"What makes the universe so hard to comprehend is that there's nothing to compare it with." -Anonymous
If I were brand new to theoretical cosmology, I might be skeptical of a whole bunch of "dark" things that I'd heard of. "Dark matter?" "Dark energy?" Come on; you've got to be kidding me! You're telling me that 95% of the Universe is not made of protons, neutrons, and electrons, like all the matter we know?
After all, I look out at the Universe, and this is what I see.
Stars, galaxies, gas and dust... normal matter, all of it. Yet all I need to do is start with two very well-supported assumptions, and you can demonstrate to yourself that the matter in the Universe must be mostly, but not completely, made up of dark matter.
The first assumption is that the Universe follows Einstein's General Relativity as its law of gravity.
The second assumption is that the Big Bang picture of the Universe works.
(And yes, you astute ones, it is unfair to place the Big Bang before inflation, as this graphic does.)
You give me these two things, and I can, unambiguously, give you dark matter.
How's that?
Because that's what the laws of physics let us do!
We start with a hot, dense, nearly (but not perfectly) uniform Universe that's expanding, and it unequivocally follows (among others) the following steps:
- It expands and cools down,
- It forms the light elements through nucleosynthesis,
- Matter starts to collapse under its own gravity, while radiation (like photons) push back against it,
- Neutral atoms form, leaving a snapshot of the Universe at that time (the CMB), and
- Gravity pulls the neutral matter together into stars and planets on small scales, and globular clusters, galaxies, galaxy clusters and superclusters on large scales.
The big distinction, of course, is that radiation doesn't push back against dark matter, but does against normal matter (i.e., protons, neutrons, and electrons).
So what does this mean for what's in our Universe?
It means that when we look out at the temperature fluctuations in the Cosmic Microwave Background (above), we can figure out whether our Universe is full of normal matter, dark matter, or both, and how much of each of them. In fact, there's an online calculator here that lets you put in your own parameters for how much normal matter, dark matter, and dark energy you want, and gives you the spectrum of CMB fluctuations. (Try it if you like!) Here are the observations that you need to match:
And can you do it with practically no normal matter?
Nope. But try it with all normal matter (and no dark matter), and guess what?
You can't do it either! If you want to make it work:
You need a mix of about 20-25% dark matter, about 4-5% normal matter, and let the rest (for a total of 100%) be dark energy.
But that's not the only large-scale piece of evidence we can test like this. There's also the large-scale-structure we find in the Universe!
Image credit: 2dF galaxy redshift survey.
And what we can do with this map is ask, if I look a certain distance away from a galaxy, how likely am I to find another galaxy? The result is known as the Matter Power Spectrum of the Universe, and it looks something like this.
Here's the thing: if the Universe were only full of normal matter (and not dark matter), the spectrum wouldn't look smooth like this! It would have certain regions where the spectrum dropped out to zero, corresponding to scales where normal matter were "pushed away" by the radiation in the early Universe.
But dark matter doesn't get pushed by radiation, and so if the Power Spectrum doesn't have these features, the Universe must have more dark matter than normal matter!
But -- and here's the most convincing part -- if you design your Universe with about 70-75% dark energy, 20-25% dark matter, and about 4-5% normal matter, you get predictions that match up perfectly with our observations!
And remember, all I needed to do this? General Relativity + the Big Bang. And the rest I figured out from the physics: by computing my predictions and comparing them with the data. And that's how we know there's dark matter! You want a Universe without dark matter, and -- at the very least -- you have to throw out General Relativity.
And trust me, you don't want to make Einstein angry...
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So where, exactly, is the dark matter and dark energy to be found?
In statistics.
Thank you for posting about this.
OK, I hope my question makes sense. I'm profoundly ignorant but super curious.
So is dark matter a defined thing whose properties can be used to make and test predictions, a term used to describe observations, or something in between? In other words, do these observations necessarily provide evidence for a "thing" called dark matter, or is the term a stand in for iignorance concerning what precisely is going on (or, again, something in between)?
But local gravitational affects seem to obey normal matter. There is no 75% discrepancy in the orbit of Jupiter. And if dark matter is not pushed away by radiation, then what makes it allergic to our solar system?
While the dark matter and dark energy models work well in explaining most major observations that defy the older standard model, so does "God did it". Until someone can show the extra gravity isn't just due to the weight of the souls of our dearly departed and how the dark matter was formed in the big bang and/or inflation, there's an awful big hole in the theory. And yes, I'm kidding on the God and the souls model.
Jeffery Boser @ 4,
Try listening to this talk:
'A Universe From Nothing' by Lawrence Krauss, AAI 2009
As`Ethan has said, you *DO NOT WANT TO UPSET EINSTEIN*!
http://www.youtube.com/watch?v=7ImvlS8PLIo
Jeffrey--
Dark matter isn't allergic to our solar system, but like any other matter in the system, the great majority of it should be part of/within the sun. In fact there's some recent work on ways to find it inside the sun. For example, it might reduce the core temperature relative to what a pure-normal-matter model predicts.
Jeffrey - that's because the matter density in the solar system is very much higher than the average matter density of the universe so the normal matter completely dominates any dark matter. I don't think you can see dark matter effects on any scale smaller than that of a galaxy.
I'm not afraid of Einstein. What's he going to do, slap me with a cosmological constant?
I don't think you can see dark matter effects on any scale smaller than that of a galaxy.
