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We know from newton laws of gravity how planets, moons etc rotate in orbit around other bodies but how does this really get setup when we know everything starting with a big bang?

As with big bang or an exploding star, it scatters the matter away in space. Assume that exploding star was the only thing in universe for simplicity (and to retrace the big bang), how would these remnants set themselves up in orbits?

zadane
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    Don't confuse the formation of solar systems with the Big Bang. Suggest you read Did the Big Bang happen at a point ? and perhaps some web pages on how planets are formed. – StephenG - Help Ukraine Apr 18 '20 at 03:52
  • @StephenG My question is not exactly about big bang, Say a star exlodes and the particles obeys newton laws of motion. Since they are set in motion, they will continue in motion for ever. But say gravity of fellow particles slows it down. Eventually say it stops to zero and then perhaps reverses. It's hard to visualize how does it sets up the orbit then instead of just collapsing on itself. – zadane Apr 18 '20 at 05:35
  • @zadane: Your question shows a misunderstanding of the Big Bang. It was not an explosion of matter outward from a point. – Christopher James Huff Apr 19 '20 at 02:21

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Suppose first that all debris have the same radial velocity in the inertial frame of the star (initial frame, before the explosion). If we are in the frame of one of that bodies, we see the others moving away also radially.

But it is a coincidence, because the only conclusion from the explosion is that momentum is conserved. If the fragments have different masses, they can have different velocities and the moment be conserved anyway.

An extreme example is an explosion totally off centre, that throws away small parts of the star. Most of them will have tangential and radial velocities and end up orbiting the remaning big part of the star.

Claudio Saspinski
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