# B Universe is accelerating expanding

1. Jun 4, 2016

### Physics_Kid

if the uverse is expanding and known to be accelerating, at what rate is this acceleration? even at small # over the billions of years, would it not be at speed of light already, and if so, how would observable mass be traveling at the speed of light?

2. Jun 4, 2016

### phinds

You confusion as a beginner to all this is understandable, but It IS WAY over the speed of light already. Even inside the observable universe it is now up to about 3c, but nothing is "traveling" at that speed, it is a recession speed with no proper motion involved. Google "metric expansion" and come back if something there isn't clear, as it might well not be. This stuff is not intuitive. Also you could do a forum search and see where this issue has already been discussed here just over 18,000 times.

3. Jun 4, 2016

### Staff: Mentor

First and foremost, expansion isn't measured as a speed. The rate at which two objects recede from each other (how quickly the distances between them increases, known as the recession velocity) can be measured as a speed, but the same rate of expansion creates different recession velocities for objects at varying distances from each other. The further away two objects are, the faster they recede from each other for any given rate of expansion.

Second, the acceleration of the expansion isn't acceleration as you and I know it based on our everyday experiences. It's not acceleration that an accelerometer would measure. In this sense its just like gravity, which is not a force in GR and doesn't accelerate objects.

Third, there is no limit to how fast objects can recede from each other due to expansion.

4. Jun 5, 2016

### Physics_Kid

yeah, the details are beyond me currently, the concepts are not. i get recession speeds, but for two objects moving in same plane 180 degrees from each other, isnt 2c the recession limit? and would it not be the case that as you approach 2c in recession it would take an infinite amount of time for light from one to reach the other?

5. Jun 5, 2016

### Staff: Mentor

Nope. In General Relativity there is no global speed limit. Two objects can recede from each other at any speed, no matter how fast. However, light speed is still important in the sense that nothing can ever beat a beam of light in a race. Light speed is a local speed limit. Remember that expansion does not accelerate objects like a rocket engine accelerates a rocket. In the frame of reference of the object it is always stationary and never feels any accelerating force.

6. Jun 5, 2016

### Physics_Kid

so in the general sense of years ago scientists thought the galaxies would eventually slow and return to a point due to gravity, but then they found it to be accelerating. dark energy may explain how that acceleration is being fueled, but how can it keep accelerating w/o limits ??

7. Jun 5, 2016

### Staff: Mentor

Well, I could tell you all about the theories and math behind it, but in the end that's simply the way it works.

8. Jun 5, 2016

### rootone

'Dark energy' is just a place-holder name for something which is observed to happening (expanding Universe).
However we don't have much idea what it could be physically, so it's properties in the future are not known.
It could turn out not to be physical entity at all but just a missing term in presently accepted mathematical descriptions, or some overlooked artifact in the way we measure things.

9. Jun 5, 2016

### Jorrie

I guess you have meant: (accelerating expansion of the Universe).

10. Jun 6, 2016

### Physics_Kid

so let me ask you this, the two competing teams (years ago) came to same conclusion about accelerating expansion. given that both teams work under the same constraints of math and physics, and, the observations were done on huge distances, is it possible that due to the observations being made over huge distances that the conclusion of accelerating was skewed by some unknown artifact within the observations?

11. Jun 6, 2016

### rootone

Oops, yes.

12. Jun 6, 2016

### phinds

Remotely possible that the earliest observations, taken on their own, were flawed in some way but given all the confirmation since, I'd say definitely not. Do you have some kind of bone to pick with the concept of accelerated expansion?

13. Jun 6, 2016

### Physics_Kid

no bones to pick. i am one of those "what if" and "what about" folks. there has been many "we were wrong" statements made over the last X years, so its not out of the realm that something as we know it right now will be slight or drastically different tomorrow, etc.