So how does that work, exactly? Is all the dark matter in the centers of galaxies? Outside and/or around them? Does dark matter exert or respond to gravitational force? Can dark energy be converted to other forms of energy?
People, this blog has an entire category called "Dark Matter". It's all explained in there.
The Einstein-as-Dr-Evil picture is so awesome I downloaded it.
I remember thinking that Ethan's multi-article argument in favor of dark matter was fairly convincing, but this reductionist argument doesn't do much for me. I guess it's nice to have as simple of a proof as possible. But to me this just says that _if_ the theory of general relativity is perfect, _then_ dark matter follows. But do I have reason to think that general relativity is perfect? If dark matter can only be seen at galactic+ scales, how do we know that general relativity doesn't need tweaking at galactic+ scales?
I was under the impression that dark matter was dark because it was beyond the event horizon of black holes. Is this not so? Or is some of the dark matter beyond event horizons but not all of it? Why doesn't radiation "push" against dark matter? Is this understood? Also, I had the hunch that dark energy was an artifact of our imperfect understanding of gravity. Guess not? Please forgive my ignorance, I'm a biologist with but two semesters of undergrad physics. Thanks for your explanations of how things work but they leave me with more questions than they answer.
If you want to see a vocal doubter of Dark Matter (who obviously doesn't read this blog ...) and a prominent student of it battle it out on stage come to Bonn University this Thursday! The debate format is explained in the commentary: The standard model comes first, then the attack, then ...?
Well, fine. :)
*reads well-written and informative series and looks at awesome pictures*
Here's where I get hung up (from Part III):
First, it seems like a third could exist, whether anyone can at present imagine it or not. Second, you can knock attempted reformulations of the theory by showing they don't fit with the data, but that doesn't mean no workable reformulations (or expansions?) are possible.
It just sounds kind of circular, like: "There are data that aren't explainable with existing theories and normal matter. I think they're explained with existing theories and dark matter. The evidence for this is...that there are data that aren't explainable with existing theories and normal matter."
Dark matter sounds perfectly plausible, but I don't know what else would....
How are the numbers for dark matter and dark energy derived?
I'm not a dark matter/cosmology expert but I'll try to answer a few of the questions above. Dark matter is a generic name for a hypothetical new particle or particles with mass and which interacts via the weak force with 'normal' matter, i.e. it does not feel the strong force and it is electomagnetically neutral. This isn't a big conceptual leap since we already have neutrinos which fit that description. Neutrinos were for a while considered a candidate for at least some of the dark matter, but with detailed calculations they don't quite fit the bill, in particular (if I recall) because they tend to be too relativistic (low mass to momentum).
Anyhow, dark matter does feel and exert gravity, like all other forms of energy, but not radiation pressure since it is electrically neutral. Recall that billions of neutrinos pass through you (and the earth) without interacting every day so this sort of thing is hard to detect. Thus the evidence is large scale behavior of galactic objects, like gravitational lensing and the rotation curves of observable matter in galaxies, and precise fits to the CMB curve shown above and such. There are ongoing 'direct detection' efforts that look for, basically, a single nucleus recoiling when it bounces off a particle of dark matter, but this is a rare event and a difficult experiment so no conclusive evidence thus far.
Dark matter and energy have nothing in particular to do with black holes, though dark matter would certainly fall into them like anything else. As for modifying general relativity, my impression is that no one has found any coherent model that explains things as well as the standard dark matter hypothesis. Besides, general relativity is very elegant and we hate to give it up without good cause.
Dark energy is currently a more general term for whatever explains the observed acceleration of galaxies away from eachother, hence could also apply to the inflationary epoch, but that was much more rapid. The simplest way to account for this is just to add a term to the general relativity equation, a cosmological constant. But that doesn't explain the possibly changing behaviour and may not fit the observations in detail. Dark energy is being converted to kinetic energy as galaxies accelerate.
As a particle physicist, I can tell you that the general characteristics of dark matter and dark energy are easy to come by. For a number of independent reasons we tend to think there are more particles than those currently known in the standard model and it is quite easy for them to be dark-matterish. For example, Supersymmetry, which solves some theoretical problems, also provides a good dark matter candidate. It is also possible to dynamically generate inflationary/dark-energy behaviour when writing down models with some simple fields. Getting the details right however, is a pain in the ass and we are all hoping for some detector evidence to help pin it down.
Ethan: "But -- and here's the most convincing part -- if you design your Universe with about 70-75% dark energy, 20-25% dark matter, and about 4-5% normal matter, you get predictions that match up perfectly with our observations!"
If you allow for invention of invisible stuff with just the right properties needed to explain away the discrepancies then of course you can match the observations.
Imagine for example if doctors could postulate invisible germs, suddenly they could easily "explain" all the human diseases by simply postulating dark bacteria or viruses with just the right properties. Would such "successes" prove that those dark bacteria and viruses are real? No.
In agreement with your analysis, yet still have a suspicion that there may be combined processes taking place, such as concentration effects due to normal matter sweeping vacancies or a complex triggering mechanism...pure speculation at present.
Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
pdf 1.52MB
An edge effect would seem plausable?
What I meant by the previous post is, I agree radiation doesn't effect dark matter directly, yet perhaps indirectly.
I was very skeptical about dark matter until Sean Carroll blogged about the Bullet Cluster a few years ago at Cosmic Variance. Now hearing that several independent observations are explained by the same mixture of dark and normal stuff, I am 90% convinced. The arguments I'm seeing in the comments here - that everything we see as evidence could be due to something else that we haven't thought of - could apply in some degree to every scientific theory. Yet science, for all its lack of philosophical certitude, seems to work a heck of a lot better than whatever is in second place.
