I don't have access to the paper, but it seems to make zero testable predictions and is therefore just mathematical fiction. If it's not falsifiable, it's not science.
EDIT: the last sentence is "Given sufficient redshift (or, equivalently, time) resolution effected by the redshift slicing, one might just find that the Hubble diagram exhibits jumps in the redshift distance relation, which would be very revealing."
So they say it's testable. However, we see the effects of "dark matter" (or whatever it really is) today affecting the spin of galaxies, so I don't see how that's compatible with the explanation of these events being "rare".
He wants to look for discontinuities in the Hubble relation. Quoting from the article:
> Looking to the future of his research, Lieu says the next step to validating his model of the cosmos could come through observations using earthbound instruments rather than something like the James Webb Space Telescope.
>"The best way to look for the proposed effect is actually to use a large ground-based telescope—like the Keck Observatory [Waimea, Hawaii], or the Isaac Newton Group of Telescopes in La Palma, Spain—to perform deep field observations, the data of which would be 'sliced' according to redshift," the researcher notes.
>"Given sufficient redshift (or, equivalently, time) resolution effected by the redshift slicing, one might just find that the Hubble diagram exhibits jumps in the redshift distance relation, which would be very revealing."
All these papers miss the fact that our main evidence for dark matter (or something like it) is the cosmic microwave background. I see no mention on how this very convoluted model can reproduce it.
The evidence is in the oscillations of the primordial plasma seen directly in the CMB. These come from the gravity pulling the plasma and pressure pushing back. Without DM they would be too shallow, DM helps by pulling the plasma gravitationally without opposing the fall with pressure of its own.
“ All these papers miss the fact that our main evidence for dark matter (or something like it) is the cosmic microwave background. ”
There is also the galactic rotation curve evidence. With Newtonian physics, the visible mass in galaxies should rotate faster toward the center and slower toward the edge. This is the rotation curve. We actually observe a very linear curve, where the outer stars and gases rotate at the same speed or even faster than the centers. A dark matter halo would provide the additional gravitational mass for cohesion.
Could this mass or gravitational impact be outside the expanding bubble of spacetime? If we are in a hole inside a block of swiss cheese, is there a way to determine if we are seeing the effects of the surrounding "cheese"?
Imagine our expanding universe to be inside a black hole. Could the "dark matter halo" be energy/matter dumped into our universe from an external source? Is the expansion of the universe actually our universe growing due to consuming its surroundings?
>All these papers miss the fact that our main evidence for dark matter (or something like it) is the cosmic microwave background. I see no mention on how this very convoluted model can reproduce it.
I am not an astronomer, but IIUC (and I may not), the first evidence for dark matter was posited by Fritz Zwicky[0] in 1933 based on the rotational velocities of galaxies, work by Vera Rubin[1] confirmed Zwicky's hypothesis in more detail decades later.
Since then Rubin's work has repeatedly been confirmed.
And while Penzius and Wilson "discovered" the Cosmic Microwave Background (CMB) a few years before Rubin published her data, (again, IIUC) the CMB was not used as a reliable tool to look for dark matter until better quality data was gathered in the 1980s and 1990s.
They didn't say it was always the main evidence, just that it now is. I'm not personally sure about that either, but it's definitely one of the major lines of evidence.
I’ve never really understood how just adding extra invisible stuff to make the equations work is justified either? It seems more likely there is simply something wrong with the equations.
The neutrino was also a particle that nobody could observe at the time when Pauli proposed it. It was just a manifestation of the energy and spin conservation.
The Higgs is the same. It is not needed, but it solves the mass problem of the weak force. It is the only scalar field so far that we have observed and it was not clear whether it would exist at all.
Quintessence and sterile neutrinos are also just pieces that make the equations of the world look prettier, but they are also candidates for dark energy and dark matter.
Pauli wrote in his famous letter:
"I agree that my remedy could seem incredible because one should have seen those neutrons very earlier if they really exist. But only the one who dare can win and the difficult situation, due to the continuous structure of the beta spectrum, is lighted by a remark of my honoured predecessor, Mr Debye, who told me recently in Bruxelles: “Oh, It’s well better not to think to this at all, like new taxes”. From now on, every solution to the issue must be discussed. Thus, dear radioactive people, look and judge."
"Measure first, discover later" can be a perfectly valid application of the scientific method. It's all about refining the resolution on reality.
The problem with dark matter/energy is that we're not guaranteed to discover anything. It might just be wrong. The neutrinos and Higgs just happened to match their initial theory, so that's a survivorship bias. We can't just assume the same will play out for dark matter. It might just be pure mathematical fiction, reflecting our ignorance and/or limitations to measurement rather than something "real" that we can zoom in on.
Or it might be at such high energies, that conventional particle accelerators will never observe it. Sterile neutrinos, axions and quintessence particles might have masses in the PeV range. We were lucky with the Higgs, and the W and Z Bosons, and the top quark.
