The Hubble space telescope has discovered seven primitive galaxies formed in the earliest days of the cosmos, including one believed to be the oldest ever detected.

The discovery, announced Wednesday, is part of the Hubble Ultra Deep Field campaign to determine how and when galaxies first assembled following the Big Bang.

"This 'cosmic dawn' was not a single, dramatic event," said astrophysicist Richard Ellis with the California Institute of Technology in Pasadena. Rather, galaxies appear to have been formed over hundreds of millions of years.

Ellis led a team that used Hubble to look at one small section of the sky for a hundred hours. The grainy images of faint galaxies include one researchers determined to be from a period 380 million years after the onset of the universe—the closest in time to the Big Bang ever observed.

The cosmos is about 13.7 billion years old, so the newly discovered galaxy was present when the universe was 4 percent of its current age. The other six galaxies were sending out light from between 380 million and 600 million years after the Big Bang. (See pictures of "Hubble's Top Ten Discoveries.")

Baby Pictures

The images are "like the first ultrasounds of [an] infant," said Abraham Loeb, a specialist in the early cosmos at Harvard University. "These are the building blocks of the galaxies we now have."

These early galaxies were a thousand times denser than galaxies are now and were much closer together as well, Ellis said. But they were also less luminous than later galaxies.

The team used a set of four filters to analyze the near infrared wavelengths captured by Hubble Wide Field Camera 3, and estimated the galaxies' distances from Earth by studying their colors. At a NASA teleconference, team members said they had pushed Hubble's detection capabilities about as far as they could go and would most likely not be able to identify galaxies from further back in time until the James Webb Space Telescope launches toward the end of the decade. (Learn about the Hubble telescope.)