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Name: Denesha
Status: other
Grade: other
Location: NV
Country: N/A
Date: N/A


Question:
If gravity pulls everything together and down why are the galaxies moving apart from each other?


Replies:
Denesha,

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.

Vince Calder



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