The three steps astronomers used to measure the Hubble constant © NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU)
Hubble is brilliant at sending us incredible images of nebulas, but it’s not just orbiting the Earth to fill an intergalactic art gallery, it also provides important clues about how the Universe works. The latest find to be discovered using the Hubble Space Telescope is that the Universe could be expanding five to nine per cent faster than previously thought, which if confirmed will provide important clues to understanding dark matter and dark energy.
Recalculating the Hubble constant
The find, published in The Astrophysical Journal, has been made possible only by improving the accuracy of the Hubble constant, developed by Edwin Hubble as a measure of the expansion of the Universe. By comparing the brightness of Cephid stars and Ia supernovas, both of which have specific qualities of brightness, a team of astronomers led by Nobel Laureate Adam Riess were able to compare these results with the stretching of light in receding galaxies to recalculate the Hubble constant. The new value for the Hubble constant when measured in the modern Universe is 73.2 kilometers per second per megaparsec, which means that the distance between stellar objects will double every 9.8 billion years.
This is all well and good until you start comparing this new figure with the results measured by ESA’s Planck mission or NASA’s WMAP, which measure the Hubble constant from the Big Bang – they both make it nine and five per cent smaller respectively. Reiss compares this to building a bridge from both sides of a river.
"You start at two ends, and you expect to meet in the middle if all of your drawings are right and your measurements are right. But now the ends are not quite meeting in the middle and we want to know why."
Still in the dark
There are a number of possible reasons why this could be happening. The most mysterious theories are that dark energy could be pushing galaxies further away from each other at accelerating pace, or that dark matter has some weird and unexpected properties that still remain to be discovered. Another theory is that a new form of dark radiation could have formed as the cosmos expanded close to the speed of light pumping more energy into the Universe. It could also mean that Einstein’s theory of gravity is actually incomplete.
The current level of uncertainty of the Hubble constant is just 2.4 per cent, but it is hoped that once the ESA Gaia satellite, NASA’s James Webb Space Telescope and the European Extremely Large Array come into service this could be reduced even further to only one per cent.