How big our Universe really is

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.

Unfortunately, the more astronomers measure this number, the more it seems to defy predictions built on our understanding of the Universe. One method of measuring it directly gives us a certain value while another measurement, which relies on our understanding of other parameters about the Universe, says something different. Either the measurements are wrong, or there is something flawed about the way we think our Universe works.

But scientists now believe they are close to an answer, largely thanks to new experiments and observations aimed at finding out exactly what the Hubble Constant really is.

"What faces us as cosmologists is an engineering challenge: how do we measure this quantity as precisely and accurately as possible?" says Rachael Beaton, an astronomer working at Princeton University. To meet this challenge, she says, requires not only acquiring the data to measure it, but cross-checking the measurements in as many ways as possible. "From my perspective as a scientist, this feels more like putting together a puzzle than being inside of an Agatha Christie style mystery."

The first ever measurement of the Hubble Constant in 1929 by the astronomer whose name it carries – Edwin Hubble – put it at 500km per second per megaparsec (km/s/Mpc), or 310 miles/s/Mpc. This value means that for every megaparsec (a unit of distance equivalent to 3.26 million light years) further away from Earth you look, the galaxies you see are hurtling away from us 500km/s (310 miles/s) faster than those a megaparsec closer.

Over a century since Hubble's first estimate for the rate of cosmic expansion, that number has been revised downwards time and time again. Today's estimates put it at somewhere between 67 and 74km/s/Mpc (42-46 miles/s/Mpc).

bbc