How supernovae help unravel the mysteries of dark energy

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

The best and most accurate observations that cosmologists have collected in recent decades show that all the matter around us, every single atom we see anywhere in the cosmos, makes up just 5 percent of everything that exists. Another 27 percent is dark matter, which holds galaxies together. And everything else – a whopping 68 percent of the universe – is dark energy, a force responsible for the expansion of the universe.

Without dark energy, the rate of expansion would decrease over time. But it is very clear that this is not the case, and the pace 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.

As much as 68 percent of the universe consists of dark energy.

It is the largest component of the universe and it 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, researchers presented data a decade in the making from 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 especially useful to astronomers because they have a very 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 things about dark energy.

a:hover]:shadow-highlight-franklin dark:[&>a:hover]:shadow-highlight-franklin [&>a]:shadow-underline-black dark:[&>a]:shadow-underline-white”>Subtle effects on a large scale

Dark energy may make up a large part of the universe, but its effects are subtle. To discover its influence, researchers must look at huge data sets showing the movements of galaxies on large scales. It will take very precise instruments to detect the kind of widespread effects that dark energy has on the movements of galaxies.

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

Dark energy may make up a large part of the universe, but its effects are subtle.

Most previous research into dark energy using supernovae was done using a technique called spectroscopy, which splits light from a supernova into wavelengths. By looking for the wavelengths of light that are absent, scientists can deduce which wavelengths have been absorbed – which tells you 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 that requires the use of a specialized telescope such as the James Webb Space Telescope.

The recent study 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 light from objects and tracked how the brightness changed over a period of a few weeks, creating data called a light curve.

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

The machine learning aspect was critical 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 might not be able to see,” Vincenzi said, and is also much faster.

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

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This impressive data set allowed researchers to understand more about the expansion of the universe than ever before, and the findings support a widely held model of the universe that is truly bizarre.

The strangeness has everything to do with the density of dark energy. To understand why that’s important, it helps to start thinking about something more familiar: the subject 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,” explains Dillon Brout of Boston University, who co-led the cosmological analysis.

“As the universe expands, the volume of the universe increases. But the amount of matter is not.”

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

This means that dark energy appears to be a property of space itself, which is why it is also called the energy of the vacuum. “If you get more space, you get more dark energy. As the universe expands, you get just the right amount of dark energy, because it is a property of space itself,” says Brout.

Dark energy is unlike anything else we know in nature, so some people are skeptical of the theory and believe there must be another explanation for the rate of expansion of the universe, such as something called general relativity is 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’ve made. The new research doesn’t definitively prove this theory to be true, but it is consistent with it.

“This has been mind-boggling to anyone who has worked in the field over the last 20 years,” Vincenzi said. “Because it is a form of energy that is very difficult to reconcile with any previous knowledge of energy and the forces that we are used to thinking about in physics.”

Dark energy can be thought of as one side of a cosmological coin, while dark matter is the other. The two forces work against each other: one pushes things apart and the other pulls them together.

“Matter and dark matter influence the universe with their gravity. Dark matter therefore tends to slow down the expansion of the universe, while dark energy tends to accelerate expansion,” says Brout. “So it really seems like a tug-of-war between dark matter and the pull of gravity, and the repulsiveness of dark energy.”

“This has been mind-boggling to anyone who has worked in the field for the past 20 years.”

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

“Dark energy dominates in the parts of the universe that are largely empty, across the vast distances between galaxies that are largely filled with empty space. In areas of the Milky Way 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 explains.

That’s part of the reason dark energy is so difficult to study: researchers have to look at the large-scale motions of galaxies to see its effects.

If this all seems counterintuitive and strange, buckle up, because there’s even more strangeness to be discovered in this story.

Although scientists know that there is an enormous amount of dark energy in the universe, its effects are relatively small. Even though it drives the expansion of the universe, which is certainly not unimportant, there has long been a problem in cosmology where its effects are weaker than theory predicts they should be: a lot of weaker.

In fact, the predictions of quantum mechanics, the most widely held theory of how matter works at the atomic scale, state that dark energy should be orders of magnitude stronger than it already is.

“If dark energy is the energy of the vacuum, the value we find is 120 orders of magnitude off the theoretical expectation of quantum mechanics. And that’s just crazy,” Brout said. “It has been called the largest discrepancy between theory and observations in all of science.”

But if dark energy were as powerful as quantum mechanics predicts, the material in the early universe would be scattered, preventing the formation of early galaxies. The development of life as we know it is demonstrably dependent 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 physics’ “most embarrassing problem.”

For dark energy researchers, however, that staggering discrepancy is what makes the subject so compelling and crucial to study.

“We measure dark matter and dark energy, which make up 95 percent of the universe,” says Brout. “And boy, if we don’t understand 95 percent of the universe, we’re going to have to search and try to understand it.”

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