# Is Dark Energy A Constant?

My understanding of dark energy is that it's like hot air bouncing around in a balloon, except the air is tiny subatomic particles that can't be seen or even detected in any way except for their apparent effect on the universe's expansion, and the balloon is gravity.

I question whether they can really tell if the universe is expanding faster now than it was a billion years ago, there are so many variables and margins of error, but if it is, wouldn't it slow down later?

The propulsion effect of dark energy would taper off like a rocket running out of steam as it cools down and gets sucked into black holes, and then no matter how far or fast the universe had expanded, over an infinite amount of time, gravity would be all that was left, wouldn't it?

I don't believe in black hole radiation, I don't think anything could ever escape the huge gravity of a massive black hole as everything has gravity, because everything distorts space by it's existence, but even if you believe that the universe ended with nothing but photons flying around, then wouldn't their gravity be the same anyway, and enough to cause the big crunch, the only logical cause for the big bang?

• Careful about statements like "I don't believe in black hole radiation". The laws of physics as we understand them require that black holes radiate. Your belief or disbelief doesn't change this fact. This site deals with the laws of physics as we understand them. If you're interested in physically motivated counter-arguments to black hole radiation, that would be on topic (we don't pretend that physics is always right, indeed it's always important to test the limits of validity of physical laws). But asking us to ignore something because you don't believe in it is akin to invoking magic. – Kyle Oman Mar 28 '14 at 20:46
• And on a friendlier note, welcome to Phys.SE :) – Kyle Oman Mar 28 '14 at 20:47
• – Kyle Oman Mar 28 '14 at 21:08

The catch with dark energy is that it has a constant energy-density, despite the expansion of space$^1$. To paint a simple picture, as space expands, more dark energy is "created" so that the energy-density of dark energy remains unchanged.

Thinking of dark energy as invisible/undetectable particles is perhaps not the most instructive way to think about it. This is a pretty good way to think about dark matter, which is a separate topic, not to be confused with this one. One idea about the nature of dark energy is that it is the energy of the vacuum. This is not vacuum in the classical sense, it is not "nothing". It is empty, but teeming with virtual particles. This starts to get to the limits of my understanding of the topic, but perhaps someone more knowledgeable can help with a comment... Thinking of dark energy as vacuum is more instructive since it makes sense that as space expands, there is more vacuum (but with constant "density"), and that you can't do things like stuff all the vacuum into black holes... what will be outside the black holes? Well... more vacuum...

$^1$ In the simplest model of dark energy... more complicated models where this is no longer true also exist.

• What I was saying is that that model is wrong for a very simple reason, the first law of thermodynamics. For dark energy density to remain constant in expanding space, it could not be the same energy that we are made out of, and I think it is the same energy, filling the vacuum, which has no energy at all, which we have never seen. – rowanman28 Mar 29 '14 at 10:35
• @rowanman28 The usual theoretical formulation for dark energy takes into consideration thermodynamic concerns, and the consensus seems to be that, with a proper treatment, the first law comes out ok. I'm not sure I know enough to be able to explain this in any great detail, but there are many accessible books on the topic. I suggest doing a bit of reading on what the theory is and why it's as widely accepted as it is before discarding it based on vague "beliefs". – Kyle Oman Mar 29 '14 at 16:52
• I have done some reading about it, online, and that's how I found this forum. It seems to me that the only way they can even tell dark energy is there at all is based on how the universe is appearing to expand faster, based on obsevations of supernovas, which may not be reliable standard candles, as it's been shown they happen in different ways, not always at the same mass. These observations might have a different meaning if we found that dark energy had a higher density in the early universe, for reasons such as gravitational redshift. They just found dark energy, they don't know. – rowanman28 Mar 30 '14 at 12:17
• @rowanman28 There's also the observation that the geometry of the universe is flat. If we count up all the energy density we can think of, we come up ~70% short of the energy density required for flatness. And putting that ~70% into dark energy is consistent with the observed accelerated expansion... – Kyle Oman Mar 30 '14 at 15:32
• Forgive me if this sounds stupid, but isn't most of the universe outside our field of vision? Doesn't the universe appear flat, or not curved because if the universe is the earth, we are looking at a small football field? The universe isn't actually flat, we just can't measure the tiny curve in our visual range. It's a sphere on the full scale, isn't it? – rowanman28 Apr 1 '14 at 10:48

Dark Energy = Cosmic Zero Vacuum

1. Dark energy is an essential part of cosmology today
2. Dark energy is just pure energy
3. In the early state of universe's evolution Dark energy was dominated
4. Dark energy is not relative, it is constant structure / stuff . . .
5. Dark energy permeates the universe
6. Dark energy distribution is always smooth
7. Dark energy does not become more dilute when the universe expands
8. Dark energy exists even if no actual particles or other stuff is around
9. Dark energy is not carried by particles or matter
10. The density (cosmological constant) of Dark energy is very low
11. The density of Dark energy is the same everywhere, remains the same over time (it can be no denser in one region than another)
12. Dark energy is very different from dark matter, which collects into objects and will be denser in some place than in others

All these proportions have the Cosmic Zero Vacuum (T=0K)

Dark Energy May Be Vacuum
Date: January 16, 2007
Source: University of Copenhagen
https://www.sciencedaily.com/releases/2007/01/070116130456.htm