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Most precise measurement of our expanding universe revealed

In an extraordinary feat of science and technology, a team of researchers has harnessed the power of 5,000 diminutive robots within a telescope perched on a mountaintop to gaze 11 billion years back in time at our expanding universe. 

This venture, made possible by the Dark Energy Spectroscopic Instrument (DESI), captures the ancient light of distant cosmic entities, providing an unparalleled window into the universe‘s nascent stages and charting its expansion to the present day.

Moreover, this ambitious project aims to unravel the mysteries surrounding dark energy, an enigmatic force that is propelling the universe to expand at an accelerating pace.

Unprecedented precision in mapping the expanding universe

By crafting the most comprehensive 3D cosmic map to date, DESI has achieved unprecedented precision in mapping the expanding universe over the last 11 billion years. 

For the first time, scientists have managed to measure the expansion history of the universe’s early epochs with a precision better than 1%, offering the clearest insight yet into cosmic evolution. 

The researchers have disseminated their findings from the initial year of data collection through multiple papers, soon to be published on the arXiv, and presented at major conventions such as the American Physical Society meeting in the United States and the Rencontres de Moriond in Italy.

New generation of dark energy experiments

“We’re incredibly proud of the data, which have produced world-leading cosmology results and are the first to come out of the new generation of dark energy experiments,” said Michael Levi, the director of DESI and a scientist at the Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab), which oversees the project. 

“So far, we’re seeing basic agreement with our best model of the universe, but we’re also seeing some potentially interesting differences that could indicate that dark energy is evolving with time. Those may or may not go away with more data, so we’re excited to start analyzing our three-year dataset soon.”

Dark energy’s influence across cosmic history

DESI’s pioneering observations are pivotal in studying dark energy’s influence across cosmic history, scrutinizing the universe under the Lambda CDM model that incorporates dark matter (CDM) and dark energy (Lambda). 

This model has been instrumental in describing the expanding universe, shaped by the interplay of matter, dark matter, and dark energy. Yet, DESI’s preliminary results, when juxtaposed with data from other studies, reveal subtle discrepancies with Lambda CDM’s predictions, sparking discussions about the necessity for model refinements or exploring alternate explanations.

“No spectroscopic experiment has had this much data before, and we’re continuing to gather data from more than a million galaxies every month,” said Nathalie Palanque-Delabrouille, a scientist at Berkeley Lab. 

“It’s astonishing that with only our first year of data, we can already measure the expansion history of our universe at seven different slices of cosmic time, each with a precision of 1 to 3%. The team put in a tremendous amount of work to account for instrumental and theoretical modeling intricacies, which gives us confidence in the robustness of our first results.”   

New standards in cosmic measurements

With an overall precision of 0.5% across the entire 11 billion years and a record precision of 0.82% for measurements dating back 8-11 billion years ago, DESI has set new standards in cosmic measurements. 

This level of precision in capturing the expanding universe during its youth is a testament to DESI’s capabilities, dwarfing the achievements of its predecessor, the Sloan Digital Sky Survey’s BOSS/eBOSS, which took over a decade to reach its conclusions.

“We are delighted to see cosmology results from DESI’s first year of operations,” said Gina Rameika, an associate director for High Energy Physics at DOE. “DESI continues to amaze us with its stellar performance and is already shaping our understanding of the universe.”

This international collaboration, involving more than 900 researchers from over 70 institutions globally, signifies a landmark in cosmological studies. 

Measuring universe expansion at various stages

According to the experts, DESI not only offers a glimpse into the structure of the cosmos, marked by galaxies woven into a vast web, but also traces the echoes of Baryon Acoustic Oscillations (BAOs), utilizing them as a cosmic ruler to measure universal expansion at various stages.

The experts found that minor disturbances in the early universe’s ionized plasma set off pressure waves, propelling the baryons into a ripple-like pattern akin to the concentric circles formed when you throw pebbles into water. 

