Hypothesis That Universe Has No Dark Matter and Is 27B Years Old
sciencealert.com
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* rotation curves of galaxies
* cosmic microwave background fluctuations
* gravitational lensing of galaxies which would otherwise have far too little mass
Whenever a "no dark matter" proposal comes along, I do a quick ctrl+f if it even tries to address all three lines of evidence. This doesn't seem to be the case here.
Don't get me wrong, this could still be an interesting approach to cosmology, but IMHO it's not ready for widespread coverage to a possibly-not-so-informed audience until it tackles all three.
[1] https://www.esa.int/var/esa/storage/images/esa_multimedia/im...
[2] https://ned.ipac.caltech.edu/level5/Sept17/Freese/Figures/fi...
Rotation curves are also not compelling evidence for LCDM because it's not predictive, eg. each galaxy needs to be fit post-hoc with a custom distribution of DM to match the rotation curve. Lensing in clusters is about the only evidence that there's some amount of DM, but the amount postulated by DM-only theories may be excessive, ie. under modified gravity, only an amount of dark matter about equivalent to normal matter is needed, not the 5x needed under LCDM.
I'd say that's pretty far from a slam dunk for DM theories over other possibilities.
I don't know what you mean by this. The peaks are completely explainable by standard LCDM, while MOND without DM totally fails to do so - no matter how much you tune it (see black vs red lines in [1]).
>pretty far from a slam dunk for DM theories
Individually, maybe. But not when you combine everything together. Then MOND is completely hopeless. You will find no paper out there that can reach the same level of predictions as DM. Of course that doesn't mean that there is guaranteed to be no modification to gravity at all. We already know it must be modified in the UV, so it's not impossible that a UV-complete description will bleed down to modified, far IR behaviour. But even if that is the case (and that's a pretty big if right now with the current theory landscape), some form of DM will still be required. And the evidence from the CMB strongly suggests that it will not be a small amount, but pretty much what we expect from original rotation curves and lensing. That's why no serious researcher in the field bothers with modifications of gravity alone, while there are many who focus only on DM.
[1] https://www.researchgate.net/profile/Bin-Wang-115/publicatio...
Sure, after more curve fitting [1]. It's just more inconvenient history that's been forgotten to favour LCDM.
> You will find no paper out there that can reach the same level of predictions as DM.
That's just wrong. LCDM's curve fitting has become great due to all of the adjustments, it's a priori predictions have not been great:
https://arxiv.org/abs/2110.06936
> some form of DM will still be required
This was always the case as I already pointed out in my previous reply, going back to at least the 90s [2]. The point is that cold dark matter has seemingly been adding epicycles in order to reduce all explanations to its one underlying cause.
[1] see section 5.3, https://arxiv.org/abs/1404.7525
What do you mean "more"? There are no new additional parameters and the curve literally fits, unlike for MOND (which doesn't fit at all unless you also introduce some form of DM).
>This was always the case
Then why would you trust any paper like this that explicitly doesn't use DM? Even if by mere chance it got a correct fit to one of the observations, you'll know immediately it could only have been a fluke since it didn't account for any DM.
I literally pointed you to the section that discusses how the a posteriori fit of LCDM to the spectrum was done.
> you'll know immediately it could only have been a fluke since it didn't account for any DM.
Right, so DM is necessary because if you used any explanation other than DM it must be wrong. Sounds perfectly scientific to me.
>DM is necessary because if you used any explanation other than DM it must be wrong
You completely missed the point. Although at this point I fathom it must be intentional. DM is necessary. End of story. You even said it yourself. By your own statement, any explanation (such as the one linked by OP) that uses no DM at all must be wrong, yes.
Of course it is much more complicated to come up with a sound theory of both, especially if you also want to retain some predictability. That's why we regularly see lone researches publish stuff like this. But it is neither the consensus nor really relevant to the field as a whole.
So you're not going to read the evidence I presented, got it.
> Although at this point I fathom it must be intentional. DM is necessary. End of story. You even said it yourself.
DM is necessary under the assumptions of LCDM and MOND. That is not the same thing as saying DM is necessary without qualification.
Could you comment on this please? Or suggest some further reading? I was under the impression that the amount of dark matter in each galaxy is adjusted to match the rotation curve.
https://iopscience.iop.org/article/10.3847/1538-4357/ad1bc6/...
By the way, they too have been postulated because collision experiments were showing some missing kinetic energy and momentum. People said "maybe there is a new particle that does not interact electromagnetically carrying away the energy", and they turned out to be correct. It's a very fruitful way of thinking. Your perceived strangeness goes away as soon as you understand all the evidence, in particular BAO and lensing, as mentioned above.
https://en.wikipedia.org/wiki/Cowan–Reines_neutrino_experime...
