South Bend news-times. (South Bend, Ind.) 1913-1938, 1918-04-20
What's Meant by
the Visible Universe?
By GARRETT II. SERVISS.

l have received two letters relat-
ing to astronomical subjects which
seem worth answering here because
they betray misunderstanding and
states of mental uncertainty that are
very common. but which ought not
10 exist. A lady asks: What COM
poses the visible universe and are
we TO infer that there are an infinite
number of universes? That word
is very pulling"
The dictionary definition of uni
verse" is the aggregate OF existing
things" or the whole creations in
contains. But the astronomer is q
little more modest, and while not
altogether discarding. in his imag-
NATIVE moments. the all-inclusive
conception above expressed. he pre-
fers, for practical purposes. to limit
his application of the word to as
much of creation as is included with
in the range Of his perceptive facul-
ties. Yet the term visible uni
verse is really intended to cover
more than the eye, or any existing
telescope can see.
Beyond the frontier os actual
visibility lies an unseen eXpanse
which sure inference teaches must
belong to the same great system
and which an increase of the power
of telescopes would bring within the
range of visibility If visible uni
verse" meant only what can at pres
ent be seen, then it's extent would
depend upon the instrument of OBJ
servation
The visible universe OF an astron-
Omer armed with a 100-inch tele-
scope would be enormously greater
than that of one who had only a
make the term cover all that we
have reason to believe would, with
sufficient optical aid. be found in
cluded in the system Of stars in
which we dwell.
The idea of a visible universe of
limited extent as distinguished from
an absolutely unlimited universe
would never have arisen il il had not
been found that when the depths TO
which the telescope penetrates be-
come very great, the number OF
stars brought into view does not in
crease in proportion TO the added
amount of space.
on this gradual thinning out OF
the stars with increase Of distance
is based the belief that the star
system TO which we belong has def
mite limits. and that, ir our tele-
scopes could be made powerful
enough they would. so TO speak.
show the bottom. The only other
explanation that has been seriously
considered for the apparent thin
ming out OF the distant stars is that
there exists in space some rare alb
sorbing medium which shuts off the
light of stars that are excessively
remote. No independently verih-
a bic evidence OF the existence OF
such medium has. however. been
offered
AS 10 other universes' outside Of
the visible universe" their exist
euce is mainly a matter OF inference.
based on probability. but recently
some astronomers have shown a dis
position to regard the spiral nebulae
as possibly outer universes. l. e..
outer starry sytems, whose stars like
| those composing our Milky Way"
are disposed in a vast. complicated
spiral ring. variegated with wonder
full loops and garlands. II so. the
singular conclusion would result
/ that universe" OF stars has a
characteristic shape. only varying in
details. Just as trees, crystals and
I other organic forms are built up on
/ fundamental plans common 70 all
! membe : Of each order.
Another inquirer says that he
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