Subtle three-dimensional ripple pattern

With the universe’s expansion and subsequent cooling, atoms stabilized into their neutral state, halting the pressure waves and solidifying these ripples across three dimensions. This process led to an increased concentration of future galaxies within these denser regions. 

Even billions of years later, this subtle three-dimensional ripple pattern, known as Baryon Acoustic Oscillations (BAOs), is observable in the distinctive spacing between galaxies.

BAO measurements serve as a universal scale for scientists. By evaluating the size of these cosmic “bubbles,” they can gauge the distance to the source of this barely discernible celestial pattern. 

Our understanding of the expanding universe 

Analyzing BAOs from various points across the universe allows researchers to segment historical data, assessing the rate of cosmic expansion at different epochs and understanding the role of dark energy in driving that expansion.

“We’ve measured the expansion history over this huge range of cosmic time with a precision that surpasses all of the previous BAO surveys combined,” explained Hee-Jong Seo, a professor at Ohio University and the co-leader of DESI’s BAO analysis. 

“We’re very excited to learn how these new measurements will improve and alter our understanding of the cosmos. Humans have a timeless fascination with our universe, wanting to know both what it is made of and what will happen to it.”

Using quasars as a backlight 

Leveraging galaxies to trace the universe’s expansion and delve into the mysteries of dark energy is a common approach, yet it has its limits due to the diminishing brightness of galaxy light over vast distances. 

To overcome this, scientists examined quasars – intensely bright galactic nuclei powered by black holes. As quasar light travels through cosmic gas clouds, it gets partially absorbed, allowing scientists to chart the distribution of dense cosmic matter through a method known as the “Lyman-alpha forest.”

“We use quasars as a backlight to basically see the shadow of the intervening gas between the quasars and us,” explained Andreu Font-Ribera, a scientist at the Institute for High Energy Physics (IFAE) in Spain who co-leads DESI’s Lyman-alpha forest analysis. “It lets us look out further to when the universe was very young. It’s a really hard measurement to do, and very cool to see it succeed.”

To carry out this analysis, researchers harnessed observations of 450,000 quasars – the most extensive collection ever used for Lyman-alpha forest studies – pushing the boundaries of BAO measurements to as far back as 11 billion years ago. 

Unprecedented insights into the early universe

By the conclusion of the DESI survey, the aim is to have mapped three million quasars alongside 37 million galaxies, offering unprecedented insights into the early universe.

DESI has pioneered the use of a “blinded analysis” in spectroscopic experiments, a method that intentionally hides the true outcomes from its researchers to avoid any subconscious confirmation bias. In this process, scientists analyze altered data without knowing the actual results, crafting the necessary computational tools in ignorance of the real data. 

Powerful tool for measuring the expansion of the universe

Only upon completion of their preparatory work do they apply their methodologies to the genuine data, unveiling the real findings.

“The way we did the analysis gives us confidence in our results, and particularly in showing that the Lyman-alpha forest is a powerful tool for measuring the universe’s expansion,” said Julien Guy, a Berkeley Lab scientist and a key figure in processing information from DESI’s spectrographs. 

“The dataset we are collecting is exceptional, as is the rate at which we are gathering it. This is the most precise measurement I have ever done in my life.”

Golden era of cosmology 

The invaluable insights garnered from DESI’s data collection aim to enhance the research capabilities of forthcoming astronomical surveys like the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope

Additionally, these findings lay the groundwork for an anticipated upgrade to DESI, dubbed DESI-II, which was recommended in a recent report by the U.S. Particle Physics Project Prioritization Panel.

Arnaud de Mattia is a researcher affiliated with the French Alternative Energies and Atomic Energy Commission (CEA) and a co-leader of DESI’s team analyzing the cosmological data.

“We are in the golden era of cosmology, with large-scale surveys ongoing and about to be started, and new techniques being developed to make the best use of these datasets,” said de Mattia.

“We’re all really motivated to see whether new data will confirm the features we saw in our first-year sample and build a better understanding of the dynamics of our universe.”


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