# Do you buy dark energy?

1. May 3, 2010

### Mu naught

Personally, I really don't buy it. To me, it seems fair more likely that either:

1) The universe is expanding but the explanation lies somewhere in the equations we already understand.

2)The universe is expanding and our theories need correction, but no new physical phenomenon that we label "dark energy" is actually at work.

3) Our observations, or the conclusion we've drawn from them, are wrong and the universe may not even actually be expanding at all.

edit - when I say expanding, I mean expanding at an accelerating rate.

I base my disbelief solely on philosophical intuition, but so long as no real explanation for what dark energy could be exists, then I think that's a fair way of addressing it. It troubles me deeply that we talk about dark energy as if it is a concretely proven phenomena, yet we have no real basis for even believing it exists, other than if we don't know the answer, it's easier to just invent an explanation. To me that isn't science, that's myth making.

To this effect, I'm also doubtful about dark matter. Not that there is unseen matter out there which has a gravitational effect, but the idea that dark matter is some unknown type of matter we've never seen before.

2. May 3, 2010

### matt.o

Basing your disbelief solely on philosophical intuition rather than evidence is not the way forward in science.

Last edited by a moderator: May 5, 2010
3. May 3, 2010

### Mu naught

You know Maxwell discovered Electromagnetism based solely on his philosophical intuition that the universe should be symmetrical. Immanuel Kant predicted the existence of galaxies before any astronomer had any inkling of such a thing. I am by no means comparing myself to these great men, but I do claim that philosophy is an incredibly useful tool - it is the foundation of science and all logical thinking. Especially in realms where the primary tools of science, mathematics and observation, fail to provide any explanation.

You claim I should base my opinions on evidence, yet you seem (judging by your rude comment) to be a believer in dark energy. What evidence do you have for this beliefs? You can claim that you observe (or at least accept that others do) that the universe is expanding at an accelerating rate, but I can just as easily claim that such an observation is far too difficult to be certain about, and that your methods and instruments are flawed.

The fact is, no one has any explanation of what or how dark energy may work, and in the end they accept it not on scientific grounds, but on philosophical ones.

4. May 3, 2010

### nicksauce

There is good evidence that the expansion of the universe is accelerating. Supernova data, CMB data, Large scale structure data, and the integrated Sachs-Wolfe effect all point in the same direction (We can go through this evidence if you like, but saying "such an observation is far too difficult to be certain about, and that your methods and instruments are flawed" is incredibly intellectually dishonest). Just because we don't know why the acceleration is happening doesn't negate all this evidence, and saying we don't know why the acceleration is happening (i.e., saying it is due to "dark energy") is the most conservative hypothesis possible.

5. May 3, 2010

### matt.o

So what? In physics, evidence always trumps philosophical musings. Aside from that, I see no logical in your argument -- you appear to be arguing from incredulity.

You can claim all you want, but until you provide some evidence that these experiments are flawed, then there isn't much point really.

Dark energy was accepted because of an overwhelming amount of evidence which pointed to an accelerating expansion, and the best explanation we have for this at the moment is dark energy. This is not to say that cosmologists are now sitting on their hands claiming they've solved everything -- quite the opposite in fact. There are numerous surveys aiming to determine the nature of dark energy, and if this is an adequate explanation for the accelerating expansion, or if some modification to GR is required on large scales, or if the Universe is not homogenous or....

Maybe you should read up on the literature before making erroneous claims about the scientists involved in these areas.

6. May 3, 2010

### sylas

This is worth emphasizing. The phrasing is unfortunate, but someone must have thought it a good idea. We already had "dark" matter, which is "dark" in the usual sense of the term... not emitting light, in contrast to luminous matter like stars or glowing gas, or planets reflecting light from their Sun.

When another unknown effect showed up (acceleration) the effect was that of a "cosmological constant", which behaves like an energy content for empty space; but otherwise we can't tell what it is. So it was, apparently, too tempting to call it "dark" energy.

But all it means, in fact, is "dark" in the sense of unknown; we don't know what it is. The thing we observe or measure is accelerating expansion. Those observations are now pretty secure (although in principle one should allow for surprises, however implausible). But, given the observed effect, acceleration, "dark energy" is simply a term to mean whatever the heck is causing this acceleration. It hardly stands as something to buy or not buy. The thing to you have to buy (or not) is acceleration.

Cheers - sylas

7. May 4, 2010

### Chalnoth

Technically, our equations already included a form of "dark energy", by necessity (the cosmological constant). We just always assumed that since its value had to be very very small, some unknown physics set its value identically to zero. Note that nobody has ever found any physics that would do such a thing.

