Is Dark Energy Just a Myth in Modern Science?

[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.

 

[matt.o]

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

 

[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.

 

[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.

 

[matt.o]

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.

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 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.

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

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.

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.

 

[sylas]

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.

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

 

[Chalnoth]

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.

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).

 

[friend]

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.

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.

 

[Chalnoth]

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.

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.

 

[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.

 

[Chalnoth]

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.

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.

 

[sylas]

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.

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.

 

[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.

 

[Jobrag]

No, I by-pass the meter and get it for free!

 

[D H]

There's really no question that it is made up of some sort of weakly-interacting massive particle.

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).

 

[Chalnoth]

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)

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.

 

[Chalnoth]

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).

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).

 

[D H]

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.

 

[diogenesNY]

Q: Do I buy dark energy?

A: Certainly! With an inflationary currency.

(rimshot!)

diogenesNY

 

[Chalnoth]

Chalnoth, you are conflating evidence that dark matter exists with speculations on what dark matter is.

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.

 

[sylas]

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.

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 , 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.

 

[D H]

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.

reference:
Katherine Freese, (2006) http://dx.doi.org/10.1016/j.nima.2005.12.132

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

 

[sylas]

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.

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

 

[IcedEcliptic]

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

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.

 

[sylas]

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.

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

 

[Chalnoth]

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

And even with dark energy, a wide range of possibilities have already been excluded. For example, one of the early explanations was "tired light": that there was something happening to the light between us and the supernovae that was causing the issue. This was first ruled out by looking at further-away supernovae that showed that there was actual deceleration before there was acceleration.

 

[AWA]

I have to side here with DH, and I think is just a matter of being cautious, at this point I think is really too bold to be as categorical as Chalnoth with respect to the causes of "dark matter" or to what has really been excluded and what's not, at the moment they are all conjectures which are valid and may eventually proved right or wrong, but only if we acknowledge them as such.
It would be better as it is been hinted, if we kept separated effects from causes. Everybody accepts the observational data about "dark matter" and "dark energy", those are raw facts.

 

[cesiumfrog]

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.

Right, what would particle physicists know about what particles likely exist?

the simple facts of [dark matter] being made up of particles that have some amount of mass and interact weakly just aren't in question.

It's a bit premature for you to be trying to conflate WIMPs and dark matter in terminology. Yes, most of the money is betting you're probably right, but for now you're simply arguing from lack of imagination rather than presenting any evidence that it actually is simply a different particle.

In the GR literature, for example, there is a significant line of research into whether dark matter is merely an artifact of astronomers trying to fit their data to approximately Newtonian models of gravity (see David Wiltshire's review papers for example).

There's also various proposals of variations to the law of gravity (after all, wasn't the form of Einstein's field equation originally just an ad hoc guess?). Or of ordinary particles in some kind of quantum state.

 

[Chalnoth]

Right, what would particle physicists know about what particles likely exist?

What I'm saying is that all of the positive evidence for dark matter, at this time, is cosmological in nature. Particle physics experiments, at the current time, have presented no evidence, so it doesn't really matter what particle physicists think.

That said, there is a crossover field called "astroparticle physics". Dark matter particle searches are inherently within this field, such as the DAMA and CDMS. And I can guarantee you that people within this field overwhelmingly agree with me: dark matter is primarily composed of some sort of weakly-interacting particle with mass. Now, I will grant that the WIMP idea, as it is used within the field, is often a bit more specific than this. However, I'm not talking about the more specific use, but instead the more general term with no more meaning than I have laid forth.

In the GR literature, for example, there is a significant line of research into whether dark matter is merely an artifact of astronomers trying to fit their data to approximately Newtonian models of gravity (see David Wiltshire's review papers for example).

The bullet cluster and CMB evidence successfully torpedo any such explanations, as well as proposals related to modifications of gravity.

Edit: Though I think you may be confusing dark matter with dark energy in relation to David Wiltshire's review papers. There has been some significant work in attempting to understand if the observed acceleration could be explained by the fact that the homogeneity and isotropy conditions aren't actually met precisely within our universe. However, to date all such attempts have turned out to not be able to explain the acceleration.

 

[Parlyne]

A couple of quick technical points that might clear up a little confusion. The term WIMP, as I understand it, was originally meant to imply "interacting under the weak force, but no other standard model forces." I presume this is not the sense of "weakly interacting" that Chalnoth presumes to be using. In this original sense, it is quite possible that dark matter is not weakly interacting - it's interactions with standard model particles could be through some other, as yet unknown, force. (In the particle physics literature, you can find quite a few such models, the most conventional of which have dark matter interacting either through scalar fields or through a new heavy neutral gauge boson, generically referred to as a Z'.) Such a new force is, still, expected to be weak compared with EM or the strong force.

A second possible point of contention regarding the particle physics is the identification of dark matter as a "particle." There's been quite a bit of discussion in the particle physics literature over the past few years about the idea of what are called "unparticle" models, which are a sort of non-local quantum field theories. The hallmark of such fields are particle-like excitations with continuous mass spectra.

These are both small technical quibbles; but, they demonstrate that the degree to which we can say that it's essentially settled that dark matter is WIMPs depends on how strictly we're interpreting what it means to have a weakly interacting particle.

 

[Chalnoth]

A couple of quick technical points that might clear up a little confusion. The term WIMP, as I understand it, was originally meant to imply "interacting under the weak force, but no other standard model forces." I presume this is not the sense of "weakly interacting" that Chalnoth presumes to be using. In this original sense, it is quite possible that dark matter is not weakly interacting - it's interactions with standard model particles could be through some other, as yet unknown, force. (In the particle physics literature, you can find quite a few such models, the most conventional of which have dark matter interacting either through scalar fields or through a new heavy neutral gauge boson, generically referred to as a Z'.) Such a new force is, still, expected to be weak compared with EM or the strong force.

A second possible point of contention regarding the particle physics is the identification of dark matter as a "particle." There's been quite a bit of discussion in the particle physics literature over the past few years about the idea of what are called "unparticle" models, which are a sort of non-local quantum field theories. The hallmark of such fields are particle-like excitations with continuous mass spectra.

These are both small technical quibbles; but, they demonstrate that the degree to which we can say that it's essentially settled that dark matter is WIMPs depends on how strictly we're interpreting what it means to have a weakly interacting particle.

Right, and let me be perfectly clear that these sorts of exotic possibilities, such as "unparticle" models would fit very much within the most general sort of definition I laid out. Yes, particle theorists are the people to ask for what sorts of specific models are out there. But those models must all be in line with the cosmological evidence that states two things:

1. It has mass.
2. It has no electromagnetic or strong force interactions.

When you look into the details of the various experiments to detect dark matter, you get further restrictions, but they're not worth going into here.

 

[diggy]

I think this is a pretty fair question. What is more believable, that our universe model in GR is wrong (or at least not the full story) or that we don't know what *actually* makes up 95% of our universe... or both.

Its been a while, but the last talk I saw on dark energy was pretty much a laundry list of "this didn't even come close to working" ideas. Dark matter is on "firmer" grounds, but no one can show you a piece of dark matter.

 

[IcedEcliptic]

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

It is not concrete, but I am not saying that dark matter does not exist. I believe that GR explains enough that such a gap would be unlikely, but that does not make Dark Matter anything but a title for a something that we are yet to detect directly, or even through interaction or decay. Sterile Neutrinos, The Tau, WIMPs, and more... what is a WIMP other than a title for "we don't know, but something is there that has mass and does not radiate." It is a guess, a conjecture to explain gaps in our observations, but everyone always has the explanation just about to be revealed, and it never works.

 

[Chalnoth]

I think this is a pretty fair question. What is more believable, that our universe model in GR is wrong (or at least not the full story) or that we don't know what *actually* makes up 95% of our universe... or both.

