Our knowledge of the world has reached a critical stage

Over the past few years, a series of controversies have rocked the established field of cosmology. In short, the predictions of the standard universe model appear to be at odds with recent observations.

There is intense debate about whether these observations are biased, or whether the cosmological model, which predicts the structure and evolution of the universe, may need to be reconsidered. Some even claim that cosmology is in trouble. Right now, we don’t know which side will win. But interestingly, we are on the verge of finding that.

To be fair, argumentation is just a common method of the scientific method. And over the years, the standard cosmological model has had its share. This model suggests that the universe is made up of 68.3% “dark energy” (an unknown substance that causes the universe’s expansion to accelerate), 26.8% dark matter (an unknown type of matter) and 4.9% ordinary atoms, accurately measured from to the microwave background – the background light from the Big Bang.

It describes very successfully a large amount of data at large and small global scales. For example, it can explain things like the distribution of galaxies around us and the amount of helium and deuterium produced in the first few minutes of the universe. Perhaps most importantly, it can also fully explain the cosmic microwave history.

This has led to it being characterized as a “concordance model”. But a perfect storm of inconsistent measurements – or “tensions” as they are known in cosmology – is now calling into question the validity of this long-standing model.

An uneasy tension

The standard model provides some ideas about the nature of dark energy and dark matter. But despite decades of intense investigation, we still seem no closer to solving what dark matter and dark energy are made of.

The litmus test is the so-called Hubble tension. This is related to the Hubble constant, which is the rate of expansion of the universe at this time. When measured in our immediate, inner universe, from the distance to the stars blowing in nearby galaxies, called Cepheids, the value is 73 km/s/Mega parsec (Mpc is a measure of distance in intergalactic space). However, when predicted theoretically, the value is 67.4 km/s/Mpc. The difference may be large (only 8%), but it is statistically significant.

Hubble’s gravity became known about a decade ago. At the time, it was thought that the investigation might be biased. For example, Cepheids, although they were very bright and easy to see, were crowded with other stars, which could make them more visible. This would have made Hubble a few percent higher than model predictions, creating tension.

With the advent of the James Webb Space Telescope (JWST), which can isolate individual stars, it was hoped that we would have an answer to this tension.

Sadly, this has not yet happened. Astronomers now use two other types of stars besides Cepheids (known as Tip of the Red Giant Branch stars (TRGB) and J-region Asymptotic Giant Branch (JAGB) stars). But while one group has reported values ​​from JAGB and TRGB stars that are very close to the value expected from the cosmic model, another group has claimed that they still see discrepancies in their observations. Meanwhile, measurements of Cepheids continue to show Hubble tension.

It is important to note that although these measurements are very accurate, they may still bias some of the effects associated with each type of measurement. This will affect the accuracy of the observations, in a different way for each type of star. Correct but incorrect measurement is like trying to have a conversation with someone who always misses the mark. To resolve disagreements between conflicting data, we need precise and accurate measurements.

The good news is that Hubble’s tug of war is now a rapidly developing story. We will probably have an answer to that within the next year or so. Improving the accuracy of the data, for example by including stars from more distant galaxies, will help solve this. Similarly, measurements of waves in spacetime known as gravitational waves will also help us pin down constants.

All this can prove a common example. Or it may imply that something is missing from it. Perhaps the nature of dark matter or how gravity acts on specific scales is different from what we currently believe. But before discounting the model, one should marvel at its unparalleled accuracy. It misses the mark by at least a few percent, while totaling more than 13 billion years of evolution.

To put it in perspective, even the clock motion of the planets in the Solar System can only be calculated with certainty for less than 1 billion years, after which it becomes unpredictable. A classic cosmological model is a strange machine.

The Hubble tension is not the only problem for cosmology. Another one, known as the “S8 gravity”, also causes problems, although not to the same extent. Here, the model has a smoothness problem by predicting that the matter in the universe should be more compact than we see – by about 10%. There are various ways to measure the “mixing” of matter, for example, by analyzing the distortion in the light from galaxies produced by the supposed dark matter intervening in the line of sight.

Currently, there seems to be a consensus in the community that uncertainty in observations should be teased out before removing the global model. One possible way to reduce this tension is to better understand the role of gas winds in galaxies, which can push out some of the matter, making it smooth.

Understanding how the dimensions of mixing at the micro-scale relate to those at the macro-scale will help. The observations may also suggest there is a need to change the way we model dark matter. For example, if instead of being made of cold, slow-moving particles, as the standard model assumes, black matter could be mixed with hot, fast-moving particles. This can slow down the growth of clumpiness in the late times of the universe, which can reduce the tension of S8.

JWST has highlighted other challenges in a typical fashion. One is that early galaxies appear to be much larger than expected. Some galaxies may be as massive as the Milky Way today, even though they formed less than 1 billion years after the Big Bang, suggesting they should be slightly larger.

However, the results against the cosmological model are less clear in this case, as there may be other possible explanations for these surprising results. The key to solving this problem is to improve the measurement of the number of stars in the galaxy. Instead of measuring them directly, which is impossible, we estimate these masses from the light emitted by the galaxy.

This step involves simplifying assumptions, which can translate into quantitative estimates. Recently, it has also been suggested that some of the light associated with the stars in these galaxies is produced by supermassive black holes. This would mean that these galaxies cannot be massive after all.

An alternative theory

So, where do we stand now? Although some of the tensions may soon be explained by more and better research, it is not yet clear whether there will be solutions to all the challenges facing the global model.

There has been a dearth of theoretical ideas on how to modify the structure though – perhaps too many, in the range of a few hundred and counting. That is a daunting task for any theorist who might want to explore them all.

The possibilities are many. Perhaps we need to change our assumptions about the nature of dark energy. Perhaps it is a parameter that varies with time, which some recent measurements have suggested. Or maybe we need to add more dark energy for example to increase the expansion of the universe at early times, or, conversely, at late times. Modifying how gravity works on larger cosmic scales (different from what is done in models called Modified Newtonian Dynamics, or MOND) may also be an option.

So far, however, none of these alternatives can explain the vast array of observations that the standard model can. Even more, some of them can help with one tension but aggravate others.

The door is now open to all kinds of ideas that contradict the most basic principles of cosmology. For example, we may need to abandon the notion that the universe is “homogeneous and isotropic” on very large scales, meaning that it looks the same in all directions to all observers and suggest there are no fixed points in the universe. Others propose changes to the theory of general relativity.

Some even think of a subtle world, one that participates with us in the act of observation, or that changes its appearance depending on whether we are looking at it or not – something we know happens in the quantum world of atoms and particles.

Over time, many of these ideas can be left to the curiosity cabinet of theorists. But at the same time, it provides fertile ground for testing “new physics”.

This is a good thing. The answer to this tension will undoubtedly come from more data. Over the next few years, a powerful combination of observations from experiments such as JWST, the Dark Energy Observatory (DESI), the Vera Rubin Observatory, and Euclid, among many others, will help us find long-sought answers.

A hint

On the one hand, more accurate data and a better understanding of the systematic uncertainty in measurements can bring us back to the comfort of standard design certainty. From its past problems, the model can emerge not only confirmed, but also strengthened, and cosmology will be a science that is accurate and correct.

But if the scales tip the other way, we will be ushered into uncharted territory, where new physics will have to be discovered. This could lead to a major paradigm shift in cosmology, similar to the discovery of the accelerated expansion of the universe at the end of the 1990s. But on this path we may have to reckon, once and for all, with the nature of dark energy and dark matter, two of the world’s great unsolved mysteries.