But -- and here's the most convincing part -- if you design your Universe with about 70-75% dark energy, 20-25% dark matter, and about 4-5% normal matter, you get predictions that match up perfectly with our observations!
But how sensitive are the models to the "dark energy" fraction? If you run those simulations without any dark energy, how close is the match?
There's been good evidence of the multiple effects of dark matter for the past four decades (galaxy rotation profiles, for example, or galaxy cluster stability arguments), but the evidence for dark energy depends on measurements that could be influenced by all sorts of things including selection effects and other unknown phenomena. I'm not sayin' there's no such thaing as dark energy, I'd just prefer to wait until the confirmation is stronger before I start taking its existence for granted.
Dark energy is being converted to kinetic energy as galaxies accelerate.
That's a good trick! How do we explain it? Is there an exchange particle? Are any fundamental constants changing while this happens?
From a layperson's perspective I tend to take these things as "the current best approximation" and look forward to empirical results that support or falsify them. The Bullet Cluster item is very interesting, in that dark matter seems to be the most parsimonious explanation for the findings.
AI @ #18: the point is, those values are theoretical predictions, that can be checked with observations. "IF we're right, THEN we should observe X quantity of dark matter relative to Y quantity of baryonic matter..." The prediction is on the table, and then we start looking around.
In point of fact, doctors postulate invisible germs all the time, and very often they find them and end up developing tests, vaccinations, and treatments. Look up the history of the work on any infectious disease. This is how scientific method works: very often from theory to hypothesis to specific prediction to observation. And this is also what makes religious extremists so darn jealous: their track record for successful predictions frankly stinks by comparison.
I think Mike G is making an effective argument here. One needs to understand that Dark Matter was first proposed in the context of measurements galactic rotation curves and velocity dispersion in galaxies. Having introduced DM as a theoretical construct to explain these observations one can then make further predictions - lensing, microlensing, supernova distances etc. which are all consistent with the theory. Some of these predictions do require additional parameters. So then the questions are a) can you find a consistent set of DM parameters that explain all the observations and if yes b) is the number of parameters small enough (compared to the number of observations) that the theory is actually predictive of anything. (And finally c. is there another theory with no more parameters that explains all the observations.) I think the answer to a) + b) is yes. So far as I understand it, you have basically only two parameters in DM - the density and the speed/temperature of the particles. Given the number of independent lines of evidence pointing towards DM that strikes me as a fairly well-constrained theory.
Okay, so I took the time to watch the Lawrence Krauss presentation. I was able to follow along for the most part.
But I still don't understand the basic problem with the lack of local observation of dark matter. There should be thirty times as much dark matter, gravitationally speaking, around as there is normal matter. So why is there zero, absolutely zero, evidence if it in the solar system. The orbits of the solar bodies, our every observation from satellites, our every experiment to test the behavior of light and the effects of relativity, not one of them suffered even a small percent of unexplained gravitational disturbance from something unseen.
Shouldn't we see dark matter structures, at least in the patterns of nearby star movement? Or does dark matter somehow not gravitationally affect itself, and does not gather like normal matter does? Or does not it not gather with normal matter like some shadow body that moves in gravitational tandem with the normal matter that it is bound to?
A few minor things bugged me, like how they knew the gravitational lensing in that image was valid.. A part of the universe a quarter of the universe's age/expansion ago, was able to sustain its structure over the course of another quarter of the universe's age/expansion to form a lens for us? Isn't it far more likely a smaller mass that was not distinguishable between us and them responsible? Or did they measure stellar explosions from those blue sections to verify they all had similar distance? I didn't get the impression that sort of measurement was possible from those observations.
Jason,
You say " But I still don't understand the basic problem with the lack of local observation of dark matter. "
To be clear I'm not even a scientist let alone a particle physicist, cosmologist or astrophysicist. So with regards trying to shed some light (pun intended) on that, I can only hope someone much more qualified then myself will jump in here and help out. Perhaps the recent ongoing experiments at the LHC will also be able to create heretofore undetected particles and confirm it's existence by direct observation.
Here's another talk that might interest you from Frank Wilczek, the 2004 Nobel Physics prize winner.
http://fora.tv/2008/09/25/Frank_Wilczek_The_LHC_and_Unified_Field_Theor…
However Re: Proof of gravitational lensing, this has been empirically verified many many times and there is no doubt whatsoever that this is a real physical phenomenon.
http://archive.ncsa.illinois.edu/Cyberia/NumRel/EinsteinTest.html
There is nothing like observation and experiment precisely matching calculations based on theoretical predictions to verify the validity of said theory. This one has been in the bag for some time now.
Cheers!
Oops that was a response to Jeffrey Boser, not Jason, sorry!
This took me a while to get my head around. The thing is, gravity alone isn't enough to get matter to clump together - it also needs to able to exert / experience friction in order to slow down and "stick". Dark matter doesn't, so it forms very large and extremely diffuse clouds around (but much larger than) galaxies. I seem to recall that the total mass of dark matter inside our solar system is expected to be about that of a basketball, but spread out more-or-less evenly all the way.
Ethan Siegel said at the end of his pictorial on the dark matter: "You want a Universe without dark matter, and -- at the very least -- you have to throw out General Relativity." Wanting to throw out General Relativity is what I always wanted since I was a teenager 50 years ago. Back then people were not to pleased with GR. Way before 95% of the universe went missing, G.C. McVittie wrote in 1965, "To say instead that gravitation is a manifestation of the curvature of four-dimensional geometrical manifolds is to account for a mystery by means of an enigma."
