# Why is dark energy necessary?

Drakkith
Staff Emeritus
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.

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

Drakkith
Staff Emeritus
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.

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.

Last edited:
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.

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
Gold Member
Dearly Missed
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.

Last edited: