B What is our understanding of dark matter?

My question is about dark matter and how we guess it is working. I do not have a huge understanding of this but given a few things I have been lead to understand I would like to ask a few basic questions to better understand.

My question will start with antimatter;
Antimatter supposedly would have gravity but it would repel. Would this lead to the expansion of the universe? Would our matter move to the outside while the antimatter moved to the inside like water and oil mixed together and floating in space? Could this lead to an impenetrable wall like cosmic microwave background radiation?

The stronger our gravity field is the slower we accumulate units of time relative to other objects. Could we theorize that antimatter would accumulate time faster the stronger its gravitational field? Is this something that would be felt throughout the universe? If so would this be the reason that for us time passes, and for the opposite everything doesnt happen all at once?

thank you for your time, I would really like to get farther in this.
 

phyzguy

Science Advisor
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Antimatter supposedly would have gravity but it would repel.
We believe that this is incorrect. Most physicists strongly believe that antimatter behaves just like matter with respect to gravity. However, it is not experimentally determined yet, and there are ongoing efforts to measure this fact. This Wiki article gives a good overview.
 

Ibix

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Antimatter supposedly would have gravity but it would repel.
As @phyzguy says, this is not the case as far as we can tell. Even if this were the case repulsive gravity would do the opposite of what dark matter does. Dark matter is our explanation for galaxies having stronger gravity than can be accounted for by visible matter, not weaker as would be the case for a hypothetical something with repulsive gravity.

You may be confusing dark matter and dark energy. Dark matter is something that we hypothesise to account for the exact rotation of galaxies. Dark energy is something that we hypothesise to account for a small discrepancy between the observed expansion of the universe and our model of its expansion of it only contained matter, dark matter, and radiation. Dark energy is, in some senses, a source of repulsive gravity. Its behaviour is not consistent with matter or antimatter.
 

ohwilleke

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My question is about dark matter and how we guess it is working. I do not have a huge understanding of this but given a few things I have been lead to understand I would like to ask a few basic questions to better understand.
Before addressing your specific questions, you really need a more solid foundation. This reply provides some of that foundation so that you can be better informed before asking more questions. As several others have noted, dark matter and dark energy phenomena have no plausible connection to antimatter and antimatter is not expected to behave as you surmise that it might.

The Evidence For Dark Matter Phenomena

The phenomena which we observe that are sometimes attributed to dark matter are indisputably real.

These come from comparisons of astronomy observations with what we predict would happen if unmodified general relativity as applied today is correct, and if the only substances in the universe entering into the stress-energy tensor of general relativity are photons and other ordinary matter of the type described in the Standard Model of particle physics.

In practice, the vast majority of ordinary matter is in the form of stars (luminous matter), interstellar gas, and interstellar dust, and there are reasonably reliable means of estimating the masses and locations of each.

There are circumstances where we have made this comparison mostly boil down to the following, although there are also some more subtle indirect indications that some phenomena of this kind exists:

* Looking at the dynamics of stars at the edges of galaxies. These stars act as if they are under a stronger gravitational pull than would be inferred from visible matter alone (which would obey Kepler's law if unmodified general relativity acting on ordinary matter was all that was present). A typical illustration of this from a source at The Ohio State University is shown below:

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* Looking at the dynamics of luminous objects in galactic clusters. These stars act as if they were under a stronger gravitational pull than would be inferred from visible mater alone.

* Looking at how gravity distorts the path of light in the vicinity of galaxies and galactic clusters. This gravitational distortion of the path of light, called gravitational lensing, is greater than would be expected from the estimated amount of matter in the galaxies in question.

The discrepancy between predicted gravitational lensing based upon only ordinary matter and the observed amount of gravitational lensing in galaxies is close in order of magnitude to the amount of inferred invisible matter needed to produce the dynamics of stars at the fringes of galaxies that is observed.

* Looking at the patterns observed in the cosmic background radiation of the universe and other global measurements of the distribution of matter and voids in the observable universe. When this data is mapped on parameter space in a particular manner, it produces a "third peak" that is consistent with matter in excess of that produced by ordinary matter that behaves in a manner different from ordinary matter being present, as shown below (via this source at Cal Tech):

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* Simulations of large scale structure formation in the universe that assume only standard general relativity and known ordinary matter don't do a good job of replicating what we see in real life.

* No anomalies have been observed with respect to the predictions of general relativity for very strong gravitational fields (e.g. merging black holes or dynamics in the immediate vicinity of neutron stars).

"Dark energy", unlike dark matter, arise from the observation that galaxies get further apart from each other over time at an increasingly fast rate, roughly as if empty space had a constant amount of mass-energy of its own per volume, in addition to the mass-energy of everything else in the universe. But, our data used to estimate its amount and potential changes in magnitude over time isn't terribly precise so it is hard to discriminate between varying explanations of it. Dark energy is a bit like repulsive gravity but isn't in any way associated with antimatter either.

