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should be temporary phenomena, enduring at most for a billion years. Oh,
we should see some spiral galaxies, but there is no way that three out
of every four large galaxies should be spirals--which is the ratio we
actually observe. Ellipticals should far outnumber spimls, but they do
not."
"Obviously, then, there's a flaw with the theory," said Keith.
Jag lifted his upper shoulders. "Indeed. We astrophysics types have
been limping by for centuries with something called 'the density-wave
model' for explaining the abundance of spiral galaxies. It proposes a
spiral-shaped disturbance that moves through the medium of a galactic
disk, with stars getting caught up in it--or even being formed by it--as
the wave rotates. But it has never been a satisfactory theory.
First, it fails to account for all the different types of spiral forms,
and, second, we don't have a good answer as to what would cause these
imagined density waves in the first place. Supernova explosions are
sometimes cited, but it's just as easy to model such explosions
canceling each other out as it is to get them to build up long-duration
waves." He paused. "We've had other problems with our galaxy-formation
models, too. Back in 1995, human astronomers discovered that distant
galaxies, observed when they were only twenty percent of the current age
of the universe, had rotational rates comparable to what the Milky Way
has today--that's twice as fast. as they should have been rotating at
that age, according to theory."
Keith thought for a second. "But if what we're seeing right now is
correct, then spiral galaxies like ours must somehow form from simple
disks, right?"
Another lift of the Waldahud's upper shoulders. "Perhaps.
Your Edwin Hubble proposed that galaxies each start as a simple sphere
of stars, gradually spin out into a flat disk, then develop arms that
open up more and more over time.
But although we now have observational proof that that sort of evolution
does indeed happen"--he gestured at the disk of stars in the glowing
frame--"we still don't have an explanation for why the evolution takes
place, or why the spiral structures persist."
"But you say three quarters of all large galaxies are spirals?" asked
Lianne.
"Wellll," said Jag, PHANTOM translating a hissing bark as a protracted
word, ."actually, we don't know much directly about the ratio of
elliptical to nonelliptical galaxies in the universe at large. It's
hard to make out structure in dim objects that are billions of
light-years away. Locally, we see that there are many more spirals than
there are ellipti-cals, and that spirals contain a preponderance of
young blue stars, whereas our local ellipticals contain mostly old red
stars. We've assumed, therefore, that any vastly distant galaxy that
showed lots of blue light--after correcting for redshift, of course--was
a spiral, and any that showed mostly red was an elliptical, but we
really don't know that for sure."
"It's incredible," said Lianne, looking at the image.
"So--so if that's how it looked six billion years ago, then none of the
Commonwealth homeworlds yet exists, right? Is there--do you suppose
there's any life in the galaxy now?"
"Well, 'now' is still 'now,' of course," said Jag. "But if you're
asking if there was any life in the Milky Way hack when that light
started its journey to us, I would say no.
Galactic cores are very radioactive--even more so than we used to think.
In a large elliptical galaxy, such as we're seeing here, the whole
galaxy is essentially the core. With stars that close together, there
would be so much hard radiation everywhere that stable genetic molecules
wouldn't be able to form." He paused. "I guess that means it's only
middle-aged galaxies that can give rise to life; young, armless ones
will be sterile."
There was silence on the bridge for a time, broken only by the gentle
hiss from the air-circulating equipment and the occasional soft beep
from a control panel. Each person contemplated the small fuzzy blot of
light that one day would give rise to all of them, contemplated the fact
that they were farther out in space than anyone had ever been before,
contemplated the vastly empty darkness all around them.
Six billion light-years.
Keith remembered reading about Borman, Lovell, and Anders, the Apollo 8
astronauts who had circled the moon over Christmas of 1968, reading
passages from Genesis back to the people on Earth. They had been the
first human beings to get far enough from the homeworld so that they
could cup it in an outstretched hand. Maybe more than any other single
event, that view, that perspective, that image, had marked childhood's
end for humanity--the realization that all their world was one tiny ball
floating against the night.
And now, thought Keith, maybe--just maybe--this image was the one that
marked the beginning of middle age: a still frame that would become the
frontispiece of volume two of humanity's biography. It wasn't just
Earth that was 'tiny, insignificant, and fragile. Keith lifted his hand
and reached out toward the hologram, cupping the island of stars in his
fingers. He sat silently for a long moment, then lowered his hand, and
allowed his eyes to wander over the overwhelming dark emptiness that
spread out in all directions.
His gaze happened to pass over Jag--who was doing exactly what Keith had
done a moment ago, using one of his hands to cup the Milky Way.
"Excuse me, Keith," said Lianne, the first words spoken by anyone on the
bridge for several minutes. Her voice was soft, subdued, the way one
would talk in a cathedral. "The electrical system is repaired. We can
launch that probe anytime you like." [ Pobierz całość w formacie PDF ]