To get rid of something questionable like General Relativity and the dark matter, you have to replace it with something better. That is what happened with the aether which was passionately believed in by people such as James Clerk Maxwell. A better theory came along i.e.Special Relativity and the near compulsive belief in the in the all pervading aether was soon dropped.
The trouble with either Newton's or Einstein's gravity theory is that both theories assume that MASS mediates the gravitational force. Exactly what is the property of mass that is able to make it magically attract other mass or warp space? This question is similar to the question: What is the property of the earth that can magically make all the objects in the heavens rotate around it in a 24 hour period that the followers of Aristotle failed to ask for 1500 years. Similar to the Ptolemaic earth-centered system, an ARTIFACT has given Newton's and Einstein's mass-based theories a life of their own. With the Ptolemaic system, the artifact that kept that theory alive for centuries was the the earth rotating on its axis every 24 hours. The artifact that has kept our ancient mass-based gravity theories alive is Prevost's Law (i.e. if a mass has temperature it has radiation leaving it or entering it).
No one has seriously settled the question as to whether it is the MASS of a body or the RADIATION leaving it that is doing or causing the gravitational attracting of other mass.
The Tully-Fisher law directly states that the luminosity of a galaxy is directly proportional to its highest orbital velocity raised to some power like 3 or 4. With this law, the truth is starring scientists right in the face. Go to my website by clicking my name and see four experiments where a 2%, 8.9%, 9.6% and a 16% increase in weight was observed with test masses placed between a 1000 W heat source and a cold source made of ice. As George Orwell said, "To see what is in front of one's nose needs a constant struggle."
@AI:
What you are ignoring here is (1) one can explain a lot of totally different observations with the same numbers, and (2) both the existence of dark matter and of dark energy are essentially expected from the viewpoint of particle physics.
And this analogy makes no sense because the doctors would have to postulate essentially a different type of "invisible germ" for every disease, or a few / a single type with a lot of quite different, even contradictory, properties. This is in no way comparable to dark matter or dark energy.
@Peter Fred:
Err, one could say analogous things about every theory in physics.
That's wrong - neither Newton's nor Einstein's theory assume that. In Newton's theory, gravity is mediated by nothing at all ("action at a distance"); in Einstein's theory, gravity is mediated by the curvature of spacetime. If, on the other hand, you didn't mean "mediate", but "mass is the source of gravity", then you are still wrong - because in Einstein's theory, not mass (directly), but energy, momentum and pressure are the sources of gravity (mass is only a source of gravity there because of its rest energy).
If you ask questions like that, you also have to throw out electrodynamics along with Newton's/Einstein's gravity - because one could equally well ask: "Exactly what is the property of charge that is able to make it magically attract other charge or warp an U(1)-bundle?"
That's simply wrong. Lots of people have tried to explain gravity with radiation-like stuff leaving and/or being absorbed by bodies - they all failed. And especially considering the electromagnetic radiation you talk about makes little sense, because it is well-known that gravity is lots of orders of magnitude weaker than electrodynamic forces - so how could the second explain the first?
Strange that all the people who design bridges, skyscrapers etc. never noticed that the mass of objects increases by up to 16% when heat flows through them - don't you think...? As long as no one else replicates your experiments (and I'm not equipped to do so), the straightforward explanation is an error somewhere in your measurements.
Astronomers may be convinced; but many physicists remain skeptical.
New York Times front page article Nov 17, 2010
A Costly quest for the Dark Heart of the Cosmos
pg A4, column 4, "And many physicists say they will not be satisfied that dark matter has been found until there is supporting data from particle accelerators and underground experiments that seek to detect the particles directly."
Why oh why do you not say, "You want a Universe without dark matter, and -- at the very least -- you have to throw out the Big Bang model"? Bias, red herring, or both.
Perhaps one day, you will address Fred I. Cooperstock's assertion,"Einstein's general theory of relativity, could account for the observed flat galactic rotation curves without the requirement for vast stores of mysterious dark matter." Yes he used a simple model for a galaxy. So?
I await CERN results on new particles (or NOT) that could be the hypothesized "dark matter". Perhaps you are a bit concerned no such particles will be found.
@OKThen: You are Thomas - aren't you?
I also await that day... but even if that assertion were right, it would still leave lots of other pieces of evidence for dark matter!
@Bjoern said "Strange that all the people who design bridges, skyscrapers etc. never noticed that the mass of objects increases by up to 16% when heat flows through them - don't you think...?"
In my paper Figure 5 shows a 1000-Watt heat-element that was suspended ~25 cm below three copper hemispheres filled with ice water. After 6.6 minutes of full power, the weight of the heat-element increased by 16%. The heat-element reached a temperature of more than 250 degrees Celsius. Skyscrapers do not get near this hot.
I stand corrected. I meant "mass is the source of gravity for both Newton's and Einstein's gravity theories." If mass and energy are equivalent due to the well-verified formula E = m*c^2, then what you call energy I call mass. Heat is measured in energy per second, so if you want to call energy as the source of gravity of General Relativity you and General Relativity are careening into my Heat-based theory of gravity. However, with your energy-based theory you are needlessly concerned with the action-at-a-distance problem and needlessly get involved with non-euclidean geometry. That action-at-a-distance problem is much more simply dealt with in my theory by the assumption that luminosity interchanged between astronomical bodies causally produces the observed attraction between them. This assumption which is obvious from a heat-based point of view is well-supported by my five experiments in my paper.
