Is Dark Energy Necessary for the Acceleration of the Universe's Expansion?

[gregtomko]

The question is entirely unanswerable without taking a mechanism into account.

Why is it that the acceleration of the expansion of the universe, related to the mass of the universe, can not be related to the energy necessary to cause it? Why does there have to be a mechanism? I know there HAS to be a mechanism to EXPLAIN it, but to just relate the ratio of energy in, to energy of acceleration, seems like a simple concept.

 

[gregtomko]

If you assume (a la Newton, in violation of GR) that you could have a static background space in which stars fuse, emit energy, acting altogether as a mixed gas of light, particles, and stars: then you would have decreasing rate of expansion not acceleration. All contributors to pressure of the gas would decrease as the universe expanded. Thus, this hypothesis is trivially counter-factual.

I would like to know what "this hypothesis" is. I am not proposing a hypothesis. I am just asking a simple question.

 

[phinds]

Why is it that the acceleration of the expansion of the universe, related to the mass of the universe, can not be related to the energy necessary to cause it? Why does there have to be a mechanism? I know there HAS to be a mechanism to EXPLAIN it, but to just relate the ratio of energy in, to energy of acceleration, seems like a simple concept.

Yeah, but it keeps coming back to the fact that without a mechanism the number is pointless. I think that folks on this board, when answering questions, natually assume that there is a POINT to the question. Searching for the value of a meaningless statistic doesn't fit the bill, and without a mechanism, your entire discussion is just a search for the value of a meaningless statistic.

 

[PAllen]

I would like to know what "this hypothesis" is. I am not proposing a hypothesis. I am just asking a simple question.

The hypothesis is that energy released by fusion from stars could account for accelerated expansion. It can't. If this was the mechanism of expansion, the prediction would be for decreased rate of expansion rather than accelerated expansion.

 

[gregtomko]

Yeah, but it keeps coming back to the fact that without a mechanism the number is pointless. I think that folks on this board, when answering questions, natually assume that there is a POINT to the question. Searching for the value of a meaningless statistic doesn't fit the bill, and without a mechanism, your entire discussion is just a search for the value of a meaningless statistic.

Seriously, the POINT of the question is what the ratio of energy is, that's the whole point. There is no other point. I am sure that the question might be meaningless. Does that mean I should not ask the question?

 

[gregtomko]

The hypothesis is that energy released by fusion from stars could account for accelerated expansion. It can't. If this was the mechanism of expansion, the prediction would be for decreased rate of expansion rather than accelerated expansion.

I am not arguing with the concept, I would just like to know where to look for the information. If the energy needed to accelerate the universe is somewhat understood, and the mass of the universe is similarly understood, and the total amount of energy released from stars through time is similarly understood, then the ratio of how much energy is available to that which is needed to explain what we see, should also be similarly understood. I am just asking what that ratio is.

 

[gregtomko]

Maybe a more clear set of questions would be helpful. Is there a range of estimates for the energy needed to cause the universe to expand as it is observed? Is there an estimate of the mass of the universe? Is there an estimate of the total energy released through fusion in the universe?

 

[Drakkith]

Seriously, the POINT of the question is what the ratio of energy is, that's the whole point. There is no other point. I am sure that the question might be meaningless. Does that mean I should not ask the question?

The ratio of energy released by stars to the energy required to do what? Accelerate the universe? How can I figure out the required energy eithout a mechanism to explain how to figure out this required energy?

 

[Drakkith]

Maybe a more clear set of questions would be helpful. Is there a range of estimates for the energy needed to cause the universe to expand as it is observed? Is there an estimate of the mass of the universe? Is there an estimate of the total energy released through fusion in the universe?

Hold on. Are you asking about the expansion itself, or the acceleration of the expansion?

 

[gregtomko]

I am asking about the energy needed to cause the acceleration of the expansion.

 

[Drakkith]

Hmm. Would there even need to be an expenditure of energy? Or just a force? It isn't that objects are getting pushed away from each other, gaining velocity in space, but that space is expanding in between them.

