I Questions on Expansion of the Universe and Newtonian Physics

[lee_st3]

TL;DR Summary

I just have a few musings about how newtonain physics could be applied to the accelerating universe.

So I know that people say that Newtonian physics can't be applied alone to the total of the cosmos, as the universe is expanding outward in contradiction to the Newtonian principles. My question has to do with the force and energy regarding the big bang. We know that the big bang was a huge release of energy and force outwards. Could the increasing acceleration of the universe be attributed to the massive force of the big bang inducing an acceleration that hasn't become negative due to the sheer force? And later in time the gravity of the universe will eventually slow the acceleration? Sorry if this is a dumb question and if you can't follow my logic, I'm new to astrophysics and am not well versed on the topic, but I was curious.

 

[Orodruin]

We know that the big bang was a huge release of energy and force outwards.

No, it wasn't. There is no "outwards" in the big bang model of the early universe. Your question is based on false premises. What you are describing is simply not how the model works.

 

[phinds]

We know that the big bang was a huge release of energy and force outwards.

Clearly, you have gotten your "knowledge" from pop-sci presentations, which DO describe (TOTALLY incorrectly) the Big Bang Singularity as an explosion in space with everything radiating out from there. This is just nonsense. Read some actual physics. Pop-sci presentation can be great fun to watch and read but they are entertainment, not education.

 

[Ibix]

No, in short.

The acceleration familiar from Newtonian physics is a very different thing from the second derivative of the scale factor, which is the "acceleration" referred to in cosmology. They have the same broad sense of increasing speed, and there isn't realy a more appropriate word than acceleration, but they really are completely distinct concepts. For example, even in a universe where there is no accelerated expansion, receding objects increase their recession speed over time.

You really do need to get to grips with the maths before you can make sensible suggestions, I'm afraid.

 

[Orodruin]

For example, even in a universe where there is no accelerated expansion, receding objects increase their recession speed over time.

That not really correct. Recession speed between comoving objects is proportional to the Hubble parameter $H = \dot a/a$ and to the distance $d = ax$, where $x$ is the fixed comoving distance. The recession speed is therefore $\dot a x$, which increases if its derivative $\ddot a x$ is positive, ie, if $\ddot a >0$ - which is the very definition of accelerating expansion.

Alternatively just differentiate the physical distance $d$ twice for the same result.

 

[Ibix]

That not really correct. Recession speed between comoving objects is proportional to the Hubble parameter $H = \dot a/a$ and to the distance $d = ax$, where $x$ is the fixed comoving distance. The recession speed is therefore $\dot a x$, which increases if its derivative $\ddot a x$ is positive, ie, if $\ddot a >0$ - which is the very definition of accelerating expansion.

Alternatively just differentiate the physical distance $d$ twice for the same result.

Ah yes - was thinking $Hd$ and forgot $d=a\chi$.

 

[Drakkith]

Could the increasing acceleration of the universe be attributed to the massive force of the big bang inducing an acceleration that hasn't become negative due to the sheer force?

No. The expansion of space is not the result of applying a newtonian-style force.

And later in time the gravity of the universe will eventually slow the acceleration?

The expansion is slowing down, but that rate of slowdown will actually decrease as time passes. In other words, the expansion is 'accelerating' as objects in the universe become more spread out and gravity between them weakens.

 

[Jaime Rudas]

So I know that people say that Newtonian physics can't be applied alone to the total of the cosmos, as the universe is expanding outward in contradiction to the Newtonian principles.

In fact, Newtonian physics doesn't contradict the expansion of the universe, as Professor Leonard Susskind explains here.