The Hubble constant calculated using gravitational lenses

The quasar HE0435-1223 seen through a gravitational lens that creates four images of it (Image ESA/Hubble, NASA, Suyu et al.)
The quasar HE0435-1223 seen through a gravitational lens that creates four images of it (Image ESA/Hubble, NASA, Suyu et al.)

A series of articles about to be published in the journal “Monthly Notices of the Royal Astronomical Society” describes various aspects of a new calculation of the Hubble constant, the value indicating the rate of expansion of the universe. A team of the H0LiCOW collaborative used the Hubble Space Telescope and other telescopes to measure the Hubble constant using the effect of gravitational lensing of 5 galaxies.

The astronomer Edwin Hubble wasn’t the first to suggest that the universe was expanding and to calculate the speed of that expansion but his studies became a reference point for astronomers and for that reason that expansion rate was called the Hubble constant. Over the years various methods have been used to calculate the value of the Hubble constant using Cepheids variables and supernovae as references. In this new research 5 galaxies were used with their gravitational lensing effect.

The choice of those 5 galaxies is due to the fact that their position is almost exactly between the Earth and 5 quasars, active galactic nuclei and consequently very bright, ideal for this type of research. The force of gravity of those galaxies curves the light that passes in its vicinity and in this way can create a magnifying glass effect that for this reason is called a gravitational lens.

An international team of astronomers of the H0LiCOW collaboration (H0 Lenses in COSMOGRAIL’s Wellspring), led by Sherry Suyu, exploited the fact that the gravitational distortions created by the galaxies used for the measurements are not completely spherical and that the gravitational lenses and the quasars are not aligned with precision.

In essence, the light of the quasar takes slightly different times to follow different trajectories around the gravitational lenses. Their brightness varies with time and the astronomers obtain images that have different delays depending on their route. Those delays are directly related to the value of the Hubble constant, therefore their measurement allows to calculate its value.

According to the H0LiCOW collaborative team, the value of the Hubble constant is 71.9 ± 2.7 kilometers per second per megaparsec. It’s a value very close to that calculated in recent years using Cepheids variables and supernovae as references but it’s much higher than that calculated using data collected by ESA’s Planck Surveyor, which is 66.93 ± 0.62 kilometers per second per megaparsec.

In simple words, according to this research and those that provided results consistent with it the universe is expanding at a faster rate than expected. This also supports another research published in June 2016 in “The Astrophysical Journal”. An important factor is that the calculation based on data collected by the Planck Surveyor was carried out by studying the cosmic microwave background radiation in the early universe’s history.

Research of this type cross path with those about dark matter and dark energy because they concern the models of the universe evolution. Scientists specializing in various disciplines are seeking information to improve these models by using telescopes, particle accelerators and other instruments that support the theoretical research. The measurements of the Hubble constant are part of the information they need.

The 5 quasars seen through gravitational lenses (ESA/Hubble, NASA, Suyu et al.)
The 5 quasars seen through gravitational lenses (ESA/Hubble, NASA, Suyu et al.)

Leave a Reply

Your email address will not be published. Required fields are marked *