The earliest known galaxies in the universe, which formed
during the universe’s "dark age" nearly 13 billion years ago, have been spied by
two teams of astronomers. The discoveries, reported separately
in this week’s issue of the journal Nature, suggest that galaxies were forming
just 700 million years after the birth of the universe. Theory
holds that the universe formed 13.7 billion years ago. when an extremely dense
concentration of mass rapidly expanded in an event known as the big
bang. The universe has been expanding ever since, so astronomers
are able to age galaxies by computing how much the wavelength of their light has
stretched-or redshified-as the expansion takes the galaxies farther from Earth.
The redder the light is, the older and more distant the galaxy is.
The detection of such ancient galaxies adds intrigue(神秘色彩) to theories of
how the very fast galaxies formed, according to astronomers.
Were there many large, young galaxies in the early universe that are
obscured from astronomers’ view by abundant gases absorbing their light Or were
galaxies rare and small way back then, as a prevailing theory suggests, and
later clumped together to form larger galaxies such as the Milky Way
"We believe that we need both these processes to explain what we see,"
Masanori lye, a professor at the National Astronomical Observatory of Japan,
said in an email. The galaxy, called IOK-1, formed about 750
million years after the big bang-60 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(稀有) 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(再次电离). 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 (银河间的) 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 comparative rarity of IOK-1 means that significant changes must have occurred in the universe over the 60 million years that ______.