Question about dark energy

Factual Questions
21

Science is a two-way street. Observations are systematized by theory; a good theory also makes additional and unexpected predictions that observations can confirm. It must work both ways. That’s why Creationism cannot be science: it can’t make original observations. Observations, however, tend to be incomplete, or misunderstood, or poorly measured. Trying to fit a theory to them is not simply slapping on a template. It may take years and numerous attempts. And the best and most all-encompassing theories - relatively and QM, say - can work for a billion things but not something else at the very edge. Explaining the universe tends to have that problem.

That said, what kaltkalt is doing isn’t science either. Coming up with random science-y words to “explain” things is not really different from saying that God did it. The only way it’s better is that applying the knowledge gained from one thing to another is a basic human trait. A good analogy is the cute mis-sayings that children make when trying to apply a newly learned rule where it doesn’t belong - “I goed” instead of “I went”. Children have to painstakingly learn every irregular verb and every exception to the rule individually. If they don’t, society places sanctions on them and calls them illiterate, which makes it difficult for them to function.

Society does not place the same sanctions on those who don’t painstakingly learn the rules and procedures of science. Without those kaltkalt fails to notice some basic things. Just because rotation can explain expansion in some circumstances doesn’t imply that it can do so for the universe. The people who do equations for a living would certainly have applied it by now if it could work. But they also know that you can’t abstract one single fact out of the whole to apply your theory to. As Chronos noted, rotation would imply non-isotropic results which aren’t seen; therefore scientists would and do dismiss rotation as an answer out of hand. Eliminating answers because they don’t square with observation is a basic part of science: it works in cosmology and it works in biology, which is another reason Creationism fails.

The lack of societal punishment for being science-illiterate is something that all scientists notice but seldom understand, mostly because, ironically, scientists tend to be social science-illiterate. This is not quite the same as the truism that everyone is ignorant, only in different ways. You don’t have to be a grammarian to be literate in English; you just need to have an understanding of how the process works. Similarly, you can understand the process of science and what makes something science and another thing woo without being a subject matter expert in any particular branch of science. Looking at the formulation of an argument is often sufficient.

It’s this understanding of how science works that removes the “faith-based” element from the discussions of dark energy and dark matter. Not knowing the ultimate answer doesn’t remove a problem from science and open it up to random religious speculation. Science is a process that every scientist understands and follows and that every other scientist can critique because they’re playing by the same rules. A rotating universe is outside the game and insisting that it’s not is like criticizing baseball for not allowing field goals. Claims like these - we’ve had hundreds of like threads in GQ - show that the one who makes the claim needs to do some painstakingly learning, not the players.

22

You must imagine that scientists study a phenomenon, like the acceleration of the expansion of the universe, and once they give it a name like “dark energy” they high five each other and go home, their work being complete.

Of course this is silly. “Dark energy” is just another name for “the cause of the acceleration of the expansion of the universe”. It is completely true that we can’t see this cause and we can’t explain it and we don’t know for sure that it even exists (it could be some systematic bias in our observations for example, but this seems unlikely and the cause systematic bias itself (if it exists) would be pretty important to figure out). That doesn’t mean we give up.

We have an observation. We know that the universe is expanding. We can explain that at one point the entire universe was contracted into a singularity and the universe proper started existing 13 or 14 billion years ago when the singularity started expanding.

But the funny thing about the expansion is that it doesn’t seem to be slowing down. If you had an explosion everything would be thrown out into space, but gravity would cause the expansion to slow down. Even if there wasn’t enough gravity to cause everything to contract into a Big Crunch, we still would see things slow down, just like things thrown into space from Earth are slowed by Earth’s gravity even if they eventually escape.

But that’s not what we see, instead of the expansion of the universe slowing slightly over time it seems that we are expanding faster and faster. This is a problem because there is no known mechanism by which this could happen. We have no idea what could cause this. We have a nickname for this cause, “dark energy”, but that doesn’t mean we think we understand it. Giving something a nickname is not the same thing as understanding it, and no actual astrophysicist puzzling over “dark energy” thinks it is.

23

We know about dark energy the same way I know about the table in front of me: by observing its effects. It reflects photons, it is impenetrable to my hand, it produces certain sensations when touched, makes certain sounds when kicked, etc. That is, it produces certain observable effects in the environment it interacts with.

