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.
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
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.
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."