Possbile Mechanisms

These phenomena are probably due to one or both of two possible mechanisms:

(1) Exotic dark matter particles. There is some sort of nearly collisionless matter, called "dark matter" (although strictly speaking "transparent matter" would be more accurate, since it does not interact with photons) that has existed in a roughly constant amount since the early days of the universe, and have a mean particle velocity that is not relativistic (relativistic dark matter particles are called "hot dark matter" which is a theory that has been ruled out).

Naively, one of the obvious approaches to dark matter phenomena is to infer a distribution of invisible matter that isn't interacting in any non-gravitational way that we can observe from the gravitational dynamics of what we do observe. Those inferences suggest that dark matter particles should be diffuse, should be located in roughly spherical halos around galaxies and immersing galactic clusters with greater concentrations around galaxies, should be long lived or have annihilation and creation quantities in near equilibrium, should not have a huge annihilation signal that can be detected (although it might have a subtle one), and should not be very clumpy. Different theories propose masses for dark matter particles from masses smaller than neutrinos to masses as large as asteroids. Most dark matter particles theories proposed dark matter predominantly made of one type of particle (because multiple types of dark matter performed worse in early simulation efforts at reproducing what is observed), but that isn't a definitively proved matter either.

Pretty much all of these theories proposed kinds of matter not found in the Standard Model of Particle Physics except primordial black hole dark matter which proposed that black holes smaller than the roughly three stellar mass minimum for their formation in modern times arose by some unknown mechanism in the early universe, which has an ever shrinking but not yet entirely ruled out parameter space. This is because unobservable non-luminous ordinary matter (a class of theories dominated by what are called MACHOs for massive compact halo objects like very dim stars and gas giants) have been largely ruled out because their behavior would be inconsistent with astronomy observations to date.

A chart presenting some of these possibilities (via Stacy McGaugh) is as follows:

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A few more possibilities are discussed in more easily accessible language in this 2015 article.

(2) Modified Gravity. General relativity does not accurately model the weak field behavior of gravity and/or inertia and needs to be modified in some respect to describe reality. Inertia based approaches sometimes see inertia as arising from Mach's Principle (i.e. from the collective gravitational pull of everything else in the universe). Fifth force explanations that don't strictly speaking modify gravity itself (arguably including Verlinde's entropic gravity) are also usually lumped into the modified gravity category.

A chart presenting some of these possibilities (from a conference presentation made by Dr. Tessa Baker, also via Stacy McGaugh) is as follows:

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Pros and Cons

There are many possible candidates for both possible mechanisms and none of them are a good description of all phenomena attributed to dark matter.

Strengths and Weaknesses of Dark Matter Particle Theories

Dark matter particle models generally do a good job of explaining the cosmic microwave background and very large scale structure of the universe, and improve the predictions of simulations of galaxy formation and the dynamics of galaxies relative to a GR and ordinary matter only prediction. But, the simplest dark matter models still don't do a great job of predicting the distribution of dark matter and its effect at the galaxy and galaxy cluster scale which is much more orderly and predictable than a simple random distribution of a single type of dark matter particle should naively be.

For example, these models tend to predict dark matter distributions in central areas of galaxies that are too cuspy, tend to predict halos that are more spherical than what observations would suggest, show less variation between galaxies with similar distributions of luminous matter than would be predicted, predict the wrong number of satellite galaxies relative to those observed, can't explain why satellite galaxies tend to align themselves in a plane, predict lower than observed velocities of colliding galaxies than are observed, can't explain anomalous gravitational dynamics in wide binary stars, and doesn't have a good way of predicting which dwarf galaxies will appear to have a lot of dark matter (most of them) and which ones will appear to have little dark matter (a few of them). These models also don't do a good job of explaining by the proportions of dark matter to ordinary matter tend to be much lower in elliptical galaxies than in spiral galaxies, or why the proportion of dark matter to ordinary matter tends to be lower in very nearly spherical elliptical galaxies than in significantly non-spherical elliptical galaxies. Further, dark matter particle theories have a very poor track record of making accurate predictions of phenomena that have not yet been observed (and even the prediction of a third peak in the CMB is somewhat less impressive when you realize that the height of that peak is more of a post-diction than a prediction of dark matter particle theories, even though it did predict that there would be a third peak). More recently, however, some problems have come up in the cosmology level predictions of dark matter, which predict that large galaxies will be formed later than they are first observed, and make different predictions regarding 21 centimeter radio signal backgrounds than are observed (which are a window into the "radiation era" of the early universe).

No direct evidence of dark matter particles has been made via either particle accelerators or observational evidence, and almost all of the parameter space of the originally anticipated expectation that dark matter particles where the lightest supersymmetric particles of supersymmetry theories has been ruled out. There is also a lot of observational data from astronomy, and high energy physics data, that greatly constrain the parameter space of other proposed dark matter particles.

One of the particular problems that has emerged is that it is increasingly clear that there must be some sort of interaction and feedback in addition to gravity to explain why dark matter is inferred to have a very tight relationship with the distribution of ordinary matter in the universe, but direct dark matter detection experiments have established that any such interaction must be extremely weak (much weaker, for example, than the interactions between neutrinos and ordinary matter).

