SAT Reading - Khan Diagnostic Quiz level 3 - reading 13

Questions 1-11 are based on the following

Passage 1 is excerpted from Tamara Davis, "If Galaxies are All Moving Apart, How Can They Collide?" ©2009 by Scientific American. Passage 2 is excerpted from David Biello, "More Often Than Not, Massive Galaxies Form by Mergers" ©2005 by Scientific American.

Passage 1

Dark energy, believed to be causing the acceleration of the

expansion of the universe, provides a constant outward force

that does not dilute as the universe expands. Pitted against
Line this relentless push is the gravitational pull from the rest of
5 the matter and energy in the universe. Early on, the universe

was much denser than it is today, and the attractive force of

gravity was winning the battle, on scales both large and

small. Clouds of gas condensed to form stars and galaxies,

and galaxies drew together to form clusters. If there had been
10 more matter around, the universe might have started to

recollapse before it ever had the chance to accelerate. But

matter and energy do dilute as the volume of the universe

increases, so dark energy slowly came to dominate. Since

about six billion years ago (about a billion years before Earth
15 formed), the expansion has, on average, been accelerating.

Nevertheless, the cosmic dance continues. Galaxies that

had been pulled together before the universe began

accelerating still have the chance to collide. Collectively they

form overdense patches of the universe in which gravity still
20 reigns. In our neighborhood the Andromeda galaxy, our

largest companion, is actually falling toward us, and we will

have our first close encounter with it in just a few billion

years' time.
Our local group comprises Andromeda, the Magellanic
25 Clouds and about 35 other galaxies, all of which lie in an

even larger cluster called Virgo. Together we will travel

through the expanding universe, and we had better learn to

like the company. [A]ny galaxies that have not yet won the

gravity war have missed their chance. The universe is now
30 split into pockets of interaction that will drift alone through

the expanding cosmos.
Like revelers on a ship, the galaxies in our group will

continue to collide and interact in myriad interesting ways,

but we will be forever separated from the revelers on other
35 ships sailing away from us in the vast universe.

Passage 2

New data seem to show that galaxies collide all the time.

In fact, the oldest and largest galaxies in the universe most

likely formed from such intergalactic combinations.
Astronomer Pieter van Dokkum of Yale University used
40 some of the longest and deepest sky surveys ever conducted

to try to determine whether the oldest, largest galaxies -

called ellipticals because they lack the swirling arms of

the spiral type, like our own Milky Way-formed from the

collapse of ancient clouds of gas or the accretion of smaller
45 galaxies bumping into each other. Of the 126 galaxies of all

varieties van Dokkum looked at, 67 showed telltale signs of

impact, such as trailing tails of stars, or a collision in

"Our study found these common massive galaxies do form
50 by mergers," Van Dokkum explains. "It is just that the

mergers happen quickly and the features that reveal the

mergers are very faint and therefore difficult to detect."
"Quickly" on a galactic scale means just a few hundred

million years-a small fraction of the 13.7 billion years the
55 universe has been in existence-and, because such collisions

rarely involve head-to-head star crashes, they leave few

traces behind except in the shape of the resulting galaxy and

a general slowing in its formation of new stars.
None of the six spiral galaxies in the survey showed any
60 after-crash damage, but that doesn't mean that our own

galaxy is free and clear. "The Milky Way will indeed undergo

a collision in the near future as we are heading toward M31,

the Andromeda Nebula," van Dokkum adds. "'Near future' in

this case is about four billion years from now though."