The "behind-the-scenes promoter" of the accelerated expansion of the universe is slowly pulling back...

Physicists have long assumed that dark energy is constant in strength, but new results from a project to map the expansion of the universe challenge that view, adding to evidence that dark energy, which is accelerating galaxies away from each other, has weakened in strength over the past 4.5 billion years.

Part of DESI. Image source: KPNO

This is the latest research result announced by the Dark Energy Spectroscopic Instrument (DESI) collaboration at the American Physical Society meeting on March 19. The research is based on three years of data collection and is more convincing than the one-year data to be released in 2024.

If the findings hold up, they could force cosmologists to revise their standard models of the universe’s history, which typically assume that dark energy is an intrinsic property of the vacuum of space that doesn’t change over time, a “cosmological constant.”

“Now I’m really paying attention,” says Catherine Heymans, an astronomer at the University of Edinburgh, UK. “This is a challenging question for physicists.”

DESI is located at Kitt Peak National Observatory near Tucson, Arizona, USA. The device uses 5,000 robotic arms to aim optical fibers at selected galaxies or quasars in the field of view. These fibers then transmit light to a precision spectrometer to measure the redshift value of each celestial body, that is, the degree to which its light waves are stretched by the expansion of the universe on the way to the earth. Researchers can use the redshift value to estimate the distance of celestial bodies from the earth, and then draw a three-dimensional map of the history of the expansion of the universe.

In this 3D map, scientists have identified remnant fluctuations from early acoustic oscillations, known as baryon acoustic oscillations (BAOs), through the density of galaxies. These oscillations existed before star formation, with an original characteristic scale of 150 kiloparsecs (450,000 light-years), and have continued to grow as the universe expands. They have now expanded 1,000 times to 150 megaparsecs, becoming the largest characteristic scale known in the universe today.

By tracking the evolution of the BAO size, researchers were able to reconstruct the historical changes in the rate of expansion of the universe. About 5 billion years ago, the expansion of the universe switched from slowing down to accelerating under the push of dark energy. Until last year, all cosmological data were consistent with the assumption that dark energy was a cosmological constant, which meant that the universe should continue to expand at an accelerating rate.

But the latest analysis from DESI shows that the current acceleration of the universe's expansion is less than in the past, which is inconsistent with the assumption that dark energy is a cosmological constant. Instead, the data show that the dark energy density (the amount of dark energy per unit space) has dropped by about 10% since 4.5 billion years ago.

The analysis included redshift data for more than 30 million galaxies and quasars, covering 11 billion years of the universe's history. Although the statistical power of the DESI data is not yet sufficient to completely deny the cosmological constant attribute of dark energy, DESI member Seshadri Nadathur of the University of Portsmouth in the UK expects that once DESI accumulates seven years of data and completes the investigation, this cosmological constant will become history.

The Euclid Space Telescope, which is scheduled to be launched in 2023, and the 8-meter Vera Rubin Observatory, which is about to be put into use in Chile, will further supplement the relevant cosmic history data.

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Source: China Science Daily

Compiled by Li Muzi

Editor: Yinuo

Proofread by Xu Lailinlin