If the Universe is about 13.7 billion years old, and the diameter of the observable universe is 93 billion light years, how can it be that wide if the universe isn’t even old enough to let light travel that far that quickly?
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The light we receive from distant galaxies doesn’t tell us about what or where they are, but about what or where they *used to be*. Images aren’t the objects themselves, but *messages* from those objects.
Let’s say some alien species on some distant galaxy decides to send a message, a broadcast visible to any other intelligence. They engineer their entire solar system, reconfiguring planets, harvesting solar and nuclear energies, and devise the greatest transmitter the universe has ever seen.
After countless eons devoted to this single task, they release their message into the void. Their mission completed in the dying days of their star, they allow it to consume them, drawing comfort in the fact that they have made their mark on the cosmos.
The message, riding waves of electricity and magnetism, races through the cold, endless voids of the spaces between galaxies. Millions, billions of years pass, as the message crawls along and the universe grows ancient.
Finally, the alien signal washes faintly, feebly over a planet known as Earth. Our scientists separate the scrap of signal from the noise, the barest whisper that was once the proudest achievement of a long-dead race. The expansion of the universe doesn’t just separates, it *spreads*, the wavelength of the radiation increasing with every passing day as it travels. Earth scientists are able to measure the amount of that stretching, that redshifting, and can reconstruct how long the signal has been propagating.
13 billion years. For 13 billion years that message has been traveling the universe. For 13 billion years its alien creators have been dead.
But 13 billion years is a time, not a distance. Earth scientists have more work to do. They must appeal to a cosmological model, the maps the expansion history of the universe, linking the tick of a clock to a growth in the size of the cosmos.
You see, the universe isn’t static: it expands, carrying galaxies ever farther apart. What were once neighbors are now distant acquaintances, falling into strangers. And in the great perversion of cosmology, the greater the distance the greater the recession. The farther apart two galaxies grow, the faster they separate. Double the distance, double the speed of separation. Triple the distance, triple the speed of separation, and so on. And endless expansion of suffocating silence.
According to their models, the creators of the signal now reside in a galaxy that sits, approximately, 45 billion light-years away. No trick of physics, no angering of Einstein’s ghost. Just the cold mathematics of inexorable expansion. What was once close is now far away – it’s just that simple.
Indeed, the alien’s home galaxy is now so far away that it sits beyond our cosmological horizon. We will receive no further messages from them, and it is lost to our view.
After decades of work, Earth scientists decode the alien tongue. The billions of souls of humanity gather to hear their first message from another intelligence, a relic of the forgotten past and a reminder that we are not truly alone:
“We’d like to speak to you about your car insurance.”
Source: I play a cosmologist on TV (and also real life)
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