Gravitational lenses measure the expansion of the universe

It’s one of the big debates on cosmology: the universe is expanding, but exactly how fast? Two available measurements produce different results. Leiden physicist David Harvey adapted a third independent measurement method using the light deformation properties of galaxies predicted by Einstein. He published his findings in Royal Astronomical Society Monthly Notices.

We have known the expansion of the universe for almost a century. Astronomers have noticed that light from distant galaxies has a shorter wavelength than nearby galaxies. The light waves appear to be elongated, or shifted towards red, which means that those distant galaxies are receding.

This rate of expansion, called the Hubble constant, can be measured. Some supernovae, or exploding stars, have a well understood brightness; this makes it possible to estimate their distance from Earth and relate that distance to their redshift or velocity. For every megaparsec away (one parsec is 3.3 light years), the speed at which galaxies move away from us increases by 73 kilometers per second.

Einstein

However, increasingly accurate measurements of the cosmic microwave background, a remnant of light in the early universe, have produced a different Hubble constant: about 67 kilometers per second.

How can it be? Why the difference? Could this difference tell us something new about the universe and physics? “This,” says Leiden physicist David Harvey, “is why a third measurement came into view, independent of the other two: gravitational lenses.”

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

HoliCOW

In 1964, Norwegian astrophysicist Sjur Refsdal had an “a-ha” moment: when the concentrating galaxy is a little off-center, one course is longer than the other. This means that the light takes longer on that path. So when there is a change in the quasar’s brightness, this blip will be visible in one image before the other. The difference could be days, or even weeks or months.

This timing difference, Refsdal showed, can also be used to define distances to the quasar and target. Comparing them with the redshift of quasars, an independent measurement of the Hubble constant is obtained.

A research collaboration within the HoliCOW project used six of these lenses to narrow the Hubble constant to around 73. However, there are complications: aside from the difference in distance, the mass of the foreground galaxy also exerts an effect. delay, depending on the exact mass distribution. “You have to model that distribution, but a lot of unknowns remain,” says Harvey. Uncertainties like this limit the accuracy of this technique.

Imagine the whole sky

That could change when a new telescope first sees light in Chile in 2021. The Vera Rubin Observatory is dedicated to imaging the entire sky every few nights and is expected to image thousands of double quasars, offering the ability to even narrow the Hubble constant. further.

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

“In that case, the individual quirks of the gravitational lenses aren’t that important and you don’t have to do simulations for all the lenses.

“In the paper, I show that with this approach, the error in the thresholds of the Hubble constant is 2% when approaching thousands of quasars.”

This margin of error will allow a meaningful comparison between the different candidates for the Hubble constant and could help to understand the discrepancy. “And if you want to go below 2%, you have to improve your model by running better simulations. My guess is that this would be possible.”