Thirteen billion years ago, it was morning in the universe. On Thursday, two independent teams of astronomers reported that they might be zeroing in on the time that bright galaxies began to light up the primordial universe.

The number of galaxies, they say in two papers published in Nature, seems to have increased drastically 700 million to 900 million years after the Big Bang. If true, the results could be a boon to theorists who would like to understand how galaxies formed and what stars they were made of.

The results also lend support to the prevailing notion of galaxy formation, which holds that luminous galaxies were rare in early times and that they grew from the assembly of smaller building blocks.

"This is a very exciting time," said Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics, who was not involved in the work. "Advanced technology allows us for the first time to observe the opening chapter of 'genesis."'

Rychard J. Bouwens and Garth D. Illingworth of the University of California, Santa Cruz, combed three years worth of data from the Hubble Space Telescope and found a tenfold increase in the numbers of galaxies over the 200 million years.

"The data demonstrate very clearly that there is quite a dramatic change in galaxy population over 200 million years," Illingworth said. The galaxies the researchers saw, he said, were one-tenth the size of the Milky Way, the solar system's galaxy, but just as bright, because they were ferociously forming new stars. He characterized those galaxies as "the seeds" of galaxies like ours or its neighbor in Andromeda.

Another team, led by Masanori Iye of the National Astronomical Observatory in Tokyo, which has been using the giant Subaru Telescope on Mauna Kea in Hawaii, found a similar jump in galaxy counts in the same period. That includes what they say is the earliest galaxy yet confirmed, just 750 million years after the Big Bang, or equivalently 12.8 billion light-years from Earth.

Astronomers compute the distances of cosmic objects in space and time by measuring how much their light has been lengthened in wavelength, or "red shifted," by their motion away from Earth because of the expansion of the universe.

Iye and his colleagues used direct spectroscopic measurements of light emitted by hydrogen in the galaxy's starlight to do that.

Illingworth and his colleague used an indirect technique, looking for the absorption of starlight by hydrogen, to derive distances and dates.

The universe was born in light 13.7 billion years ago in the standard Big Bang model. But after it had cooled for protons and electrons to combine into hydrogen atoms, the cosmos went cold and dark.

How the dark ages ended is a matter of hot debate. From 300 million years to 1 billion years after the Big Bang, figures from NASA's Wilkinson Microwave Anisotropy Satellite show that the hydrogen inspace was re-ionized and split back into electrons and protons by radiation from stars or, perhaps, black holes.

Calculations suggest that the first stars to form, out of hydrogen and helium produced in the Big Bang, would be 100 times as massive as the Sun and would rapidly explode, scattering heavier elements like oxygen, carbon, nitrogen and iron — the stuff of planets and life — into space to serve as material for a new generation of stars.

The stars forming in the newly discovered galaxies are probably of the second type, Illingworth said.

"It is not known which type of stars ionized the universe or when the transition between the two populations took place," Loeb said.

Comparing data from surveys like the ones being reported with theoretical models could help resolve such questions, as well as sharpen theories of galaxy formation.

Alan Dressler of the Carnegie Observatories in Pasadena, Calif., cautioned that it would take bigger telescopes and observations of even smaller objects to discern what occurred in the early universe. "This is a good solid step," he wrote in an e-mail message. "However, it doesn't come close to settling anything."

Nor is the Japanese record likely to last long. Richard Ellis of the California Institute of Technology said in an e-mail message that he had used the Keck Telescope on Mauna Kea and a quirk of Einsteinian gravity to find protogalaxies even farther in the past, less than 500 million years after the Big Bang. These objects, too feeble and small for the Hubble to have seen them, have been amplified by the gravitational fields of intervening galaxies.