One of the biggest cosmological discoveries in history was Edwin Hubble's 1925 realization that the universe is not a static place — it's expanding. Now, astronomers have used the Hubble Space Telescope, plus a collaboration of telescopes in space and on the ground, to take the most precise observations of the rate of expansion to date, but they used a strange quirk of spacetime to do it.

Strong cosmic lensing occurs when light travels from a distant point in the universe and encounters a massive object along the way. Massive objects, like galaxies, cause spacetime to bend and warp, as predicted by Einstein's general relativity. If the alignment is just right between us and the distant source of light, the massive object in between can create a spacetime "lens" that magnifies and distorts the passage of light through space.

Multiple lensed images and distorted arcs are commonly seen in deep space imagery, showcasing the phenomenon. It's a bit like holding a magnifying lens in front of a candle; get the positioning right, and the candlelight intensifies and distorts. These natural lenses have been used by Hubble in the past to amplify its magnification potential as part of the Frontier Fields project, seeing deeper into space than its conventional optics will allow.

But these cosmic lenses in spacetime can be used for other astronomical purposes, and one of them, as revealed by new research published in the journal Monthly Notices of the Royal Astronomical Society, is to test a fundamental constant that describes the universe's relentless — and accelerating — expansion. The research was carried out by the wonderfully named H0LiCOW collaboration.

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An interesting detail with cosmic lenses is that they're not perfect. In other words, light from the same distant source (like an ancient quasar) may take different paths along different regions of warped spacetime. Rather than a single lens, there are many different lenses clustered together of different magnifications. (This is especially true if the lens is being caused by an uneven galaxy or galaxy cluster.) In this scenario, Hubble will see multiple images of the same distant quasar, and as each image has passed through a different lens, they've taken a different length of time to be observed from Earth. Some examples are shown here:

Five lensed quasars and the foreground galaxies studied by the H0LICOW collaboration (ESA/Hubble, NASA, Suyu et al.)

Now, for the bright quasars observed by Hubble, it is known that these highly active galactic cores flicker in brightness and these flickers will be delayed from lensed image to image. Using flicker time delay as a measure, this new research has been able to get a very precise measurement of the rate of cosmic expansion, confirming previous measurements of the Hubble constant — a number that defines the rate of cosmic expansion.

"Our method is the most simple and direct way to measure the Hubble constant as it only uses geometry and General Relativity, no other assumptions," said astronomer Frédéric Courbin, of the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland.

By using this method, the researchers have been able to measure the Hubble constant to a precision of 3.8 percent — the most precise measurement yet made. "An accurate measurement of the Hubble constant is one of the most sought-after prizes in cosmological research today," said collaborator Vivien Bonvin, also from EPFL.

Previous measurements of the Hubble constant have used Cepheid variable stars as "standard candles" to measure cosmic distances and derive the rate of expansion — these stars vary in brightness in a predictable way, making them ideal beacons to study. This new study is in agreement with these previous measurements of the Hubble constant, only more precise, and confirms that the universe is expanding faster than cosmic models predict, compounding a mystery.

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Observations made by the European Planck observatory of the cosmic microwave background (CMB) radiation at the furthest-most reaches of our universe agree with theories of universal evolution. Planck's measurements of the Hubble constant represent the ancient universe shortly after the Big Bang, whereas the Hubble Space Telescope measurements represent the state of our universe billions of years later and show a universe that is expanding faster than expected — a mismatch that suggests something isn't right with our understanding about how the cosmos works.

"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 lead researcher Sherry Suyu, of the Max Planck Institute for Astrophysics in Germany, the ASIAA in Taiwan and the Technical University of Munich.

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