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Dark energy is one of the biggest mysteries in science, and now we are one step closer to understanding it

More than a decade ago, the Dark Energy Survey (DES) began mapping the universe as part of an effort to find evidence that would help us understand the nature of the mysterious phenomenon known as dark energy. I am one of over 100 scientists involved in the preparation of the final DES measurement results, which were recently published at the 243rd meeting of the American Astronomical Society in New Orleans.

Estimates suggest that dark energy makes up about 70% of the observable universe, but we still do not understand what it is. Although its nature remains elusive, the impact of dark energy is felt on a grand scale, primarily in the accelerated expansion of the universe.

The announcement made in New Orleans may bring us closer to a fuller understanding of this form of energy. Among other things, it gives us the opportunity to test our observations against the concept called the cosmological constant, introduced by Albert Einstein in 1917 as a way to counteract the effects of gravity in his equations to achieve a universe that neither expands nor contracts. Einstein later removed it from his calculations.

However, cosmologists later discovered that the universe is not only expanding but also accelerating. This observation was attributed to the mysterious quantity called dark energy. Einstein’s concept of the cosmological constant could explain dark energy if it had a positive value (allowing it to correspond to the accelerating expansion of the cosmos).

The DES results are the culmination of decades of work by researchers worldwide and represent one of the best measurements of the elusive parameter called “w,” which denotes the “equation of state” of dark energy. Since the discovery of dark energy in 1998, the value of its equation of state has been a fundamental question.

This state describes the relationship between pressure and the energy density of matter. Everything in the universe has an equation of state. Its value indicates whether the substance is gaseous, relativistic (described by Einstein’s theory of relativity), or behaves like a liquid. Clarifying this parameter is the first step toward understanding the true nature of dark energy.

Our best theory predicts that w should be exactly minus one (w=-1). This forecast also implies that dark energy is the cosmological constant proposed by Einstein.

Deception of Expectations

The equation of state equal to minus one tells us that with an increase in the energy density of dark energy, negative pressure also increases. The higher the energy density in the universe, the stronger the repulsion—in other words, matter pushes against other matter, leading to a constantly expanding accelerating universe. This may seem somewhat strange as it contradicts everything we observe on Earth.

The work utilizes the most accurate sensor we have for studying the history of the universe’s expansion: Type Ia supernovae. They are a type of star explosion and serve as a sort of cosmic yardstick, allowing us to measure remarkably large distances in the universe. These distances can then be compared with our expectations. The same method was used 25 years ago to detect the existence of dark energy.

Now, the difference lies in the size and quality of our supernova sample. Using new methods, the DES team obtained data over a range of distances 20 times greater, enabling one of the most precise measurements of w, yielding a value of -0.8.

At first glance, this is not exactly minus one as we predicted. This may indicate that it is not the cosmological constant. However, the uncertainty in such measurements is significant, so with a 5% probability or a 20-to-1 odds, we can still allow for minus one. The level of uncertainty is still high enough to refrain from making definitive statements, but it is an excellent start.

For the detection of the subatomic particle Higgs boson in 2012 at the Large Hadron Collider, the probability of error was anticipated at a ratio of one million to one. By the way, our measurement may signal the end of “Big Rip” models, in which the equations of state are more negative than minus one. According to such models, the universe expands infinitely, faster and faster, and eventually tears apart galaxies, planetary systems, and even spacetime itself. It’s comforting to see that they turned out to be erroneous.

As usual, scientists want more data, and plans are already in motion. The DES results suggest that our new methods will be useful for future experiments with supernovae within the European Space Agency’s “Euclid” mission (launched in July 2023) and the new Vera Rubin Observatory in Chile. Soon, this observatory’s telescope is expected to capture the first image of the sky post-construction, providing insights into its capabilities.

Next-generation telescopes will be able to detect thousands more supernovae, helping us conduct new measurements of the equation of state and shed even more light on the nature of dark energy.

Robert Nicol, Executive Dean of the Faculty of Engineering and Physical Sciences at the University of Surrey. He is also an astronomer and cosmologist with over 30 years of experience in major astronomical sky surveys.

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