dark energy

Does dark energy refer to energy that is yet to be discovered or a mystery form of energy for which we have yet to build detectors?
I know they are various forms of energy (electromagnetic, kinetic etc etc)
How confident are scientists that there is not some other, as yet undetectable, form of energy?

There are people on this board much more capacble of answering this, but here’s my quick response:

Dark energy is a gap-filler. It’s necessary to explain observations of the universe, particularly with regard to its expansion. Scientists have been able to explain some of its characterisitcs but there is still much that isn’t understood about it and it can only be detected indirectly.

Scientists really aren’t in the business of ruling new things out. Anything ‘new’ found may in fact be what we’re looking for with regard to dark energy, could be part of it, or could be something else entirely.

I don’t see the distinction between these two. One discovers something by building detectors for it, and if one has not yet built a detector for something, one cannot discover it.

In any event, the only evidence we have for the existence of dark energy is the effects we observe it having on the evolution of the Universe. No form of energy that’s ever been detected on any smaller scale than that has the right properties to account for what we observe.

Just think of Dark = Unknown.

Dark energy is just a catch-all term, recognizing that several different types of fields can cause the apparent accelerated expansion of the Universe. As Chronos says though what dark energy isn’t is something that we currently recognize in some other form (or at least if it does then it’s behaviour is vastly altered).

If you’re asking along the lines of “Do we know more or less what Dark Energy is, and we just haven’t figured out exactly where it is; or do we really not know what kind of thing it is?” then the answer is more the second: We know there’s something causing this effect, but we’re really not sure whether future scientists who know more about it would use the word “energy” to describe it.

Does that help?

From what we understood of the Universe the rate at which it expands, after the Big Bang, should be slowing down because of gravity working against it. To prove this and hopefully work out the eventual fate of the universe - would it reach a steady state or all come back together in a Big Crunch - astronomers started to make more and more accurate measurements of the rate of expansion of objects at different distances (distance equates to age due to the constancy of the speed of light). To their surprise this showed that the rate of expansion is actually increasing! To account for this there must be some other source of energy not previously accounted for which pushes the Universe apart. Perhaps because the term Dark Matter had already been coined to account for some other anomaly, this mysterious force has been termed Dark Energy. Physicists know it’s there and some of the properties it must have, but they don’t know what it is.

So they have mathematically determined that it must exist but they don’t know what it is and they certainly can’t detect it. Is that right?

More or less.

But rather than say “can’t detect it”, it’s probably more accurate to say “don’t know how to detect it directly”.

We’ve certainly detected it indirectly or else we wouldn’t know it’s there. We just don’t know exactly what it is nor are our current tools/knowledge up to the task of detecting it directly.

Sometimes I feel it was unfortunate that the term, dark energy, stuck, so to speak. But other proposed alternatives such as negative energy or null energy are just as poorly descriptive. We are caught in the conundrum of describing realms that our language is not equipped to deal with. This is why we have such fanciful names for quantum entities. ‘Spin’, ‘color’, and ‘charm’ have genuine and quantifiable values in the language of quantum dynamics, but they are merely placeholders for things that we can only approximately perceive. So, we reuse other terms in order to make a facile analogy.

In fact, we’re sort of worse off than just not knowing what it is. Most theories of particle physics predict that the Universe should be filled with a substance (if you can call it that) called the vacuum energy, which would have the same qualitative properties that we expect of the dark energy. So one might be tempted to say “OK, that’s what it is, we know what the dark energy is”. But then one must ask whether we have the right amount of it, as predicted from particle physics. Well, our current theories of particle physics aren’t yet good enough to actually calculate a number for that, but we can at least estimate it. But when we compare this estimate from particle physics with what we get from observations of the Universe, we find that it’s ridiculously, ludicrously, absurdly off. The estimated prediction from particle physics is about 120 orders of magnitude too high. Note, that’s “orders of magnitude”, not “factor”: As in, the factor by which it’s off would take 120 digits to write. It really isn’t possible to overstate just how bad that is.

So if dark energy is a placeholder required to explain our observation of accelerating expansion
and we have no clue what dark energy is

Is it possible that our notion of accelerating expansion is an incorrect inference from observations made and that we should be looking at other possible interpretations of our primary data?

Is anyone seriously asking this question? (I ask because 120 orders of magnitude error seems a bit much to suggest that we are on the right track.)

I can’t even think of any physical quantities that “span” that many magnitudes. Even going from plank length to observable size of universe doesn’t span 120 orders of magnitude. It really is impossible to overstate just how bad being off by 120 orders of magnitude is. I mean, you could say “the size of the observable universe is about the same size as a plank length” and still be less wrong than estimating dark energy from known particle physics.

Expansion has been measured by a survey of Type-1a supernova. These are short lived but very bright events - about the same brightness as an entire galaxy making it possible to observe them from very large distances and therefore way back in time. In addition the brightness of these is very predictable so by measuring the brightness observed it is possible to calculate the distance to a high degree of accuracy. Correlating this distance with the red-shift, which is a measure of the expansion which has occurred since the event, it was possible to plot an expansion history for the universe.

I know that. (Although I am not sure how we get increasing rate of expansion from that. It seems to be a violation of calculus.) My question is whether in the midst of all this there are some uncertainties or hidden assumptions which, if revisited with a fresh mind might hypothesise a universe where there is not 120 orders of msgnitude between what we observe and what we predict.

j_sum1 writes:

> . . . whether in the midst of all this there are some uncertainties or hidden
> assumptions . . .

There are assumptions and uncertainties in our understanding of absolutely everything. There are many reasons to think that the present structure of physical theory, with relativity and quantum mechanics and the Big Bang and dark matter and dark energy and cosmic inflation and the Standard Theory of particles, will turn out to someday be shown to be not quite right and will have to be reorganized and corrected. Dark energy isn’t much worse than everything else in this respect.

There are at least three or four different sorts of completely independent cosmological measurements, plus a large number of partly-independent ones, that all agree very closely on the same parameters. If it’s all just a misunderstanding, it certainly isn’t a simple misunderstanding.

Right, there could be some source of error that is biasing all these different sorts of measurements the same way. But calling the error source “dark energy” isn’t horrible. I mean, there’s no such thing as phlogiston, but the theory of phlogiston lead to experiments to determine the properties of phlogiston, and those experiments proved that phlogiston didn’t exist.

So an incorrect theory suggests experiments that lead to results that disprove the theory, and those observations provide data that leads to a more correct theory.

Dark energy be fundamental, it may be as a result of fields some of which we can see their effects or it may be something else.


The fact that the predicted vacuum energy density is 120 orders of magnitude larger than the observed dark energy density isn’t quite the deathblow to vacuum energy as dark energy that it may seem. It may be that once additional terms are added in they almost cancel out the predicted value to a much smaller one (though why they would very almost, but not quite cancel would need some 'splaining) or maybe there’s something we just don’t understand about vacuum energy on this scale.

Lots to respond to but little time at the moment. Thanks Chronos, AF and WW.
I will pick off the easy one first (and hope I am not hijacking this thread too much.)

Ok analogy up to a point. And I have no problem with naming it Dark Energy. If we review where we have gotten to; we know some of the properties of DE and we know that it is similar to the vacuum energy which is still not largely understood. We just have 10^120 too little of the stuff. Compared with phlogiston where we found we had none at all.
The question is at what point do we begin to look for other explanations? For me, I would think that point is pretty close. And the first place to look is to check our progress to this point to make sure we actually know what we think we know.

There must be other models for the universe out there surely??