The dark energy that is already within our equations stems from General Relativity. The basic idea is this: if we want a theory of space-time curvature based upon an action principle, the only coordinate-independent quantity that can enter the action is the curvature and powers of the curvature. Thus a natural thing to do is to make an expansion:

$$L = a_0 + a_1 R + ...$$

More detailed estimates might take into account the square and higher powers of the curvature, but it turns out that just this first-order expansion is enough to produce General Relativity. The first term, $a_0$, is the cosmological constant. The second term, $a_1 R$, leads to the non-trivial form of Einstein's equations.

Now, when making expansions like this, you typically hope that the first few terms hold nearly all of the information, and the later terms have less and less impact. This definitely seems to be the case here, as only going up to the second term is enough to reproduce all of the tests of gravity which we have done to date (the higher-order terms, if they are non-zero, would be most significant at short distances, which is one reason why many experiments are looking at gravity at short distances). Quantum theories of gravity, such as string theory, predict that these higher-order terms will be non-zero (but they may be very, very small).

Hopefully it now seems a bit odd that the first term in the expansion, the constant, would be identically zero. This is why the existence of some dark energy is no surprise at all. However, the fact that it is very small is a surprise. In fact, it had to be very small for our universe to exist at all (which is why people thought it was zero anyway).

8. May 4, 2010

### friend

I find this statement very interesting. Can you point to some article or paper on the arXiv that goes into more detail about this? Thanks.

9. May 4, 2010

### Chalnoth

Well, this is more textbook stuff than arXiv stuff: it was all figured out many decades ago. If you really want to get into it, you'd have to delve into a GR textbook, such as Weinberg's.

10. May 5, 2010

### AWA

Mu naught, if what you mean is that terms like "dark energy" and "dark matter" can be translated as: "we don't have a clue about how to fit certain observations like accelerated expansion and galaxies spiral arms speed in our model" you are on the right track. But you have to be patient, they are working on it.

11. May 5, 2010

### Chalnoth

I would strongly disagree with that characterization. The cause of the accelerated expansion is still fairly uncertain (though many these days seem to be coalescing on it being the cosmological constant), but dark matter is not. Dark matter is evidenced by a wide body of mutually-corroborating evidence stemming from the very early universe to today. There's really no question that it is made up of some sort of weakly-interacting massive particle.

12. May 5, 2010

### sylas

I would have put it as follows:

The translation is: "we do know how to fit observations into our models, and the implications are that there's something out there: non-luminous matter, and also a very small constant energy density contribution to the cosmos from empty space".

What we are working on is finding out more about those things that are implied by the models.

13. May 6, 2010

### AWA

Chalnoth, I'm interested in your point of view about this: do you relate in any way the pioneer anomaly with dark matter effects? being both accelerations of the same sign(atractive)
Sylas ,we basically say the same thing but thank you for rephrasing.

14. May 6, 2010

### Jobrag

15. May 6, 2010

### D H

Staff Emeritus
Particle physicists may beg to differ. WIMPs currently remain within the realm of the purely conjectural. Dark matter is a play on two meanings of the word 'dark': (1) It doesn't interact electromagnetically; it truly is dark. (2) We don't have much beyond educated guesses as to what dark matter is; our models are dark on the subject. Once we know what dark matter is we will probably call dark matter by some more appropriate name. Dark matter is a placeholder term for a very real effect with a so-far unknown explanation. The same goes for dark energy, only more so. The effect is real but what causes it, who knows? Physicists like to be able to answer the question "what makes that happen?" For now, the answer is we don't really know. We do know that the effects are real.

There is nothing wrong with "we don't know" as an answer. If we knew the answer to life, the universe and everything there wouldn't be as much need for physicists. Fortunately, we don't know all the answers (yet).

16. May 6, 2010

### Chalnoth

I strongly suspect that the Pioneer anomaly has more to do with the spacecraft itself or with how we are measuring its position than with fundamental physics.

17. May 6, 2010

### Chalnoth

If particle physicists beg to differ, which I doubt, that is their prerogative. But the evidence is outside their area of expertise, so it doesn't exactly count for much. The evidence for dark matter is entirely within the realm of astrophysics and cosmology, evidence that particle physicists usually don't get deeply involved with.

But I strongly suspect that the vast majority of particle physicists who have looked into this matter much at all would agree with me here: dark matter is made of some form of massive particle that interacts weakly with itself and with other matter (crucially, it doesn't interact electromagnetically).

18. May 6, 2010

### D H

Staff Emeritus
Chalnoth, you are conflating evidence that dark matter exists with speculations on what dark matter is. Nobody disagrees with the evidence (well, almost nobody, and those who do are mostly quacks). The evidence is just too solid and crosses many lines. Speculations that dark matter is some weakly interacting massive particle remain just that: speculations. Stealing your own words, those speculations by astronomers and cosmologists are outside their area of expertise, so they don't exactly count for much.

