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Every point mass is attracted to each other with a force $F= \frac{GMm}{r^2}$. I know nothing about theories behind expanding universes but, why wouldn't the universe eventually shrink down to a minuscule space?


marked as duplicate by Chris, Qmechanic Feb 26 '18 at 4:54

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    $\begingroup$ Possible duplicate of How can the universe expand if there is gravitation? $\endgroup$ – Stéphane Rollandin Feb 25 '18 at 23:13
  • $\begingroup$ I understand that the universe is expanding currently due to the initial, extremely high velocities from the Big Bang. But can the theory by which the universe eventually stops expanding and shrinks down be discarded anyway? Or is it feasible? $\endgroup$ – Nico de Ory Feb 25 '18 at 23:16
  • $\begingroup$ @NicodeOry, I'd say one Big Bang deserves another, but there isn't really a settled cosmological model that rules anything out and there are a variety of theories to account for our limited observations about how the universe evolves. Cosmological models in general tend to raise as many questions as they answer. Ultimately no matter what we observe and what theories we conceive, we can never be assured that we know all of reality, and there always remains the potential for so-called "new physics" whose effect takes place over scales that dwarf all life on Earth. $\endgroup$ – Steve Feb 25 '18 at 23:38
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    $\begingroup$ Newtonian physics is not enough to describe the expansion of the universe. You need general relativity and specifically you need a thing called the Friedmann–Lemaître–Robertson–Walker metric. $\endgroup$ – StephenG Feb 25 '18 at 23:52

If I understand your question correctly, you are noting that gravitation is purely attractive and are wondering why that doesn't just pull everything together into a small blob of matter.

One reason is centrifugal force. Imagine two masses connected by a spring, spinning around like a bolo. The spring is trying to pull them together, but unless there is friction to slow down the spinning, centrifugal force will keep the two apart. They will just keep spinning around without getting closer to each other. Think of gravity as being the spring.

Another reason is conservation of energy. If masses fall toward each other, they move faster and faster as they fall. If they have the slightest sideways motion they will not collide and they'll fly past each other and continue until they are as far apart as they were in the beginning. If there are multiple masses it's more complicated, but the bottom line is that in order to concentrate some of the masses into tight orbits around a central point, it is necessary for another portion of the masses to be moved to very high orbits -- simply for conservation of energy.


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