Dark Energy Strength in Great Voids of Galaxies

[Herbascious J]

TL;DR Summary

How strong is dark energy out in the deepest, emptiest voids of the cosmos and how does it compare in strength to the positive gravity coming from the matter in the universe in general.

Assuming dark energy is fairly, uniformly distributed through out the cosmos, how strong is it, or how much energy is associate with it, out in the deepest, emptiest voids in space? I'm specificlaly refering to the great voids in between the great walls of galaxy clusters. I'm making the assumption that gravity in the cosmos is at it's weakest in these places and will be the most "over powered" by dark energy. The idea behind this question is to think of how the cosmos has a general gravitational field strength, even in the darkest places, because at least at one time, the galaxy walls were attracting each other, and slowing down expansion over all. So there should be a non-zero gravitational field between them. However, now that dark energy is taking over, the tables have turned and expansion is accelerating. So, how does the strength of dark energy in these dark regions compare to the gentle positive gravity which I assume is present there as well. Specifially, how do they compare in strength, and then, how do they compare in energy or energy content, If that is a relevant way of framing the question? I'm a little unsure about asking about it in terms of energy content, so that can be ignored if it is not correct.

 

[PeterDonis]

The answer to your question is simple in terms of relative energy densities. Consider the total (averaged over large scales) energy density of the universe now as 1. Then dark energy is about 0.7, dark matter is about 0.26, and ordinary matter (the stuff we, and all the galaxies and stars and planets and other things we can see, are made of) is about 0.04. That's why dark energy currently dominates the dynamics of the universe (i.e., why the expansion is currently accelerating): because it's the largest component of the average energy density.

These relative ratios change over time because they behave differently as the universe expands. The density of matter (both ordinary matter and dark matter) decreases as the universe expands. But the density of dark energy stays the same. So a few billion years ago, the density of matter (ordinary plus dark) was larger than the density of dark energy, and at that time, and before that time, the expansion of the universe was decelerating, because the matter dominated the dynamics. (And long before that, in the very early universe, radiation, which is currently negligible, about 0.0001 on the scale I described above, was dominant, and the expansion decelerated even more than it did during the time matter was dominant.)

 

[Herbascious J]

So if I imagined two force vectors out in space, one pushing two galaxy cluster walls apart, the other pulling them, would the force be .7 and .3 approximately? Or is that a pour way of framing it?

 

[PeterDonis]

if I imagined two force vectors out in space

You would be imagining incorrectly. The expansion, and its acceleration or deceleration, is not a result of anything pushing on anything. Gravity is not a force in GR, and galaxies and galaxy clusters in general are in free fall, feeling no force at all.

The average distribution of matter and energy in the universe (of all kinds) affects the motion of galaxies and galaxy clusters on large scales because it affects the spacetime geometry of the universe. If matter (or radiation) dominates, the expansion decelerates because the spacetime geometry curves in a way something like a cup or a wine glass, with the "bottom" of the cup or glass being the early universe and the top being the universe now; it expands as you go up (i.e., forward in time), but the shape is such that the expansion rate (how fast the circumference of the cup increases as you go up) decreases.

If dark energy dominates, on the other hand, the shape is more like a trumpet horn, where the expansion rate (how fast the circumference of the horn increases as you go up) increases.

The relative energy densities tell you which kind of stress-energy dominates.

 

[Herbascious J]

Ok, that is very clear, thank you. Can dark energy also be thought of the same way as gravity is described in GR? So like a spacetime curvature? Then they both have an energy content, and that is .7 compared to .3 approximately?

 

[PeterDonis]

Can dark energy also be thought of the same way as gravity is described in GR? So like a spacetime curvature?

Dark energy is a kind of stress-energy. It is a source of spacetime curvature like any other stress-energy.

 

[RpiDav]

Dark energy is a kind of stress-energy. It is a source of spacetime curvature like any other stress-energy.

I know it's just a typo. But for reference sake, the 0.4 in post#2 should be 0.04

 

[PeterDonis]

the 0.4 in post#2 should be 0.04

Oops, yes, thanks! Fixed now.