"Dark energy" is ripping the universe apart, but we aren't exactly sure how

For the past quarter of a century, scientists have been puzzled by the composition of our universe. Atoms, molecules, gas, dark matter, dark energy…it’s a lot to comprehend, even for the brightest minds.

Traditional matter, such as atoms and molecules, which make up everything we can see, only accounts for a mere 5% of the cosmos.

Another 25% is believed to be “dark matter,” an elusive substance that cannot be directly observed but can be detected through its gravitational influence on normal matter.

However, the biggest enigma lies in the remaining 70% — “dark energy.”

The enigma of dark energy

Discovered in 1998, this unknown form of energy is thought to be responsible for the universe’s accelerated expansion rate.

In a new study, scientists have evaluated the properties of dark energy in unprecedented detail, revealing intriguing possibilities about its nature.

To grasp the significance of this study, we must delve into the past. Over a century ago, Albert Einstein formulated the General Theory of Relativity, which showed that the universe should either be expanding or contracting.

Einstein found this concept unsettling and introduced the “cosmological constant” — a form of energy inherent in empty space — to counteract the force of gravity and maintain a static universe.

Einstein, Hubble and the expanding universe

However, when researchers like Henrietta Swan Leavitt and Edwin Hubble provided evidence that the universe was indeed expanding, Einstein abandoned the cosmological constant, calling it his “greatest mistake.”

In 1998, two independent teams of researchers made a startling discovery — the universe’s expansion was not only continuing but was also accelerating.

This finding hinted at the existence of something akin to Einstein’s cosmological constant, which we now refer to as dark energy.

Since then, scientists have employed various techniques, including studying supernovae and other cosmic phenomena, to unravel the enigma of dark energy.

Previous studies indicated that the density of dark energy within the universe remained constant, suggesting a consistent strength as the universe expanded.

The Dark Energy Survey

The Dark Energy Survey (DES) represents a monumental international effort, bringing together over 400 experts in astrophysics, astronomy, and cosmology from more than 25 leading institutions.

Orchestrated primarily by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, this collaboration has made significant strides in mapping the cosmos.

Utilizing the Dark Energy Camera, a cutting-edge 570-megapixel digital camera developed by Fermilab and funded by the DOE Office of Science, DES has successfully mapped nearly one-eighth of the night sky.

This remarkable instrument was mounted on the Víctor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory, part of NSF’s NOIRLab, in 2012. Over six years, DES scientists diligently collected data across 758 nights, contributing to a better understanding of the universe.

At the heart of the DES’s mission is the quest to comprehend dark energy and measure the universe’s expansion rate. Employing four distinct techniques, including the supernova method pioneered in 1998, DES scientists are inching closer to unraveling these cosmic mysteries.

Measuring the unmeasurable with supernovae

The measurement of dark energy and its intricate properties has been a challenging endeavor. Scientists have employed a concept called “standard candles” to gauge the composition of the universe and its rate of expansion.

To illustrate this concept, imagine standing on a long road at night with multiple light poles. Although each pole possesses the same light bulb, the ones farther away appear dimmer due to the proportional fading of light with distance.

By comprehending the brightness of the bulbs and measuring their apparent luminosity, astronomers can calculate the distance to each light pole.

Astronomers commonly employ a specific type of exploding star, known as a Type Ia supernova, as a standard candle.

These white dwarf stars typically accumulate matter from a neighboring star until they reach a critical mass of 1.44 times that of our Sun, leading to an explosive event.

By measuring the rate at which the explosion fades, scientists can determine its initial brightness, enabling them to calculate its distance from Earth.

Rich Kron, DES director and spokesperson, who is also a scientist at Fermilab and the University of Chicago, explains the significance of these findings.

“As the universe expands, the matter density decreases. However, if the dark energy density remains constant, then the total proportion of dark energy must be increasing as the universe’s volume expands. This has profound implications for our understanding of the cosmos.”

Pioneering a new approach

The Dark Energy Survey (DES) recently made significant strides in understanding our universe through the standard cosmological model known as ΛCDM, or Lambda Cold Dark Matter.

This model, which is central to our current understanding of cosmic evolution, primarily focuses on the density and type of matter, as well as the behavior of dark energy.

One of the critical methods employed by DES is the supernova technique, which effectively constrains two vital parameters: the matter density and a quantity known as ‘w’. This ‘w’ parameter is instrumental in determining whether the density of dark energy remains constant over time.

In the realm of cosmology, a constant dark energy density is a pivotal concept. If this is accurate, the value of ‘w’ should be –1, according to the ΛCDM model. However, the recent findings by the DES collaboration have sparked new discussions in the scientific community.

Results after a decade of hard work

After a decade of meticulous effort, the team, led by dedicated Ph.D. students and postdoctoral fellows, revealed their supernova results.

This moment was both a professional and emotional milestone, as described by Tamara Davis, a professor at the University of Queensland and co-convener of DES’s supernova working group: “I was shaking. It was definitely an exciting moment.”

