Page 4254 - 1970S

ASTRONOMERS
(Continuedfrom page 6)
thus the escape velocity would need
to be greater. For the expanding
universe, a similar reasoning ap–
plies: There is the possibility that
the expansion will stop and even–
tually reverse itself, or that it will
continue indefinitely.
Obviously, the deciding factor is
whether there is enough mass in the
universe so that gravi tational attrac–
tion will eventually overcome the
expansion. The amount of mass in
the universe is clearly related to the
average density of the universe, and
it turns out that the "critica! den–
sity" needed to eventually stop the
universe's expansion is only about
four hydrogen atoros per cubic me–
ter. This number may seem incred–
ibly small , and, indeed, it represents
a far better vacuum than the rnost
sophisticated scientific instruments
can produce. But the universe is in–
conceivably large, so the total
amount of matter represented by
such a density is very great. Now, íf
the actual density of the universe is
smaller than the critica! value, then
the universe wiJI expand forever;
conversely, íf the actual density is
higher than the critica! value, the
universe will eventually contract,
and everythíng will be squeezed in
what sorne astronomers call the "big
crunch."
Measurlng the Mass of the Universe
But how do we find the mass of
the universe? Probably the.
most obvious method is to
simply count up all the galaxies we
can see and estímate their mass. The
astronomer J. H. Oort did just that
about fifteen years ago, and he
found that the mass of all the matter
in galaxies was only about one per–
cent of the amount needed to
"close" the universe. Since that
time, many researchers have at–
tempted to find "the missing mass."
Astronomer J . R. Gott and others
have made retined measurements of
the mass of galaxies. Their "dynam–
ical" method implies that galaxies
may have a great deal of mass that
telescopes cannot see. Yet, even
The PLAIN TRUTH December 1978
with this more exact method, the
density of the universe is still no–
where near the critica! value. In fact,
the density they find is only about
five percent ofthe critica! value.
Other studies indicate that galax–
ies are associa ted with an amount of
mass sorne ten times larger than the
mass in the visible parts of the gal–
axies themselves. But this mass is
still at least ten times too small to
stop the expansion of the universe.
Of course, there is always the possi–
bility that additional mass exists be–
tween the clusters of galaxies, but so
far no one has been able to detect
an appreciable amount.
Another method for estimating
the mass and density of the universe
is based on the abundance of the
element deuterium-a form of hy–
drogen with one proton and one
neutron. Deuteríum was presum–
ably made in the early history of the
universe, as were a number of other
elements. But the abundance of
deuterium today is thought to be
directly related to the density of the
early universe. In other words, if we
knew the present abundance of deu–
terium, we could calculate the origi–
nal density of the universe, which
would in turn give us a good idea of
what the present density is. Re–
cently, astronomers have used satel–
lites to measure the amount of
deuterium in interstellar space, and
they find that the density corre–
sponding to the observed amount of
deuterium is very low, indicating
that the universe will expand for–
ever.
Similar studies all seem to point
to one conclusion. The expanding
universe began at a definite moment
in time about 18 billion years ago
and will apparently never contract.
The universe will thus end, not with
a big crunch, but with a whimper.
Telescoplc Time Machine
But if all these measurements
indica te that the density of the
universe is small , and that the
universe will thus expand forever,
then why is it that astronomers have
often considered the universe to be
oscillating and thus closed?
The answer is that for years Sand–
age and other astronomers used the
"classical" method of determining
the deceleration of the universe.
This method was simply a plot of
dimness (distance) of galaxies
against red shift (velocity). lf the
universe acts like an explosion, we
would expect a straight line for such
a plot, the objects farthest away
having the greatest velocity (red shift).
However, when we view distant
galaxies, we are actually looking
back in time. Like a time machine,
telescopes reveal the galaxies as
they were bi llions of years ago, trav–
eling at very high velocities because
they haven' t yet been slowed down
by the gravitational attraction of the
rest of the universe. Therefore, the
distant galaxies should show a de–
viation from t·he expected red shift ,
i.e. , their red shifts should be too
high for their distance, when coro–
pared with nearby galaxies.
By measuring these deviations,
Sandage was able to calculate the
amount of deceleration the universe
has experienced. The universe ap–
peared to be slowing down rapidly,
and ultimately it was expected to
reverse its motion and collapse (and
possibly begin another expansion).
Unfortunately, Sandage's method
makes a very tenuous assumption:
It assumes that the brightness of
galaxies does not change over their
lifetimes. But if galaxies evolve and
get old, then they were probably
brighter in the past, when their stars
were still young and bright. There–
fore, the distant galaxies we see may
actually be farther away than we
thought. This implies that their red
shifts may not be disproportionately
higher than nearby galaxies and
that the universe has not been slow–
ing down appreciably. When Sand–
age's calculations take this etfect
into account, he obtains a value for
the deceleration near zero. So again,
the universe wiU apparently never
contract, but will continue to ex–
pand. According to the latest avail–
able evidence, the stars and galaxies
of the universe will disperse forever
until a ll is darkness and emptiness.
The lmmortal Universe Dead?
his result - that the universe
ad only one (bright and awe–
ome) beginning and that it
39