How supernovae are helping to unlock the mysteries of dark energy

The largest component of the universe is something we know almost nothing about.

The best and most precise observations that cosmologists have collected for decades show that all the matter around us, every atom we see anywhere in the cosmos, represents only 5 percent of everything that exists. Another 27 percent is dark matter, which holds galaxies together. And everything else (a staggering 68 percent of the universe) is dark energy, a force responsible for the expansion of the universe.

Without dark energy, the expansion rate would slow down over time. But it's very clear that's not the case, and the rate of expansion is actually increasing. There must be some kind of force driving that expansion, and that unknown force is what we call dark energy.

It is the largest component of the universe and is a mystery. But for a certain type of scientist, that makes studying it an irresistible challenge.

At a meeting of the American Astronomical Society earlier this month, The researchers presented data after a decade of preparation. of the largest and most uniform sample of supernovae ever collected. The data was part of the Dark Energy Survey, an international collaboration of more than 400 astronomers working together to unravel the mysteries of dark energy.

The analysis focused on a variety of supernovae called Type 1a. These are particularly useful to astronomers because they have a highly predictable brightness, making them invaluable as mile markers that can be used to accurately measure distances. By using these supernovae to calculate the distance to distant galaxies, scientists can measure how fast the universe is expanding and hopefully learn more about the strange matter of dark energy.

Dark energy may make up a large portion of the universe, but its effects are subtle. To detect its influence, researchers must look at huge data sets showing the large-scale motions of galaxies. Very precise tools are required to be able to detect the kind of widespread effects that dark energy has on the motions of galaxies.

“To make these super-precise measurements, you need the best cameras and the best telescopes available, on Earth or in space,” explained Maria Vincenzi of Duke University, who co-led the cosmological analysis of the DES supernova sample. “Building these types of instruments is such a monumental effort that it is something that cannot be accomplished by a single group or with the resources of a single university.”

Most previous research on dark energy using supernovae was done using a technique called spectroscopy, in which light from a supernova is split into wavelengths. By looking for wavelengths of light that are absent, scientists can infer which wavelengths have been absorbed, indicating the composition of an object.

This is extremely useful for obtaining detailed information about an object, but it is also a very expensive and time-consuming process and requires the use of a specialized telescope such as the James Webb Space Telescope.

Recent research took a different approach. “We tried to do things in a completely different way,” Vincenzi said. They used a technique called photometry, in which they observed the light from objects and tracked how the brightness changed over a period of a few weeks, producing data called a light curve.

They then fed these light curves into a machine learning algorithm, which was trained to identify the particular supernovae they wanted: Type 1a supernovae.

The machine learning aspect was key because the differences between the light curves of supernova types can be subtle. “The machine learning algorithm can see things that even a very well-trained eye couldn't see,” Vincenzi said, in addition to being much faster.

That allowed the group to identify a huge sample of about 1,500 of these supernovae in the five-year data set, collected with a single instrument called the Dark Energy Camera mounted on the Victor M. Blanco Telescope in Chile.

With this impressive data set, researchers were able to understand more than ever about the expansion of the universe, and the findings support a widely held model of the universe that is truly strange.

The rarity has to do with the density of dark energy. To understand why this is important, it's helpful to start by thinking about something more familiar: matter.

“As the universe expands, the volume of the universe increases. But the amount of matter is not. It is a constant of the total matter. So if the volume increases and the matter is constant, the density will decrease,” explained Dillon Brout of Boston University, who co-led the cosmological analysis.

So far, so good. But dark energy is not like that: it has a constant density over time. “As the universe expands, the density does not decrease. You get a correspondingly larger total amount of dark energy,” Brout said.

That means that dark energy appears to be a property of space itself, which is why it is sometimes also known as vacuum energy. “If you get more space, you will get more dark energy. “If the universe increases in size, you get just the right amount of dark energy, because it is a property of space itself,” Brout said.

Dark energy is unlike anything we know of in nature, so some people are skeptical of the theory and believe there must be some other explanation for the expansion rate of the universe, like something about general relativity being incorrect or incomplete.

But cosmologists increasingly agree that this theory of the constant density of dark energy over time, called Lambda cold dark matter, is the best explanation we have for the observations we have made. The new research doesn't definitively prove this theory to be true, but it is consistent with it.

“This has been mind-blowing for everyone working in this field for the last twenty years,” Vincenzi said. “Because it is a form of energy that is very difficult to reconcile with any prior knowledge about energy and forces that we are used to thinking about in physics.”

Dark energy can be thought of as one side of a cosmological coin, with dark matter being the other. The two forces counteract each other: one separates things and the other unites them.

“Matter and dark matter influence the universe with their gravity. So, dark matter has a tendency to slow down the expansion of the universe, while dark energy has a tendency to accelerate it,” Brout said. “So it's really like a tug of war between dark matter with the attraction of gravity and the repulsion of dark energy.”

This model means that as time goes by and the universe expands, there is more and more dark energy. At earlier points in the history of the universe, its physics was dominated by dark matter because its size was smaller and the density of matter was greater. As the universe has gotten larger, dark energy has come to dominate.

“Dark energy dominates in the parts of the universe that are mostly empty, in the great distances between galaxies that are mostly filled with empty space. In regions of the galaxy that are filled with much more matter or dark matter, such as in a galaxy or in the solar system, we do not feel or see the effects of dark energy,” Brout explained.

That's part of the reason dark energy is so difficult to study: Researchers need to observe the large-scale motions of galaxies to see its effects.

If all of this seems contradictory and strange, then buckle up, because there are even more oddities to discover in this story.

Although scientists know that there is a huge amount of dark energy in the universe, its effects are relatively small. Although it is driving the expansion of the universe, which is not inconsequential, there is a long-standing problem in cosmology where its effects are weaker than theory predicts they should be: a lot weaker.

In fact, predictions from quantum mechanics, the most widespread theory of how matter operates at the atomic scale, claim that dark energy should be orders of magnitude stronger than it is.

“If dark energy is vacuum energy, the value we find is 120 orders of magnitude below the theoretical expectation of quantum mechanics. And that's crazy,” Brout said. “It is sometimes considered the largest discrepancy between theory and observations in all of science.”

But if dark energy were as powerful as quantum mechanics predicts, then it would have spread material throughout the early universe. prevent the formation of early galaxies. Arguably, the development of life as we know it depends on the relative weakness of dark energy.

This discrepancy in the apparent value of the cosmological constant, which is part of general relativity, is an important question for cosmology. It has even been described as 'physical'”most embarrassing problem.”

For dark energy researchers, however, that puzzling discrepancy is what makes the topic so attractive and critical to study.

“We're measuring dark matter and energy, which makes up 95 percent of the universe,” Brout said. “And boy, if we don't understand 95 percent of the universe, we have to look and try to understand it.”