Given how unfathomably large the universe is, it is perhaps understandable that we haven’t yet cracked all its secrets. But there are actually some pretty basic features, ones we used to think we could explain, that cosmologists are increasingly struggling to make sense of.

Recent measurements of the distribution of matter in the universe (so-called large-scale structure) appear to be in conflict with the predictions of the standard model of cosmology, our best understanding of how the universe works.

The standard model originated some 25 years ago and has successfully reproduced a whole plethora of observations. But some of the latest measurements of large-scale structure, a topic which I work on, indicate that the matter is less clustered (smoother) than it ought to be according to the standard model.

This result has cosmologists scratching their heads looking for explanations. Some solutions are relatively mundane, such as unknown systematic errors in the measurements. But there are more radical solutions. These include rethinking the nature of dark energy (the force causing the universe’s expansion to accelerate), invoking a new force of nature or even tweaking Einstein’s theory of gravity on the largest of scales.

At present, the data cannot easily distinguish between different competing ideas. But the measurements from forthcoming surveys are poised to take a giant leap forward in precision. We may be on the cusp of finally breaking the standard model of cosmology.

The early universe

To understand the nature of the current tension and its possible solutions, it is important to understand how structure in the universe formed and subsequently evolved. Much of our understanding comes from measurements of the cosmic microwave background (CMB). The CMB is radiation that fills the universe and is a leftover relic from the first few hundred thousand years of cosmic evolution after the Big Bang (for comparison, the universe is estimated to be 13.7 billion years old).

Scientists discovered the CMB by accident in 1964 (garnering them a Nobel prize), but its existence and properties had been predicted years earlier.

In excellent agreement with some of the earliest theoretical work, the observed temperature of the CMB today is an incredibly chilly 3 Kelvin (-270°C). However, at very early times, it was sufficiently hot (millions of degrees) to enable the fusion of all of the light elements in the universe, including helium and lithium, into heavier ones.

The CMB’s spectrum (light broken down by wavelength) suggests it must have been in thermal equilibrium with matter in the past – meaning they had the same distribution of energies. Matter and radiation can only reach thermal equilibrium in very dense environments. So measurements of the CMB convincingly demonstrate that the universe was once an extremely hot and dense place, with all the matter and radiation packed into a very small space.

As the universe expanded, it quickly cooled. And as it did so, some of the free electrons that existed at the time were captured by protons, forming atoms of hydrogen. This “era of recombination” happened around 300,000 years after the Big Bang. After this point, the universe was suddenly less dense so the CMB radiation was “released” to travel without impediment, and it has not significantly interacted with matter since.

As the radiation is very old, when we make measurements of the CMB today, we are learning about the conditions of the early universe. But detailed mapping of the CMB tells us a great deal more than this.

A key insight from CMB maps obtained with the Planck telescope is that the universe was also exceptionally smooth at early times. There was only a 0.001% variation from place to place in the density and temperature of the matter and radiation in the universe. If there had been more extreme variation, that matter and radiation would have been much more clustered.

These variations, or “fluctuations”, are of fundamental importance to how structure subsequently evolved in the universe. Without these fluctuations, there would be no galaxies, no stars or planets – and no life. A very interesting question is, where did these fluctuations come from?

Our current understanding is that they are a result of quantum mechanics, the theory of the microcosmos of atoms and particles. Quantum mechanics shows that empty space has some background energy which allows sudden, local changes, such as particles popping in and out of existence. The quantum nature of matter and energy has been verified to remarkable accuracy in the laboratory.

These fluctuations are thought to have been blown up to large scales in a very rapid period of expansion in the early universe called “inflation”, although the detailed mechanism behind inflation is still not fully understood.

Over time, these fluctuations grew and the arrangement of matter and radiation in the universe became more clustered. Regions that were slightly denser had a stronger gravitational pull and so attracted even more matter, which increased the density, which strengthened the gravitational pull, and so on. Regions of slightly lower density lost out, becoming emptier with time – a cosmic case of the rich getting richer and the poor getting poorer.

