Robert Nichol: More than a decade 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 a hundred scientists who contributed to the final DES measurement, which was just released at the 243rd meeting of the American Astronomical Society in New Orleans.
It is estimated that dark energy makes 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 being felt on a large scale. Its main effect is to accelerate the expansion of the universe.
The announcement in New Orleans could bring us closer to a better understanding of this form of energy.
Among other things, it gives us the opportunity to test our observations against an idea called the cosmological constant, which was introduced by Albert Einstein in 1917 as a way to counteract the effects of gravity in his equations for creating a universe. to achieve something that neither expands nor contracts. . Einstein later removed it from his calculations.
However, cosmologists later discovered that not only was the universe expanding, but that the expansion was also accelerating. This observation was attributed to the mysterious entity called dark energy.
Einstein’s concept of the cosmological constant could actually explain dark energy if it has a positive value (allowing it to adapt to the increasingly rapid 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 measurements yet of an elusive parameter called ‘w’, which represents the ‘equation of state’ of dark energy. Since the discovery of dark energy in 1998, the value of the equation of state has been a fundamental question.
This state describes the relationship between pressure and energy density for a substance. Everything in the universe has an equation of state.
Its value tells you whether a substance is gaseous, relativistic (described by Einstein’s theory of relativity) or not, or behaves like a liquid. Working out this figure is the first step in 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 assumes that dark energy is the cosmological constant proposed by Einstein.
Subverting expectations
An equation of state of minus one tells us that as the energy density of dark energy increases, the negative pressure also increases. The more energy density in the universe, the more repulsion there is – in other words, matter pushes against other matter.
This leads to an ever-expanding, accelerating universe. It may sound a bit bizarre, because it goes against everything we experience on earth. The work uses the most direct probe we have into the expansion history of the universe: Type Ia supernovae.
These are a kind of star explosions and they act as a kind of cosmic benchmark, allowing us to measure astonishingly large distances far into the universe. These distances can then be compared with our expectations.
This is the same technique used 25 years ago to detect the existence of dark energy. The difference now is in the size and quality of our sample of supernovae.
Using new techniques, the DES team has twenty times more data, over a wide range of distances. This allows for one of the most accurate measurements of w ever, yielding a value of -0.8.
At first glance, this is not the exact minus one value we predicted. This could indicate that this is not the cosmological constant. However, the uncertainty in this measurement is large enough to allow minus one at a 5% chance, or a bet of only 20 to 1. This level of uncertainty is not yet good enough to say either way, but it’s an excellent start.
The 2012 detection of the subatomic particle Higgs Boson at the Large Hadron Collider required a one-million-to-one chance of being wrong. However, this measurement could spell the end of ‘Big Rip’ models, which have equations of state more negative than one.
In such models, the universe would expand faster and faster indefinitely, eventually pulling apart galaxies, planetary systems, and even space-time itself. That is a relief.
As usual, scientists want more data and those plans are already in full swing. The DES results suggest that our new techniques will work for future supernova experiments with ESA’s Euclid mission (launched July 2023) and the new Vera Rubin Observatory in Chile.
This observatory should soon use its telescope to take a first image of the sky after construction, giving an impression of its capabilities.
These next-generation telescopes could find thousands of additional supernovae, giving us new measurements of the equation of state and shedding even more light on the nature of dark energy.
Robert Nichol, Pro-Vice Chancellor and Executive Dean, University of Surrey
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