Rue the day that I get someone to finally replicate my experimental results. Then its London-bridges-falling-down time and this 300-year-old, ludicrous dream that mass is the source of gravity will come to naught like the 1500 year-old, ludicrous dream that the earth was the center of the universe came to naught in the 17th century.
@Fred
The experiment of heating something and weighing it before and after has been conducted many times. Nobody has replicated your experimental results because they are false.
Though of course because energy and mass are the same thing, your samples do get more massive as they gain heat energy in proportion to E=mc^2. This effect is just very tiny. But anywhere there's enough energy, then gravity will be noticeably effected because it's energy that creates gravity, and matter is just a form of energy just like heat is.
@Peter Fred:
Is the temperature the important thing, in your opinion? Or the temperature difference between the two sides? Or the amount of heat flow through the body? Or what?
Well, you then still ignore that in General Relativity, momentum and pressure are also sources of gravity.
Pardon?!?!? Where did you get that nonsense from? Heat is measured in Joules! Say, why should I take anything you say seriously if you don't even know these absolute basics?
There is no action-at-a-distance problem in General Relativity. Another piece of evidence that you don't know what you are talking about...
Feel free to derive Newton's law of gravity from this assumption...
Bjoern
Focus on the idea that "many physicists.. will not be satisfied that dark matter has been found until there is supporting data from particle accelerators." NYTimes
I suggest that the idea of "dark matter" merits the "hypothetical" disclaimer as much as the "hypothetical graviton"; hence "hypothetical dark matter."
This isn't a particle popularity contest to see if physicists elect "dark matter" to a new standard particle model because the big bang model requires it.
When experiment confirms(or disputes) the existence of a new particle; the consequence will ripple through physics and astrophysics.
I expect new particles from CERN; but mostly surprises not support for current theories. There's a lot of chaff in current astronomical, string and particle theories
I further think that Ethan is being ingenuous when he suggests that the big bang model is stronger and more credible than general relativity.
correction, I meant "disingenuous"
Cooperstock's assertion is based on general relativity calculations; whereas galactic "dark matter" assertion is classical Newtonian gravity. Why avoid that discussion?
First, let's get the starting premises straight: I have no qualms whatsoever that we're seeing something real. The math all adds up, and the physics will follow. Clearly, visible matter is only a small proportion of everything that we see.
But I have serious issues with calling this invisible something "matter". It may be matter; it may not be. But calling it "matter" suffers the same problem as any other naming process: the name you choose biases what you go looking for and what evidence you will use to go looking for it.
The problem I haven't seen answered about dark "matter" is the following: What properties does it have such that it exerts a gravitational influence (like other forms of matter), yet cannot coexist with the normal matter that we see all around us? That should be an obvious question, since if it interacts with normal matter, it would be all around us. ("You're soaking in it.") Yet none of our equations for matter, whether at a microscopic (quantum) or macroscopic (Newtonian) level show an error term of 500% (the ratio of the presumed amount of dark matter to the amount of normal matter).
So whatever this missing mass happens to be, its essential property is that it segregates independently of normal matterâin fact, that it actively dissociates itself from normal matter. That's convenient, because it means we don't have to throw out our entire model of matter. But it makes it hard to consider this stuff to really be matter, at least in the classical sense.
Has anyone tried rewording the problem in such a way that it is phrased in non-matter terms? For example, imagine the following metaphor: imagine a universe in the form of a foam rubber that originated as liquid rubber squirted through a pinhole under gas pressure (i.e., the big bang) from another universe into our universe. As the rubber expands, what would you expect to see? A structure composed mostly of void (air pockets in the case of the ball, empty space in the case of our universe), with thin layers of matter (rubber) forming the walls of those voids. In fact, this is precisely what we see when we look at the large-scale structure of the universe: bubbles surrounded by concentrations of matter.
I propose this metaphor not because I believe that it's correct, but rather because it leads to a whole new way of thinking about the problem. Instead of having to invoke really weird types of matter that don't fit into any of our existing models, it leads us to examine an entirely different solution set, such as the possibility of something like "branes" existing at a large scale and being responsible for the current "foamy" structure of space. They may not be the physicist's branes, but rather something more like a real-world membrane, like the walls of the pores in the rubber ball or the soap that forms a soap bubble.
Is that right? Who knows? But it's a hypothesis that can be tested by means of mathematical modeling. Dark matter, so far as I've read, doesn't generate such testable hypotheses that would let us fit the new form of matter into any existing or proposed new model.
So let's stop calling this stuff "matter" until we have a good reason to believe that it's made up of dark protons, dark neutrons, and dark electrons. That may end up being the right description, but until we have a reason to believe this (i.e., a model that states how these new particles relate to existing classical particles), it biases our thinking. If we're wrong, that bias will greatly delay finding the real answer.
I like the observation made by Geoff about "testable hypotheses", so I'll add my own.
Why, now, can't any of the new "theories" be easily proven or disproven? Three of the biggest mysteries seem to be string theory, dark energy, and dark matter. Contrast with ancient theories (note no quotes needed) such as special relativity and general relativity, both of which could be verified or falsified quite simply.
All of this seems to be nothing more than "tenure track" academic nonsense, conveniently unconcerned about practical reality.
