How fast is the universe expanding?
A galaxy cluster one megaparsec away from us is probably moving away from us at about 70 km/s. That’s Hubble’s law:
The basic idea is that new empty space is being created at a (roughly) constant rate throughout the universe. Therefore, the further away an object is from us, the more empty space is being created between it and us, because there’s more space. And so it appears to be moving faster.
That means stuff forty-five billion lightyears from us appears to be moving away from us at the speed of light. But that’s an illusion. It’s not really moving.
Think about somebody baking raisin bread in the oven. As the bread bakes, it expands. The raisins don’t move, but they appear to get further away from each other.
That’s why it can look like things are going away from us faster than light, when they’re really not.
(The rate of creation of empty space used to be considered constant, but we believe it’s changing. We don’t know what’s causing that change.,… but we’ve given it the name dark energy. https://en.wikipedia.org/wiki/Dark_energy)
If you want a more complete description, including some about the history of Hubble and his law, you could read this article I wrote a while back.
Source: Jonah Miller responds to a question in the Science on Google+ community. The best answers to questions will be posted as part of our #Askascientist series. Do you have science questions for us? Use the Science Outreach category to ask the science community.