19. May 6, 2010

### diogenesNY

Q: Do I buy dark energy?

A: Certainly! With an inflationary currency.

(rimshot!)

diogenesNY

20. May 6, 2010

### Chalnoth

There really aren't any reasonable proposals other than some sort of weakly-interacting massive particle, however. Now, bear in mind that the mass range is currently all over the place, as are the precise properties of the interactions. But the simple facts of it being made up of particles that have some amount of mass and interact weakly just aren't in question. These are basic requirements for dark matter to have the observed properties it does, and are not tied to a specific model, but incorporate a wide variety of them.

21. May 6, 2010

### sylas

Um. I would have said there was pretty good evidence that the great majority of dark matter is not baryonic, given the fit of models for nucleogenesis with light atoms. Also I believe that very light weight particles are likely to be relativistic and hence be more like a radiation energy term than a matter term. The main contenders for dark matter are "WIMPS" and "MACHOS"; and the evidence -- not speculation -- seem to make MACHOS a very long shot indeed except perhaps for a small proportion of the unseen matter in the universe.

I can't speak for cosmologists or for particle physicists. But from what I've seen (which is not comprehensive) I am with Chalnoth in doubting that particle physicists are as you say.

Do you have any basis for declaring that particle physicists as a group would disagree with Calnoth that some kind of WIMP is by far the most likely contender? Isn't the fit of low atomic number nucleons to nucleogenesis models sufficiently good to stand as evidence that dark matter is not baryonic? And doesn't that mean some other particle is involved? Which would have to be massive, and weakly interacting?

This isn't really my field, but I've had a brief look at what is more easily available, and mostly it suggests Chalnoth is right. Dark matter pretty much has to be mostly some kind of new weakly interacting massive particle. Here, for example, is a recent review style reference:
• Katherine Freese, (2006) http://dx.doi.org/10.1016/j.nima.2005.12.132 [Broken], in Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 559, Issue 2, Proceedings of the 11th International Workshop on Low Temperature Detectors - LTD-11, 15 April 2006, Pages 337-340
This looks like a particle physics person.

This is only one reference, of course. But references are recommended in our guidelines and may help to give more substantive progress.

Cheers -- sylas

PS. Crossed posts with Chalnoth.

Last edited by a moderator: May 4, 2017
22. May 6, 2010

### D H

Staff Emeritus
Sylas and Chalnoth, I think we are talking at cross purposes here. WIMPs are, in the vernacular of PhysicsForums and elsewhere, "Beyond the Standard Model." Particle physicists do not yet have a viable model for these particles (but they have some guesses), nor have they yet found conclusive evidence that they exist (but there are some potential hits. Note well: I said yet. As far as particle physicists are concerned, these things right now are in the realm of the hypothetical.

Something is out there. Dark matter certainly does point out that the Standard Model most likely is not complete. The connection between cosmology and particle physics is one of the more surprising developments of the last half century. That said, the connection is, right now, hypothetical.

This paper is on the arxiv; see http://arxiv.org/abs/astro-ph/0508279.

Last edited by a moderator: May 4, 2017
23. May 6, 2010

### sylas

This seems to be a distinction with meaning. Sure, we don't have a viable model for them as yet, but the evidence that they exist is strong.

What's wrong with "hypothetical"? And what do you mean by that? Isn't there a difference between something which is purely hypothetical and something for which there is good empirical evidence... even if not enough to nail down a full description.

Previously you said "purely conjectural". I'm not sure what you mean by that either; I would say they are a lot better than pure conjectural, and that there is evidence to constrain what properties they are likely to have.

Previously, you said "Chalnoth, you are conflating evidence that dark matter exists with speculations on what dark matter is." Is that really true? I doubt he is. When we speak of what something "is", doesn't that come down to giving a description of some kind? And although we don't have a complete description, we do surely have enough to constrain their likely properties, in precisely the way Chalnoth has described above?

Thanks for the arxiv link! That's handy.

Cheers -- sylas

24. May 6, 2010

### IcedEcliptic

How can dark matter and energy be discussed in these terms? They are names given for the hypothetical cause of an effect that is observed. There is nothing in the way of observation for real candidates, just a hole we fill with "dark", or an expansion driven by "dark". That is the razor's edge of hypothetical and pure conjecture.

25. May 6, 2010

### sylas

Um, no. Dark energy might be a bit that way, but dark matter is much more concrete, in the sense of having a range of observations and data giving useful constraints on real properties. Hence there is a strong trend in the literature to say that dark matter is most like in the form of WIMPS rather than MACHOS or other possibilities. It's not all settled of course, but it is one heck of a lot more than pure conjecture. Scientists are not just conjecturing, but testing the alternative ideas and reaching conclusions on the basis of available empirical observations.

Cheers -- sylas