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 expended 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 redshifted—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 first 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-l means that the universe must have
changed significantly over the 60 million years that separate the two epochs,
the team suggests. lye 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 m
protons and electrons. The ionized hydrogen then became more transparent,
allowing the galaxies’ light to pass through lye 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. lye 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. According to the passage, the "hierarchical" theory of galaxy formation suggests that big, bright galaxies formed as ______.