expanding/accelerating universe

[bwanaaa]

In this TED talk
http://www.ted.com/talks/sean_carroll_distant_time_and_the_hint_of_a_multiverse.html

Sean Carroll mentions that in 1998 we had data that showed the universe's expansion is accelerating. I thought Hubble showed that much longer ago?

what is the data to which he refers?

tnx.

 

[TastesLikeChicken]

He's probably referring to the Type 1a supernova observations that kicked off the whole accelerating universe debate.

 

[DirkGently1]

Just to be clear, it's not whether the expansion is accelerating but the rate of change that's been somewhat debated in the last few decades. When i was a lad it was said that the rate of acceleration was decreasing, leading to speculation of a possible 'Big Crunch', whereas now it's believed the rate of acceleration is increasing, leading to speculation of 'Dark Energy' providing the driving force for this change.

Correct me if i'm wrong as i don't study cosmology as much as i used to.

 

[Sunny129]

Just to be clear, it's not whether the expansion is accelerating but the rate of change that's been somewhat debated in the last few decades. When i was a lad it was said that the rate of expansion was decreasing, leading to speculation of a possible 'Big Crunch', whereas now it's believed the rate of expansion is increasing, leading to speculation of 'Dark Energy' providing the driving force for this change.

Correct me if i'm wrong as i don't study cosmology as much as i used to.

fixed . remember, the rate of change in the Univere's expansion velocity IS acceleration or deceleration. other than the terminology, i believe you're correct...

 

[A5]

fixed . remember, the rate of change in the Univere's expansion velocity IS acceleration or deceleration. other than the terminology, i believe you're correct...

I think the debate currently is about the rate of change of the acceleration (the jerk), though. I'm not an astrophysicist though, so I'm probably wrong

 

[Sunny129]

I think the debate currently is about the rate of change of the acceleration (the jerk), though. I'm not an astrophysicist though, so I'm probably wrong

well even if the debate were over the rate of acceleration (as opposed to the rate of expansion), i don't know what that would really tell us about the universe...that is, i don't quite see the importance in knowing the rate at which the acceleration of the expansion of the universe changes. in fact, i don't even know what the 3rd derivative of a position function is by definition, other than to simply define it as the rate of change in acceleration.

either way, all one needs to know in order to determine that the expansion of the Universe is either accelerating (resulting in an infinitely dark/diffuse and infinitesimally dense universe) or decelerating (the consequences of which would be the "Big Crunch") is the change in its rate of expansion...which would in fact be acceleration, not a change in the rate of its acceleration.

 

[silverpig]

well even if the debate were over the rate of acceleration (as opposed to the rate of expansion), i don't know what that would really tell us about the universe...that is, i don't quite see the importance in knowing the rate at which the acceleration of the expansion of the universe changes. in fact, i don't even know what the 3rd derivative of a position function is by definition, other than to simply define it as the rate of change in acceleration.

either way, all one needs to know in order to determine that the expansion of the Universe is either accelerating (resulting in an infinitely dark/diffuse and infinitesimally dense universe) or decelerating (the consequences of which would be the "Big Crunch") is the change in its rate of expansion...which would in fact be acceleration, not a change in the rate of its acceleration.

It's called "the jerk" as the other poster said

And it would tell us about the rate of uh, creation, of dark energy.

Say dark energy has a certain energy density, and this leads to a negative pressure between two objects, causing them to accelerate apart from each other. If the energy density of dark energy is increasing, then this acceleration is becoming greater.

 

[bwanaaa]

Say dark energy has a certain energy density, and this leads to a negative pressure between two objects, causing them to accelerate apart from each other. If the energy density of dark energy is increasing, then this acceleration is becoming greater.

so does anyone know which data indicate the increasing amount of dark matter?

 

[silverpig]

so does anyone know which data indicate the increasing amount of dark matter?

I don't think there is enough data to even make that an educated guess. It's just a thought exercise for now.

 

[Sunny129]

It's called "the jerk" as the other poster said

And it would tell us about the rate of uh, creation, of dark energy.

Say dark energy has a certain energy density, and this leads to a negative pressure between two objects, causing them to accelerate apart from each other. If the energy density of dark energy is increasing, then this acceleration is becoming greater.

ok, so if i understand you correctly, aside from the derivative of acceleration being used to calculate by definition the rate of change in acceleration, it is in this context being used to infer the rate of change in the energy density of dark energy?

i'm still trying to wrap my head around what a change in acceleration means in the physical world. given that its referred to as "the jerk," i have to assume that it is an even more abrupt change in velocity than that which is caused by a constant, unchanging acceleration. is this at all accurate?

TIA,
Eric

 

[silverpig]

ok, so if i understand you correctly, aside from the derivative of acceleration being used to calculate by definition the rate of change in acceleration, it is in this context being used to infer the rate of change in the energy density of dark energy?

i'm still trying to wrap my head around what a change in acceleration means in the physical world. given that its referred to as "the jerk," i have to assume that it is an even more abrupt change in velocity than that which is caused by a constant, unchanging acceleration. is this at all accurate?

TIA,
Eric

First no one is able to measure the change in the rate of acceleration of the universe yet. There just isn't enough data.

1. Standing still (constant position)
2. Moving in one direction at a constant speed (constant velocity)
3. Accelerating uniformly (constant acceleration)
4. Starting to accelerate (jerk)

1 and 2 you are familiar with so I will leave them alone. With 3, you are accelerating constantly at a uniform rate. You will feel a constant and unchanging force propelling you forward (F=ma, m and a are constant, so F is constant). You feel this constant force when your car is in the middle of a turn, when a plane accelerates down the runway, and when an elevator stops. By the equivalence principle (google it), constant acceleration feels exactly like constant gravity.

Jerk is exactly what it sounds like. You get jerked around.

Imagine you are in a drag race car. At the start line your position, velocity, acceleration, and jerk are all 0. You're just sitting there. Then you hit the gas and start to move. Your head snaps back as you begin to accelerate. This is jerk. Your acceleration goes from 0 to some relatively constant level (say 8 m/s^2) quite quickly. During the ramp up to this acceleration level, you experience jerk.

Once the engine is applying its force to the road through the wheels and is established, jerk goes to zero (for simplicity) and you just feel a constant force pushing you back in the seat. This is constant acceleration.

 

[Sunny129]

thanks for the explanation...i guess i have a much better understanding of "jerk" than i thought i did. i think my mental block stemmed from the fact that even though i was already aware that acceleration can change, i was not picturing how the change occurs in real-time. i wasn't thinking about the fact that acceleration cannot jump from one value to another instantaneously, and that a change in acceleration must take place over some time interval...the concept of "jerk" makes much more sense now.