Researchers from the University of Tokyo (Japan) recently managed to measure the expansion of the universe using innovative techniques and new data from state-of-the-art telescopes.
As reported in Astronomy and Astrophysics, their method takes advantage of the way light from very distant objects takes multiple paths to reach us. Differences in these paths help improve models of what happens on the largest cosmic scales, including expansion.
There is a major unresolved controversy in cosmology regarding the expansion rate of the universe, and its resolution could reveal new physics. Astronomers are constantly looking for new ways to measure this expansion in case there are unknown errors in the data from traditional markers such as supernovas.
The universe is big and it is getting bigger. Although its size is not known, it is known how stretchy it is. However, it is not simple, because the expansion appears faster the farther away we look. For every 3.3 million light-years (or 1 million parsecs), we see objects at that distance moving away at an increasing speed of about 73 kilometers per second. In other words, the expansion rate of the universe is 73 kilometers per second per megaparsec (km/s/Mpc), also known as the Hubble constant.
There are different ways to determine the Hubble constant, but until now, they have all relied on so-called distance scales. These are phenomena such as supernovas or Cepheid variable stars, which are thought to be understood enough that their presence, even in other galaxies, allows us to obtain precise measurements of them, including their distances. After observing enough of these stars over the decades, the Hubble constant became increasingly limited. However, there has always been some doubt about this method, so cosmologists welcome improvements.
In their latest paper, a team of astronomers, including project co-author Professor Kenneth Wong and postdoctoral researcher Eric Baek from the University of Tokyo’s Center for Early Universe Research, successfully demonstrate a method known as time-delayed cosmography that they believe can relax the dependence on distance scales and should have ramifications in other areas of cosmology.
“To measure the Hubble constant using time-delay cosmography, you need a very massive galaxy that can act as a lens,” Wong says. “The gravity of this ‘lensing’ bends light from objects behind it, so we see a distorted version of themselves. This is called the gravitational lensing effect. If conditions are right, we will see multiple distorted images, each of which will take a slightly different path to reach us, with different timings.”
By looking for identical changes in these slightly outdated images, we can measure the difference in the time they took to reach us. Combining these data with estimates of the mass distribution of distorted galactic lensing allows us to calculate the acceleration of distant objects more accurately. “The Hubble constant we measured is within the ranges supported by other estimation methods.”
As for why researchers would go to such lengths just to find a number they already know, it has to do with something important to understanding the history of the universe, which currently remains unresolved. The 73 km/s/Megapass value for the Hubble constant is correct based on observations of nearby objects, but there are other ways to measure the rate of cosmic expansion that can also look at data from the distant past, especially the radiation permeating the universe from the Big Bang, also known as the cosmic microwave background (CMB).
Therefore, when researchers use the CMB to calculate the Hubble constant, they get a lower value of 67 kilometers per second/Megaparsec. This discrepancy is called the Hubble strain, and the work of Wong, Pike and their collaborators helps clarify its nature, as there is still a question about whether it could be more than just the result of experimental error.
“Our measurement of the Hubble constant is more consistent with other current observations and less consistent with measurements of the early universe. This is evidence that Hubble stress can arise from real physics and not just from an unknown source of error in different ways,” Wong says. “Our measurements are completely independent of other methods, both in the early and late universe, so if there are systematic uncertainties in these methods, we should not be affected by them.”
“The main goal of this work was to improve our methodology, and now we need to increase the sample size to improve accuracy and determine the Hubble tensor with certainty,” Pike explains. “Currently, our accuracy is about 4.5%, and to accurately determine the Hubble constant to a level that conclusively confirms the Hubble lineage, we need to achieve an accuracy of 1% to 2%.”
The team is confident that such subtle improvements are possible. The current study used eight time-delayed lensing systems, each covering a distant quasar (a supermassive black hole that accumulates gas and dust, giving it an intense glow), and new data from state-of-the-art space and ground-based telescopes, including the James Webb Space Telescope. The team aims to increase the sample size, in addition to improving other measurements and eliminating any unjustified systematic errors.
One of the biggest sources of uncertainty is that we don’t know exactly how mass is distributed in lens galaxies. It is generally assumed that the mass follows a simple profile that matches the observations, but this is difficult to be sure, and this uncertainty can directly affect the values we calculate, says Wong.
The Hubble pedigree is important because it could signal a new era in cosmology that reveals new physics. Our project is the result of decades of collaboration between multiple observatories and independent researchers, highlighting the importance of international cooperation in science.