DES survey reveals clues about how complex the universe is  Urania

A decade of research reveals results consistent with standard cosmological models, but leaves open the possibility of more complex explanations.

In 1998, astronomers discovered that the universe is expanding at an accelerating rate, as a result of a mysterious force called… Dark energyWhich makes up about 70% of the universe. This discovery was surprising because it contradicts the prevailing theory that assumes that the expansion of the universe must slow down due to gravity.

Astronomers have made a breakthrough by counting certain types of exploding stars, called starbursts, from observations Type Ia supernovaeWhich led to them receiving the Nobel Prize in Physics in 2011.

25 years after the initial discovery Dark Energy Survey (DES) Publish the results of the analysis using the same technique to further explore the secrets of dark energy and the expansion of the universe. During a presentation at the 243rd Meeting of the American Astronomical Society on January 8, 2024, in condition Submitted for publication in the Astrophysical Journal, Masao Saku of the University of Pennsylvania's College of Arts and Sciences and other DES astronomers presented results confirming consistency with Standard cosmological model The universe assumes accelerated expansion. However, the presenters point out that their findings are not definitive enough to rule out a more complex model.

It was exciting to be able to 'unpack' our results and take a look at the latest and greatest quantitative measurements Dark matter And dark energy and we see that this effort reaches its end – Sako said. It was definitely an emotional moment for me.

Unique approach to analysis
To understand the nature of dark energy and measure the expansion rate of the universe, DES scientists perform analyzes using four different techniques, including the supernova technique used in 1998.

Sacco and his team began working on DES in 2010, and their main contribution to the project was developing a scanning strategy for the supernova technology. This involves selecting areas of the sky to observe, establishing an observing rhythm, and simulating a full survey to determine exactly what to expect from the imaging data.

The researchers explain that this technique requires data from Type Ia supernovae, which occur when a dead star is extremely dense, known as White dwarfIt reaches critical mass and explodes. Since the critical mass is the same for all white dwarfs, all Type Ia supernovae reach roughly the same level of brightness. Therefore, by comparing the apparent brightness of Supernova Ia as seen from Earth with its actual brightness, astronomers can determine the supernova's distance from us.

There is a strong relationship between the distance of a supernova and its proximity RedshiftIt is a measure of how quickly it is moving away from us. Combining these two values ​​reveals how quickly the universe expanded when the supernova exploded. This allows astrophysicists to compare this combined rate with the current expansion rate to determine whether the density of dark energy has remained constant or changed over time.

As the universe expands, the density of matter decreases – said Richard Krohn, DES Director. But if the density of dark energy is constant, this means that the total percentage of dark energy should increase with increasing volume.

The culmination of a decade of effort
DES collaborator Tamara Davis, a professor of astrophysics at the University of Queensland in Australia, notes that the standard cosmological model, known as… L-Clean Development MechanismIt is the leading theory that describes how the universe works. It tells us how the universe evolves using only a few features, such as the density of matter and the behavior of dark energy – She said.

Davis explained that the supernova method limits itself well to two factors: the density of matter and… ThWhich shows whether the density of dark energy is constant or not. According to the standard cosmological model, the amount of dark energy in the universe is constant, meaning it does not weaken as the universe expands. If true, the parameter is represented by a letter Th It should be -1.

The results showed that w = -0.80 +/- 0.18. Along with supplementary data from Planck telescope, Th It got closer to -1, but didn't get -1 within error.

Th It's not exactly at -1, but it's close enough to be consistent with -1 – said Davis. A more complex model may be needed, as dark energy can change over time.

Astrophysicists will need more data to reach definitive conclusions. Unfortunately, DES will not be able to provide this data as the review stopped collecting data in January 2019. However, DES collaborators are continuing their analysis using other techniques. The supernova team, made up of several graduate students and postdoctoral students, will soon be using all available observations from DES.

Combining supernova information from DES with data from other spacecraft will provide a better understanding of our cosmological model – said Davis.

The supernova team will soon be using all available DES observations. Soon, other experiments, such as the Vera Rubin Observatory, will pick up where the DES program left off, likely performing the same types of analyses. We are pioneering these technologies that will have a direct impact on the next generation of supernova research Crone said.

A pioneering new approach
The 2018 supernova analysis made several improvements over the first DES result published in 2018, which included only 207 supernovae and three years of data. Davis and Sacco say the team of scientists, led by several DES researchers, improved their analysis at every step. Although this collaboration took many years of work, it halved the methodological uncertainty and significantly reduced the statistical uncertainty.

One of the many improvements was that Sacco's team and others involved in the DES research used more advanced machine learning techniques to help classify supernovae. With data from 300 million observed distant galaxies, the DES Sako working group has identified 1,499 Type Ia supernovae, making it the largest and most detailed sample of supernovae from a single telescope ever collected.

The 1998 Nobel Prize-winning astronomers used just 52 supernovae to determine that the universe is expanding at an accelerating rate. This is a truly enormous increase compared to the situation 25 years ago – said Davis.

The DES supernova data are also more accurate than previous studies because they were observed using four colors of light filters, a significant improvement over the Nobel Prize-winning supernova samples that used only one or two filters.

Even as more advanced research on dark energy emerges, DES scientists have stressed the importance of having theoretical models to explain dark energy in addition to their experimental observations. All of this is really uncharted territory Crone said. We have no theory that puts dark energy in a framework that applies to other physics that we understand. Right now, we at DES are working to constrain how dark energy works in practice, with the hope that we can later sort out some of the theories.

Even if we measure dark energy very precisely, that doesn't mean we know what it isDavis said. Dark energy is still waiting to be discovered.

Agnieszka Novak

more information:

Source: University of Pennsylvania

In the illustration: Kepler's supernova remnant (Type Ia). Source: Image provided by NASA/CXC/SAO/D. Patno. (Optical) DSS

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