The latest measurement of the Hubble constant is higher than previous values, prompting scientists to believe there may be “new physics” beyond the standard model of cosmology yet to be discovered.
Named after the physicist Edwin Hubble, the Hubble constant is a measurement of how fast the cosmos is expanding. It provides a crucial estimate of the universe’s scale, age, and density and can be used to probe the properties of dark energy and dark matter.
Hubble noticed that galaxies were moving away at a speed proportional to their distance from Earth – also known as Hubble’s Law. The Hubble constant is the value of the proportional change in Hubble’s Law.
Estimates of the Hubble constant have changed over time. The initial figures were too high, and the approximations have vastly improved since Hubble’s day. A new paper published in the Monthly Notices of the Royal Astronomical Society shows the latest value is accurate to 3.8 per cent.
A large international team of physicists estimated the Hubble constant using a programme they call H0LiCOW (for H0 [abbreviation for Hubble constant] Lenses in COSMOGRAIL’s Wellspring). Data taken from telescopes on Earth and in space was used to look at the gravitational lensing effect between three massive galaxies wedged in between distant quasars and planet Earth.
The light from quasars is bent around the galaxies, creating multiple images of the background quasar.
Einstein’s theory of general relativity states that mass causes light to bend: the path of the light is bent by bodies' gravitational fields. Since the mass distribution around the surrounding galaxies is different, some light is bent more and takes longer to reach Earth.
The relationship between the time delays and the distance of the galaxies allows researchers to arrive at a new value for Hubble’s constant.
Previous measurements derived from the cosmic microwave background (CMB) show Planck’s Hubble constant value at 67.8±0.9 kilometres per second per megaparsec – lower than the one calculated by the H0LiCOW team at 71.9±2.7 kilometres per second per megaparsec.
“When people use the CMB from the Planck satellite, they are able to fit their data with a relatively simple model – the standard model of cosmology. But one of the things that these CMB experiments don’t give you – if you take them by themselves – is the Hubble constant.
“If you look at their results, they can give very precise measurements of Hubble’s constant after making strong assumptions based on the standard model of cosmology,” Professor Christopher Fassnacht, coauthor of the paper and researcher at University of California, Davis, told The Register.
The Hubble constant depends on specific properties of the universe. Changes in parameters – such as the number of types of neutrinos, or their masses, or the amount of dark energy – give a different picture of the cosmos and shift the constant.
The small discrepancies in the measured values of Hubble’s constant could mean the assumptions in the standard model of cosmology are wrong, Fassnacht explained. It could mean that there’s something beyond the standard model that scientists don’t know about yet.
“The Hubble constant is crucial for modern astronomy, as it can help to confirm or refute whether our picture of the Universe – composed of dark energy, dark matter and normal matter – is actually correct, or if we are missing something fundamental,” said Professor Sherry Suyu, coauthor of the paper and researcher at the Max Planck Institute for Astrophysics, Germany.
H0LiCOW’s result is closer to the value calculated by the trio of physicists who won the Nobel Prize in 2011.
Professor Saul Perlmutter from the Lawrence Berkeley National Laboratory, professor Brian Schmidt, vice-chancellor of the Australian National University, and professor Adam Riess from Johns Hopkins University discovered that the universe is expanding at an accelerated pace. ®