This is an extremely misleading measure, though, because it's a measure that changes with time. We know the primary makeup of our universe a few seconds after reheating, for example: in the very early universe, nearly all of the energy density was made up of radiation. Not long after that, and until a few billion years ago, nearly all of the energy density was normal matter and dark matter. A few billion years from now, nearly all of it will be dark energy.

Its been a while, but the last talk I saw on dark energy was pretty much a laundry list of "this didn't even come close to working" ideas. Dark matter is on "firmer" grounds, but no one can show you a piece of dark matter.

It should be noted that the first idea on "dark energy" is still the most in accord with experiment: that it's a cosmological constant. And as for dark matter, just because it's hard to detect directly doesn't mean we can't be extremely confident it exists.

 

[IcedEcliptic]

This is an extremely misleading measure, though, because it's a measure that changes with time. We know the primary makeup of our universe a few seconds after reheating, for example: in the very early universe, nearly all of the energy density was made up of radiation. Not long after that, and until a few billion years ago, nearly all of the energy density was normal matter and dark matter. A few billion years from now, nearly all of it will be dark energy.


It should be noted that the first idea on "dark energy" is still the most in accord with experiment: that it's a cosmological constant. And as for dark matter, just because it's hard to detect directly doesn't mean we can't be extremely confident it exists.

I may be miscommunicating in this case; I personally believe that dark matter is composed of WIMPs, or Sterile Neutrinos, or both. I do not believe GR has such holes in it, that we're just "off the mark." Dark energy, I do not know. It seems there is a positive cosmological constant, and vacuum expectation energy would explain both, but it is far from being shown to be. I am disputing your certainty, not your conclusions.

 

[Chalnoth]

I may be miscommunicating in this case; I personally believe that dark matter is composed of WIMPs, or Sterile Neutrinos, or both. I do not believe GR has such holes in it, that we're just "off the mark." Dark energy, I do not know. It seems there is a positive cosmological constant, and vacuum expectation energy would explain both, but it is far from being shown to be. I am disputing your certainty, not your conclusions.

Slight caveat: I think we can be quite certain that GR is wrong on some level, but we can also be extremely confident that the way in which it's wrong cannot explain dark matter. Our confidence that the way in which it's wrong doesn't explain dark energy is a little less certain, but it still seems likely that the answer is not a modification of gravity.

 

[diggy]

This is an extremely misleading measure, though, because it's a measure that changes with time. We know the primary makeup of our universe a few seconds after reheating, for example: in the very early universe, nearly all of the energy density was made up of radiation. Not long after that, and until a few billion years ago, nearly all of the energy density was normal matter and dark matter. A few billion years from now, nearly all of it will be dark energy.

Then we will know what makes up approximately 0% of the universe -- much better :-)

I think the point is that if you model y=a*x, and measure x, you can only infer y through the model. That is all the OP is saying, and he is correct, imo. You however are taking the model as gospel, which is dangerous even if you (not necessarily you specifically) are right.

 

[Chalnoth]

You however are taking the model as gospel, which is dangerous even if you (not necessarily you specifically) are right.

Hardly. I'm just saying we have cause to be a lot more confident about specific features of the models than many people in this thread seem to think. The evidence for dark matter, in particular, spans a wide variety of mutually-corroborating data sets. From the data we have available to us today, there just isn't much of any wiggle room left.

Dark energy is much more up in the air. It could still be a modification of gravity, though that is seeming increasingly unlikely. The possibility of it just being a misunderstanding of how to deal with inhomogeneities properly is exceedingly far-fetched now. That leaves a cosmological constant or some time-varying field with the right properties (specifically having a pressure/energy density ratio close to -1 for late times, and not having too much energy density at early times).

 

[diggy]

Hardly. I'm just saying we have cause to be a lot more confident about specific features of the models than many people in this thread seem to think. The evidence for dark matter, in particular, spans a wide variety of mutually-corroborating data sets. From the data we have available to us today, there just isn't much of any wiggle room left.

