Earliest-known galaxy, spotted by Webb telescope, is a beacon to cosmic dawn

NASA's James Webb Space Telescope has spotted the earliest-known galaxy, one that is surprisingly bright and big considering it formed during the universe's infancy - at only 2% its current age.

Webb, which by peering across vast cosmic distances is looking way back in time, observed the galaxy as it existed about 290 million years after the Big Bang event that initiated the universe roughly 13.8 billion years ago, the researchers said. This period spanning the universe's first few hundred million years is called cosmic dawn.

The telescope, also called JWST, has revolutionized the understanding of the early universe since becoming operational in 2022. The new discovery was made by the JWST Advanced Deep Extragalactic Survey (JADES) research team

This galaxy, called JADES-GS-z14-0, measures about 1,700-light years across. A light year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km). It has a mass equivalent to 500 million stars the size of our sun and was rapidly forming new stars, about 20 every year.

Before Webb's observations, scientists did not know galaxies could exist so early, and certainly not luminous ones like this.

"The early universe has surprise after surprise for us," said astrophysicist Kevin Hainline of Steward Observatory at the University of Arizona, one of the leaders of the study published online, opens new tab this week ahead of formal peer review.

Read: James Webb Telescope found galaxies that shouldn't exist

"I think everyone's jaws dropped," added astrophysicist and study co-author Francesco D'Eugenio of the Kavli Institute for Cosmology at the University of Cambridge. "Webb is showing that galaxies in the early universe were much more luminous than we had anticipated."

Until now, the earliest-known galaxy dated to about 320 million years after the Big Bang, as announced by the JADES team last year.

"It makes sense to call the galaxy big, because it's significantly larger than other galaxies that the JADES team has measured at these distances, and it's going to be challenging to understand just how something this large could form in only a few hundred million years," Hainline said.

"The fact that it's so bright is also fascinating, given that galaxies tend to grow larger as the universe evolves, implying that it would potentially get significantly brighter in the next many hundred million years," Hainline said.

While it is quite big for such an early galaxy, it is dwarfed by some present-day galaxies. Our Milky Way is about 100,000 light years across, with the mass equivalent to about 10 billion sun-sized star.

Read: James Webb video zooms through the cosmos to the Cartwheel Galaxy

The JADES team in the same study disclosed the discovery of the second oldest-known galaxy, from about 303 million years post-Big Bang. That one, JADES-GS-z14-1, is smaller - with a mass equal to about 100 million sun-sized stars, measuring roughly 1,000 light years across and forming about two new stars per year.

"These galaxies formed in an environment that was much more dense and gas-rich than today. In addition, the chemical composition of the gas was very different, much closer to the pristine composition inherited from the Big Bang - hydrogen, helium and traces of lithium," D'Eugenio said.

Star formation in the early universe was much more violent than today, with massive hot stars forming and dying quickly, and releasing tremendous amount of energy through ultraviolet light, stellar winds and supernova explosions, D'Eugenio said.

Three main hypotheses have been advanced to explain the luminosity of early galaxies. The first attributed it to supermassive black holes in these galaxies gobbling up material. That appears to have been ruled out by the new findings because the light observed is spread over an area wider than would be expected from black hole gluttony.

It remains to be seen whether the other hypotheses - that these galaxies are populated by more stars than expected or by stars that are brighter than those around today - will hold up, D'Eugenio said.