Strengths and Weaknesses Of Modified Gravity Theories

Modification of gravity theories, in contrast (which is currently a minority view), demonstrate that there is a very systematic relationship between observed dark matter phenomena and particular distributions of ordinary matter. The earliest and best known of these models, call MOND (for modified gravity) is a non-relativistic toy model theory devised in 1983 by Dr. Milgrom which predicts that at a certain critical value of the strength of the gravitational field that is very weak, that gravity is stronger than predicted by general relativity. This is not a complete theory because is it valid only for weak gravitational fields for which general relativity's predictions are more or less indistinguishable from Newtonian gravity (except for gravitational lensing).

But, it does an excellent job of predicting to the extent of observational precision, the rotation curves and dynamics of all gravitationally dominated phenomena from solar system scale phenomena (apart from general relativistic tweaks which are very nearly infinitesimal) to the dynamics of wide binary star systems to those of the largest galaxies. It also have a built in mechanism to determine when a dwarf galaxy is or is not expected to have dynamics that reflect dark matter phenomena called the external field effect, which has born out in a couple of test cases. It can be generalized in theories such a TeVeS (tensor-vector-scalar gravity) to capture the strong field behavior of general relativity, although this theory, in turn doesn't have a perfect track record and fails in many of the circumstances (like galactic clusters) in which toy model MOND also fails.

The strongest evidence in favor of a MOND-like explanation for dark matter phenomena is that it has also repeatedly made accurate predictions of not yet observed dark matter phenomena, while dark matter particle theories have a miserable record of failed predictions or of not making predictions at all. The cosmic microwave background prediction for dark matter is pretty much the only big phenomena that was predicted rather than merely post-dicted by dark matter particle theories.

MOND has its own shortcomings, however. It underestimates the magnitude of dark matter phenomena in systems larger than galaxies, such as galactic clusters, it doesn't always do a great job of predicting the dynamics of fringe starts in spiral galaxies outside the galactic plane, and by virtue of its status as a toy model that isn't fully relativistic, and also by virtue of the fact that modeling this non-linear set of gravitational rules is much more difficult than modeling general relativity with a combination of analytic solutions to simplified situations and Newtonian gravity approximations with dark matter particles, it doesn't really make many meaningful cosmology scale predictions at all.

The predictions of modified gravity theories other than MOND about cosmology and very large scale structure tend to be only superficially developed with back of napkin grade predictions when they are present at all, although it does appear generically that modified gravity theories that reproduce galactic rotation curves tend to speed up galaxy formation relative to GR alone or GR plus dark matter particle predictions.

Other modified gravity theories, however, such as TeVeS, MOG, and conformal gravity, to name just a few, have performed better than the toy model of MOND in areas where MOND has shortcomings, although they are less well studied, and some of them have their own flaws. For example, some modified gravity theories have some fields that propagate at less than the speed of light which is inconsistent over most of the possible parameter space with recent data from the collision of a black hole and a neutron start observed in both visible light and with gravitational wave detectors.

While some observations like the Bullet Cluster dynamics (involving two colliding galaxies) and "no dark matter dwarf galaxies" that have been touted as strong proof against modified gravity theories actually do no such thing, an if anything, tend to favor modified gravity theories relative to dark matter particle theories, it is also true that just as there is no dark matter particle theory yet developed with wide acceptance, that is a good fit to all of the evidence, there is also no modified gravity theory yet developed with wide acceptance, that is a good fit to all of the evidence.

Intermediate Gray Areas Between Dark Matter Particles And Modified Gravity.

There are also many theories like axion-like dark matter particle theories (which imagine dark matter as Bose-Einstein condensates in many situations), self-interacting dark matter theories (which add both new particles and new forces), and modified gravity theories that attribute dark matter phenomena to the quantum gravity effects of a theory in which gravity is quantized either via gravitons (since in these theories gravitons are themselves particles) or entanglement of ordinary particles (a non-gravitational mechanism for the effect seen) or quantization of spacetime (none of the above), which blur the line between dark matter particle theories and modified gravity theories.

There are some theories which propose a common origin for "dark energy" phenomena as well. The leading explanation of dark energy is a modified gravity theory which involves adding the cosmological constant (also called "lambda") to Einstein's equations of general relativity. But, alternative theories propose a dark energy substance (such as "quintessence"), propose interactions between matter and dark energy that give rise to dark matter phenomena, or propose a single substance (such as a "Chaplygin gas" a.k.a. dark fluid) that sometimes acts like dark matter and sometimes act like dark energy.

There are also theories that proposed that dark matter phenomena are mostly gravitational along the lines of MOND, but are supplemented by dark matter that is predominantly located in galactic clusters and is less constrained in its parameter space as a result (and could even be some special form of ordinary matter), with a gravitational cosmological constant explanation of dark energy as well.
 
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