First, I doubt the Bullet cluster can be explained by modified gravity.
Second, even if so, modified gravity theories have a major problem with explaining observations on all scales. Hints of dark matter has been observed on a wide range of length scales, and while you may be able to fit one of the observations with modified gravity the same fitting won't generally work on the other scales.
Seeing as how I'm not a mind reader, I guess I'll have to wait to see the paper hit Arxiv before I place any bets on that one.
Here's an earlier cosmological overview paper on this topic.
As far as I can tell it does seem to "fit" just fine. What exactly was your complaint with his overview presentation?
It seems to me that putting all your eggs in the Lambda-CDM theory (or any cosmological theory that is based on metaphysical constructs) has serious scientific drawbacks. His mathematical presentation of this modified gravity theory looks mathematically sound to me. It looks just as sound as any Lambda model I've ever looked at, and it requires a lot less faith in unseen and unproven forces of nature. I'm certainly looking forward to seeing how they explain the lensing data from the bullet cluster analysis.
Wait and see. Anyway, I think the words by Douglas Clowe are not so strong.
The 'Bullet Cluster' greatly constrains the nature of a universe devoid of 'Dark Matter'. Too strong to survive, IMO. Liking DM is optional. Consider how many years it took to detect' the atom.
There is a physical difference however. In the case of the atom, we could in fact "detect" the presence of matter in controlled laboratory conditions, and we knew there were different forms of matter based on their chemical properties. The atoms could be manipulated in controlled scientific conditions. Dark matter however is not like that. Nobody knows what "dark matter" is made of, or if it even exists because it is a mathematical construct that is based upon pure observation of objects that are light years away from us, and that we could never hope to "control" in any scientific sense. There is no real "testing" that can go on because it's a mathematical hypothesis at this point in time, not a physical entity that we can play with in a lab.
The modified gravity theory is also a purely mathematical construct that is designed to explain the same pure observations of objects and events that are light years away from us. Like dark matter theory, there is no obvious way to "test" this idea in a controlled scientific experiment.
Both of these mathematical theories seem explain these distant observations, each in their own unique mathematical way. How then do we determine which theory is the "better" scientific theory?
The decision to label on theory as "better" than another is ultimately a subjective judgment call. The way I personally make that choice is for me to see how well the idea conforms to what I can observe, and how few (or how many) metaphysical entities it relies upon. For my money, the fewer the metaphysical entities it requires, the better. I'm much more open to modified gravity theory, and theories that attempt to concentrate (normal) mass along the axial plane of the galaxy, to explain distant movements, than I am open to concepts about "dark matter". I've never seen any dark matter show up in any controlled scientific lab test. I therefore have no physical evidence that it even exists in nature.
This particular modified gravity theory does not require that I put faith in a metaphysical entity, and therefore I personally find it more attractive than a 'dark matter' explanation for the same events and observations. Both ideas seem to solve the problem mathematically, but dark matter theories require that I put faith in something that nobody can demonstrate actually exists in nature.
Hopefully at the LHC.
According to the most popular dark matter models (WIMPs) you wouldn't have, since the particles have masses around and above the electroweak scale. (100 GeV-10 TeV.)
Could it be black dwarfs, white dwarfs, planets ..anything except stars out there that we can not see then they play the role of dark matter? I think by now, scientists only observe stars and consider them matter. How about other stuff?
The mainstream [itex]\Lambda[/itex]CDM model has two types of Dark Matter, baryonic and non-baryonic DM.
The total amount of DM is about 27% of the total density, and therefore mass, of the universe. The Big Bang in the standard model can only produce about 4% of the total density as baryonic matter, mainly ordinary hydrogen and helium, (Note the visible baryonic matter, stars and nebulae, comprise only 0.3% of the total), and therefore about 23% of the total has to be some unknown non-baryonic species. Neutrinos, axions and other exotic particles have been candidates at one time or another.
There are a number of different types of observations: galaxy rotation curves, galaxy cluster dynamics, cluster gravitational lensing, large scale structure formation rates and the analysis of the CBM fluctuations, which is evidence that something is out there - but what?
Alternatively if all these observations are all to be explained by a modified theory of gravitation then that theory would have a mountain to climb, and explaining the Bullet Cluster is a particularly difficult case for such a theory.
Until the DM particle(s) have been identified in the laboratory (LHC?), their properties measured and found to be concordant with astronomical and cosmological constraints, we shall not really know what we are talking about.
