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

 

[Chalnoth]

No. In this case, the energy density is reasonably well-defined, but the volume is not. There is no one answer for, "What is the volume of our universe?"

 

[Quarlep]

But the best answer will be the observable universr

 

[Chalnoth]

Why?

 

[Quarlep]

Physics work with observable think.

 

[Chalnoth]

Sure, but there's more than one relevant length scale to choose from. We could pick the future horizon, or the comoving distance to the CMB, or the comoving distance to the past horizon. There are other potential choices as well, and no way to say which one is better.

 

[Quarlep]

Lets suppose I will going to write a theory.And I will going to use radius of the universe.I can't write "theres a infinite possibility"so I will write the observable one.In your logic "The only thing that I know is nothing" We can't be sure hundred per cent about anything

 

[marcus]

Hi Q,
what you say makes perfect sense to me. "Observable universe" has a conventional meaning in cosmology.
You just, to be clear, should include the adjective "observable".
Try to avoid just saying "radius of the universe" (although people do say that when they really mean the observable region).

If you say "radius of the observable universe" then I think most professional cosmologists would understand and would not quibble. It is not really ambiguous. One understands what you mean in this context is the so-called PROPER distance at this present moment in universe history.
I will tell you what the cosmologists mean by "proper" distance at some given moment.

But first I want to mention that "comoving" distance is just another word for the proper distance at THIS very moment...the proper distance NOW. It is very useful. You can give a label to every bit of matter we know about according to how far it is now and that label will not change. Even if at some time in the past that bit of matter was closer to us, it still has that "comoving" distance label. this is an extremely useful label. The proper distance that something is now.

 

[Quarlep]

Thanks

 

[PeterDonis]

If you say "radius of the observable universe" then I think most professional cosmologists would understand and would not quibble.

It's worth clarifying, though, that this is because "radius" also has a conventional meaning here, namely, the distance evaluated in a surface of constant comoving time. That does not mean this is the only physically relevant distance involved; as Chalnoth pointed out, there are others.

 

[Quarlep]

I ll be carefull using terms in cosmology

 

[marcus]

It's worth clarifying, though, that this is because "radius" also has a conventional meaning here, namely, the distance evaluated in a surface of constant comoving time. That does not mean this is the only physically relevant distance involved; as Chalnoth pointed out, there are others.

That's right, I was just getting to that
I want to make clear to Quarlep (unless you would like to) what is technically meant by proper distance at a given moment in cosmic time.
Q, this is very interesting. You may already know this. the universe has a criterion of STILLNESS. We can talk about the class of observers who are at rest with respect to the CMB. Because the CMB is a uniform soup of light which is the same temperature in all directions. But if you move in some direction the doppler effect of your motion will make it slightly warmer in that direction ahead of you and slightly colder behind you.
This can be measured and data can be corrected for the solar system and Earth's motion! We can correct our data so as to be from the standpoint of an observer which is at rest with respect to the uniform ancient gas and ancient light coming from it.

This is also the same as being at rest relative to the expansion process itself. If you are at CMB rest (ie. ancient light rest) then the expansion looks the same in all directions. Galaxies are not receding faster in one direction than in another. CMB rest really is UNIVERSE rest. It is a really beautiful fact that this is defined.

And this means we can define a preferred universe TIME which is the time measured by the class of observers all over the universe who are at rest.
They can in principle agree on time (after adjusting for some slight gravitational effects) so there is an unambiguous definition. (jargon=professional slang, these are sometimes called "comoving" observers.)

proper distance is what you would measure if you could pause the expansion process at some moment of history long enough to have time to measure it

 

[Quarlep]

Ok I understand it

 

[Chalnoth]

I find it very interesting that terms which we usually think of as being absolute here on Earth, such as relative velocity or energy, tend to lose their specificity when describing what happens in a curved space-time. It turns out to be quite difficult, and counter-intuitive, to come up with absolute measurements that are useful in General Relativity.

For example, the relative velocity between two different objects is well-defined in General Relativity if those two objects are passing the same point in space-time. If the two objects are far away from one another, we lose our ability to unambiguously state the relative velocity. This means, for example, if you were to ask me, "How fast is that galaxy moving away from us?" then the most honest answer would be, "Well, it depends. There are a few choices, and there's no way to say which is better." This is why the speed of light limitation in General Relativity only applies to objects passing one another: it cannot apply to far-away objects because that's not a well-defined quantity in General Relativity.

The same goes for energy, and for similar underlying reasons.

A related discussion, on the conservation of energy, can be found here:
http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

 

[Quarlep]

You are saying there's a few ways to calculate universe observable radius and these ways takes us different solutions.
Cause General relativity don't allow us to measure it

 

[marcus]

I ll be carefull using terms in cosmology

I know. I think you have been being careful, actually.
You are working in a language which is not your first language. And also there are these technical terms. Like "proper".

Measuring a distance at some given moment in time, by some conventional means like radar, or a long string, or laser beams etc. Measuring always takes time. But distances are changing. So you have to imagine PAUSING the expansion process long enough to measure. But then you have to have a preferred cosmic time---a "universe standard time"---so you can say I want to measure the distance NOW. That "NOW" has to have universal meaning throughout the region where you are measuring.

But usually you can just say "distance" and professionals will understand you mean proper distance ( at whatever the appropriate time is).
People of good will understand each other partly using context and rarely need to quibble.

So anyway, we were talking about the volume of the observable universe. Does anybody find that there is some ambiguity? Should we clarify or add some modifiers here?

In an earlier post I used the socalled "particle horizon" as the conventional meaning of the radius of the observable universe. It is the comoving distance (distance NOW) of the farthest matter we can in principle have gotten light or other signal from. A rough estimate is 46 billion LY. That is the current radius of the current observable region. The region grows as there is more time for light etc to come in from farther matter.

So I made a rough estimate of the volume of the (until now) observable portion of the universe assuming approximate flatness:
(4/3)πR³