The structure of the universe may be “smoother” than expected, raising big questions for the Standard Model of cosmology

Given the unfathomable size of the universe, it is perhaps understandable that we have not yet uncovered all of its secrets. But there are actually some pretty fundamental features that we once thought could explain, and that cosmologists are having increasing difficulty understanding.

Current measurements of the distribution of matter in the universe (so-called large-scale structure) appear to contradict the predictions of the Standard Model of cosmology, our best understanding of how the universe works.

The Standard Model emerged about 25 years ago and has successfully reproduced a wide range of observations. But some recent measurements of large-scale structures, a topic I’m working on, suggest that the matter is less bundled (smoother) than it should be according to the Standard Model.

This result leaves cosmologists scratching their heads and looking for explanations. Some solutions are relatively trivial, such as unknown systematic errors in the measurements. But there are more radical solutions. This includes rethinking the nature of dark energy (the force accelerating the expansion of the universe), invoking a new force of nature, or even optimizing Einstein’s theory of gravity on the largest scale.

Currently, it is not easy to distinguish between different competing ideas based on the data. But the measurements from the upcoming surveys are likely to make a huge leap forward in terms of precision. We may be on the verge 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 the 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 remnant of the first few hundred thousand years of cosmic evolution after the Big Bang (for comparison, the age of the universe is estimated to be 13.7 billion years).

Scientists discovered the CMB by accident in 1964 (earning them the Nobel Prize), but its existence and properties had been predicted years before.

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

The spectrum of the CMB (light broken down by wavelength) suggests that it must have been in thermal equilibrium with matter in the past – meaning they had the same energy distribution. Matter and radiation can only reach thermal equilibrium in very dense environments. Measurements from the CMB convincingly show that the universe was once an extremely hot and dense place where all matter and radiation was contained in a very small area.

As the universe expanded, it cooled rapidly. Some of the free electrons present at the time were captured by protons and formed hydrogen atoms. This “era of recombination” occurred about 300,000 years after the Big Bang. From that point on, the universe suddenly became less dense, so the CMB radiation was “freed up” to travel unhindered, and since then it has had no significant interaction with matter.

Because the radiation is very old, measurements of the CMB today tell us something about the conditions of the early universe. But detailed mapping of the CMB tells us much more.

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

These variations or “fluctuations” are fundamental to the subsequent structural evolution of the universe. Without these fluctuations there would be no galaxies, no stars or planets – and no life. A very interesting question is: where do these fluctuations come from?

According to our current understanding, they are a result of quantum mechanics, the theory of the microcosm of atoms and particles. Quantum mechanics shows that empty space has a certain background energy that allows sudden, local changes, such as the appearance and disappearance of particles. The quantum nature of matter and energy has been confirmed in the laboratory with remarkable accuracy.

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

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

The fluctuations increased so much over time that galaxies and stars began to form, with the galaxies distributed in and along the familiar filaments and nodes that form a “cosmic web.”

The standard explanation

The rate at which fluctuations increase over time and how they accumulate in space depends on several factors: the nature of gravity, the components of matter and energy in the universe, and the way these components interact (both with each other as well as with each other). ).

These factors are summarized 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), which assumes that the universe is broadly homogeneous and isotropic – meaning it looks the same to every observer in every direction.

It also assumes that the matter and energy in the universe consists 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 theory ) exists constant, referred to as lambda).

Since its creation about 25 years ago, the model has successfully explained a variety of large-scale observations of the universe, including the [detailed properties of the CMB].

And until recently, it also provided excellent fits for a variety of measurements of the clustering of large-scale structures in later times. In fact, some measurements of large-scale structures are still very well described by the Standard Model, and this could provide an important clue to the origin of the current stress.

Remember that the CMB shows us the accumulation of matter (the fluctuations) at early times. So we can use the Standard Model to evolve this over time and predict what it should theoretically look like today. If there is agreement between this prediction and the observations, it is a very strong indication that the components of the Standard Model are correct.

That₈‘ Tension

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

In other words, there is a mismatch between the early and late fluctuations: the later fluctuations are not as large as expected. Cosmologists refer to this collision as “S₈ Tension,” as S₈ is a parameter we use to characterize the accumulation of matter in the late-period universe.

Depending on the data set in question, the probability that the voltage is a statistical coincidence can be as low as 0.3%. From a statistical perspective, however, this is not enough to definitively rule out the Standard Model.

However, there is clear evidence of tension in a number of independent observations. And attempts to explain it away due to systematic uncertainties in the measurements or modeling have simply not been successful so far.

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

If the voltage actually points us to a flaw in the Standard Model, it would mean that something is wrong in the basic components of the model.

This would have enormous consequences for fundamental physics. For example, the tension may indicate that something is wrong with 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 mere gravity).

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

Note, however, that any modifications to the Standard Model would also need to take into account the many observations of the universe that the model successfully explains. This is not an easy task. And before we jump to conclusions, we need to be sure that the tension is real and not just a statistical fluctuation.

The good news is that upcoming measurements of large-scale structures with Desi, Rubin Observatory, Euclid, Simons Observatory, and other experiments can confirm whether the stress is real with much more precise measurements.

They can also extensively test many of the proposed alternatives to the standard model. It may be that in the next few years we will have rejected the Standard Model of cosmology and fundamentally changed our understanding of how the universe works. Or the model could be confirmed and more reliable than ever. It’s an exciting time to be a cosmologist.

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

This article is republished from The Conversation under a Creative Commons license. Read the original article.