# How much energy do we need to make an expanding universe?

1. Apr 27, 2015

### Quarlep

I know that universe can be have three different futures.But for lastes theories universe is growing faster then we thought.I want to ask how much energy we need to make this observable universe.

2. Apr 27, 2015

### Staff: Mentor

I don't understand your question. Are you asking how much total energy is in our observable universe, either now or at some point in the past? There really isn't a well-defined answer to this question.

3. Apr 27, 2015

### Quarlep

We know that universe should collapse or at least be stable but we know thats not true.If potantial energy is high or equal then kinetic energy we get this two solutions.But of course theres an energy number to calculate which one is bigger you calculate kinetic energy and potantial energy and then you can see which one is bigger and you can find universe future.We know universe is expanding so without dark energy universe should collapase but with dark energy universe started to expand so dark energy make difference between closed,flat,and open universe.So myself I made a conclusion dark energy make differences between flat universe and expanding universe.But theres how much dark energy can we know that.Thats my question answer.

Last edited: Apr 27, 2015
4. Apr 27, 2015

### Staff: Mentor

Why?

There is no well-defined "potential energy" for the universe.

This is not correct. It is perfectly possible for a universe with no dark energy in it to expand forever.

5. Apr 27, 2015

### Quarlep

I know that solution in Friedmann eqution of it but you know that there a lot of evidences about dark energy.

6. Apr 27, 2015

### Quarlep

If theres a solution for that why physicist believe dark energy

7. Apr 27, 2015

### Quarlep

Without dark energy what would be happen

8. Apr 27, 2015

### Staff: Mentor

The universe's expansion would be decelerating instead of accelerating. If we assume that everything else is held constant, then the expansion would still continue forever (as it is expected to do in our actual universe with dark energy).

9. Apr 27, 2015

### Quarlep

What means everything held is constant

10. Apr 27, 2015

### Quarlep

Without dark energy means k equal negative in F equation that gives us a universe expands forever.

11. Apr 27, 2015

### Quarlep

Lats observations says there cant be collapse so to possible solution left.Again we need dark energy For make universe how its look like

12. Apr 27, 2015

### Staff: Mentor

Quarlep, I am really confused as to what you are asking. Are you asking what the actual dark energy density is in our universe, and how we know?

13. Apr 27, 2015

### Quarlep

Friedmann equations didnt gave the observable provement so that we create dark energy.And dark energy makes our universe observable provement.I mean dark energy solves the observable problems isnt it ?

14. Apr 27, 2015

### Chalnoth

Dark energy is a relatively small adjustment to late expansion rates (last few billion years or so). There's no reason to believe that dark energy was ever "created". The simplest model is the cosmological constant, which has been a component of General Relativity pretty much from the beginning of the theory.

But yes, it solves observational problems. Every scientific model or theory ever conceived was designed precisely to solve observational or theoretical issues. I don't know why you think pointing this out is important.

15. Apr 27, 2015

### Chalnoth

As far as we know, zero.

The total energy of a universe is somewhat ambiguous. However, it is possible under certain special circumstances to write down an energy for the universe as a whole (this would be an expanding universe with closed spatial curvature). And that energy turns out to be identically zero.

16. Apr 27, 2015

### Quarlep

It changes somethink isnt it.So theres two universe model I was asking energy diffrences between them and thats dark energy

17. Apr 27, 2015

### Staff: Mentor

There aren't two models of the universe; there are an infinite number of possible models, both with and without dark energy. One of those models is our current best fit to observational data; that model happens to include dark energy. What other model are you proposing to compare it with?

18. Apr 27, 2015

### Quarlep

Friedmann model without cosmological comsatnt

19. Apr 27, 2015

### Staff: Mentor

Which Friedmann model without a cosmological constant? There are an infinite number of them, all with different values for parameters like the density of matter, the density of radiation, etc. Which one do you want to use?

20. Apr 27, 2015

### Quarlep

The one wwlhich the our universe through history