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

[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.

 

[PeterDonis]

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.

 

[Quarlep]

We know that universe should collapse or at least be stable but we know that's not true.If potantial energy is high or equal then kinetic energy we get this two solutions.But of course there's 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 there's how much dark energy can we know that.Thats my question answer.

 

[PeterDonis]

We know that universe should collapse or at least be stable

Why?

If potantial energy is high or equal then kinetic energy we get this two solutions.

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

without dark energy universe should collpase

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

 

[Quarlep]

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

 

[Quarlep]

If there's a solution for that why physicist believe dark energy

 

[Quarlep]

Without dark energy what would be happen

 

[PeterDonis]

Without dark energy what would be happen

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).

 

[Quarlep]

What means everything held is constant

 

[Quarlep]

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

 

[Quarlep]

Lats observations says there can't be collapse so to possible solution left.Again we need dark energy For make universe how its look like

 

[PeterDonis]

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?

 

[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 isn't it ?

 

[Chalnoth]

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 isn't it ?

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.

 

[Chalnoth]

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.

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.

 

[Quarlep]

It changes somethink isn't it.So there's two universe model I was asking energy diffrences between them and that's dark energy

 

[PeterDonis]

So there's two universe model I was asking energy diffrences between them and that's dark energy

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?

 

[Quarlep]

Friedmann model without cosmological comsatnt

 

[PeterDonis]

Friedmann model without cosmological comsatnt

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?

 

[Quarlep]

The one wwlhich the our universe through history

 

[PeterDonis]

The one wwlhich the our universe through history

But there isn't one; that's the whole point. The model that best matches the observational data has dark energy in it. That means our universe contains dark energy, and always has.

 

[phinds]

The one wwlhich the our universe through history

That's one that includes dark energy.

 

[Quarlep]

Without dark energy we can't tell what will going to happen.

 

[marcus]

Friedmann model without cosmological comsatnt

Hi Quarlep, I think I understand your question.
Some scientists believe that there is no "dark energy" but simply a cosmological constant, like what Einstein wrote in his equation.
It is on the lefthand side and belongs there like a constant of integration. A constant curvature which is allowed by the symmetry of the theory.

If I understand what you are asking, you are saying that you understand how Friedmann model with cosmological constant can fit the data.
But then you ask what if there is no basic built-in constant curvature Λ, what if there is an actual DARK ENERGY FIELD INSTEAD!?? THEN HOW MUCH ENERGY INPUT DOES THAT REPRESENT??

If I understand you correctly and that is what you are asking, then I have to say it seems to me like a very reasonable question for you to ask. It is a huge, mind-boggling amount of energy and the amount in any given region keeps growing as the region expands. No practical agency that I can imagine could be supplying that incredible amount of energy to the expanding universe, to keep up with the expansion. So it seems reasonable to ask about it.

I will try to answer, assuming that is what you are asking about. Probably, in my humble opinion, "dark energy" is simply a modern physicists myth or fairy story and probably there is simply a curvature constant in the GR equation and nothing like what we are used to considering an energy is needed. But if there is an actual "dark energy" let us try to say how much would be in some reasonably large region of space.

as I recall the constant energy density needed to cause that Λ curvature, if energy caused it, is 0.7 nanojoule per cubic meter. So let us multiply that by a cubic light year and see how many joules.
And remember that with ordinary expansion that volume must be expanding so it must be containing more and more "dark energy" coming from nowhere

Maybe google calculator will do it
0.7e-9joule per m^3*cubic light year

 

[Quarlep]

Exactly

 

[marcus]

I'm glad to think I understood your question! When I type in that thing at the end of the previous post, google says:
0.7e-9 (joule per (m^3)) * (cubic light year) =
5.92712685 × 10³⁸ joules

So that is the amount in a cubic light year. And roughly how many cubic light years are there in the observable universe, radius 46 billion LY (that is the particle horizon, the current distance to the farthest matter we can have gotten signals from)
4/3*pi*(46 billion LY)^3

When I put this in:
0.7e-9 joule per m^3*(4/3)*pi*(46 billion light years)^3
google gives back:
0.7e-9 (joule per (m^3)) * (4 / 3) * pi * ((46 billion light years)^3) =
2.41660865 × 10⁷¹ joules

So I guess that must be the answer

 

[Chalnoth]

The one wwlhich the our universe through history

This statement makes no sense. Prior to the 1990's, the parameters on our models for the expanding universe had gigantic error bars. We really didn't know much of anything about how that expansion rate has changed over time.

Now we know that it is impossible to fit the data we observe without some form of dark energy (or other, more exotic addition to the theory).

 

[Quarlep]

I'm glad to think I understood your question! When I type in that thing at the end of the previous post, google says:
0.7e-9 (joule per (m^3)) * (cubic light year) =
5.92712685 × 10³⁸ joules

So that is the amount in a cubic light year. And roughly how many cubic light years are there in the observable universe, radius 46 billion LY (that is the particle horizon, the current distance to the farthest matter we can have gotten signals from)
4/3*pi*(46 billion LY)^3

When I put this in:
0.7e-9 joule per m^3*(4/3)*pi*(46 billion light years)^3
google gives back:
0.7e-9 (joule per (m^3)) * (4 / 3) * pi * ((46 billion light years)^3) =
2.41660865 × 10⁷¹ joules

So I guess that must be the answer

Thank you so much

 

[Quarlep]

This statement makes no sense. Prior to the 1990's, the parameters on our models for the expanding universe had gigantic error bars. We really didn't know much of anything about how that expansion rate has changed over time.

Now we know that it is impossible to fit the data we observe without some form of dark energy (or other, more exotic addition to the theory).

Without dark energy we can say nothing as I said before but with dark energy we can fit the data with the observable one.

 

[Chronos]

You are asking an ambiguous question, quarlep. If there was a mathematically valid way to define energy globally, it would already have been done.

 

[Quarlep]

Marcus gave me an answer

 

[Chalnoth]

Marcus gave me an answer

The answer Marcus gave is one of many possible answers.

 

[Quarlep]

He calculate dark energy energy isn't it

 

[Chalnoth]

He calculate dark energy energy isn't it

He gave one possible calculation. Just because it's possible to write down some numbers doesn't mean there aren't other ways.

With a well defined question, there may be many ways to arrive at an answer, but that answer will always be the same. In this case, different methods will give very different answers.

 

[Quarlep]

Are you talking about different approches to dark energy like general relativity or QED