyears away when the light starting heading towards us. The light has only travelled 13 billion lightyears, but we can calculate the rate of expansion of the universe based on redshift, and so we know that the origin of the 13 billion year old photons reaching us is currently 45 billion lightyears away.

Suppose you’re standing by the road, and a car drives past you. After it’s, say, 10 meters past you, someone in the back seat throws a bottle out of the window in your direction. The bottle is travelling towards you at 1 m/s. After 10 seconds, the bottle reaches you, but at this point the car itself is 100 meters away. The bottle couldn’t have travelled 100 meters in 10 seconds, and indeed it didn’t – it only travelled 10 meters. But you can then take that bottle, and read the note inside, and learn things about the occupants of the car who now *are* 100 meters away.

It’s funny, but they say the universe expands so fast that it’s possible..

Nothing can travel faster than light, but add universe expanding, and suddenly that rule is half broken.

After having read all the responses to this question, I will say that there are some inconsistencies that I still cannot reconcile. First, let me explain my understanding, then I’ll pose some questions that perhaps one of you brainiacs can answer.

1. The Big Bang took place approximately 13.5 billion years ago. 2. Just before the bang, all the matter/energy that existed in the universe was condensed down into a golf ball (or smaller) bit of matter. 3. When the bang happened, that matter was ejected out and now comprises all the observable and unobservable matter/energy that exists today. 4. Matter, energy, photons, etc. are all constrained by the speed of light. 5. If matter originated at the center of that golf ball sized mass and is constrained by the speed of light in its speed away from the point of origin in space/time of that bang, then the maximum distance from that central point would be 27 billion light years. 6. Even if space were expanding between two bits of matter, then due to the relative position of those bits of matter from one another, they would appear to be moving away from one another at a speed that is greater than the speed of light. 7. The only thing that would make sense to me is if the Big Bang happened and all matter and energy was ejected instantaneously at a speed far greater than the speed of light. Then, at a later time, the laws of physics took hold and the speed of light was constrained.

Question time!
1. How can two bits of matter in our universe be a greater distance apart than 27 billion light years? 2. Even if the fabric of space time were expanding and accelerating, wouldn’t the relative distance between us and another object on the other side of the universe necessarily have to move and speeds well beyond the speed of light? 3. What is earth’s distance from the point of origin of the Big Bang? 4. Are we on the outer edge of the universe, or somewhere in the middle? 5. Was the Big Bang one large explosion like a stick of dynamite, or was it more like a firework which released all of its matter and energy over the course of some quantifiable time?

The only thing that would make sense to me about a photon being observed at a distance greater than 27 billion light years away would be if the star that emitted that photon was already at a distance from us here on Earth (say, 20 billion light years away) instantaneously after the Big Bang happened; and was traveling away from us here on earth at the speed of light. Then the next question would be, what tool would be able to measure or even observe that photon?

Too many questions… too few answers…


This is due to the expansion of space. If a very distant object emitted light a long time ago, that light would have to travel across expanding space to reach us. By the time the light hits our telescope, the distance between us and the object has increased because of the expansion of space. In the circumstance you mentioned, the light has not actually travelled for 45 billion years because that clearly violates causality. It is just that the distance between us and the object that emitted the light is now 45 billion light years due to the expansion of space. But how can we determine this distance just by measuring light? This increase in distance can be detected by analysing the redshift in the light. Because of general relativity, we know that light gets redshifted as it moves through expanding space. We also know the spectra that elements emit when hot. If we analyse the pattern of redshifted wavelengths we are seeing, we can tell which elements initially emitted this light and compare the observed wavelengths to ones that we know these elements emit. This information is plugged into the equations of general relativity to determine the actual distance between us and the object that emitted the light.


Stars far away from us are moving away but the space in between is also expanding. There is a certain distance away where space is expanding such that the distance is increasing faster than the speed of light. There are things that are becoming invisible and will never be visible to us ever again.

Isn’t light years a measurement of distance, not time?

I used to wonder this too and actually Kurzezazaza just recently made a [video that discusses this](https://www.youtube.com/watch?v=uzkD5SeuwzM)

Space, time and distance are all intertwined. Think of the age of the universe like the distance the crow flies from a to b. That’s 13.9 billion years.
Now think of the light from the furthest star that that’s traveling towards you, traveling over the bumpy hills of warped space time going up and down and round bends and curves before it reaches your eyes. That’s 45 billion years.