The same goes for dark energy, whose effect is the uniform acceleration of all matter in the universe (unless bound up by stronger influences) away from each other. This does not entail any claim as to its nature, it is merely an observation of its effect, exactly analogous to how calling something a table does not assume anything about what, exactly, tables are made off—we certainly knew about tables before we knew anything about the atomic composition of matter, for instance.

As for your spin idea, it doesn’t produce the same phenomenology as dark energy. As an analogy, picture a pizza baker spinning pizza dough: it expands radially outwards, that is, from a point in its center; from any other point on the pizza, things towards the rim will move away faster, and things towards the center more slowly, in contradiction with the observed uniformity of acceleration. Plus, the existence of a nonvanishing angular momentum of the universe as a whole would lead to certain anisotropies in the cosmic microwave background, which aren’t observed (this of course just puts an upper limit on the size of the effect, as there could be an anisotropy our detection capacity isn’t good enough to find yet, but already we can say that if there’s any rotation, it’s got to be tiny).

Additionally, the current ideas on the origin of the universe include an inflationary epoch, where it grew exponentially in an extremely short amount of time. One effect of this is providing an explanation for the observed flatness of the universe, as any primordial curvature was smoothed out by this; but additionally, any primordial angular momentum would similarly be greatly reduced—picture an ice skater suddenly holding out her (exponentially long) arms.

Furthermore, I’m not quite clear on how exactly you think the idea of an angular momentum of unknown origin would be more economical than the idea of a uniform (negative) pressure; both are relatively straightforward and simple additions to current models (granted, dark energy could also be something complicated, but current observations are in line with the simplest hypothesis you could think of, the so-called cosmological constant), and both would seem to be in need of additional explanation.

24

This is it in a nutshell. Any rotation introduces an asymmetry that isn’t observed in the Universe. And that’s even before you go on to consider whether rotation can cause a dark energy-like effect. Some people have mentioned the Godel dust solution (which was my first thought too), but it’s worth mentioning that cosmological constant in the Godel solution, whose value is precisely chosen to keep it stationary, has exactly the same value as the cosmological constant in the Einstein static Universe, which serves exactly the same purpose. Or in other words the rotation in the Godel solution has no effect on the expansion. This is a clear example of how the effects of rotation are nothing like the effects of dark energy.

25

This is rather disingenuous. I think astronomers truly were very surprised when their calculations concerning such things as the dynamics of galactic rotation suggested that most of the mass of the universe is invisible to telescopes. You, personally, may be sufficiently used to the hypothesis of dark matter and all the various speculative hypotheses about what it might be that it no longer seems arbitrary or surprising to you, but that is a fact about your history and your psychology, not about the scientific situation. Dark matter remains an unproven and unexplained hypothesis designed to explain some rather recondite facts. It does explain these facts rather well, so the scientific focus has shifted from trying to find alternative explanations for them to trying to find explanations for the dark matter itself, but that is less an established fact about nature than a fact about the current state of astronomical research.

26

The problem with talking about dark matter is that the term means two different things, depending on context: In one sense, dark matter is the kind of matter most familiar to us, and we are in fact made of dark matter. The discovery that there’s more of that out there is hardly surprising, and not at all mysterious, though of course most of it is expected to be in the form of Jupiter-like objects, not people.

But there are also observations and calculations that show us that not all of the Universe is composed of the sort of matter we’re familiar with. The matter we’re familiar with is mostly (in terms of mass) composed of protons and neutrons, but we can deduce the density of protons and neutrons in the Universe from, for example, the relative abundances of isotopes of the light elements. And we can also measure how much gravitational mass there is in the Universe. And the amount of matter made up of protons and neutrons, or baryonic matter, falls far short of the total. So there must be some sort of non-baryonic matter out there, and a lot of it. Now, this non-baryonic matter must be dark matter, since we’ve never seen any of it, but beyond that, we know very little. And so often, the term “dark matter” is taken to refer exclusively to this stuff which really is genuinely mysterious.

The mysterious dark matter, meanwhile, is unproven only in the same sense that everything in science is unproven. There are multiple independent lines of evidence that all point to the same results concerning it, and while alternate mechanisms have been proposed to explain one or two of the observations, nothing else explains all of them. If it looks like dark matter, and quacks like dark matter, we really ought to just call it dark matter.

27

“Just because rotation can explain expansion in some circumstances doesn’t imply that it can do so for the universe.”

