Technological Experiments to Relieve (Cosmic) Tension

Thanks to a dizzying growth in tools for observing and measuring the universe, as well as a number of new advances (primarily the “discovery” of what we call dark matter and dark energy), all within the context of general relativity, the 2000s Early on, nothing seemed capable of doing this.

Although we know there is still much to discover, the apparent consistency between our observations, calculations, and theoretical frameworks suggests that our understanding of the universe will continue to grow significantly.

However, thanks to increasingly sophisticated observations and calculations, an apparently small “glitch” in our understanding of the universe is proving capable of jamming seemingly perfectly oiled gears. Initially, it was thought that the problem could be solved through more precise calculations and measurements, but this was not the case.

The “cosmic tension” (or Hubble tension) is the difference between the two ways we calculate the so-called Hubble parameter H₀which describes the expansion of the universe.

Hubble parameters can be calculated through the following two paths:

• Astrophysical observations are defined as local objects, that is, objects that are not very far away from us: the speed at which objects at different distances are moving away can be calculated. Expansion and H₀ In this case it is calculated by comparing speed and distance.
• The calculations are based on data from the Cosmic Microwave Background (CMB), a faint and extremely distant radiation that dates back to the early days of the universe. The information we gather at this distance allows us to calculate the expansion rate and Hubble parameter of the universe.

The H values ​​provided by these two sources are not exactly equal, but are very close and consistent₀, it seemed at the time that the two methods showed good consistency. bingo.

Around 2013, we realized “the numbers didn’t add up.” “The differences that appear may seem small, but given that the error bars on either side are getting smaller, the gap between the two measurements is getting larger,” Khalife explained.

The initial two values ​​of H₀In fact, it’s not too precise, and since the “error bars” are large enough to overlap, hopefully future finer measurements will eventually overlap. “Then the Planck experiment came along and gave very small error bars” compared to previous experiments, but still maintained a difference, dashing hopes of an easy solution to the problem.

Planck is a satellite launched into space in 2007 to collect unprecedentedly detailed images of the CMB. Results released a few years later confirmed that the disparity was real, and modest concerns turned into a serious crisis. In short: compared to the oldest and most distant universe, the nearest and most recent parts of the universe we observe tell a different story, or rather appear to follow different physical principles, which is a very different story. possible possibilities.

Many believe that if this is not a measurement problem, then it may be a flaw in the theory. The currently recognized theoretical model is called ΛCDM. ΛCDM is primarily based on general relativity—the most extraordinary, elegant, and repeatedly observed theory of the universe proposed by Albert Einstein more than a century ago—and takes into account dark matter (explained as cold and slow-moving) and dark energy as the cosmological constant.

Over the past few years, various alternative models or extensions of the ΛCDM model have been proposed, but so far none have been shown to be convincing (and sometimes even trivially testable) to significantly reduce “tension”.

“It’s important to test these different models to see what works and what can be ruled out, so we can narrow the path or find new directions,” Kaleff explained. In their new paper, he and his colleagues lined up 11 such models based on previous research, bringing some order to the theoretical jungle that has been created.

The models were tested using analytical and statistical methods on different data sets from the near and distant universe, including the latest results from SH₀ES (Supernova H₀ Equation of State) collaboration with SPT-3G (the Antarctic Telescope’s newly upgraded camera that collects CMB).This work was published in Journal of Cosmology and Astronomical Particle Physics.

Three selected models that proved to be feasible solutions in previous studies were ultimately eliminated by the new data considered in this study. On the other hand, the other three models still seem to be able to reduce tension, but that doesn’t solve the problem.

“We found that these can reduce tensions in a statistically significant way, but only because they have very large error bars and the predictions they make are too uncertain by the standards of cosmological research,” Kaleff said.

“There’s a difference between resolving and reducing: these models statistically reduce tension, but they don’t resolve it,” meaning that none of them predict large values ​​of H₀ Just look at the CMB data. More generally, in terms of tension reduction, none of the tested models proved to be superior to the other models studied in this work.

“Through our tests, we now know which models we should not consider for resolving tensions, and we also know which models we might consider in the future,” Khalife concluded.

This work can form the basis for developing models in the future, and by constraining them with increasingly precise data, we can get closer to developing a new model for our universe.