/tangent I learned a fun fact! The original proposal for the neutrino experiment wanted to detonate a nuclear bomb. The reactor was a last-minute change.
- "Eminent physicists enthused about the plan. It was approved by Reines’s employer, the government-funded Los Alamos laboratory in New Mexico. Work began on the detector, nicknamed El Monstro, and on the construction of the shaft. At the last minute, Reines and Cowan transferred the experiment to a nuclear reactor, but not because of environmental or safety concerns. They had worked out that although the reactor would deliver a flux of neutrinos three orders of magnitude lower than that from the bomb, it offered a better option for distinguishing signal from noise."
https://www.physics.uci.edu/node/14126 ("The singing UCI Nobel Laureate who nearly bombed Nevada")
While the probability that neutrinos will interact with other kinds of particles is very low, it is finite.
Therefore we have neutrino detectors, even if they are very big and very expensive.
So we can actually view neutrinos in the same way as we view anything else. We can imagine even building a video camera that would provide images of the spatial distribution of the neutrino flux, i.e. an image of the sky based on the incoming neutrino rays.
Such a neutrino video camera would be made like a compound insect eye, in order to separate by angle the incoming neutrinos with shields. Unfortunately such a video camera for neutrinos would need to have a planetary size, because shields that could block a large fraction of the neutrinos coming from different directions than the one desired for reception would be humongous.
For neutrino sources that are much more luminous (in neutrino flux) than their background, e.g. supernova explosions or the Sun, there is an alternative way to separate the neutrinos that come from a specified direction, besides shielding. One can use 2 neutrino detectors located at a great distance between them and count only the neutrino fluxes recorded in both detectors. Unfortunately the great distance needed for good angular resolution and the size of the individual neutrino detectors still make impossible the construction of a neutrino receiver that could be oriented in any direction, at will.
This picture in my opinion "views" dark matter just as much as you would "view" neutrinos: https://upload.wikimedia.org/wikipedia/commons/thumb/a/a8/1e... (the bullet cluster)
Regardless, I find the discussion on the meaning of ill-defined laymen language tiring while we probably agree on the actual facts.
Like another poster has already written, the status of dark matter is like that of the neutrinos when their existence had been postulated, but before means to detect them by alternative methods have been devised.
In the beginning, the existence of neutrinos has been presumed because in certain reactions the conservation of the energy and of the linear momentum and of the angular momentum was not observed.
So every time when there were missing energies and momenta, it was assumed that they had been carried away by neutrinos.
In the same way, nowadays whenever there is an extra gravitational force, it is assumed that there exists some dark matter that is the source of the extra force.
Until another means of determining the existence of dark matter is discovered, this does not constrain enough the properties of the dark matter. For any mathematical model of the gravitational forces, when we see that it does not match reality we can add a fictitious distribution of dark matter computed to match the observations.
In fact, for the postulated neutrinos even before their first detection we had much more knowledge about them, because they had to satisfy many distinct conservation laws, so we knew that they must be fermions with null electric charge and h_bar/2 spin and that their mass must be much smaller than the mass of the electron.
On the other hand, we are really in the dark about dark matter. We know nothing about any properties of any constituents of dark matter.
Well, dark matter got its name because we couldn't find direct observation. It's just a nickname for «unexplained gravitational effect from invisible source», the closest explanation we have is non-radiating matter.
Next physics might explain this without any additional matter, but for now, that's all we have.
As the authors state in the summary "it remains to be seen if the new model is consistent with the CMB power spectrum, the Big Bang nucleosynthesis of light elements, and other critical observations".
Maybe this model will be contradicted by further work or maybe it will be confirmed.
In any case such models must be searched and studied, because the explanations based on deus ex machina, i.e. on dark matter and dark energy, are not good enough, because they lack predictive power. Adding an arbitrary and unconstrained distribution of gravitational field over the entire Universe can match an arbitrary mathematical model of the gravitation, so it cannot be used as an argument that the current standard model is valid.
Only if another means of detecting the dark matter, except by its gravitational influence, were discovered, than that could constrain the distribution of the dark matter, so that the supposition of its existence would be converted into a valid model with predictive power.
That's how nearly all successful models started out: being able to explain a part that the established models can't, and then later being expanded into other areas. But of course, only a small minority of such models ever make it to being the dominant/accepted one.
I'm all in favor of this process, the only thing I'm weary about is how it's communicated to the public. I know so many people who read such an article and come away with "dark matter isn't real after all", and I don't really know what to do about it. Media outlets don't really have the incentives to communicate the amount of caution that is appropriate towards such a model, and even if they did, it's somewhat hard to do well.
Is it the journal or the researcher (or me) being obtuse here? A new theory can not just ignore observations of localized structure such as galaxy rotation speeds (and be taken seriously). If dark matter is removed, the theory must account for why galaxies rotate faster than merely their visible matter allows.
