Apparently there is not enough observable mass within a galaxy for it to be held together under gravity, so astrophysicists have proposed the existence of dark matter which provides the missing mass holding galaxies together.
Recent observations show that the galaxies in the observable universe are not only moving away from each other, they are accelerating away from each other in defiance of their gravitational attraction. Astrophysicists have proposed the existence of dark energy which causes this acceleration.
Questions: are dark matter and dark energy the new “ether” or have they actually been observed to exist?
What reason is there to believe the current gravitational theory must be correct? It makes correct predictions on the scale of the solar system but incorrect predictions on larger scales. It seems that corrections like dark matter/energy must be introduced simply to make observation fit existing gravitational theory.
What if there is no such thing as dark matter/energy, but rather that the most current gravitational theory is wrong? Since the observed large-scale behavior of the universe does not agree with predictions of current gravitational theory, is there any reason to cast doubt on the time since the big bang? Or whether there even was a big bang?
Disclaimer: I am not a crank amateur astrophysicist. These are honest layman’s questions.
Milgrom’s Modifed Newtonian Dynamics (MOND) is probably the most prominent alternative. It was devised specifically to explain away the galaxy rotation problem without resort to dark matter. (Dark energy is something else entirely.)
That Wiki page states that MOND has passed almost all its tests so far, but I recall reading a number of articles in places like New Scientist that indicated other trials that it has not worked well for.
A major problem with MOND is that it has no good theoretical basis. It’s mostly an ad hoc solution. I’ll let the pros tackle that.
Scientists are very well aware of the problems behind dark matter (and dark energy, too). None of the alternatives are as well developed as the older, standard theories. Remember that any hypothesis has to answer every single question that gravity throws at us. MOND and its rivals may answer one better, but that doesn’t mean that they’re the better answer overall. It’s a very hot research subject now, so one side or the other should see its predictions confirmed by more and more tests over the next few years.
Here is a cite giving some evidence for dark matter. Gravity bends light, and if a large mass is between us and some light source, a distant galaxy, for instance, gravitational lensing can create multiple images of the distant light source. We can compute the mass of the galaxy or cluster causing the lensing. Observation shows only 10% of the mass observable - the rest is dark matter.
Here is a book on the subject which I read. It is pretty good.
This is not quite accurate. For the directly observed matter in galaxies, they seem to be rotating too fast. It’s as if the planets in the Solar System were rotating 50% faster than they are now, but not even consistently. E.g., the inner ones rotating a bit too fast and the outer ones a lot faster. Without dark matter, the galaxies could still exist, just not rotating as fast. Galaxies have plenty of mass to hold together at lower rates.
Although to be fair, dark matter (or as it is more explicitly known, cold dark matter or CDM, in contrast to radiating matter like stars and hydrogen clouds) is merely mass that we can’t see. It need not fundamentally be something exotic; it is merely mass that isn’t radiating in the electromagnetic spectrum, and in fact there is at least a small percentage of CDM that is exactly that; dust, small heavy objects, brown dwarfs, et cetera, that are dense enough that they don’t absorb and reradiate EM energy in a distinguishable amount.
However, it seems clear that CDM is not all just normal mass, and the part that is not seen is known as missing dark matter (MDM). There are several reasons for believing that MDM is real; first is that in order to make the mechanics work with our observations of large scale cosmological objects (galaxies, clusters, sheets, and filaments) the distribution would have to be broad and even, which would result in energy being absorbed and reradiated in detectable bands. (All elemental matter radiates energy in specific frequency bands corresponding to the energy difference between electron orbitals, and at least with lighter weight elements these bands are distinct i.e. the Balmer series for hydrogen.) Modeling with mass too clumped together doesn’t work; it would nucleate larger structures and we’d see a less even distribution. The same goes for large structure black holes. At one point the idea that very tiny black holes (quantum singularities) might be responsible for the missing mass-energy was considered, but current understanding of black hole thermodynamics indicates that such structures wouldn’t be stable and would radiate away. It was also hoped that neutrinos, a product of many fusion reactions which interact gravitationally but only very weakly with electromagnetic fields, would be adequate to explain much of the mass, but the estimated mass of neutrinos and their average momentum just doesn’t fit.
