From Abigail Beall of BBC FUTURE...
Let's start by saying the Universe is big. When we look in any direction, the furthest visible regions of the Universe are estimated to be around 46 billion light years away. That's a diameter of 540 sextillion (or 54 followed by 22 zeros) miles. But this is really just our best guess – nobody knows exactly how big the Universe really is.
That is because we can only see as far as light (or more accurately the microwave radiation thrown out from the Big Bang) has travelled since the Universe began. Since the Universe burst into existence an estimated 13.8 billion years ago, it has been expanding outwards ever since. But because we don't know a precise age for the Universe either, it makes it tricky to pin down how far it extends beyond the limits of what we can see.
One property that astronomers have tried to use to help them do this, however, is a number known as the Hubble Constant.
"It's a measure of how fast the universe is expanding at the current time," says Wendy Freedman, an astrophysicist at the University of Chicago who has spent her career measuring it. "The Hubble Constant sets the scale of the Universe, both its size and its age."
It helps to think about the Universe like a balloon being blown up. As the stars and galaxies, like dots on a balloon's surface, move apart from each other more quickly, the greater the distance is between them. From our perspective, what this means is the further away a galaxy is from us, the faster it is receding. READ MORE
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