Gravitational lenses could hold the key to a better estimate of the universe’s expansion

The universe is expanding but astrophysicists aren’t sure how fast the expansion is taking place, not because there are no answers, but rather because the answers they might give disagree.

Now, Simon Birrer, a postdoctoral fellow at Stanford University and the Kavli Institute for Particle Physics and Astrophysics at the Department of Energy’s SLAC National Accelerator Laboratory, and an international team of researchers have a new answer that could, once perfected with more data, help resolve the debate.

This new response is the result of revisiting a decades-old method called time lag cosmography with new hypotheses and additional data to derive a new estimate of the Hubble constant, a measure of the expansion of the Universe. Birrer and colleagues published their findings on November 20 in the journal Astronomy and astrophysics.

“It is the continuation of a great ten-year commitment of success by a great team, with a reset in some key aspects of our analysis,” said Birrer, and a reminder that “we should always reconsider our assumptions. Our recent work. it is exactly in this spirit. “

Distance, speed and sound

Cosmologists have known for nearly a century that the cosmos is expanding and have established two main ways to measure that expansion during that time. One method is the cosmic distance scale, a series of steps that help estimate the distance to distant supernovae. By examining the light spectrum of these supernovae, scientists can calculate how fast they are moving away from us, then divide by distance to estimate the Hubble constant. (The Hubble constant is usually measured in kilometers per second per megaparsec, reflecting the fact that space itself is growing, so that more distant objects are moving away from us faster than closer objects.)

Astrophysicists can also estimate the constant from ripples in the cosmic microwave background radiation, or CMB. Those ripples come from sound waves traveling through plasma in the early universe. By measuring the size of the ripples they can deduce how long ago and how far the CMB light we see today was created. By drawing on well-established cosmological theory, researchers can then estimate how quickly the universe is expanding.

However, both approaches have drawbacks. Sound wave methods rely heavily on how sound traveled in the early universe, which in turn depends on the particular mix of types of matter at that time, how long the sound waves have traveled before leaving their imprint on the CMB, and from the assumptions about the expansion of the universe since that time. Meanwhile, the cosmic distance scale methods put together a series of estimates, starting with radar estimates of the distance to the sun and parallax estimates of the distance to pulsating stars called Cepheids. This introduces a chain of calibrations and measurements, each of which must be sufficiently precise and accurate to ensure a reliable estimate of the Hubble constant.

A lens from the past

But there is a way to measure distances more directly, based on what are called strong gravitational lenses. Gravity bends spacetime itself and with it light travels the cosmos. A special case is when a very massive object, such as a galaxy, bends light from a distant object around it so that the light reaches us along multiple different paths, effectively creating multiple images of the same background object. A particularly beautiful example is when the distant object varies over time, for example, as do accreting supermassive black holes, known as quasars. Since light travels slightly different amounts of time along each path around the moving galaxy, the result is multiple slightly out of sync images of the same flicker.

This phenomenon is more than cute. As early as the 1960s, students of Einstein’s theory of gravity, general relativity, demonstrated that they could use strong gravitational lenses and bending light to more directly measure cosmic distances, if they could measure relative time along each path with sufficient accuracy and if they knew how matter was distributed in the slow galaxy.

Over the past decade, Birrer said, the measurements have become accurate enough to take this method, time-delayed cosmography, from idea to reality. Subsequent measurements and a dedicated effort by the H0LiCOW, COSMOGRAIL, STRIDES and SHARP teams, now under the joint organization TDCOSMO, culminated in an accurate measurement of the Hubble constant at approximately 73 kilometers per second per megaparsec with an accuracy of 2 %. This is in agreement with the estimates made by the local distance scale method, but in tension with the measurements of the cosmic microwave background according to the assumptions of the standard cosmological model.

Mass distribution hypothesis of the galaxy

But something was wrong with Birrer: the galaxy structure models on which previous studies were based may not have been accurate enough to conclude that the Hubble constant was different from estimates based on the cosmic microwave background. “I went to my colleagues and said, ‘I want to conduct a study that doesn’t build on these assumptions,'” Birrer said.

In their place, Birrer proposed investigating a range of additional gravitational lenses to make a more observational estimate of the mass and structure of slow galaxies to replace the previous hypotheses. The new avenue Birrer and the team, TDCOSMO, were undertaking was deliberately kept blind – meaning the entire analysis was performed without knowing the resulting result on the Hubble constant – to avoid experimenter bias, a procedure already established in the previous analysis of the team and an integral part of moving forward, Birrer said.

Based on this new analysis with significantly fewer assumptions applied to the seven time-lag slow galaxies that the team analyzed in previous studies, the team came up with a higher value of the Hubble constant, about 74 kilometers per second for megaparsecs, but with greater uncertainty – Enough so that their value was consistent with both high and low estimates of the Hubble constant.

However, when Birrer and TDCOSMO added 33 additional lenses with similar properties – but without a variable source to work directly for time-delayed cosmography – used to estimate the galactic structure, the estimate of the Hubble constant dropped to about 67 kilometers per hour. second for megaparsec, with an uncertainty of 5%, in good agreement with the estimates of sound waves such as that of the CMB, but also statistically consistent with the previous determinations, given the uncertainties.

This substantial change doesn’t mean the debate over the value of the Hubble constant is over, far from it, Birrer said. First, his method introduces new uncertainties in the estimate associated with the 33 additional lenses added to the analysis, and TDCOSMO will need more data to confirm its results, although that data may not be far in the future. Birrer: “While our new analysis does not statistically invalidate the mass profile assumptions of our previous work, it demonstrates the importance of understanding the mass distribution within galaxies,” he said.

“We are now gathering data that will allow us to retrieve most of the accuracy we had previously gained based on stronger assumptions. Looking further ahead we will also have images of many more galaxies with lenses from the Rubin Observatory Legacy Survey of Space and it is time. to draw on to improve our estimates. Our current analysis is only the first step and paves the way for the use of these next datasets to provide a firm conclusion on the remaining problem. ”