Cosmic lenses calculate faster than expected expansion of Universe

An artist's impression of the NASA/ESA Hubble Space Telescope in its orbit 600 km above the Earth

An artist’s impression of the NASA/ESA Hubble Space Telescope in its orbit 600 km above the Earth. Image Credit: European Space Agency

An international group of astronomers – including one from the University of Portsmouth – has used galaxies as giant gravitational lenses to calculate a faster than expected expansion of the Universe.

The team used the Hubble Space Telescope and other telescopes in space and on the ground to observe five galaxies to calculate the independent measure of the Hubble constant, the unit of measurement used to describe the expansion of the Universe.

The findings agree with other measurements of the Hubble constant in the local Universe, which used stars and supernovae as points of reference.

However, they differ from those made by the European Space Agency (ESA) Planck satellite, which measured the Hubble constant for the early Universe by observing the cosmic microwave background.

This means there is a serious discrepancy, which lies at the heart of our astronomical understanding.

Astrophysicist Dr Thomas Collett, from the University of Portsmouth’s Institute of Cosmology and Gravitation, is a member of the H0LiCOW collaboration – the group of astronomers who conducted the research.

He said: “We’ve measured how fast the Universe is expanding today, and it’s faster than we had expected. When we take measurements from the edge of the Universe made by ESA’s Planck satellite and extrapolate to today, we expect the expansion to be five per cent slower than measured from these lenses.

“Astronomers using other methods have independently got similar results that agree with ours, so this discrepancy with Planck might be a sign that the standard cosmological model is missing something new and important.”

The H0LiCOW group - Dr Thomas Collett is second from the left

The H0LiCOW group – Dr Thomas Collett is second from the left

Dr Collett developed algorithms to account for the effect of the weak deflections caused by the other galaxies near the line of sight of the primary lens. He was also responsible for combining the measured distance with the Planck constraints to test which extensions to the standard cosmological model might explain the discrepancy between the two results.

He said: “While the value for the Hubble constant determined by the ESA Planck satellite fits with our current understanding of the cosmos, the values obtained by the different groups of astronomers for the local Universe are in disagreement with our accepted theoretical model of the Universe.”

The H0LiCOW collaboration is led by Sherry Suyu, Max Planck @TUM professor at the Technical University Munich and the Max Planck Institute for Astrophysics in Germany.

Professor Suyu said: “The expansion rate of the Universe is now starting to be measured in different ways with such high precision that actual discrepancies may possibly point towards new physics beyond our current knowledge of the Universe.”

The study targeted massive galaxies positioned between Earth and very distant quasars – incredibly luminous galaxy cores. Strong gravitational lensing causes the light from the more distant quasars to be bent around the huge masses of the galaxies. This creates multiple images of the background quasar.

Dr Collett explains the method: “Each of the images of the background quasar travels on a different path past the lens galaxy and through the Universe. These paths have different lengths, so the light takes a few weeks more or less to reach us. These ‘time-delays’ can be measured because the background quasar flickers. Once we have measured the time-delays (which has taken the team several years of monitoring) we can convert them into a measurement of the Hubble constant using our model of how the matter is distributed in the lens.”

Co-lead Frédéric Courbin from École Polytechnique Fédérale de Lausanne (EPFL), Switzerland, said: “Our method is the most simple and direct way to measure the Hubble constant as it only uses geometry and General Relativity, and no other assumptions.”

Using the accurate measurements of the time delays between the multiple images, as well as computer models, has allowed the team to determine the Hubble constant to an impressively high precision of 3.8 per cent.

Professor Suyu added: “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.”

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