[Q] Universe expansion

[kristoferzz]

I was watching Discovery Channel and guys there said that nobody has any idea why universe expansion is getting faster. Also they copmared the universe with baloon. It gave me an idea..I dont know if it could be so..maybe it is stupid idea, but it seems logical to me, maybe sombody can comment.

 

So here it is:

 

Universe is expanding faster and faster because the gravity gets weaker between galaxies as they are getting farer away from each other. The same as the baloon rubber gets thinner when baloon gets bigger and it is more easier to blow air in. At least in baloons I have blown.. in empty baloon is harder to blow in than in one already big... :) So in this case there is no need for some mistical "dark energie" that overwhelms gravity and fuels expansion..just the blow from the Big Bang still going :hyper:

[Tormod]

But for how long is it easier to blow air into the (larger) balloon?

[modest]

Universe is expanding faster and faster because the gravity gets weaker between galaxies as they are getting farer away from each other. The same as the baloon rubber gets thinner when baloon gets bigger and it is more easier to blow air in. At least in baloons I have blown.. in empty baloon is harder to blow in than in one already big... :) So in this case there is no need for some mistical "dark energie" that overwhelms gravity and fuels expansion..just the blow from the Big Bang still going :)

The attractive nature of gravity would slow down expansion (known as the deceleration parameter). The attractive force counteracts the inertia of the expansion. When things get further apart, as you say, gravity gets weaker and there is less slowing down of expansion. As the attractive force of gravity goes to zero (when distance becomes quite large) the deceleration (or the slowing down of expansion) goes to zero. But, such a thing would never cause acceleration of expansion, only zero deceleration.

 

You might imagine a universe where there is no gravity—everything just coasts with inertia away from everything else. In that case there would be no acceleration of expansion. There also would be no deceleration of expansion—there would just be expansion. Gravity getting weaker, even to the point of zero, doesn't cause acceleration in the rate of expansion.

 

To accelerate expansion one needs the cosmological constant so that deceleration not only goes to zero, but reverses and becomes acceleration.

 

In other words, an attractive force pulling two things together can't make the speed between them increase even if the force goes to zero.

 

Would that make sense?

 

 

 

If you're looking for a more technical answer, what you are describing is a Friedmann universe with a cosmological constant of zero and a density below the critical density which could be any of the lines from [math]\Omega_M=0[/math] to [math]\Omega_M=1[/math] (the straight-looking lines):

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source

For acceleration of expansion you need the line that curves upward where [math]\Omega_{\Lambda}=0.7[/math] where [math]\Omega_{\Lambda}[/math] represents the cosmological constant. This reflects the fact that the force of gravity is always negative (negative meaning attractive) unless there is a cosmological constant term:

[math]\mbox{Force of Gravity} = -{\frac{GM}{r^2}}+\frac{c^2{\Lambda}r}{3}

[/math]

where [math]r[/math] is the distance between things. The second term with [math]\Lambda[/math] is the cosmological constant term. With that term it becomes possible for things far enough apart to be repulsed from one another rather than just having an attractive force between them. Without the second, cosmological constant, term gravity would always be negative no matter how big [math]r[/math] got. It would always be an attractive force and never a repulsive one.

 

 

~modest