That’s why I asked here. I’m not sitting here posting this with a gleeful smile thinking I’ve solved a mystery that many people smarter than myself are currently working on. I’m just asking the question on an internet forum meant for questions. I fully expected someone to explain why my idea wouldn’t work rather than get a Nobel prize. I just want to know why the thought i had wouldn’t work. I have yet to receive a good answer that addresses everything mentioned in my OP, by the way. Someone mentioned the isotropic nature of the expansion and how rotation arguably wouldn’t show that - though I addressed that issue in the 4th paragraph of my OP which it seems nobody read.

I’m still sure my question is properly answered in the negative. That being said, I’ve read quite a lot about DM and DE and they’re both just vague placeholders that don’t even rise to the level of a hypothesis used to explain certain astrological phenomena that we currently cannot explain. Hence the word “dark” - they can’t be seen directly, can’t be detected, and are just illusory concepts to explain something that we know about but currently cannot explain with known data, facts and scientific concepts. So we plug in DM or DE as a temporary placeholder. When the mysteries are solved, I guarantee you there will be explanations that don’t involve actual DM or DE, and we’ll look back on these as the “Dark Days” of astrophysics, before these mysteries were satisfactorily solved and Nobel prizes were given to those who solved them.

But to say science knows DM and DE exist as actual invisible, undetectable matter or energy, respectively, with the same degree of confidence that atoms or evolution exist is simply wrong. In fact it’s very wrong.

28

Although most of your concern in this thread has been about Dark Energy, I have to ask:

What is it you’re skeptical about with regards to Dark Matter?

The rates of galactic rotation pretty well confirm that galaxies are much more massive than is apparent from what we can see, and that this extra mass is distributed more-or-less uniformly throughout space.

29

Also, I’m an absolute nonbeliever and few people hate religion and faith-based ideas more than I do. So any inference that I’m some religionist trying to poke a hole in science so i can masturbate to my fantasies of “intelligent design” is gravely misguided and erroneous.

30

Like you said my question here involves dark energy, but yes i’m skeptical of dark matter too. I agree rates of galactic rotation imply much more matter than we currently identify. I just don’t think missing, invisible matter (that’s up to 90% of the entire universe’s mass) is the answer. No idea what is. I just have a feeling one day we’ll solve the mystery and the solution won’t be invisible, undetectable matter that is still affected by gravity.

31

One thing we are sure of is that dark matter cannot be caused by anything as large as unaccounted brown dwarfs. The gravitational lensing effects observed aren’t possible with clumps of dark matter any larger than a pebble.

So, in the battle between machos and wimps, the wimps are winning.

32
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kaltkalt,

For the sake of argument, let’s accept your theory. Let’s agree that the expansion of the universe is caused by “rotation in all directions.” The question then becomes how is that rotation getting faster? What is powering the acceleration of the rotation? What is that energy and where is it coming from?

You’re still left with the problem of energy coming from somewhere that we don’t understand yet and your theory doesn’t get us any closer.

34

If you’ve read “quite a lot” about DM and DE then surely you know that there have been thousands of papers published about them analyzing every aspect of what is known about the distribution of mass within and among galaxies, the evidence for and against certain types of particles and mass structures, the movement of galaxies within the universe and stars within galaxies, and the wealth of math that has been done to examine every known fact. That seems much more than a placeholder to me.

And the mere fact that you use the word “astrological” instead of “astronomical” is more than sufficient to dismiss every word you have to say and make us care less about your “feelings” on the subject.

35

Well, who’s to say that the usual conservation laws apply in trans-universal cosmology? In ordinary physics, if something spins and expands, the rate of spin decreases. (The familiar figure-skater analogy.) But when it comes to entire cosmoses spinning? Can we be sure? Maybe it’s spinning, expanding, and thus spinning faster!

(Also…could it be spinning in higher dimensions, like Flatland spinning in the third dimension? Wouldn’t that address the “where is the center?” problem?

(No, not proposing any of this at all seriously; just riddling.)

36

Your question has been answered: rotation introduces an asymmetry that we don’t observe. Symmetry and rotation go hand in hand. For example: the Schwarzschild solution is the spherically symmetric vacuum solution in generally relativity for zero cosmological constant, it describes non-rotating black hole. If you relax the spherical symmetry condition into an axisymmetric symmetry condition you get the Kerr solution, which describes a rotating black hole.