The DES findings indicated that ‘w’ equals –0.80 +/- 0.18 when considering supernovae alone. This value intriguingly suggests that ‘w’ is not exactly –1, but close enough to be consistent with it.

When these results are combined with complementary data from the European Space Agency’s Planck telescope, ‘w’ aligns with –1 within the error margins. This finding opens the door to potential revisions in the current cosmological model.

“w is tantalizingly not exactly on –1, but close enough that it’s consistent with –1,” Davis noted. “A more complex model might be needed. Dark energy may indeed vary with time.”

The journey to a definitive conclusion about dark energy and the universe’s expansion is ongoing, and more data is needed. While the DES stopped collecting data in January 2019, the wealth of information gathered is still being analyzed.

“More than 30 people have been involved in this analysis, and it is the culmination of almost 10 years of work,” said Maria Vincenzi, a research fellow at Duke University who co-led the cosmological analysis of the DES supernova sample.

“Some of us started working on this project when we were barely at the beginning of our Ph.D., and we are now starting faculty positions. So, the DES Collaboration contributed to the growth and professional development of an entire generation of cosmologists.”

Advancements in measuring dark energy

The 2018 analysis was a pioneering effort where DES scientists combined spectral data and photometry — a method involving different filters to identify peak light fluxes — to categorize and redshift supernovae.

However, acquiring spectra was challenging due to the extensive observing time required on large telescopes. This obstacle would be impractical for future surveys like the Legacy Survey of Space and Time (LSST) at the Vera C. Rubin Observatory.

Addressing this challenge, the latest DES study introduces an innovative approach using photometry with four filters. This method not only identifies and classifies supernovae but also measures their light curves more precisely.

Follow-up spectroscopy of host galaxies using the Anglo-Australian Telescope provided exact redshifts for each supernova. This advancement in methodology is a leap forward from the Nobel-winning samples, which relied on one or two filters.

AI machine learning joins the effort

The use of advanced machine-learning techniques in this study was crucial. Among data from approximately two million distant galaxies, DES identified several thousand supernovae.

The final sample comprised 1,499 type Ia supernovae with high-quality data – the largest and deepest collection from a single telescope to date. Tamara Davis, a key contributor to the project, highlighted this achievement, saying, “It’s a really massive scale-up from 25 years ago.”

While the photometric approach has some drawbacks, such as increased classification uncertainty compared to spectroscopy, its ability to handle a much larger sample size compensates for these limitations.

Next generation of supernova surveys

The DES’s innovative techniques are setting a precedent for future astrophysical analyses. Upcoming projects like Rubin’s LSST and NASA’s Nancy Grace Roman Space Telescope are expected to build on this foundation.

“We’re pioneering these techniques that will be directly beneficial for the next generation of supernova surveys,” said Rich Kron, DES director and spokesperson.

Dillon Brout, an assistant professor at Boston University who co-led the cosmological analysis with Maria Vincenzi, expressed excitement about this achievement.

“This new supernova result is exciting because this means we can really tie a bow on it and hand it out to the community and say, ‘This is our best attempt at explaining how the universe is working.’ These constraints will now be the gold standard in supernova cosmology for quite some time.”

Dark energy theoretical models needed

In addition to these experimental observations, DES scientists emphasize the necessity for theoretical models to understand dark energy.

“All of this is really unknown territory,” Kron remarked. “We do not have a theory that puts dark energy into a framework that relates to other physics that we do understand. For the time being, we in DES are working to constrain how dark energy works in practice with the expectation that, later on, some theories can be falsified.”

The team is integrating the supernova results with other DES techniques to further refine the cosmological model. “Combining the DES supernova information with these other probes will even better inform our cosmological model,” said Davis.

Despite these advancements, the mystery of dark energy remains. “Even if we measure dark energy infinitely precisely, it doesn’t mean we know what it is,” Davis added.

“Dark energy is still out there to be discovered.” This balance of experimental breakthrough and theoretical exploration encapsulates the exciting and evolving nature of cosmological research.

What the future holds

In summary, The Dark Energy Survey, a collaborative effort by over 400 scientists, has significantly advanced our understanding of the universe’s composition, particularly the elusive dark energy which accounts for about 70% of the cosmos.

Researchers used state-of-the-art telescopes and technology to study Type Ia supernovae, exploding stars that serve as cosmic distance markers. By analyzing the brightness and decay rate of these supernovae, the team measured the universe’s expansion rate with unprecedented precision.

This survey marked a major leap in cosmology, using approximately 1,500 Type Ia supernovae to refine our knowledge of the universe’s expansion.

The survey’s findings have shed new light on the nature of dark energy, indicating that its density, represented by the value ‘w’, is -0.80 ± 0.18.

This deviates from the -1 value predicted by Einstein’s cosmological constant, suggesting that dark energy might be more complex and dynamic than previously thought.

The implication of this discovery is incredibly significant. It opens the possibility that dark energy could evolve over the universe’s lifetime.

This revelation challenges existing cosmological models and paves the way for future research to explore and understand the mysteries of the universe in greater depth.

The full study was published in the Astrophysical Journal.

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