THE UNIVERSE IS EXPANDING. AND WE CAN MEASURE THE SPEED OF ITS EXPANSION. THE TROUBLE IS, DEPENDING ON HOW WE MEASURE THAT SPEED, WE KEEP GETTING DIFFERENT RESULTS
The first anomaly concerns the speed at which the Universe is expanding. Astronomers determine this in two ways and herein lies the problem: the two methods yield different values.
The obvious method is to observe galaxies (the basic building blocks of the Universe) in the nearby Universe and measure how fast they’re moving away from us. They’re scattering like pieces of cosmic shrapnel in the aftermath of the Big Bang, the titanic explosion in which the Universe was born 13.82 billion years ago.
A second way of determining the expansion rate is to deduce it in the early Universe and extrapolate it to today. The primordial expansion rate is encoded in the cosmic background radiation, the ‘afterglow’ of the Big Bang, which is still around us today and accounts for 99.9 per cent of the photons, or particles of light, in the Universe.
The problem is that the expansion rate measured locally is eight per cent greater than deduced by extrapolating from the early Universe. There are those who believe there is a cosmic ingredient we’ve overlooked which has speeded things up over the past 13.82 billion years. But extraordinary claims require extraordinary proof. Does the evidence stack up?
The first thing to say is that measurements of the cosmic background radiation by the European Space Agency’s (ESA) Planck satellite have ushered in the age of precision cosmology and are considered the gold standard of astronomical observations. Thanks to Planck, not only have we learnt that the Universe is 13.82 billion years old, but also that it consists of 68.3 per cent dark energy, 26.8 per cent dark matter and 4.9 per cent ordinary atoms.
Given astronomers’ faith in Planck, all attention has focused on the local expansion measurements. These, however, are fraught with difficulties.
The expansion rate is characterised by the so-called Hubble constant. In 1929, the American astronomer Edwin Hubble discovered that the further away a galaxy is from us, the faster it’s moving away. The Hubble constant (H0) connects these two quantities, so that a galaxy that’s D megaparsecs more distant than another galaxy (where one megaparsec, or Mpc is equal to 3.26 million light-years) is receding at H0x Dkilometres per second faster. The speed a galaxy is moving away is encoded in its
