I will explain what I mean.
Assumptions to help simplify: Point A is stationary, point b is moving away from it. And the expanding universe is moving point b away 1 light year in a year at point “a’.
Year 0 we see nothing
Year 1 we see point “b” 1 ly away
In year 1 point “b” is already 2 years away because universe expanded.
So it will take 2 years for the light from point “b” to hit observers eye in point “a” (year 3)
In that 2 years point “b” would have moved 2 more light years away.
In year 3 point “b” is observed as being 2 ly away but is 4 ly away.
In year 7 point “b” would be observed as being 4 years away. But in Year 7, the point “b” would actually be 8 ly away.
Based on this, how do we know age of universe is 13B+ light years and observable universe radius is 46B+ light years?
Since we are only at 13B years old, how does the light reach us from 46B light years away?
Also, since we haven’t been observing the process from beginning how do we know if a star was moving away from us at x light years per year or w/e? Things can change directions in those billions of years as galaxies merge or w/e, right?
In: Physics
It’s precisely because the universe is expanding, and it’s expanding faster than light. Everyone knows that nothing can travel faster than light, and it’s a common but incorrect assumption that this applies to space as well, but it doesn’t. Things in space can’t move faster than light, but space itself can and in fact is expanding faster than the speed of light. We can see this because the farther an object is from us, the faster it is receding, and at a point we call the Hubble radius, things are receding from us faster than light because space itself is expanding faster than light.
In a static universe, if light emitted 13.8 billion years ago just reached us now, that would mean that object emitted that light when it was 13.8 billion light years away and it is still 13.8 billion light years away. But we know the the universe is not static, and is in fact expanding faster the farther away from us you go. That means the object that was 13.8 billion light years away when it emitted that light is now much farther away than that. If we plot the distance vs velocity of these objects, we see that the most distant possible object that could emit light that we are now seeing is roughly 46.6 billion light years away, which is the radius of the observable universe. In other words, the sphere in light emitted from objects has had enough time to reach us.
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