Gravitational Attraction, Galactic Expansion
If gravity pulls everything together and down why are the
galaxies moving apart from each other?
What we call gravity is the result of the attraction masses have for each other.
In the case of the Earth, this would be the attraction that the mass of the Earth
have for everything that is on it.
However, just like on the Earth, we can apply enough force to counter the force of
the gravitational pull of the Earth. For example, we can throw a ball up and away
from the Earth and for a few moments have that ball be moving away from the Earth.
Similarly, we can fire a rocket and have a spacecraft move permanently away from
the Earth and the solar system.
So, some ways we can think about the fact that galaxies are moving farther apart is
that: (1) there was an initial push that got the galaxies moving apart (like the Big
Bang) and the gravity that brings the galaxies together has not quite overcome this
initial push (just like a ball will keep moving away from the Earth for a while),
or (2) there is something that is constantly pushing the galaxies apart (vacuum may
be expanding or tends to get bigger, or there is something in vacuum that pushes
things apart) much like a long range satellite that is constantly being pushed away
from the solar system.
Greg (Roberto Gregorius)
The short, but true, answer is no one really knows. Originally, astronomers thought
the Universe would "slow down" and come to a halt, then others asked whether the
galaxies would not only come to a halt but reverse their expansion and collapse.
Still later Hubble found experimentally that galaxies were moving further apart
faster. Even later yet the expansion was found to be even faster than it "should
be". This begged for an explanation involving invisible "dark" matter and "dark"
energy. Or alternatively, maybe the laws of gravity were different at large
distances. The answer(s) have not yet been resolved.
Click here to return to the Astronomy Archives
Update: June 2012