They discovered that the Universe can expand at different rates.

25 years ago, the international scientific community was stunned by a discovery that was as extraordinary as it was unexpected: not only was the Universe getting larger, but its rate of expansion was accelerating. That is, the Cosmos is not only becoming larger and larger, but also growing faster and faster. At the time, astrophysicists believed that the opposite was happening and that the expansion of the universe was slowing down due to gravity, so the discovery came as a huge surprise to everyone. Additionally, the “blame” for this accelerated expansion lies with a mysterious entity called dark energy, which accounts for about 70% of the total mass of the Universe.

The revolutionary discovery, achieved through the observation of specific types of supernovae, was awarded the Nobel Prize in Physics in 2011. And now, a quarter of a century later, using the same technique, more than 400 scientists from 25 international institutions working on the DES (Dark Energy Survey) have just published the results of an unprecedented analysis. And they found that while their data is broadly consistent with the current cosmological model, the so-called Lambda Cold Dark Matter (ΛCDM) model, the mysterious dark energy may not always be the same over time as previously thought. , which would force him to partially change. The discovery, presented during the 243rd meeting of the American Astronomical Society, held a few days ago in New Orleans, will soon be published in The Astrophysical Journal.

Largest sample of supernovae

This time, the results are based on more than five years of study of 1,500 different supernovae (only 52 were analyzed in 1998) observed with the Dark Energy Camera, a 570-megapixel digital camera built at Fermilab and mounted on the Victor M. Blanco Telescope at the Inter-American Cerro Tololo Observatory in Chile. This is the largest sample of supernovae collected using a single instrument.

DES scientists use four different methods to conduct their analysis, although this time they decided to do it the same way they did 25 years ago, although, yes, with an almost thirty times larger sample of supernovae. The method requires data from a special class of supernovae, “Type Ia,” which occur when an extremely dense dead star, a white dwarf, absorbs the mass of a companion star until it reaches a critical mass and explodes. Because the critical mass is almost the same for all white dwarfs, all Type Ia supernovae always have the same true brightness, making them excellent distance markers. So when astrophysicists compare the apparent brightness of two Type Ia supernovae observed from Earth, they can determine their relative distance from us.

Thanks to growing samples of these supernovae, astrophysicists can trace the history of cosmic expansion. To do this, they combine for each supernova its distance and the speed at which it is moving away from the Earth due to the expansion of the Universe. In this way, they can also determine the density of dark energy at a certain time in the past and know whether it remained constant or not.

changes over time

To locate 1,500 Type Ia supernovae, DES researchers observed more than two million distant galaxies over 758 nights with a dark energy camera, searching for the unmistakable glow of these stellar explosions. The resulting data was then combined with data from the European Space Agency’s Planck telescope. And among the results emerged the intriguing possibility that dark energy does not always have the same density.

“There are compelling indications that dark energy is changing over time,” says Tamara Davis of the University of Queensland in Australia and co-coordinator of the DES Supernova working group. We found that the simplest dark energy model, ΛCDM, does not fit best. It’s not that far off that we can rule it out, but in the quest to understand what’s accelerating the expansion of the Universe, it’s an intriguing new piece of the puzzle. “A more complex explanation may be required.”

When the results were announced, Davis admitted: “I was shaking. “It was definitely an exciting moment.”

Of course, scientists will need more data to come to a definitive conclusion. However, DES will not be able to provide this because the study stopped accepting new data in January 2019. So we’ll have to wait for those who will provide future dark energy surveys, such as the Legacy Survey of Space and Time, LSST, which will be conducted at the Vera K. Rubin Observatory, scheduled to become operational later this year.

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