Abstract: https://dx.doi.org/10.3847/1538-4357/ad1bc6
Direct PDF Link: https://iopscience.iop.org/article/10.3847/1538-4357/ad1bc6/...
All I know is that dark energy != dark matter
"Despite the name, dark energy isn’t like dark matter, except that they’re both invisible. Dark matter pulls galaxies together, while dark energy pushes them apart." - 0
[0] - https://www.cfa.harvard.edu/research/topic/dark-energy-and-d....
Here's a page focused on explaining this stuff:
https://www.astro.ucla.edu/~wright/tiredlit.htm
- "The tired light model does not predict the observed time dilation of high redshift supernova light curves. This time dilation is a consequence of the standard interpretation of the redshift: a supernova that takes 20 days to decay will appear to take 40 days to decay when observed at redshift z=1."
Tired light would affect the energy of photons, but not their density. Blackbody radiation at a given temperature not only has a specific spectrum, but also a specific energy density. A putative tired light effect would not preserve the latter matching the spectrum.
Note that the cosmic background radiation we see today is blackbody radiation (including its energy density) to very high precision.
My second belief is that popular science articles are not good at explaining nuance, and tend to focus on human factors. Both of these are fine for lay audiences but should probably not be used to judge the health of the scientific process itself
For Dark Matter we have existence proofs. It MUST exist, in some form, we just don't know what it is. It will take another century before we're at 180 years ...
DM and MOND are referring to different ideas of matching the anomolous observations, not the observations themselves. And each is really a family of different theories, as opposed to one singular theory each.
As an aside, MOND is not doing so well these days. One of the leading proponents of MOND has published a paper showing a need for substantial revision to the theory following observations of wide binary stars [1].
[1] https://academic.oup.com/mnras/article/527/3/4573/7342478
Our current best understanding of gravity is encapsulated by the theory of General Relativity (GR; Einstein 1915). This reduces to Newtonian dynamics in the weak-field non-relativistic limit...
Scientific knowledge has a structure, similar to a structure of a math theory where axioms and definitions are used to prove theorems which in their turn are used to prove even more statements and so on. You can trace a path from any statement to axioms and definitions. But empirical sciences have structure more complex than that of math. They rely on empirical evidence which is limited by abilities of instruments, and they are subject to change when instruments become better. Sometimes they reject some hypotheses not due to contradicting evidence, but because there were several competing hypotheses and one of them had more supporting evidence.
If you start asking questions why physics thinks that X is true, and ask a lot of them not accepting any statement without asking such question then you'll get a lot of answers (conditionally on a benevolent physicist willing to answer them all), and you'll get eventually the structure of physics.
But then comes some freak and claims he proved Einstein wrong, while it is clear to physicists that he doesn't understand the structure of the theory. To get a simple example I can resort to a calculus and... Imagine a student that when asked to prove that sin(x)/x -> 1 when x->0, "proved" it with a help of L'Hôpital's rule by taking derivatives of nominator and denominator. L'Hôpital's rule doesn't forbid it explicitly, and it may seem to be ok, but the trouble is the derivative of sin is proved with reference to a limit of sin(x)/x when x->0. So it becomes a circular reasoning and if you see how student does it you know that the student doesn't understand a thing about calculus, his attempts to pretend to know calculus are pathetic. Similar mistakes one can make in any discipline, though in them the structure or reasoning behind the theory is more complex, in math you can rely on pure logic, you need no Occam's Razor or something like. In social sciences you need to know all the history of your chosen domain of knowledge, how it evolved and why so. In physics it is simpler but still you need to understand at least some decisions made in the past (or maybe all of them, I'm not a physicist, so I'm just speculating).
One can propose any insane theory contradicting to Einstein, Newton or anyone else and not to be yelled at, but you need to state your starting position carefully with regard to a structure of a scientific knowledge. Like if you want to derive statement sin(x)/x->1 when x->0 by resorting to L'Hôpital's rule then you need first to build your own calculus with blackjack and hookers from scratch in a way that make it to be a non-circular reasoning. And probably you will need to explain your motivation why are you doing that, because people will be asking about it a lot.
I agree that science is meant to be challenged, but a worthy challenger must produce a better theory. The consensus is that DM fits the available evidence best. The case about dark energy is way less settled and way less clear due to lack of evidence. The standard model (i.e. a cosmological constant) fits best while having the least number of parameters, but I'd say that most cosmologists agree that it is not the final answer. It is expected that another theory with more free parameters will prevail, but current observations are not precise enough to decide which ones can be ruled out.
its a pretty easy rubric to tell what you're currently doing on any topic
ask yourself what information would alter your belief, is reproducibility a factor in your belief at all, does your belief conveniently exempt itself from reproducibility in some way, such as a reward for maintaining your belief ... perhaps after you die?
