Ask a Physicist Answers
With the latest telescopes, we can look at some galaxy's some 13 billion light years away, correct?
And that is much older than the earth is thought to be. I also heard that we have seen what the universe looked like only a short time from its birth. So how can all this be true? If nothing can travel faster than light and the light from that moment in time would have passed the spot the earths current location long ago, how can we see that light? What am I missing? I can not fathom how we beat the light from the early universe to this spot. Something doesn't add up for me, so can someone break it down for me?
Asked by: Mark S. from Florida
Mark, as with many questions that stump people, the answer is not to be found down this rabbit hole. One has to take a step back and examine the premise, and realize that the question stumps you because it's the wrong question.
Keep in mind that before the Big Bang, space did not exist. There was nothing at all, in fact. The Big Bang created space. So there was no "spot" existing yet that would eventually be filled with the Earth! That "spot" only existed within the infinitesimally small speck of everything contained within the seed universe.
After the bang part of the Big Bang, space started expanding, quickly, in all directions. The location where Earth would eventually be was embedded within this rapidly growing spacetime, as was all of the residual light energy from the Big Bang (the "cosmic microwave background" or CMB).
Galaxy Cluster MACS J0647.7+7015, a galaxy seen as it was only 420 million years after the Big Bang.
Image Credit: NASA/ESA/M Postman/D Coe/CLASH Team
So it's not that the light from the Big Bang somehow "passed" the location where Earth would someday form, because the light from the Big Bang was everywhere already, and is everywhere still. It envelopes us the way air surrounds you in a room.
For a visual, imagine a balloon that isn't yet inflated. Draw a dot on it - that's Earth. Now inflate the balloon. As the balloon expands, the surface of the balloon, which we'll understand to be like early-light-filled spacetime, doesn't "pass" the black dot marking the location of Earth.
Instead, the dot stays surrounded on all sides by the surface of the balloon, despite the fact that the balloon is changing shape and size. The surface of the balloon moves, but the Earth-dot moves with it.
As the balloon (universe) expands, time progresses, and stars and galaxies start to form. Maybe one forms as a blue dot on the opposite side of the inflating balloon. Now, the speed of light is the limit dictating how fast the light from this new blue dot can travel along the surface of the balloon to the black dot where Earth is.
Since it takes time for the light to get from the blue dot to the black dot - let's say 10 million years - what we're essentially seeing is the blue dot as it looked 10 million years ago. That galaxy is going to keep producing light for billions of years, so we'll be seeing light constantly from that galaxy, but the light is always what the galaxy looked like 10 million years before right now. (Of course, if that galaxy or whatever it is only gave off light for the duration of a single second and we weren't looking in the right place at the right time, we'd miss it.)
This example isn't perfect (consider that the balloon is expanding out into already existing space, while space expanded out into a void; also, the balloon is actually a two-dimensional case whereas the universe is three-dimensional). But it hopefully gives you an idea of why asking "how do we see the light if it passed us" stumps you for the wrong reason: in short, the light from the Big Bang didn't pass us!
Kelly Chipps (AKA nuclear.kelly)
Department of Physics
Colorado School of Mines