To reiterate: rotation introduces asymmetry full-stop.

I did read your 4th paragraph and the above still applies. To spell it out: the only way something can rotate at the same rate in all directions is if it doesn’t rotate at all, just as the only way something can move (translationally) at the same rate in all directions is if doesn’t move at all. The simple addition of extra dimensions doesn’t change this or alter the symmetries of the subspace which we’re interested in.

The evidence for dark matter is good. There’s independent lines of evidence. The only viable alternatives to dark matter currently are ones which make fundamental changes to basic physics, e.g. modify gravity at a Newtonian level and even then they don’t fit with observation as well as dark matter. We may not know all the properties that dark matter must have, but we know some of them and to say it is not even a hypothesis is a very basic misunderstanding.

Dark energy is a catch-all term to describe several classes of spactime fields which can explain the accelerated expansion. Only these fields or something that conspires to create something very closely resembling one of these fields can explain the accelerated expansion of the Universe.

37

Well, nobody says it’s undetectable; in fact, if it were, we wouldn’t be talking about it, since we’d never have detected it. However, if there is some sort of stuff responsible for the observations regarding galactic rotation course etc., then we know it’s invisible, which just means that it doesn’t interact electromagnetically. This isn’t, in itself, anything new: neutrinos, for example, are invisible in that sense as well.

These two statements are directly in conflict. First, you say you don’t accept faith-based ideas; then, you propose an idea about the universe that you seem to believe, but is based on nothing but a ‘feeling’. But what is a belief in something you do not have rational grounds for if not faith-based?

Most of the time, scientists try to extend their current models in the most conservative way feasible. Often, this is justified by appealing to ‘Ockham’s razor’, but this runs the danger of obscuring why it’s the sensible thing to do; many people flaunt this principle as a kind of passed-down from on high unquestionable argument-winner, but its applicability ultimately just rests on a desire for ensuring the predictivity of their models: otherwise, you could always add on ad-hoc hypothesis to be consistent with any further observations.

Both dark energy and dark matter, or rather, the hypotheses advances to explain the origin of the effects subsumed under these names, are such conservative extensions: dark energy, as modelled by a cosmological constant, is basically the simplest addition to the equations of general relativity you could think of; some even say that the theory is more natural with its addition (going through the derivation, there’s no reason to leave it out, and you generally work with the principle of including anything not expressly forbidden), leaving just its value as a further explanandum.

The same goes for dark matter: our observations look as if there was additional stuff out there we can’t see or account for, so the simplest solution is to assume that that’s the way it is, regardless of any ‘feelings’ one might have on the matter. Once these explanations are ruled out, only then does one in general feel licensed to move to more exotic ideas, such as modifying the laws according to which stuff moves.

38

I do not believe in astrology.

It’s entirely plausible for someone to look at a theory (and I see DE and DM more of placeholder explanations than actual theories) and get the feeling it’s not right without resorting to faith or religion. Some of you act as if I’m suggesting rotation is the answer without proof and demanding you accept it based on faith. It’s not even my theory. It’s just a question. Chill out. If the answer is no and if why not is adequately explained, okay then.

As for the acceleration, isn’t DE meant to explain the fact that the expansion of the universe is accelerating? Not that it’s accelerating faster than it should otherwise be accelerating. I very well could be wrong about this, but I thought we knew the universe was expanding but we expected it to be slowing down due to gravity, and in fact it’s speeding up (i.e. accelerating). Not that we expected it to be accelerating at X mph but it’s actually going 2X mph going up to 3X mph exponentially, which is what at least one of you is describing.

Dark Energy is meant to explain the accelerated expansion of the universe, not the ever increasing acceleration of the expansion of the universe. It explains expansion at m/s^2 not m/s^3. I’m nearly certain of this.

If something were being moved by centripetal force, wouldn’t it accelerate outwards? As in m/s^2?

39

kaltkalt writes:

> If something were being moved by centripetal force, wouldn’t it accelerate
> outwards? As in m/s^2?

No, it wouldn’t. Centripetal force is at a constant velocity.

40

ok, assuming that’s right then it answers my question. I thought it was like gravity, an accelerative force. I was thinking about those donut-like space stations that spin around to simulate gravity. Isn’t that accomplished by centripetal force? If it spins around at the correct speed it can simulate the acceleration of gravity on earth.