What we’re left with in order to get a simple answer to the question, is some type of unobservable mass that is spread out among normal matter allowing spinning structures like spiral galaxies to appear like a frozen continuum rather than a fluid (i.e. the spiral arms are distinct rather than being constantly twisted as the inside stars move faster than those on the rim). Applying this mass distribution, though, gives a very simple, if not directly observable, answer that corresponds nicely with celestial mechanics as modified by general relativity, and thus, is a reasonable assumption.
Dark energy is somewhat more nebulous; in essence, we’re seeing the space between large scale objects (clusters and larger structures) expand, and that expansion appears to be accelerating. There is no physical reason for this, or at least, not if assumptions about the curvature of spacetime being neutrally flat or slightly negative is true; like a baseball thrown up in the air, it should have only the initial energy released during inflation and should be slowing as it expands, but instead, it appears (from current inferences of observed large structures near the cosmic event horizon) to be accelerating. Dark energy is just some kind of matrix of anti-gravitationaly energy that is pushing space apart.
Niether MDM nor dark energy is, or indeed, can be directly observed, so they are indeed placeholders for something we don’t understand. In a sense, they could be considered the new “luminiferous aether”. (I personally prefer “atramentous corporeity” and “crepuscular palpitation” respectively, as I feel they offer a somewhat baroque flavor to a field that has becoming increasingly dry and tending toward acronyms in its jargon.) Like the aether of Newton’s time, it is undetectable except by apparent effect, the measure of which corresponds to the value that it needs to modify in order to have the anticipated effect. In other words, its existence is predicated by a closed circle of logic needed to fulfill its desired properties, and so yes, it is a bootstrapped justification.
Someone brought up MOND as an alternative. While the various MOND theories are also bootstrapped, it is also the case that there are a large number of properties in MOND that have to be arbitrarily set in order to get the theory to fit observation, and more refined observations often result in a significant realignment of these parameters. Furthermore, there is no physical or consistent rationale for why MOND mechanics are so complex. Whereas general relativity is a globally consistent theory that is (at least in concept) very simple (i.e. spacetime is distorted by the presence of mass-energy per a single metric tensor), MOND is a very complex theory that has to be tuned in order to fit one set of data, and retuned in order to fit another. It is inelegant and even if it is pointing to a more expansive solution than GR, itself is strictly thrown together in order to try to match an observation, instead of providing novel predictions that precede observations.
MOND is just interesting, but not currently useful as a workable theory for any practical use, whereas general relativity is used regularly for prediction to very high precision on local cosmological scales, and the simple inclusion of fairly evenly distributed MDM provides a very simple and workable fudge factor on larger scales. Our expectation is that physical mechanics, although complicated in their interactions and sometimes explicitly unpredictable in their specific solutions (for instance, the prediction of motion of a single particle in a cloud of energetic gas), follows a few very simple rules that underly reality, and advances in the 19th and 20th centuries have all led to justify this assumption. A new theory that is arbitrary and complex, like MOND or M-Theory, is probably itself a stand-in for some more simple fundamental theory.
There is no direct evidence for the non-baryonic dark matter or for dark energy, but there are multiple different lines of completely independent evidence for both, which give consistent answers. When you’ve just got one kind of observation giving you a result, you can easily argue that there might be something wrong with the observation, but when you’ve got three or four different methods all giving you the same answer, you start to take it much more seriously.
Others have already mentioned MOND, which would explain away some (but not all) of the observations which indicate the existence of dark matter. It’s not quite crackpottery, but nobody but Milgrom (the model’s originator) really thinks it has any chance of being correct.
On the dark energy front, though, the effect that’s currently called dark energy can in fact be modeled as a very simple change to what we know of the laws of gravity. This is in fact the original ways such effects were modeled, all the way back to Einstein, as the cosmological constant in Einstein’s field equations. The primary reason it’s now described as a substance of sorts, rather than as a modification to the fields, is that particle physics theories independently predict the existence of a substance which would have qualitatively the right properties to fill the bill. The only problem there is that, while the particle physics models aren’t yet well-developed enough to actually calculate a value for the dark energy, they can be used to make a rough estimate, and that rough estimate is insanely ludicrously horribly wrong.