@Geoff Hart
Re the misleading name problem by using the term 'matter' the same objection could be directed at the word 'dark'. It's actually spooky and invisible, not so?
(Yes I know 'dark' can refer to energy levels etc).
There have been speculations by some that DM might be a form of negative matter with anti gravitational properties, but this does not appeal to the great majority of physicists, simply on the grounds that you would need a huge amount more of the stuff to achieve the observed effects.
It's probably safer to continue to assume that the halo like concentrations of DM that surround galaxies are strongly attracted to its baryonic matter, after all, the calculations still work, don't they?
Bjoern: "What you are ignoring here is (1) one can explain a lot of totally different observations with the same numbers, and (2) both the existence of dark matter and of dark energy are essentially expected from the viewpoint of particle physics."
1. No, one can only fit some cosmological observations and that's that. If we could see it anywhere else I would agree with you, but so far they are pretty much single purpose fixes: dark matter - unexplained positive curvature, dark energy - unexplained negative curvature. Of course this curvature manifests itself in many observations but they are not independent.
Also dark matter and dark energy are not just two parameters, they are fields whose local density you can arbitrarily fine-tune to suit your needs - there are infinitely many configurations you can try to fit your data. In other words those two together can fit any observed curvature arbitrarily well.
2. This is manifestly false. There is nothing in SM that could explain them. Speculative theories with no empirical support don't count.
Al noted: "Also dark matter and dark energy are not just two parameters, they are fields whose local density you can arbitrarily fine-tune to suit your needs"
"Fields" may be precisely the right way to think about these things, since it doesn't require us to invoke strange new states of matter unnecessarily (thereby violating Occam's principle). The advantage of thinking of the phenomenon in terms of fields is that you can describe and parameterize the phenomenon, thereby allowing you to investigate it by framing testable hypotheses. Once you have enough data from these tests, then you can begin ruling in or ruling out whether we're really looking at weird matter, weird energy, membranes, or something else entirely.
Another paradox I'd like to see explained by the dark matter theorists:
Premise 1: dark matter exerts enough of a gravitational pull to shape entire galaxies (i.e., it acts like conventional matter when it comes to gravitation).
Premise 2: We're soaking in it, but just can't see it (because it's "dark").
Conclusion 1: All our models of gravity (which are based on measurements of the attraction between masses composed of normal matter) and of chemistry (protons and electrons and the like) and of particle physics (e.g., the mass of atoms and their components) are wrong, because we have ignored 80% of the existing matter in these calculations. Back to the lab, boys and girls! *G*
Alternate conclusion: We're not soaking in dark matter because it either doesn't exist, or because it dissociates itself from normal matter in such a way that it doesn't affect our lab-based observations or theory-based predictions about the behavior of normal matter.
Alternate alternate conclusion: The large-scale structure of the universe is shaped by some sort of complex field (something as simple yet complex as the way magnetic fields are twisted in the sun), and matter accumulates in the areas of lowest potential energy within that field.
I like the last one a lot because, despite invoking the need for some really whacky new type of field, it doesn't require us to throw out existing models of matter, which work pretty damned well at both Newtonian and Einsteinian scales. WIMPs are all very well, but the "massive" part is a big red flag that something is wrong with that explanation: mass = gravity. If there are so many WIMPs left over from the big bang (80% of all matter), why are there no WIMPs anywhere we can see them? Wouldn't they be attracted to stars and planets, just like other forms of matter?
Interesting stuff in the last Scientific American about this, but it doesn't answer any of the big questions.
@AI:
With "only some" you mean "only a few", or what? Well, then we should discuss how many things are needed until one can sensibly talk about "a lot"...
Hogwash. Dark Matter also explains e. g. the following things: (1) rotation curves of galaxies, (2) motion of galaxies in galaxy clusters, (3) the CMBR power spectrum - see Ethan's article just above! And Dark Matter was not introduced into cosmology in order to "fix" the "unexplained positive curvature", it was introduced into
cosmology because astronomers already knew from (1) and (2) that there probably is such stuff out there!
Pardon?!? Dark Energy has nothing to do with "unexplained negative curvature"; what are you talking about? (and there are several pieces of evidence for the existence of Dark Energy - additional to the two which Ethan discusses here are e. g. (1) the WMAP results, (2) the SN Ia data, (3) the observations of the integrated Sachs-Wolfe effect)
Again, hogwash, especially with respect to Dark Energy. Dark Energy is constant throughout the universe, i. e. it has the same density everywhere! And Dark Matter consists of particles with mass and hence obeys gravity - hence there you also can't simply set the density as you wish, but you have to start from primordial density fluctuations (which you also can't choose as you wish, there are also lots of constraints for those) and then do simulations in order to see where the particles end up. I. e. the local density of Dark Matter is determined by the laws of nature, you can't choose it willy-nilly.
Err, I didn't claim that there is something in the SM that could explain them. I also pointed out that this is expected from the viewpoint of particle physics - simply because it is clear in Quantum Field Theory that the SM is not the end-of-all, but that there have to exist other particles. What additional particles exist is still speculative, right - but that there have to be additional particles is pretty clear in particle physics.
@Bjoern said," Pardon?!?!? Where did you get that nonsense from? Heat is measured in Joules! Say, why should I take anything you say seriously if you don't even know these absolute basics?"