 

[gregtomko]

It isn't that objects are getting pushed away from each other, gaining velocity in space, but that space is expanding in between them.

What is the difference between objects gaining separation in space, and space expanding between objects?

 

[Drakkith]

What is the difference between objects gaining separation in space, and space expanding between objects?

For one, an object cannot exceed the speed of light as measured by traveling through local space. (Non-expanding space around massive objects) However, two galaxies can be receding from one another at a rate greater than the speed of light because neither are traveling through local space anywhere close to that speed. Instead space itself is expanding between them, carrying them apart.

 

[gregtomko]

two galaxies can be receding from one another at a rate greater than the speed of light.

I thought relativity excluded that possibility. Doesn't time skew as the rate of those galaxies separation increases?

 

[gregtomko]

How can I figure out the required energy without a mechanism to explain how to figure out this required energy?

Just the simple equation of F=ma. There is a mass of the universe. There is an acceleration of the universe. There can be an expected F on the universe. From that force, a quantity of energy needed to satisfy the F=ma relationship can be calculated.

 

[Drakkith]

I thought relativity excluded that possibility. Doesn't time skew as the rate of those galaxies separation increases?

No, because they are not traveling through local space at near the speed of light. If we could cut away all the space between us and that galaxy it would be traveling very close to our own speed.

Just the simple equation of F=ma. There is a mass of the universe. There is an acceleration of the universe. There can be an expected F on the universe. From that force, a quantity of energy needed to satisfy the F=ma relationship can be calculated.

There is no acceleration on the mass itself, the acceleration is only causing the rate of expansion to increase. IE how fast a volume of space expands to a certain size, say double it's current volume.

 

[gregtomko]

There is no acceleration on the mass itself, the acceleration is only causing the rate of expansion to increase.

If the stars aren't actually accelerating away from each other, I can see how there would be no way to calculate the energy needed. That is definitely where my confusion arises. That was the whole basis of the question. Thanks so much for your input! I think you cleared this up.

 

[gregtomko]

If the red shift data on the expansion of the universe was considered as the acceleration of the stars away from each other, is it known to any degree of accuracy, how much energy would be required to cause that acceleration? Just straight forward F=ma if it assumed that the stars are accelerating how the red shift makes them appear.

 

[gregtomko]

I think found part of the answer I was looking for. The gravitational binding energy of our sun is 6.9E41 J. The energy output at its current rate, if you average that over its lifetime, which is smaller than the total output, would be something like 4E46.

 

[gregtomko]

Is radiation pressure and neutrinos a significant portion of the total energy output of stars, or most of it, or very little?

 

[Drakkith]

So is radiation pressure and neutrinos a significant portion of the total energy output of stars, or most of it, or very little?

Almost all of it. One might include the solar wind too.

 

[gregtomko]

I know it is inconsequential, but how did the word "So" get placed at the beginning of the previously quoted text?

 

[Drakkith]

I know it is inconsequential, but how did the word "So" get placed at the beginning of the previously quoted text?

Not sure. I was probably typing or quoting something else and then I deleted most of it.

 

[twofish-quant]

I am asking about the energy needed to cause the acceleration of the expansion.

The question doesn't make any sense. In order for the question to have an answer, you need to define "energy." For ordinary situations, you can define energy as "that thing that I measure which is conserved when I do certain things". When you accelerate something, potential energy turns into kinetic energy and you can define a number that stays constant.

When you are talking about cosmology, it turns out that things that stay constant under "ordinary situations" don't stay constant, and so there isn't a unique and obvious definition of "energy."

There isn't even a unique and obvious definition of distances.

Things get weird once you leave Kansas, and things that work in Kansas don't work elsewhere. For example F=ma. Light has zero mass, yet you can make it accelerate. So when you start talking about cosmology, things like F=ma just don't work anymore, and you have to use some new and different rules.

The reason that F=ma and energy works is because in Kansas, the laws of physics are time invariant. Objects in Kansas behave the same way yesterday as they do today, and so you can define this thing called energy that comes from the time invariance. The universe as a whole is not time invariant, so there is no obvious number that you can define that is called energy.