For example, as another alternative, if the expansion rate of the early universe were different to that of the standard model, due to the action of some as yet unknown scalar field for example, then the amount of baryonic matter able to be produced in the BB would change and such a scenario might provide a third possibility; that all DM is in fact baryonic in nature. But then you would have to explain where it all is today and why it cannot be 'seen'!
The mainstream [itex]\Lambda[/itex]CDM model is the best thing we have going, at the moment.
Big Bang Nucleosynthesis puts upper limits on the amount of baryonic (i.e. ordinary) matter in the universe. This can also be seen from observations of the CMB.
Thus, ordinaty matter can only make up a fraction of the total amount of dark matter needed. The major part must be non-baryonic.
Edit: I see Garth was faster...
Thanks Garth and El. I can say one word by now: Wait.
Experimental proof is the gold standard. Dark matter is not unlike neutrinos [which also took a long time to experimentally confirm], only more elusive. I am extremely confident it will be detected, but not by the LHC. It sprang, IMO, from energies far beyond the capabilities of terrestrial laboratories. There are a number of very good experiments currently underway that take different approaches. I'm confident one or more will succeed - very likely by the end of this decade. See:
Can you even describe a LHC test that might "falsify" the notion of DM?
That's quite an energy range wouldn't you agree? Should we be able to be a bit more specific? It seems to me that WIMPS are but one "possible" theory related to DM, so this problem still seems to come back to the same issue.
Is there any controlled scientific LHC test that you would accept as a falsification of the existence of DM? I ask this because I know of no controlled scientific test that demonstrates that DM exists in the first place, and there is at least one other theory on the table that doesn't requires DM at all. If we can't empirically demonstrate that DM even exists in nature, and we can't falsify the idea in any controlled scientific test, and it isn't necessary to use DM to describe events in the universe, doesn't that put the concept of DM *outside* of the realm of science?
Controlled experimental evidence is indeed the gold standard of science. In science it is also true that the burden of proof is always on the one making the claim.
But there is a very significant difference between neutrino theory prior to their detection in controlled experiments and DM theory today. In the case of neutrinos, it was a series of controlled scientific tests that led us to hypothesize the existence of neutrinos. In these controlled scientific tests, we found evidence that there was a small amount of missing energy in certain known (and identified) particle decay reactions. If neutrinos did not exist to account for this missing energy, then the law of conservation of energy would have been violated. It was a known law of physics, and controlled scientific testing that led us to hypothesize the existence of neutrinos. More importantly we already knew which particle decay reactions created them, and we knew where them came from. That allowed us to create additional tests that could verify their existence.
With DM however, there are no controlled tests that demonstrate they exist, and no laws of physics that require that they exist. More importantly however, pure observation of objects that are light years away do not contain any control mechanism of any sort. There is also more than a single way to account for the lensing patterns in the Bullet Cluster data, and there is more than one way to account for the movements of galaxies, some of which do not require DM at all. We can't even be sure that DM exists based on these observation, and we have no idea where DM comes from, or how to test for it. More telling however is the fact that we have absolutely no way to falsify the concept.
Which scientific test would falsify the DM theory? If the theory cannot be falsified, and there is no empirical evidence that DM even exists, then what makes DM theory superior (in a scientific sense) from modified gravity theory?
Just to keep the pot boiling....
Durrer and Maartens' overview article for the dark energy issue of GRG published on today's physics ArXiv: Dark Energy and Dark Gravity
LHC could falsify many of the most popular models. If in the future energies up to ~10 TeV will be probed without finding anything, then WIMPs are in big trouble. Of course people can after that always try to build coco-models which would still avoid experimental constraints, but probably with a lot af fine-tuning (maybe even of the order needed for modified gravity theories!).
The WIMP hypothesis rests not only on its ability to explain all "dark matter"-observations (on all observed scales) and correctly account for the formation of large scale structure, but also on motivations from pure particle physics. We expect there to be new physics around the electroweak scale, and if one of these new particles turns out to be stable it automatically (if it was in thermal equilibrium in the early universe) would give a relic density of exactly the order needed to explain all dark matter. (This is the so called "WIMP miracle".)
I do not find it very unlikely that one of the expected new particles would happen to be stable. (Like e.g. the neutralino in the MSSM.) At least I find it much more likely than the curve fitting (and abandonness of the well tested GR) going on in modified gravity theories. However, that is of course a personal taste. If LHC or any direct or indirect detection experiments won't find anything within a decade, I will probably abandon my current position.
This is just wrong. See my post above.
