How supernovae are helping to uncover the mysteries of dark energy

-Gudstory

How supernovae are helping to uncover the mysteries of dark energy -Gudstory

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

The best and most accurate observations gathered by cosmologists over decades show that all the matter around us, every single atom we see anywhere in the universe, makes up only 5 percent of all that exists. The other 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 rate of expansion would slow down over time. But it is quite clear that this is not the case, and the rate of expansion is actually increasing. There must be some force driving that expansion, and we call that unknown force dark energy.

A staggering 68 percent of the universe is dark energy.

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

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

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

DES Collaboration/NOIRLab/NSF/AU

subtle effects on a large scale

Dark energy may make up a large part of the universe, but its effects are subtle. To detect its impact, researchers will have to look at huge datasets that show the movements of galaxies on a large scale. To be able to detect the widespread effects of dark energy on the movements of galaxies requires very precise instruments.

“To make these super precise measurements, you need the best cameras and the best telescopes available on the ground or in space,” explained Maria Vincenzi of Duke University, who co-led the cosmological analysis of the DES supernova sample. “Creating these types of devices is such a huge effort that it’s something that can’t be done by any one group or by the resources of any one university.”

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

Most of the previous research on dark energy using supernovae was done using a technique called spectroscopy, in which the light from the supernova is divided into wavelengths. By looking for the wavelengths of light missing, scientists can guess which wavelengths were absorbed – which tells you the composition of an object.

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

Recent research took a different approach. “We tried to do things a whole 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, generating data called light curves.

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

The machine learning aspect was important because the differences between light curves of supernova types can be subtle. “Machine learning algorithms can see things that even a very well-trained eye can’t see,” Vincenzi said, “and it’s also very fast.”

This enabled the group to identify a huge sample of about 1,500 of these supernovae in a five-year dataset, which was collected from an instrument called the Dark Energy Camera mounted on the Victor M. Blanco Telescope in Chile.

Victor M. Blanco Telescope in Chile
CTIO/NOIRLab/NSF/AURA/D. Munizag

a property of space itself

With this impressive dataset, researchers were able 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.

This strangeness is about the density of dark energy. To understand why this is important, it helps to start by thinking about something more familiar: matter.

“As the universe is expanding, the volume of the universe is increasing. But not the quantity of the substance. It is a constant of the entire substance. So if the volume is increasing and the matter is stable, the density will decrease,” explained Dylan Brough of Boston University, who co-led the cosmological analysis.

“As the universe is expanding, the volume of the universe is increasing. But not the quantity of the substance.”

so far so good. But dark energy is not like that – its density remains constant over time. “As the universe expands, the density does not decrease. You get a correspondingly larger 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 sometimes known as the energy of the vacuum. “If you get more space, you get more dark energy. If the universe grows in size, you get the right amount of dark energy, because it’s a property of space itself,” Brough said.

Dark energy is unlike anything we know in nature, so some people doubt this theory and believe there must be some other explanation for the rate of expansion of the universe, such as general relativity being inaccurate or incomplete. Something about.

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

Vincenzi said, “This has been shocking to everyone who has worked in this field for the last twenty years.” “Because this is a form of energy that is very difficult to reconcile with any previous knowledge of energy and the forces that we think about in physics.”

cosmic tug of war

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

“Matter and dark matter affect 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 speed it up,” Brout said. “So it’s really like a tug of war between dark matter with the pull of gravity and the repulsion of dark energy.”

“This has been shocking to everyone who has worked in this field for the last twenty years.”

This model means that as time passes and the universe expands, there is more and more dark energy. At earlier points in the universe’s history, its physics were dominated by dark matter because its size was smaller and the density of matter was higher. As the universe has grown larger, dark energy has become dominant.

“Dark energy dominates the parts of the universe that are mostly empty, the vast distances between galaxies that are mostly filled with empty space. In regions of the galaxy that are filled with a lot of matter or dark matter, such as the Milky Way or the Solar System, we cannot feel or see the effects of dark energy, Brough said.

This is why dark energy is so difficult to study: Researchers need to look at the large-scale movements of galaxies to see its effects.

a big discrepancy

If all this sounds counterintuitive and strange, buckle up, because there’s even more weirdness to be uncovered in this story.

Although scientists know there is a lot of dark energy in the universe, its impact is relatively small. Even if it is driving the expansion of the universe, which is hardly irrelevant, there is a long-standing problem in cosmology where its effects are weaker than theory predicts – excess weak.

In fact, predictions from quantum mechanics, the most widely believed theory of how matter operates on the atomic scale, suggest that dark energy should be orders of magnitude stronger than that.

“If dark energy is the energy of the vacuum, the value we get is 120 orders of magnitude lower than the theoretical expectation of quantum mechanics. And that’s just nonsense,” Brout said. “This is sometimes said to be the largest discrepancy between theory and observations in all of science.”

But if dark energy were as powerful as quantum mechanics predicts, it would scatter material throughout the early universe, preventing early galaxies from forming. The evolution of life as we know it is certainly 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 a major question for cosmology. It has also been called “the most embarrassing problem” in physics.

However, for dark energy researchers, that surprising anomaly is what makes the subject so compelling and important to study.

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

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