You are right. I have equated radiation with heat and I should have equated it with heat transfer or luminosity. I used to call my theory a luminosity-based theory. I could have just as well called it a radiation-based theory. I did not want to do that because most people are aware of the radiation pressure studies and the fact that sun pushes comet tails away from it as noted by Kepler. This mistake of mine has made me recall the difference in results between a Crookes Radiometer and a Nichols Radiometer. Using a radiometer is not a good way to study the all important question "Is radiation attractive or repulsive." This question is not easily settled. I worked for ten years full-time with somewhat limited resources at my own expense to obtain the results of the five experiments that I show in my paper. Without cold source in addition to a hot source, I have found the weight-increase effect is elusive and not easily repeatable. However, with replenishing the cold source with ice with each new run, I have found that with subsequent runs, the weight-increase effect can be repeated. But it will not necessarily be the same percentage weight-increase as the previous run.
The pressure at the center of the earth can be estimated by (Pressure = g*rho*r) where (g) is earth's surface gravity (rho) is its density and (r) is its radius. So if (g) decrease by 0.08% on its dayside, it will have a tremendous effect at the center of the earth because (rho)is 10^3 and (r)is 10^6 . If there is not a concomitant decrease on earth's night side, there will be a pressure IMBALANCE between the solid dayside hemisphere and the solid night side hemisphere that could produce the 10^22 Newtons needed to centripetally accelerate the earth towards the sun at 0.006 m/s^2 (see my paper for a fuller explanation.
The Nobel laureate Allais observed a diurnal variation in the earth's surface gravity (g) in ~1959 and no one has bothered to replicate his work just as they have not payed any attention to my work. Only a slight diurnal variation in (g)is all that is needed to account for orbital motion of a planet around the sun and no one realizes this. They are far more interested in defending the standard model where 95% of the universe is missing. However since luminosity is as ubiquitous as mass and if it is attractive and not repulsive as some experiments, including mine show, we have a great, simple, close-to-experience way to deal with the flat rotation curves and the fact of cosmic acceleration (see my paper).
For "Dark Matter: a debate" see http://www.astro.uni-bonn.de/~pavel/kroupa_SciLogs.html. This site also contains a listing of recent Nature papers on the current status of the standard cosmological model. There is also an entry about the Bullet Cluster and its relevance for the Standard Cosmological Model. Concerning the notion that one should not anger Einstein: Well, Einstein might not have wanted to anger Newton and would then not have transformed physics. Always remember that any scientific theory must account for all observed phenomena if it is to be a successful description of nature. This is why everyone knows that we do not have a final theory of everything, since quantum mechanics and classical physics remain disjoint, as does gravitation and the three fundamental interactions of nature (electromagnetic, strong and weak). Also, simplicity is never a good argument: We discard Newtonian mechanics on the sub-atomic scale although quantum mechanics is a much more complex and a non-intuitive description of nature. Finally, since the Cold-Dark Matter Cosmological Model was designed to account for sub-8Mpc scale physics, and since it fails, we have a difficult issue at hand, which is why there is a serious argument.
I like the last one a lot because, despite invoking the need for some really whacky new type of field, it doesn't require us to throw out existing models of matter, which work pretty damned well at both Newtonian and Einsteinian scales. WIMPs are all very well, but the "massive" part is a big red flag that something is wrong with that explanation: mass = gravity. If there are so many WIMPs left over from the big bang (80% of all matter), why are there no WIMPs anywhere we can see them? Wouldn't they be attracted to stars and planets, just like other forms of matter?
@tütüne son:
You do know that the "WI" in WIMP means "weakly interacting" - don't you?
Peter Fred, how do you measure mass? Perhaps your scales deform in the heat?
And if you replenish ice to the heat sink, how can that not change the mass of the heat sink!?!
What is the property of electric charge that is able to make it magically attract other electric charge or warp the electric field?
Mass is the charge of gravity.
Gravitation waves = gravitons are radiation. They're just not the electromagnetic radiation (photons) that you're looking at.
Easy: we've done all the easy stuff already, what's left is the difficult stuff. :-|
Premise 2: we can't "see" it even for very wide meanings of "see", because it only interacts with normal matter by gravity. (And perhaps very weakly by the weak force, which is... weak.) Neutrinos interact with the rest of normal matter only by gravity and the weak force; billions pass through every square centimeter of us and the rest of the Earth every second, and we detect a few per day or so, because the weak nuclear force is just so weak.*
Conclusion 1: due to gravity, dark matter clusters together with itself and with normal matter, but very weakly so, because -- not noticing the electromagnetic force and even the strong force -- it does not experience friction.
Prediction 1 from Conclusion 1: when galaxies collide, the normal matter should slow down much faster than the dark matter. The normal matter should actually, you know, collide, while the dark matter should just pass through and swing back and forth a couple of times. This means that normal and dark matter should separate. We should therefore get a gravitational lens without normal matter in it.
Confirmations of Prediction 1: the Bullet Cluster and that other cluster... it hasn't got a name, and I forgot its number, but it's mentioned in the Wikipedia article on the Bullet Cluster.
Prediction 2 from Conclusion 1: the density of dark matter in our solar system should not be noticeably higher than in the interstellar space elsewhere in the galaxy. As a result, dark matter should not have noticeable consequences below the galaxy scale.
Confirmation of Prediction 2: there's no evidence to the contrary, but we already knew that. Don't forget how utterly microscopic solar systems are compared to the whole galaxy.
* Gravity and electromagnetics diminish with the square of the radius. The strong nuclear force diminishes with the seventh power of the radius, so it is considered negligible for distances above 10-15 cm. The weak nuclear force is considered negligible at distances above 10-18 cm.