But I know how people work pretty damn well. I will tell you, right now, with 100% certainty: dark matter is modern witch craft. Bunch of nerds got together and convinced themselves of magical forces that can’t be seen, heard, felt, or touched because their models of the universe don’t work without it.
Might be 200 years until we figure it out, but there is an error somewhere and scientists are too proud to admit that they ain’t got a clue. How’d the moon get in the sky? Well I guess someone had to put it up there. How do birds fly? I guess God made them float on account of their lack of sin. Why don’t our models of gravity work? I guess the universe is made up of weird, invisible stuff. Humans don’t like not knowing. So they make wild theories about stuff no one can explain. Someone will come along and figure it out eventually.
Every piece of evidence for hundreds of years proved this point. Every scientist knew it to be true. And yet I ask you today, does an object in motion desire to rest OR does it stay in motion until acted upon by an outside force?
It wasn’t until later that people had the tools to say, hey, wait a minute, this isn’t right. There is more here than we understand.
Which of these scenarios seems more likely: 85% of the matter and 95% of the universe is made up of stuff that we can’t detect, doesn’t interact with anything and doesn’t follow the rules of everything else OR there is something going on that we don’t yet understand?
I guess I have to ask: why do you presume there exists an undetectable force that makes your equations work and not that the equations don’t account for everything?
Seems silly. You would never believe 85% of software bugs were caused by unseen, undetectable stochastic physics errors that only manifested as a system crash. The fact that doing the same thing over and over again produces the same result certainly doesn’t cement that theory in my mind.
FWIW, the standard model already has plenty of particles that don't interact with EM, so you can't act like that's prima facie unreasonable. The story of the discovery of neutrinos is particularly relevant.
Anyway, feel free to read around in this thread for the multiple other lines of evidence that lead to dark matter. Or read the Wikipedia article on it. The information is out there, if you want to know. Or you can keep making up strained analogies involving the number 85%. No one can stop you.
Dark matter, I believe, is one such example. It’s a proxy for lack of knowledge, no different than rain spirits being called to water the crops. It orients and aligns, allows people to continue without the weight of the unknown burdening them.
As I stated above, that doesn’t mean fake, or conspiratorial, any more than taxation is a conspiracy to steal your money. These beliefs are just tools we use as humans to lubricate interaction and establish agreed upon norms. Everything from the Law, to Philosophy, to things like Human Rights are mythological. That doesn’t mean they are wrong or immoral, but it doesn’t make them true either. Humans, objectively, only have the rights we communally grant them.
When you drive your car you choose to follow the rules of the road. To do otherwise is both foolish and dangerous. We all agree on them and act in accordance. At any moment though, someone could swerve and kill you - even while you sit in your office or walk down the street. We just tend to not think about that unfortunate truth because of the burden it would entail. That is mythology working.
It’s doesn’t change reality though.
The controversy comes up because it has become pop science to discuss alternative proposals to DM/DE without properly conveying why DM/DE are the leading candidates for their associated measurement abnormalities. So you get a lot of people who functionally know nothing about the topic arguing that DM/DE are illogical because according to them scientists are just fudging the data.
This is a thing even with the more informed/STEM-minded crowd of HN.
Because if not, that's basically game over directly no?
I'm just a layman, but at this point it seems to me there's a strong chance there's two things going on, not just one. As in, we likely have actual dark matter, say sterile neutrinos, but we also have something else like quantum corrections to gravity.
Some more discussion:
The difficulties of quantum gravity today are mainly experimental and not theoretical.
If by "the difficulties of quantum gravity today are mainly experimental" you mean that if we could go and study the behavior of quantum mechanics near the event horizon of a black hole we might have solved the problem already, you'd be right, but that's kind of tautological.
My guess is that the classical picture of the black hole interior is completely wrong but of course you can't know because you can't look inside and then tell us what you saw.
The following paradox is never taken seriously: if I watch an object fall into a classical black hole I will never see it actually cross the horizon. That makes sense for a classical black hole because it lasts forever, but a semi classical black hole does not last forever so the viewer on the outside probably sees the object that fell in get destroyed when the black hole explodes at end, not to mention destroyed by the extreme tidal forces that will exist around the hole at the end of its life.
If the classical picture of what falling in looks like from the outside makes no sense than I think the idea that you fall through the horizon in a finite proper time and then hit the singularity in the future of that really makes no sense. I think the outside observer who sees you torn apart by tidal forces near the apparent horizon and turned into radiation is probably right.
That to me is a much more important “paradox” than the silly bet about information loss because if you throw out unitarity you throw out quantum mechanics. That people can pooh-pooh firewalls but believe in the no-unitarity fairy is just beyond me. (e.g. my PhD thesis was about an alternate way to compute quantum mechanical density of states and unitarity turned out to be the most important guiding principle in that)