Dark energy is one of the biggest puzzles in science, and now we’re one step closer to understanding it

More than ten years ago, the Dark Energy Survey (DES) began mapping the universe to find evidence that could help us understand the nature of the mysterious phenomenon known as dark energy. I am one of more than 100 participating scientists who helped produce the final DES measurement, which was just released at the 243rd meeting of the American Astronomical Society in New Orleans.

Dark energy is thought to make up almost 70% of the observable universe, but we still don’t understand what it is. Although its nature remains mysterious, the impact of dark energy is felt on a large scale. Its main effect is to stimulate the accelerating expansion of the universe.

The New Orleans announcement may bring us closer to a better understanding of this form of energy. Among other things, this allows us to test our observations against an idea called the cosmological constant, which was introduced by Albert Einstein in 1917 as a way of counteracting the effects of gravity in his equations to achieve a universe that neither expands nor shrinks. Einstein later removed it from his calculations.

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

The DES results are the culmination of decades of work by researchers around the world and provide one of the best-ever measurements of an elusive parameter called “w,” which stands for 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 ratio of pressure to energy density for a given substance. Everything in the universe has an equation of state.

Its value tells you whether a substance is gas-like, relativistic (described by Einstein’s theory of relativity) or not, or whether it behaves like a liquid. Working out this figure is the first step towards truly understanding the true nature of dark energy.

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

Read more: Euclid spacecraft will change the way we see the ‘dark universe’

Subverting expectations

A minus one equation of state tells us that as the energy density of dark energy increases, so does the negative pressure. The greater the energy density in the universe, the more repulsion there is – in other words, matter pushing against other matter. This results in an ever-expanding accelerating universe. It may sound a bit strange as it goes against everything we experience on Earth.

The work uses the most direct probe we have of the universe’s expansion history: Type Ia supernovae. These are a type of stellar explosion, and they act as a kind of cosmic yardstick that allows us to measure amazingly large distances far out in the universe. These distances can then be compared to our expectations. This is the same technique that was used to discover the existence of dark energy 25 years ago.

The difference now is in the size and quality of our supernova sample. Using new techniques, the DES team has 20 times more data over a wide range of distances. This allows one of the most precise measurements of w, giving a value of -0.8

At first glance, this is not the exact minus one value we predicted. This may mean that it is not the cosmological constant. However, the uncertainty of this measurement is large enough to allow minus one at a 5% chance, or betting odds of only 20 to 1. This level of uncertainty is still not good enough to tell either way, but it is great start.

The discovery of the Higgs Boson subatomic particle in 2012 at the Large Hadron Collider required a million-to-one chance of being wrong. However, this measurement may signal the end of “Big Rupture” models that have equations of state that are more negative than one. In such models, the universe would expand infinitely at faster and faster rates—eventually tearing apart galaxies, planetary systems, and even space-time itself. That’s a relief.

As usual, scientists want more data, and those plans are already underway. The DES results suggest that our new techniques will work for future supernova experiments with ESA’s Euclid mission (launching July 2023) and the new Vera Rubin Observatory in Chile. This observatory should soon use its telescope to take the first image of the sky since construction, which will give an idea of ​​its capabilities.

These next-generation telescopes could detect thousands more supernovae, which will help us make new measurements of the equation of state and shed even more light on the nature of dark energy.

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