The mystery of the expansion of the universe has been solved? Take a look at the second type of dark energy

Original author | Davide Castelvecchi

Researchers have found a clue to a previously unknown early cosmic matter that may explain why the universe seems to be expanding faster than theory predicts.

Dark energy is the ubiquitous but unusually mysterious matter in the universe, driving the accelerated expansion of the universe. Now, cosmologists have found signs of a second type of dark energy that may have existed for the first 300,000 years after the Big Bang.

Not long ago, two independent studies were published on the arXiv preprint server[1,2], and researchers used data collected by the Atacama Cosmological Telescope (ACT) in Chile from 2013 to 2016 to find the first first signs of this "early dark energy". If the findings are confirmed, they could be used to solve a long-standing puzzle surrounding the data of the early universe that seems to contradict the currently measured rate of cosmic expansion (see: How fast did the universe expand?). The latest measurements have confused scientists even more). However, these data are still preliminary and cannot be used to determine whether early dark energy really existed.

Data from the Atacama Cosmological Telescope (ACT) show that there were two types of dark energy | in the earliest stages of the universe Giulio Ercolani/Alamy

Silvia Galli, a cosmologist at the Astrophysical Institute in Paris, said: "Given the multiple reasons, we must be careful about whether this can be regarded as a discovery of new physics. ”

The two preprint papers were from the ACT team and an independent group, respectively. The authors of both articles acknowledge that the data is not strong enough to detect the presence of early dark energy with high levels of confidence. But they say further observations from the ACT and another telescope, the South Pole Telescope (SPT), located in Antarctica, will soon provide a more rigorous test. "If that's true — if there was indeed early dark energy in the early universe — then we should be able to see a strong signal." Colin Hill, co-author of the ACT team paper [1] and a cosmologist at Columbia University in New York, said.

Draw the cosmic microwave background

Both the ACT and the South Pole Telescope were designed to map the cosmic microwave background (CMB), a primitive radiation sometimes referred to as the afterglow of the Big Bang. CMB is a pillar of cosmologists' understanding of the universe. By mapping the subtle variations of the CMB in the sky, researchers have found tangible evidence to support the Standard Model of Cosmology. The Standard Model of cosmology describes the evolution of the universe made up of three major substances: dark energy; dark matter, which is as mysterious as dark energy—the main cause of galaxy formation; and ordinary matter—which accounts for less than 5 percent of the universe's total mass and total energy.

The most cutting-edge CMB map was currently drawn by the Planck mission, a mission carried out by the European Space Agency from 2009 to 2013. Calculations based on data from the Planck mission can predict — assuming that the Standard Model of cosmology is correct — how fast the universe is expanding now. But over the past decade, observations of supernova explosions and other methods have also led to increasingly accurate measurements of the rate of expansion of the universe, which is 5 to 10 percent faster than theoretical predictions [3].

Theoretical physicists believe that the Standard Model may have contributed to this discrepancy and suggest drastic adjustments to it. Two years ago, Marc Kamionkowski, a cosmologist at Johns Hopkins University in the United States, proposed adding an element to the Standard Model.[4] The "early dark energy" they suggested increased — precisely the idea they and other teams had been working on for years — was a fluid that permeated the entire universe and gradually disappeared in the hundreds of thousands of years after the Big Bang. Kamionkowski said: "This is not a very convincing point of view, but it is the only model we can explain. ”

Early dark energy wouldn't be as powerful as the "normal" dark energy it is today to allow the universe to expand at an accelerated rate, but it would have cooled the plasma produced in the Big Bang faster. This affects our interpretation of CMB data — especially measurements of the age of the universe and how fast it expands, because these measurements are based primarily on how far sound waves can travel in plasma before they cool into gas. To carry out such calculations, the Planck mission and other similar astronomical observations take advantage of some of the features that remain in the sky after the conversion process.

The two newest studies found that ACT's CMB polarization plot fits better with models containing early dark energy — more than it fits the Standard Model. Hill said that if the CMB is explained using data from models containing early dark energy and ACT, the age of the universe is now 12.4 billion years, about 11 percent younger than the 13.8 billion years calculated by the Standard Model. Correspondingly, the universe is now expanding 5 percent faster than the Standard Model predicts — closer to what current astronomers have calculated.

The differences persist

Hill said he was previously skeptical of early dark energy, but he was surprised by his team's findings. Vivian Poulin is an astrophysicist at the University of Montpellier in France and co-author of another study using ACT data[2]. He was pleased that their team's analysis was consistent with that of the ACT team. Kamionkowski said: "The main authors of the paper are a very insistent, conservative group of people who really understand the data and measurement results. ”

But Galli cautioned that the ACT data appeared to be at odds with the calculations of her Planck team. While polarization data for the ACT may support early dark energy, it's unclear whether other major datasets, such as the CMB temperature map, do the same. For these reasons, she said, cross-validation using the South Pole Telescope is an essential step. She herself is involved in the project.

Wendy Freedman, an astronomer at the University of Chicago who has been involved in completing some of the most precise measurements of the expansion of the universe, says the results based on ACT data are still interesting. "Be sure to use different models, she says, and compare them to the Standard Model."

bibliography

[1] Hill, J. C. et al. Preprint at https://arxiv.org/abs/2109.04451 (2021).

[2] Poulin, V., Smith, T. L. & Bartlett, A. Preprint at https://arxiv.org/abs/2109.06229 (2021).

[3] By Valentino, E. et al. Class. Quantum Grav. 38, 153001 (2021).

[4] Poulin, V., Smith, T. L., Karwal, T. & Kamionkowski, M. Phys. Rev. Lett. 122, 221301 (2019).

The original article was published in the News section of Nature on September 17, 2021, under the title of New type of dark energy could solve Universe expansion mystery

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