David MarjanoviÄ said:"Peter Fred, how do you measure mass? Perhaps your scales deform in the heat?"
Click on my name and the site will take you to my paper on my radiation-based gravity theory. There you will see that I measure the mass with a sensitive force sensor that was purchased from Vernier Software. The copper test mass was under my desk. The force sensor is above my desk housed in a wooden box. There is a small hole in my desk top. A thin wooden dowel is attached to the force sensor which in turn is attached to the test mass. I put ice in the wooden box to make the temperature in the box decrease while the test mass is heated by a heat element.
See Figure 5 in my paper where it shows the temperature inside the wooden box goes down while the temperature of the test mass increases. With the precautions that I have taken, it is hard to come up with an argument as to how the observed increase in weight is somehow due to overheating of the force sensor.
The heat sink containers are attached to the underside of the desk top. They are not weighed--only the test mass is weighed. In two of the experiments the test mass hovers over the heat source. With these a 1.9% and a 9.6% increase in weight was observed.
David MarjanoviÄ said, "What is the property of electric charge that is able to make it magically attract other electric charge or warp the electric field?"
My god man; do you think our esteemed scientists have answered this question? Coulomb's law and Newton's gravity are operational definitions or ad hoc formalisms. They await to be further interpreted just as the ad hoc formalism--Planck's constant--that was developed to solve the ultraviolet catastrophe awaited to be further interpreted as it was by Einstein in 1905 and Bohr in 1913. I think I have done a tolerable job in further interpreting Newton's Law of Universal Gravitation. Radiation is measured in units of power. We have plenty of experience in relating power to acceleration. We have no experience in relating mass to acceleration. We only have Newton's ad hoc formalism to relate mass to acceleration.
When we run into trouble relating mass to acceleration such as with the flat rotation curves, all we can up with is to assume that there is some sort of unseen matter causing the unexplained acceleration. If some one at the time of Cavendish(1790) had taken ten years out and with his own resources found, as I have done, that it is the radiation emanating from mass the does the gravitational attracting then when the flat rotation curves are discovered, it would be easy to think about the possibility that light bending comes into play due to the peculiar disc shape of galaxies that would make the light in the further reaches of galaxies fall off as 1/r rather than 1/r^2. This extra light at the outer reaches of a galaxies could then be tested to see if it could explain the flat rotation curves. We already have hints that this is the case from the Tully-Fisher relation L=k*V^4 and from Renzo's Rule.
As you might be aware of, Newton was very much concerned with just why his mass-based gravity theory worked. He was especially concerned with how a force stemming from mass operating through a vacuum was able to attract other mass. As we all know sunlight and starlight can easily travel through a vacuum. So if one has a radiation-based gravity theory, this action-at-a-distance problem simply goes away.
A good many to today's scientists think that this action-at-a-distance problem was solved with Einstein's space-time warp idea. But others, like the the text book writer G.C. McVittie think otherwise. He wrote in 1965:
"To say instead that gravitation is a manifestation of the curvature of four dimensional geometrical manifolds is to account for a mystery by means of an enigma."
Pavel Kroupa said, "Concerning the notion that one should not anger Einstein: Well, Einstein might not have wanted to anger Newton and would then not have transformed physics."
With Michelson-Morley experiments the failure to detect the aether presented a "serious anomaly" which all the high-profile theoretical physicists at the time seemed incapable of dealing with. To deal with this anomaly, Einstein in 1905 came up with a disturbing, unwanted and bewildering idea of "time dilation". That idea and the equations that came with it were found to be capable to explain the energy-velocity relation of the relativistic electrons that were being accelerated at that time.
My take from this piece of history and others similar to it is that we should expect for an unwanted, disturbing and bewildering idea to come to the for ground in order to deal with the observational or experimental "serious anomaly" that has appeared on the scene.
With our advance in astronomical equipment, cosmology in the last forty years, has unexpectedly come across two "serious anomalies" that so far our high-profile, well-trained theoretical cosmologists do not seem to be able to deal with. Judging from the "solutions" to past serious anomalies we should expect that what will come to the for ground to solve problem will be a disturbing difficult-to-understand idea like time dilation.
I have offered the disturbing and unwanted idea that "radiation is gravitationally attractive" in order to deal with the unexpected observation of flat rotation curves and cosmic acceleration. It has not been easy to figure out how this idea of "attractive radiation" could explain known gravitational phenomena. This task has taken a 30 year full-time, self-financed effort. During this time I also had to figure out how to demonstrate experimentally that "spreading luminosity" is gravitationally attractive.
I forgot to mention that the the amateur Copernicus came up with the disturbing idea of "heliocentric motion" in order to deal with the difficulties of the 1500 year old, well-known theory of geocentric planetary motion. This idea took him 30 years to flesh out. Also the amateur Darwin came up with the disturbing idea of "natural selection" in order to deal with the "origin and variety of the species problem" that flew in the face of the Genesis account of creation. It took him 20 years to work out how the idea of natural selection could explain the origin and variety problem.
Was the large scale structure of the universe predicted or in any recorded sense anticipated prior to its observation?
What if we remove the concept of dark energy and dark matter.
Assume that General Relativity is missing a variable. Match the observations to find the missing component, and try to account for the variation within the formula. I believe the variation should show up somewhere between 10 to -30 and 10 to -34.
Maybe the assumption of anything traveling a strait line is the beginning of the error, and that motion at the Planck Length is limited.