Thanks for all the great answers. The references will keep me busy for a while.
Interesting—I misunderstood the problem. The rotation of a galaxy is far too slow to observe directly, but is there a not-too-difficult way to describe how the measurement is made indirectly?
We can only make inferences about the movement of stars in our galaxy based upon measurements of lateral velocity and inferences about radial movement, and based on that, the average speed of stars in orbit around the rotational axis of the galaxy looks wonky. However, we can directly observe other spiral-type galaxies, and while we can’t literally watch them spin, we can develop an estimate of the mass (based upon typical densities) and the rotational rate based upon averaged kinetic energies, which will give us an order of magnitude estimate of what the resulting rotational speed should be. In any case, we see spiral structures (arms, bars, and webs) that shouldn’t be possible; like a drop of food coloring in a draining sink, the stars should rapidly spin out and make a big, roughly homogeneous smear, forming a lenticular shape (which some older galaxies do become). Instead, spiral galaxies form large and relatively long-lasting structures (hundreds of millions of years) that appear to rotate as if nearly frozen in a very viscous fluid continuum, i.e. missing dark matter.
There are even larger structures (clusters, superclusters, and filaments) composed of collections of galaxies which can’t possibly have formed by the influence of the visible mass alone; the amount of average rotational momentum that was combined when the converged should have had them spinning each other off into space. Instead, they appear bound together as if stuck in a Jello salad with an invisible-flavor gelatin matrix.
Stranger, I must admit that “atramentous corporeity” and “crepuscular palpitation” are much more steampunkish than YATLA (Yet Another Three Letter Acronym).
Your posts were quite interesting.
I have a simple question: are neutrinos part of what cosmologists call ‘dark matter’?
Is it possible that the effects of unknown mass come from matter no longer observable because of it’s distance due to expansion of the universe? Would the gravity of mass once within observable distances in the past still be measureable once that mass leaves the observable distance?
Although neutrinos are definitely dark matter by definition (very weakly interacting with matter and almost not at all with photons) and are all around us (the current estimate is 337 neutrinos per cubic centimeter that are residual matter from the early formation of the universe) the actual mass of neutrinos is not nearly large enough–even by the most favorable estimates by about an order of magnitude–to explain the behavior of large scale structures, though in some models they are definitely a component of the formation of structures, providing energy via transferring their thermal momentum to regular matter.
There are a number of other candidates for MDM that incorporate theoretical weakly interacting particles, but we currently have not detected these particles. It is possible we might see them, or at least, their decay products, from experiments run in the LHC, but only if they fall into the low end of their estimated energies.
No, or at least, not likely. We measure the influence of gravity by observing the motion of stars and larger structures (as we can’t observe it directly) and from that it is clear that the anticipated effects of gravity as formulated by general relativity using only the mass we can observe is insufficient to explain the observed mechanics.
Interesting. Do we have evidence of objects emitting light ‘winking out’ as they leave the observable range? I assume this is much tougher to detect with gravity. Would univeral expansion cause dense areas like a galaxy to concentrate as the effects of external gravity diminish, or was this too weak a force to matter anyway?
No, because observable range is continually increasing at the speed of light. Further, for most objects the practical limit is not the horizon, but simply the fact that more distant objects are dimmer, so you’d just see (eventually) a fading out. Of course, this happens extremely slowly, such that telescopes are getting better much faster than things get dimmer, so what we really see is that every time we make a better telescope, or look at the same spot for a longer time, we see things that were too faint to see before.
From our point of view everything that is too far away for us to see has always been too far away. The only thing we see from the early universe is the cosmological background radiation in the microwave band. But that’s everywhere. There is nothing to wink out. You can’t think of stars hurtling past a cutoff point.
Gravity is mostly too weak to have anything other than local effects, although “local” can mean stupendously large groups of galaxies. In addition, the universe is presumably isomorphic, or identical at large scales everywhere. That means that the overall external gravity on everything is balanced out so that it wouldn’t have a particular directional effect.