The fluctuations grew to such an extent over time that galaxies and stars started to form, with galaxies being distributed in and along the familiar filaments and nodes that make up a “cosmic web”.

The standard explanation

The rate at which fluctuations grow over time, and how they are clustered in space depends on several factors: the nature of gravity, the constituent components of matter and energy in the universe, and how these components interact (both with themselves and with each other).

These factors are encapsulated in the standard model of cosmology. The model is based on a solution to Einstein’s general theory of relativity (our best understanding of gravity) that assumes the universe is homogeneous and isotropic on large scales – meaning it looks the same in every direction to every observer.

It also assumes that the matter and energy in the universe are composed of normal matter (“baryons”), dark matter consisting of relatively heavy and slow-moving particles (“cold” dark matter) and a constant amount of dark energy (Einstein’s cosmological constant, denoted Lambda).

Since its origin approximately 25 years ago, the model has successfully explained a great many observations of the universe on large scales, including the [detailed properties of the CMB].

And until very recently, it also provided excellent fits to a variety of measurements of the clustering of large-scale structure at late times. In fact, some measurements of large-scale structure are still very well described by the standard model and this may be providing an important clue as to the origin of the current tension.

Remember that the CMB shows us the clustering of matter (the fluctuations) at early times. So we can use the standard model to evolve that forward in time and predict what it should, theoretically, look like today. If there is a fit between this prediction and observations, that is a very strong indication that the ingredients of the standard model are correct.

The ‘S₈’ tension

What has changed recently is that our measurements of large-scale structure, particularly at very late times, have significantly improved in their precision. Various surveys such as the Dark Energy Survey and the Kilo Degree Survey have found evidence for inconsistencies between observations and the standard model.

In other words, there is a mismatch between the early time and late time fluctuations: the late-time fluctuations are not as large as expected. Cosmologists refer to this clash as the “S₈ tension”, as S₈ is a parameter that we use to characterise the clustering of matter in the late-time universe.

Depending on the particular data set, the chance of the tension being a statistical fluke may be as low as 0.3%. But from a statistical point of view, that is not enough to firmly rule out the standard model.

However, there are strong hints of the tension in a variety of independent observations. And attempts to explain it away due to systematic uncertainties in the measurements or modelling have simply not been successful to date.

For example, it had previously been suggested that perhaps energetic non-gravitational processes, such as winds and jets from supermassive black holes, could inject enough energy to alter the clustering of matter on large scales.
However, we have shown using state-of-the-art cosmological hydrodynamical simulations (called Flamingo) that such effects appear to be too small to explain the tension with the standard model of cosmology.

If the tension is indeed pointing us to a flaw in the standard model, this would imply that something in the basic ingredients of the model is not correct.

This would have huge consequences for fundamental physics. For example, the tension may be indicating that something is wrong about our understanding of gravity, or the nature of the unknown substance called dark matter or dark energy. In the case of dark matter, one possibility is that it interacts with itself via an unknown force (something beyond just gravity).

Alternatively, perhaps dark energy is not constant but evolves with time, as early results from the Dark Energy Survey Instrument (Desi) may indicate. Some scientists are even considering the possibility of a new (fifth) force of nature. This would be a force of similar strength to gravity that operates over very large scales and would act to slow the growth of structure.

But note that any modifications of the standard model would also need to account for the many observations of the universe that the model successfully explains. This is no simple task. And before we jump to grand conclusions, we must be sure that the tension is real and not simply a statistical fluctuation.

The good news is that forthcoming measurements of large-scale structure with Desi, the Rubin Observatory, Euclid, the Simons Observatory and other experiments will be able to confirm if the tension is real with much more precise measurements.

They will also be able to thoroughly test many of the alternatives to the standard model that have been proposed. It may be that within the next couple of years we will have ruled out the standard model of cosmology and profoundly changed our understanding of how the universe works. Or the model may be vindicated and more reliable than ever. It’s an exciting time to be a cosmologist.

Ian G. McCarthy, Reader of Astrophysics, Liverpool John Moores University

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