The dominant model of the universe is collapsing | Mint

In Arizona, at Kitt Peak National Observatory, a telescope has spent three years creating a three-dimensional map of the sky. Examining the light from millions of galaxies, the Dark Energy Spectroscopic Instrument (DESI) has found something surprising.

DESI, as its name suggests, is a tool to investigate the nature of dark energy, a mysterious entity that makes up 68% of everything in the universe and that tears space apart in a repulsive version of gravity. Although they don’t know what it is, scientists have until now assumed that the density of dark energy has been the same since the very beginning of the universe, 13.7 billion years ago. But early results from DESI suggest that this assumption may be wrong. Perhaps, say DESI scientists, the density is changing over time. “It’s very bizarre,” says Dragan Hütterer of the University of Michigan, who was involved with the work. If the findings prove correct, it would put cosmology in a crisis.

The study of dark energy is surprisingly new. Direct evidence of its existence didn’t emerge until 1998, when scientists discovered that extremely bright exploding stars, called supernovae, were moving away from Earth much faster than previously thought. Their conclusion: Not only was the universe expanding, but the expansion was accelerating. “People didn’t expect that,” says Adam Riess of Johns Hopkins University, who shared the 2011 Nobel Prize in physics for the discovery.

Because it is difficult to study directly, the true nature of dark energy is still not well understood. The leading hypothesis is that it is the energy inherent in the vacuum of empty space. According to quantum theory, the vacuum is not actually empty, it is filled with countless pairs of particles and antiparticles that emerge from the void, only to annihilate each other. These interactions generate a “vacuum energy” that, on the scale of the universe, can tear space apart. The idea is not without its problems – when physicists try to calculate how much this vacuum energy density would be, they get values ​​between 60 and 120 orders of magnitude higher than what is currently supported by observational evidence – a failure known as the vacuum disaster. “The consensus is that [catastrophe] “This will require fundamental new insights,” says Dr. Huterer.

Vacuum catastrophes aside, dark energy has now become one of the two central pillars of the standard model of cosmology, the best scientific explanation of the evolution of the universe. The other pillar is dark matter, an invisible form of matter that makes up 27% of the universe. Regular matter, which forms stars and galaxies, accounts for a mere 5%. The standard model says that, after the Big Bang set the universe’s expansion in motion, gravitational attractions between atoms first led to the formation of stars and galaxies, while also acting as a brake on the universe’s overall growth. However, as the amount of empty space grew, so did the amount of dark energy and, eventually, it took over as the primary influence on the universe’s evolution, leading to the accelerated expansion that Dr. Riess observed a quarter of a century ago.

This expansion of the universe is expected to continue forever, with galaxies eventually moving out of sight of one another, an event known as the Big Freeze. But if, as DESI suggests, the density of dark energy can change, other scenarios come into play: ever-denser dark energy could one day tear apart atoms and even the fabric of spacetime, an event known as the Big Rip. Conversely, decreasing density dark energy could cause matter and gravity to once again take hold of the universe, sending the universe into a reverse Big Bang, known as the Big Crunch. (Earthlings need not worry too much—the Sun will swallow the innermost planets of the solar system before either fateful event occurs.)

DESI’s initial findings were announced at the American Physical Society’s annual meeting in California in April, followed soon after by a number of papers published on arXiv, a preprint server. These papers included data from the first year of DESI’s five-year survey. Faced with the task of capturing an invisible target, DESI had to find creative, indirect ways to search for signs of dark energy. The instrument’s main task is to map the distribution of galaxies in space. This map contains traces of sound waves that passed through the early universe. These patterns have evolved as the universe expanded due to dark energy. Analyzing the most distant imprints gives cosmologists a way to look back in time, allowing them to chart the evolution of dark energy over billions of years.

time of great crisis

The DESI results not only show that the density of dark energy has changed over time. According to Dr. Hütterer, what happened is even more strange: the density kept increasing until about 4 billion years ago and then began to decrease (see chart). No one can explain why this happened.

If the DESI team’s results are correct, it would mean a complete reevaluation of what dark energy might be. [dark] “The energy changes over time, it’s not vacuum energy anymore,” says Bhuvanesh Jain, a cosmologist at the University of Pennsylvania. Alternative proposals already exist, centering on a dark-energy field called quintessence, which pervades all of space and can change over time. However, Dr. Jain says the DESI results as they stand now point to something more complex than the simplest quintessence model.

This would also mean that the standard model of cosmology, in its current form, is dead. So, it is no wonder that the DESI results are causing concern. But these are not the only troubling flaws in the model. For example, some astronomers have noticed that matter in the nearby universe is less agglomerated than it should be according to the standard model and that the early universe does not seem to be as homogeneous as it should be according to the standard model’s predictions.

Furthermore, over the past decade different teams have measured different values ​​for the Hubble constant, the rate at which the universe is currently expanding (it is named after Edwin Hubble, an American astronomer who discovered that galaxies are moving away from Earth at a velocity proportional to their distance). This would mean that cosmologists don’t really understand the historical expansion of the universe – or, by extension, how dark energy has behaved over that time. However, recent observations from the JamesWebb space telescope collected by Wendy Freedman of the University of Chicago and her team seem to suggest that these values ​​can be reconciled, meaning there is nothing unexpected in the behaviour of dark energy. However, the results have not yet been published in a scientific journal, so not all sides in the debate are convinced.

All these problems have led some cosmologists to advocate radical solutions – for example, adopting more flexible notions of dark energy, or working on alternatives to the standard model of cosmology. Some go so far as to say that Albert Einstein’s general theory of relativity, on which the model is based, has reached its limits. “We know that sooner or later, it will fail. It happened to Newton, it will happen to Einstein too,” says Andreu Font-Ribera, a cosmologist at the Institute for High Energy Physics in Barcelona and another member of the DESI team. This would not mean that Einstein was wrong, but only – even if it is a small consolation – completely right. Just as Isaac Newton’s law of universal gravitation was shown to be an approximation of general relativity under the right conditions (i.e., at relatively short distances and in low gravitational fields on and around Earth), general relativity could also turn out to be the limiting case of some deeper, as-yet-undiscovered theory.

At the moment, all talk of changing the standard model of cosmology, let alone general relativity, is driven by hints and conjecture. But as the next generation of telescopes and observatories begin to generate data, a new, more complete picture of dark energy’s role in the universe may emerge. For example, the Vera Rubin Observatory in Chile will also map the expansion of the universe over time and create a map of the evolution of the universe over the past several billion years. It will begin observing the sky next year. The European Space Agency’s Euclid, a space telescope, is already in orbit and creating its own map of galaxies. It is also aiming to track dark energy through measurements of the universe’s expansion. “You feel like the clues are almost there,” says Dr. Rees. “I’m just waiting for someone very smart to put these puzzle pieces together.”

Curious to learn about the world? Sign up to enjoy our mind-broadening science coverage Just ScienceOur weekly subscriber-only newsletter.

© 2024, The Economist Newspaper Limited. All rights reserved. From The Economist, published under licence. Original content can be found at www.economist.com.