How does the Universe's expansion relate to Newton's 1st law

In summary, the universe's expansion does not directly relate to Newton's 1st law. Instead, it is governed by the Einstein equations in General Relativity, which describe the behavior of spacetime itself. This means that the expansion of space is not caused by a specific force, but is a natural consequence of the Einstein equations.
  • #1
Pjpic
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How does the universe's expansion relate to Newton's 1st law.

In other words, is all the expansion due to an unknown cause at the big bang and the 4 forces now act on it? Or maybe is the expansion not really due to a force (as is gravity) so the 1st law doesn't apply?
 
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Pjpic said:
How does the universe's expansion relate to Newton's 1st law.

In other words, is all the expansion due to an unknown cause at the big bang and the 4 forces now act on it? Or maybe is the expansion not really due to a force (as is gravity) so the 1st law doesn't apply?
It really doesn't apply. General Relativity and Quantum Mechanics are both important for understanding the behavior of the very early universe, neither of which were conceived of when Newton wrote that law.
 
  • #3
Hi,

Newtons 1st law is in GR replaced by the 'law' that point particles follow geodesics, the straighest possible paths in spacetime. However, this is a statement about stuff in spacetime, not about spacetime itself. Spacetime itself obeys in GR the Einstein equations. The expansion of space is perfectly described by these Einstein equations.

So, short answer: it does not.
 
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1. How does the Universe's expansion relate to Newton's 1st law?

Newton's 1st law, also known as the law of inertia, states that an object at rest will stay at rest and an object in motion will remain in motion with a constant velocity unless acted upon by an external force. In the case of the Universe's expansion, this law applies in the sense that the objects (galaxies, planets, etc.) in the Universe will continue to move in a straight line with a constant velocity unless acted upon by a force, such as gravity.

2. Why is Newton's 1st law important in understanding the Universe's expansion?

Newton's 1st law is important in understanding the Universe's expansion because it helps us to understand the motion of objects in the Universe. It explains why objects will continue to move outward from each other, as the expansion of the Universe causes an increase in distance between them, unless acted upon by a force.

3. How does the expansion of the Universe affect the motion of objects?

The expansion of the Universe causes objects to move further away from each other, resulting in an increase in their distance and a decrease in their relative motion. This is because the expansion is causing the space between objects to increase, rather than the objects themselves moving through space.

4. Does the expansion of the Universe violate Newton's 1st law?

No, the expansion of the Universe does not violate Newton's 1st law. This law still applies to objects within the Universe, but it does not apply to the expansion of space itself.

5. How does dark energy play a role in the Universe's expansion and Newton's 1st law?

Dark energy is believed to be the force behind the accelerating expansion of the Universe. It acts as a repulsive force, pushing objects further away from each other and causing the expansion to accelerate. This does not violate Newton's 1st law, as the objects in the Universe are still subject to the law and will continue to move in a straight line unless acted upon by a force, in this case, dark energy.

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