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 universe can expand faster than light can travel. There’s no contradiction.
Don’t think of expansion as some blastwave from the big bang radiating into nothingness. Every point in the universe that ever will exist already exists, and has existed since the big bang. All that’s happening is that the distance between any two given points is increasing.
The universe is expanding.
This means the stuff at the edge of the observable universe is a lot further away now than it was 13+ billion years ago, when it emitted the light we’re seeing.
We can’t see *anything* as it is right now. You’re reading what this message was on your screen a nanosecond in the past (probably several nanoseconds if you’re on a desktop PC).
This is a common question. You need a mental leap here – light cannot travel faster than the ultimate limit, in a vacuum. BUT, space itself is expanding during these 13.8 billion years.
Objects aren’t just moving *through* space – the fabric of space (which is part of the same bang as the matter) has been stretching at the same time.
The fabric of space-time can expand much faster than the speed of light. During the early universe (before stars and galaxies) there was an expansion period where the space-time of the universe expanded exponentially faster than the speed of light (by orders of magnitude).
This pushed all the “materials” that would later condense to form stars and galaxies farther away then light would be able to travel to go to the other areas.
The observable universe is 93 billion light years, but that’s only what we can “see” there is much much more universe we can never “see” because of the early expansion period during the birth of our universe pushed it outside that boundary.
Think of the universe like a blanket, and people are pulling on the edges of the blanket from all sides. It’s a stretchy blanket. The blanket isn’t getting bigger when people stretch it, but all of the threads are farther apart.
Scientists are mostly sure that the blanket won’t rip some day.
Imagine the universe is a balloon that is slowly inflating, now imagine your beam of light is an ant crawling along it’s surface.
Lets say the ant moves 10 inches in some frame of time, and the balloon also expands to twice its size in that same time, the ant will be more than 10 inches away from where it started from.
If you ignore the expansion of the universe – the ant appears to be moving faster than it should be able to move.
If you consider that expansion though, the ant isn’t moving any faster or slower… the space around the ant is moving away from it.
It’s the same deal with light in space, the space between us and that light has expanded, which makes the distance between us and that light farther than the light itself has actually travelled.
If the universe if 13.7 billion years old, *was* there a 13.8 billion years ago? What existed before the universe? 😵💫
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)
Imagine a blanket with stars printed on it. Now let’s say you can roll a ball from one star to another. The ball can only go up to a certain speed and no faster.
Now imagine you pull on the corners of the blanket, stretching it out. The stars are now further apart.
Now imagine rolling the ball while simultaneously stretching the blanket out. While the ball is rolling, the distance between the stars is getting longer.
The stars are the source and destination. The ball is the light. The blanket is space itself.
Imagine the universe is flat with 4 corners (like a piece of paper, but made out of material that can stretch infinitely). Now on each of those 4 corners, you have a tether point it can be pulled by. Also imagine that the speed of light is 100kph and nothing can move faster than 100kph. Now imagine that something pulls at each of those corners at 90kph. Those corners will all be moving away from each-other at 180kph, twice out imagined speed of light, but none are moving faster than the speed of light. Now imagine that bit of paper is actually a balloon and the whole surface is being pulled in opposing directions at speeds less than the speed of light, but faster than half the speed of light, and you sort of have it.
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