Gravity lenses measure universe expansion

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This is one of the big cosmology debates: the universe is expanding, but how fast is it really? The two available measurements give different results. Leiden physicist David Harvey adapted an independent third measurement method using the light warping properties of galaxies predicted by Einstein. They published their findings Monthly notice of the Royal Astronomical Society.

We have known about the expansion of the universe for almost a century. Astronomers noted that light emanating from distant galaxies has shorter wavelengths than galaxies. Light waves appear to be drawn or red, meaning that those distant galaxies are moving away.

This expansion rate, called the Hubble constant, can be measured. Some supernovae, or explosions of stars, are a well-understood glow; This makes it possible to estimate their distance from Earth and that distance becomes related to their redshift or speed. For each megaparsec of distance (a crossover is 3.3 light years), the speed with which galaxies move away from us increases with 73 kilometers per second.


However, increasingly accurate measurements of the cosmic microwave background, remnants of light in the very early universe, a distinct Hubble continuation: about 67 kilometers per second.

How can this happen? Why the difference? Can this difference tell us anything new about the universe and physics? “This”, says Leyden physicist David Harvey, “has come into view when a third measurement, independent of the other two: gravitational lens.”

Albert Einstein’s theory of general relativity predicts that the concentration of mass, such as a galaxy, can bend the path of light, just as a lens does. When a galaxy is in front of a bright light source, light bends around it and can reach Earth by different routes, providing two, and sometimes even four, images of the same source.


In 1964, Norwegian astrophysicist Sjur Refsdal had an “a-ha-ha” moment: when the lensing galaxy is slightly off, one route is longer than the other. This means that light takes longer than that. So when the brightness of the quasar changes, this blip will appear in one image before the other. The difference can be days, or even weeks or months.

This time difference, Refsdal has shown, can also be used to reduce distances for quasars and lenses. Comparing these with the redshift of quasars gives you an independent measurement of the Hubble constant.

A research collaboration under the Holycove project used six of these lenses to stabilize the Hubble by about 73. However, there are complexities: In addition to the difference difference, the mass of the foreground galaxy also has a delay effect depending on the exact effect. Mass distribution. “You have to model that distribution, but a lot of unknowns remain,” Harvey says. Such uncertainties limit the accuracy of this technique.

Imaging the entire sky

This may change when a new telescope in 2021 sees light for the first time in Chile. The Vera Rubin Observatory is dedicated to imaging the entire sky every few nights, and it is expected that there will be an image of thousands of double quasars, which also provide an opportunity to narrow the Hubble constant. Further.

“The problem is that modeling all those foreground galaxies individually is computationally impossible,” says Harvey. So instead, Harvey devised a method to calculate the average effect of a full distribution of up to 1,000 lenses.

Harvey says, “In that case, the individual quirks of the gravitational lens are not important, and you don’t have to do simulations for all lenses. You just have to make sure that you model the entire population.”

“In the paper, I show that with this approach, the error in the Hubble constant threshold is at 2% when you reach thousands of quars.”

This error margin will allow a meaningful comparison between multiple Hubble continuous candidates, and may help in understanding the discrepancy. “And if you want to go below 2%, you have to improve your model by doing better simulations. I guess it will be possible.”

Astronomers measure universe expansions, receive signs of ‘new physics’ (update)

more information:
David Harvey, a 4 percent measurement of H0, using a cumulative distribution of strong lensing time delays in twice the imaged Kaiser. Monthly notice of the Royal Astronomical Society (2020). DOI: 10.1093 / mantra / staa2522

Provided by Leiden Institute of Physics

Quotes: Gravitational lenses measure universe expansion (2020, November 17) Retrieved November 17, 2020, from

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