Unraveling the Mysteries of Spacetime: How Physicists Know

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Physicists deduce that spacetime was created at the Big Bang by observing galaxies moving away from each other, indicating the expansion of spacetime itself rather than existing space filled with mass. The notion that only mass was created contradicts the first law of thermodynamics, as it implies a pre-existing universe governed by the same physical laws. Additionally, if mass were created in an existing universe, a definitive center would be evident, and gravity would cause a collapse rather than expansion. The discussion highlights multiple lines of evidence supporting a hot, dense origin of the universe. Overall, the expansion of spacetime is fundamental to understanding the universe's beginnings.
Jonnyb42
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How in the heck do physicists know that spacetime was created at the big bang just by observing galaxies moving away from each other? Couldn't that mean that that's when the MASS was created?
 
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You made the classic mistake of thinking that it's motion of galaxies moving away from each other into existing space. It's not. It's not space was there, and boom stuff was created and now it's flying away from each other like an explosion. It's spacetime itsself that is expanding.

You idea that it could just be the mass that was created violates the 1st law of thermodynamics, which if the Universe was existing before hand follows the same phyical laws that govern us. Not only that, but if just the mass was created in the existing universe then there would be a definitive centre that could be found. Also if all the matter were created in 1 single spot, gravity would dominate and it would collapse back in on itsself.

Those are three of the big problem with the idea that it was just the 'stuff' that was created within the universe, not the universe itsself.

There are also many other peices of evidence that point to a small dense hot beginning to the universe.
 

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