Dark energy is much more up in the air. It could still be a modification of gravity, though that is seeming increasingly unlikely. The possibility of it just being a misunderstanding of how to deal with inhomogeneities properly is exceedingly far-fetched now. That leaves a cosmological constant or some time-varying field with the right properties (specifically having a pressure/energy density ratio close to -1 for late times, and not having too much energy density at early times).

Fair enough.

New question, if somewhere/somehow we were able to prove that either dark matter or dark energy was non-existent, i.e. the our universe model was no longer consistent with GR and evidence, would you still be an advocate of the other missing quantity?

 

[Chalnoth]

Fair enough.

New question, if somewhere/somehow we were able to prove that either dark matter or dark energy was non-existent, i.e. the our universe model was no longer consistent with GR and evidence, would you still be an advocate of the other missing quantity?

Yes. Why wouldn't I?

 

[twofish-quant]

New question, if somewhere/somehow we were able to prove that either dark matter or dark energy was non-existent, i.e. the our universe model was no longer consistent with GR and evidence, would you still be an advocate of the other missing quantity?

If you show that the existence of dark energy or dark matter was inconsistent with observations and general relativity, it could be that GR is wrong, or that we fundamentally misunderstand our observations.

Also I think you are setting the bar way too high. Dark energy is a crazy, stupid, silly explanation for how the universe works, and dark matter only slightly less so. All you have to do in order to convince me that dark energy and dark matter is not the best explanation is to come up with something slightly less crazy, stupid, and silly.

It's harder than it sounds...

The other thing is that whether dark energy is a good explanation or not is not some sort of religious or political issue. Give me one paper to read, and I might think it's a nutty idea, and then two weeks later, I might change my mind, and then two weeks later, I might change my mind again.

 

[twofish-quant]

I base my disbelief solely on philosophical intuition

That's a bad, bad way of doing physics. People have come up with alternative explanations for the observations, and right now the alternative explanations are worse than "dark matter."

One thing that is very educational is to go through the original paper that presented evidence that the universe is accelerating. They went through every possibility that people could think of for attributing this to some sort of observational weirdness and came up with reasons why that didn't work.

It troubles me deeply that we talk about dark energy as if it is a concretely proven phenomena

Except that we don't. A lot of what you read in theoretical papers are of the form "if dark energy exists then..." The point of talking about something that you don't know exists as if it did was so that you can then figure out what the consequences of it existing are.

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.

That's science. Invent an explanation. See if it works. If it doesn't, invent another explanation. Repeat.

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.

The problem is that if you assume that dark matter is "normal matter" you run into a lot of problems. Galaxies clump in the wrong way, and all the elemental abundances are wrong. People have tried really, really hard to come up with explanations of this, but the explanations are even weirder than dark matter.

 

[twofish-quant]

You know Maxwell discovered Electromagnetism based solely on his philosophical intuition that the universe should be symmetrical.

Intuition is wonderful for creative inspiration, but science involves matching intuition with cold hard facts.

You claim I should base my opinions on evidence, yet you seem (judging by your rude comment) to be a believer in dark energy.

For things at the frontiers of science, I hate the term "believe" or "disbelieve". Right now, it seems to me that dark energy is the best explanation anyone has come up with for what we are seeing. It's possible that this evening I'll read some paper on the Los Alamos Preprint Server and change my mind about this.

What evidence do you have for this beliefs?

There are review papers that people can point you to. Personally I'd start with wikipedia, since the articles there are often surprisingly good,

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.

You can claim that. You can't *easily* claim that.

http://adsabs.harvard.edu/abs/1998AJ...116.1009R

Go through Section Five, where they go through all of the obvious explanations for the results and knock them down. If you can think of something they didn't include, feel free to mention it.

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.

[/QUOTE]

You come up with an explanation, figure out the consequences, fit to observations. Right now dark energy is the "least bad" of the explanations.

 

[twofish-quant]

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.

Same here. If the Pioneer anomaly were due to fundamental physics, then we should be able to see weird things happening with other things.