 

[twofish-quant]

Also you can't define energy. You can define pressure. The difference is that to define energy, you have to add up things over a large distance and that turns out to be tricky. Pressure you can measure at a single spot, and the the amount of pressure that you need to cause expansion is far, far larger than the pressure you get from ordinary processes like radiation.

 

[d0wnl0w]

I mean in order for one star to accelerate another star of equal mass at the speeds they are accelerating. It would take the entire mass of one star turned into energy directly focused on the other. Right? So the answer to your question is E = (all the mass in the universe) c^2. I have no idea if that makes any sense just trying to help.

 

[marcus]

I am asking about the energy needed to cause the acceleration of the expansion.

Hmm. Would there even need to be an expenditure of energy? Or just a force? It isn't that objects are getting pushed away from each other, gaining velocity in space, but that space is expanding in between them.

What is the difference between objects gaining separation in space, and space expanding between objects?

For one, an object cannot exceed the speed of light as measured by traveling through local space. (Non-expanding space around massive objects) However, two galaxies can be receding from one another at a rate greater than the speed of light because neither are traveling through local space anywhere close to that speed. Instead space itself is expanding between them, carrying them apart.

I thought relativity excluded that possibility. Doesn't time skew as the rate of those galaxies separation increases?

No, because they are not traveling through local space at near the speed of light. If we could cut away all the space between us and that galaxy it would be traveling very close to our own speed.

There is no acceleration on the mass itself, the acceleration is only causing the rate of expansion to increase. IE how fast a volume of space expands to a certain size, say double it's current volume.

If the stars aren't actually accelerating away from each other, I can see how there would be no way to calculate the energy needed. That is definitely where my confusion arises. That was the whole basis of the question. Thanks so much for your input! I think you cleared this up.

This is a good discussion about something that confuses a lot of us. I put the quotes all together so I could reflect and maybe add some comments, or others could comment. Actually I hit the wrong key and lost my first set of comments, so I'll just post this and try to return to it later.

It is right that a largescale uniform pattern of expanding distances is not like ordinary motion. Nobody gets anywhere. It does not involve ordinary kinetic energy (except in the small local random motion of galaxies which we can neglect). Accelerating the expansion of geometry does not involve inputting kinetic energy. You can consider the galaxies as sitting still and just the distances between all of them increasing by some percentage per unit time.

Actually maybe I don't need to say more because if you read what Drakkith is saying here he is getting the important idea across very clearly. You don't have to worry about putting in kinetic energy to the galaxies because they are not going anywhere. The distances between them are just expanding, by a small annual percentage which amounts to 1/140 of one percent per million years.

If you pick two galaxies at random from all those we can see with the Hubble telescope then typically the distance between them will be so great that even 1/140 of one percent growth in a million years means the distance is increasing faster than c. But this is of no great concern. It is just result of the small percentage expansion in geometry that commonly features in solutions to the Einstein Field Equation (EFE). The EFE governs how geometry evolves and how it interacts with matter. It's our basic law of gravity (having replaced Newton's), well-tested, accurate and the best we have so far. It develops singularities at very high density and people are working on ways to fix that. It says basically that gravity=geometry and to describe gravity properly you need to describe how geometry evolves (both of its own accord and in interaction with matter.)
Whatever universe you live in, if you buy the EFE then you are likely to get a little bit of distance expansion (or contraction) into the bargain. Can't think of any way to say this better, at the moment, than what Drakkith already said. Good conversation. Thanks to Greg T for asking the questions.

Also Twofish is making an important point about the absence of an energy conservation law in expanding geometry. I guess part of that point is that for small distances, even say the size of the galaxy, or the distances to the nearest galaxies, those distances are so small that the percentage expansion is negligible. 1/140% per million years is like nothing. So to good approximation we can neglect expansion of distances and treat geometry as static. And we therefore have energy conservation (likewise to the same good approximation.) It is at larger distance scales where that static approximation is no longer good that we have to acknowledge problems with the definition and conservation of energy.