Reading Comprehension

After evidence was obtained in the 1920s that
the universe is expanding, it became reasonable
to ask: Will the universe continue to expand
indefinitely, or is there enough mass in it for the
mutual attraction of its constituents to bring this
expansion to a halt? It can be calculated that
the critical density of matter needed to brake the
expansion and "close" the universe is equivalent
to three hydrogen atoms per cubic meter. But the
density of the observable universe-luminous matter
in the form of galaxies-comes to only a fraction
of this. If the expansion of the universe is to stop,
there must be enough invisible matter in the
universe to exceed the luminous matter in density
by a factor of roughly 70.

Our contribution to the search for this "missing
matter" has been to study the rotational velocity
of galaxies at various distances from their center
of rotation. It has been known for some time that
outside the bright nucleus of a typical spiral galaxy
luminosity falls off rapidly with distance from the
center. If luminosity were a true indicator of mass,
most of the mass would be concentrated toward
the center. Outside the nucleus the rotational
velocity would decrease geometrically with distance
from the center, in conformity with Kepler's law.
Instead we have found that the rotational velocity
in spiral galaxies either remains constant with
increasing distance from the center or increases
slightly. This unexpected result indicates that the
falloff in luminous mass with distance from the
center is balanced by an increase in nonluminous
mass.

Our findings suggest that as much as 90
percent of the mass of the universe is not radiating
at any wavelength with enough intensity to be
detected on the Earth. Such dark matter could be
in the form of extremely dim stars of low mass,
of large planets like Jupiter, or of black holes,
either small or massive. While it has not yet been
determined whether this mass is sufficient to
close the universe, some physicists consider it
significant that estimates are converging on the
critical value.

Q1
The authors' suggestion that "as much as 90 percent of the mass of the universe is not radiating at any wavelength with enough intensity to be detected on the Earth" (lines 34-37) would be most weakened if which of the following were discovered to be true?

(A) Spiral galaxies are less common than types of galaxies that contain little nonluminous matter.
(B) Luminous and nonluminous matter are composed of the same basic elements.
(C) The bright nucleus of a typical spiral galaxy also contains some nonluminous matter.
(D) The density of the observable universe is greater than most previous estimates have suggested.
(E) Some galaxies do not rotate or rotate too slowly for their rotational velocity to be measured.

OA: A

I don't understand why D is incorrect. If the density of the observable universe is twice previously estimated, then the suggestion of author would be that - "as much as 45 percent of the mass of the universe is not radiating at any wavelength with enough intensity to be detected on the Earth"

Another issue with the option A is that it attacks the premise of the argument whose conclusion is -"as much as 90 percent ....Earth". In GMAT CR we are supposed to assume that GMAT writers don't make errors in premises. Why it has been assumed that authors didnot consider the fact of the frequency of distribution of the spiral galaxies.

Density really doesn't effect because the original invisible matter is estimated(as per the passage) based on ratio of expected velocity to actual velocity at a given distance; It doesn't matter how much is the visible matter, invisible matter will be 90%;

Ve = expected velocity; Me = original estimated mass; // based on original density De;

Vr = real velocity; Mr = Real mass;

Vr/Ve = Mr/Me ; Mr = Me( Vr/Ve); or Me/Mr = Ve/Vr = 10% ( = 100-90%)

Invisible Mass = Mi = Mr- Me ; Mi/Mr = (Mr-Me)/Mr = 1- (Mr/Mr) = 1- Ve/Vr = (Vr-Ve)/Vr = 90% == This formula is independent of density;

To understand more:

Now say if original De is not the actual De ; Say Da = actual density; So Me = Volume * Da = Me1 ;

Mr = Me1( Vr/Ve); ==> (Mr-Me1)/Mr = (Vr-Ve)/Ve;

Payal,

Thanks for your comments - your second suggestion makes perfect sense.

However not yet convinced with your first answer. Here are my reasons:

Mass and Density are not independent but they are closely related. We can say that a piece of iron is heavier than a similar piece of wood because it has higher density. Now coming to the passage.



From the 1 st paragraph, last sentence we can infer that

critical density for universe to stop expanding = 70 * luminous density.

From the 3rd para, 1 st sentence we can infer that

dark mass in universe = 9 * luminous mass in universe

Now as the question says that if the observable universe has higher density it would mean that that the observable universe has a higher mass i.e higher luminous mass

So from second equation we can say that dark mass in universe = 4.5 * luminous mass in universe

Here am assuming that the rate of expansion of universe is small compared to the size of universe thus taking the size of universe = const.