Iye and his colleagues used the Subaru Telescope Facility in Hilo, Hawaii, to find the most distant galaxy known in the universe.
The galaxy, called IOK-1, formed about 750 million years after the big bang60 million years closer to the event than the previous record holder.
Given the number of galaxies found during a later epoch about 810 million years ago, the researchers had expected to find as many as six galaxies like IOK-1.
But the comparative rarity of objects like IOK-1 means that the universe must have changed significantly over the 60 million years that separate the two epochs, the team suggests.
Iye and colleagues believe that they are witnessing the last phase of a process known as reionization.
According to Iye, about 380,000 years after the fiery hot big bang, the universe cooled so much that protons and electrons recombined to form neutral hydrogen.
This is known as the beginning of the dark age of the universe, because neutral hydrogen absorbs the light from stars.
As more galaxies started to form about 300 million years later, the hot stars heated the intergalactic medium and gradually reionized the neutral hydrogen back to protons and electrons.
The ionized hydrogen then became more transparent, allowing the galaxies' light to pass through.
Iye said the new results support the idea that neutral hydrogen was still abundant 750 million years after the big bang, blocking even older galaxies from view.
"We are starting to see the last phase of cosmic reionization, or the dawn of the cosmic dark age," he said.
Iye added that the discovery also supports the "hierarchical" theory of galaxy formation, which suggests that big, bright galaxies formed as smaller galaxies collided and merged.
"The epoch we have probed is yet in this critical stage," he said.
The second study in Nature also supports the hierarchical theory.
Astronomers at the University of California, Santa Cruz, found a large jump in the number of formed galaxies between 700 million and 900 million years after the big bang.
Their technique is based on analysis of data collected by the Hubble Space Telescope.
The method cannot precisely age galaxies, but it does give a good estimate, the team says.
The Santa Cruz researchers detected only one galaxy formed at around 700 million years after the big bang but hundreds of galaxies 200 million years later.
"There must have been a lot of merging of smaller galaxies during that time," Garth Illingsworth, a co-author of the study, said in a press statement.
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