'If the model is correct, then you would imagine that the two values – what you measure today locally and the value that you infer from the early observations would agree,' says Freedman. When astronomers try to measure the Hubble Constant by looking at how nearby galaxies are moving away from us, they get a different figure. This Standard Model is one of the best explanations we have for how the Universe began, what it is made of and what we see around us today.īut there is a problem. Using these disturbances, it is then possible to measure how fast the Universe was expanding shortly after the Big Bang and this can then be applied to the Standard Model of Cosmology to infer the expansion rate today. Two competing forces – the pull of gravity and the outwards push of radiation – played a cosmic tug of war with the universe in its infancy, which created disturbances that can still be seen within the cosmic microwave background as tiny differences in temperature. These radio signals, first discovered by accident in the 1960s, give us the earliest possible insight into what the Universe looked like. We can still see this light today, but because of the distant parts of the universe zooming away from us the light has been stretched into radio waves. Most descriptions of the Hubble Constant discrepancy say there are two ways of measuring its value – one looks at how fast nearby galaxies are moving away from us while the second uses the cosmic microwave background (CMB), the first light that escaped after the Big Bang. Part of the problem is that the Hubble Constant can be different depending on how you measure it. Today's estimates put it at somewhere between 67 and 74km/s/Mpc (42-46 miles/s/Mpc). Over a century since Hubble's first estimate for the rate of cosmic expansion, that number has been revised downwards time and time again. 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. 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. 'From my perspective as a scientist, this feels more like putting together a puzzle than being inside of an Agatha Christie style mystery.' 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.

'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. Either the measurements are wrong, or there is something flawed about the way we think our Universe works.īut 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. 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. Unfortunately, the more astronomers measure this number, the more it seems to defy predictions built on our understanding of the Universe. From our perspective, what this means is the further away a galaxy is from us, the faster it is receding. 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. It helps to think about the Universe like a balloon being blown up. 'The Hubble Constant sets the scale of the Universe, both its size and its age.' '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. One property that astronomers have tried to use to help them do this, however, is a number known as the Hubble Constant. 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. Since the Universe burst into existence an estimated 13.8 billion years ago, it has been expanding outwards ever since.

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. But this is really just our best guess – nobody knows exactly how big the Universe really is.

That's a diameter of 540 sextillion (or 54 followed by 22 zeros) miles. When we look in any direction, the furthest visible regions of the Universe are estimated to be around 46 billion light years away. Let's start by saying the Universe is big.

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