It is easily detectable. The universe is clearly expanding and will do so indefinitely. We just have no clue why. The word energy shouldn’t even really be in there as it implies we know there’s energy missing. We don’t even know what we’re missing, but we do know exactly how much of it we’re missing.
The universe is clearly expanding.
It’s not just expansion that needs to be considered. It’s acceleration.
The word energy shouldn’t even really be in there.
Energy is a pretty good word. It requires energy to accelerate mass.
Rephrasing my question: If we have no feasible sources of that energy that we can attribute to this phenomenon, shouldn’t one of our avenues of research be to consider whether acceleration is indeed implied by our primary data?
Chronos, I am not suggesting that our present understanding was arrived at “simply”. I am really probing for other models. I am skeptical that our pursuit of DE will yield any results at all. (With the little I have read, I think there is logical reason to be skeptical.) This line of reasoning necessarily puts question marks over everything we think we know. I infer from Assymptotically Fat’s comments that he(?) thinks there are some concepts that are not really open for question. I admire his confidence but I don’t share it. Then again, I am undoubtedly less well-versed in the available data and current discussion on the matter.
Just explain a little why dark energy isn’t like much else we’ve seen except vacuum energy: cosmological models are one of general relativity’s greatest successes and in these models, so long as the energy density of the Universe is greater than zero and the pressure is greater than equal to zero, then the rate of the expansion of the Universe must always be decreasing. That means the rate of expansion is either going to contraction and an eventual big crunch, to zero as time goes to infinity or to a positive constant rate as time goes to infinity.
Therefore for the rate of the expansion of the Universe to be increasing as, is observed, there must be something else introducing a negative pressure-like term. The simplest thing that does this is the cosmological constant/vacuum energy which introduces an extra term with constant positive energy and constant negative pressure.
The reason it is unlikely is that the Universe isn’t expanding at an accelerating rate is simply observational data has shown us that is the case.
I wouldn’t be too quick to throw out vacuum energy as an explanation for dark energy either, in the current most favoured models dark energy appears a cosmological constant and in general relativity the cosmological constant and vacuum energy are equivalent. There is the cancellation problem, however if dark energy isn’t vacuum energy then it may be need that vacuum energy exactly cancels which may not be any more difficult to explain than it almost, but not quite, cancelling.
It’s not too difficult to imagine the vacuum energy canceling out entirely: There are plenty of cases in physics where a bunch of positive and negative terms all turn out to balance exactly. It’s canceling out almost but not quite entirely, to over a hundred decimal places, that’s freaky. Personally, I’m more inclined to say that the vacuum energy predicted by particle physics really does cancel out entirely, and that the dark energy is due to some other phenomenon, possibly a time-varying one.
Be careful with that certainty. The fact that we have no clue why also means that we can’t be confident it will continue. Maybe some day, the dark energy will go away entirely, or even reverse its effects. With as little as we know about it, there’s very little justification to say that it won’t. How would it do that? We don’t know, but that’s hardly surprising, given that we don’t know how it does what it’s doing currently, either.
Well, “dark energy” isn’t exactly a model. It’s a name for a known unknown. It isn’t the case that physicists label this expansion “dark energy”, put down their pencils, high five each other, and head out for brewskis.
It is an indication that either our observations have some consistent source of error that somehow biases all our observations in exactly the same way, or there is some new sort of physics going on that we don’t understand. This is how we come to understand new sorts of physics–looking at observations that we don’t understand, trying to eliminate sources of error, and when we can’t do that trying to figure out what is going on.
Noticing this expansion is like Roentgen noticing that his photographic plates were fogged when they were placed next to a sample of pitchblende. He didn’t know why this happened, and figured there was some mistake. But he couldn’t find the mistake, and eventually decided that there had to be some mysterious phenomenon behind it, even though it didn’t make sense to him.
We have an observation. It doesn’t fit our theories of physical law. Therefore either the observation is wrong, or the theory is wrong, or both. The problem with the theory being wrong is that in almost every other sort of observation our theories predict the behavior of matter and energy extremely precisely. So it’s not the case that we’re totally wrong about everything. We’re just confused about this one phenomenon. It is very likely that when we understand this phenomenon we’ll be able to figure out other unexplained things as well, and that will be the confirmation that our new theory is very likely correct.
If we just posit a cosmological constant with value x then we don’t have a theory, we’ve just restated the results of our observation. If our new theory about the reason for the accelerating expansion doesn’t lead to solving new problems then it’s just a restatement of our ignorance.
j_sum1 writes:
> There must be other models for the universe out there surely??
Well, no. Not in the sense that anyone has figured out one that explains the acceleration of the expansion of the universe in any way that’s more enlightening than dark energy. Could there be such a model? Of course. Newton revolutionized physics by showing that all the new facts about the universe discovered by Galileo and Copernicus and Kepler and others could be explained in a simpler, more comprehensive way with some fairly simple physical laws that he proposed. Einstein revolutionized physics again by showing that some new facts about the universe discovered by various people could be explained by relativity and quantum mechanics in a simpler, more comprehensive way.
I suspect then that someone will eventually revolutionize physics once more by showing that the whole complicated structure of present-day physics, with relativity, quantum mechanics, the Big Bang, dark energy, dark matter, cosmic inflation, the Standard Model of particle physics, and whatever else needs to be included can be explained in a simpler, more comprehensive way with a better overall theory. I can’t prove this, of course. It’s possible that the universe really is that complicated and can only be explained with dozens of complicated, almost unrelated theoretical constructions. I suspect not though. But can dark energy be replaced with some simple idea? No, not one that anyone knows of.