Try looking at this issue from a skeptics perspective for a second. These tests you describe are essentially a fishing expedition. Your (they're) "fishing" for anything in an energy range that is quite vast, and covers several orders of magnitude. That's not at all like the neutrino postulation process where we could define our theoretical numbers with some precision, and we even had identified the particle physical interactions that were the likely release point of neutrinos. Even after we cover this entire energy range, we still can't be sure about what other "whack-a-mole" versions of DM might crop up. I find it a tad disconcerting that we're hoping for a "miracle". I need some hard evidence here.
Well, that seems like a very ironic case of finger pointing from where I sit, especially since GR has essentially been abandoned in Lambda-CDM theory as I percieve it. GR as Einstein taught GR to his students, had no dark matter component, nor any dark energy component, and it certainly had no inflation component. Even Einstein's famous addition of a constant to GR he later rejected as his greatest blunder. In Lambda-CDM theory, the GR related to known types of mass that Einstein referred to in GR only makes up about 4% of the whole picture. From my perspective Lambda-CDM theory has abandoned 96% of GR in favor of a theory that relegates the GR that Einstein described (GR related to known forms of mass) to bit player status.
If I could see evidence from a controlled experiment, like beta decay reactions in the case of neutrinos, that required that we posit a theory like dark matter to uphold known laws of physics, then I might be willing to "wait and see" as you are willing to do. As it is, I simply don't see any actual physical evidence from any controlled tests that demonstrates that DM exists, or even hints that DM exists in nature.
All I've seen thus far to support the theory of DM is another mathematical theory related to what *might* be happening to objects that are light years away us and that are based on pure observations that are lacking any sort of control mechanisms. It's a bit like pointing to that same event in the sky and claiming that "something-I-never-heard-of-before" did it. From a scientific point of view, I am forced to take the skeptics approach. If the LHC comes up with any actual direct physical evidence that DM actually exists in nature, and affects objects in nature, then I"ll have to abandon my current position that DM does not exist. Skepticism is a part of the scientific method. The burden of proof in science is always on the one making the claim. If others believe that DM can affect objects at a distance, let them first show us that DM can effect objects in a lab. I'll keep an open mind on this subject, but I'll need to see some hard physical evidence that demonstrates that DM even exists before I feel any sort of confidence in the idea that DM can effect objects at a great distance.
No, "they" are not "fishing" within that energy range. It will be covered in a systematic way.
After we have searched that energy range we will hopefully have found something. If not, WIMPs are dead to me.
I don't think you got my "miracle". I am not saying we have to wait for a "miracle" to find dark matter, but that the "miracle" is already there on theoretical grounds. The "miracle" gives us a chance of solving two problems at the same time (hierarchy problem and dark matter.)
Note that I am not saying dark matter (in the form of WIMPs) exist. Only that I find it a reasonable explanation which is much more likely than modified gravity.
GR is not abandoned in LCDM, and I cannot see why you are under that impression. The energy-momentum tensor is just a collection of everything that contributes to the energy density of the universe, and not just the forms of energy that happened to "be known" when Einstein forumlated his theory. To say that the inclusion of dark matter abandons GR is just as saying that the inclusion of neutrinos abandons GR.
Dark matter is just as much "matter like" as ordinary matter and should hence be included as matter in the energy-momentum tensor.
Regarding the cosmlogical constant: The cc is part of GR. The symmetries of GR allows for it and there is no reason a priori to put it to zero.
I guess you are quite alone with your perspective.
The hints that DM exists in nature are plenty (galaxy dynamics, cluster dynamics, large scale formation dynamics, hiearchy problem). Each one of these scales may separately be solved by other things than WIMPs (e.g. modified gravity theory, another modified gravity theory, again another modified gravity theory, and divergence canceling physics without stable particles.), but the strength of WIMPs lies in the simultaneous solution.
As you say there are yet no laboratory experiments that demonstrate the existance of dark matter. I am not saying WIMP dark matter definitely exists, but just that it is the most reasonable explanation we have come up with.
And I don't think anyone claims that the existance of particle dark matter is definite.
And that is exactly what we currently are trying to do.
The point here is that the search for WIMPS is unlike the search for the neutrino. In the case of the neutrino, we already had some idea of where these particles came from, and we understood the source of the particle in question. We also understood how the theorized particle tied back into particle physics.
In the case of the fabled "WIMP" particle, we have no known particle interaction source, and there is no known particle that has these properties. The is no known particle interaction that results in "missing" mass or energy that might explain where WIMPS come from. Essentially we're just dragging a huge net through a very wide energy spectrum, and hoping to find something.
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