Dark matter and dark energy might be a phenomenon in the GUT regime, which we will not observe with particle accelerators. We need more information about the Higgs, because it sticks out like a sore thumb of all the other particles. It is our first glimpse into the underlying fabric of the universe, as the Higgs field is non-zero everywhere and a scalar field. No other (known) particle behaves like that. But a future collider is expensive...
just my 2 cents
good plot for a scifi movie: aliens measure a different vacuum value of the Higgs field and discover sub-space communications ;-)
Well, the point is actually pretty simple. You start with an equation that works for some galaxies. Then you find a galaxy where it doesn't work - you adjust the equation and now it works again. But then you find a new galaxy where it still doesn't work, you adjust again. You try this for 5, 10, 20 galaxies, just adjust the equation - but at some point there is a limit where the equation doesn't bare any more adjustment, and you still have hundreds of other galaxies that don't work.
So, the alternative that starts being simpler is a single simple equation that works for all galaxies, but allow each galaxy to have varying amounts of stuff in it with mass, but that doesn't interact electromagnetically. Right now, this is the simplest solution we have that fits all observations well.
Is it the right answer? We won't be sure unless we can detect particles that fit the necessary characteristics, and a theory that explains the distribution of these particles in different kinds of galaxies. Unfortunately, the models we have allow these particles to be arbitrarily hard to detect, at the level that we can't really rule them out even if we had a particle accelerator the size of the Earth that didn't find them.
Now, in principle a different equation could exist that has the same solutions as the current equations where they work, and different solutions where they don't work, without adding O(number of galaxies) extra parameters. But just like the dark matter particles, unless we stumble upon it, we can't know if it exists or not.
It bears mentioning that the situation is even more constraining than this, because you're not just looking at galactic dynamics - you're looking at galaxy _cluster_ dynamics, and gravitational lensing measurements, and the CMB, and large-scale structure formation, and whatever else.
Dark matter is not Fermi's elephant, as invoked elsewhere in the thread. It's more like the story of the blind men and the elephant - except that the blind men recognise that their individual observations, taken together, admit a coherent explanation.
It doesn't, and the problems have only become more problematic over time, but it's the least bad hypothesis that's broadly accepted. I suspect a generational succession is required for new paradigms to be contemplated.
There are many researchers proposing simpler, novel, and testable solutions that seem to go unnoticed. For example, I'm a fan of Alexandre Deur's work. He has some simple and elegant solutions that I've never seen discussed even though they appear "obvious". For example, from 21 years ago: https://arxiv.org/pdf/2004.05905
That paper is suggesting that one of the reasons why galaxies are spinning faster than some calculations expect is because they're failing to account for the gravitational lensing of gravity itself, which bends gravity down towards the disk.
That paper focuses on rotation curves, like all DM skeptics. I can only assume because this problem is understandable with high school level math. But that's neither the only nor the best evidence for DM. If your new hypothesis doesn't even mention the CMB power spectrum, it's not really worth listening, sorry. And to be taken seriously, it has to explain at least most of the data. DM does that, everything else does not.
I'm just a layman, but in this[1] paper from 2023 Deur and his collaborators took his model[2] and applied it to the Hubble Tension problem. This paper does mention fitting the CMB well (as I understand it), and the model having no Hubble Tension.
I know his work has been contentious in the past, and that his past work has used multiple models that are not entirely compatible for different problems, weakening his claims.
That said, at least from my armchair it seems like a worthwhile direction to pursue.
The phrase "Dark Matter" literally means we don't know and therefore until something testable is postulated and tested (to be fair i believe some candidates have fallen by the wayside over the years as measurement has improved), it's principally equivalent to plugging in a giant X and giving it properties not unlike Fermi's famous elephant curve fitting comment.
Just FYI I have a PhD in cosmology, so no need to explain to me what "Dark Matter" does or doesn't mean, but thanks anyway. It sounds like you saw that video by Angela Collier about how Dark Matter is a set of observations, and while I think it's a good video, it's a bit disingenuous to pretend that working scientists put theories of dark matter and theories of modified gravity in the same category. I know Collier literally says that MOND is a DM theory, but I respectfully disagree, as this does not reflect the reality of the language researchers use. Even if you didn't see that video, my point still stands.
Basically, our equation isn't working, and roughly speaking the equation has gravity on the left hand side and matter content on the right hand side. Matter tells spacetime how to curve and spacetime tells matter how to move, is the old motto. Because the equation isn't working, we have two options: modifying the left hand side or modifying the right hand side (or both). In my perception, researchers refer to the first option as theories of modified gravity, and the other option as theories of dark matter.
Putting both options into one category is over simplifying the situation and isn't helpful.
> I suspect a generational succession is required for new paradigms to be contemplated.
There are a constant stream of new paradigms contemplated (including this one!)
The problem is that they’re contemplated, tested and found wanting.
The notion of dark matter (and dark energy, which is a completely different animal) isn’t hanging around because of stubborn professors or a lack of imagination, it’s because nothing better has come along yet.
The good thing about this theory is that it seems easily testable. Maybe it’ll be different.
This is basically the first and most common objection laypeople have. (In fact, this comes up in every single thread on HN about DM, not exaggerating.) Believe it or not, scientists have thought of that as well. They tried, and nothing fits the data as well as the standard model.
The trope is so common that there even is an xkcd for it:
Nobody said it's "true", whatever that even means for a model. The claim is that it's the best anyone has been able to come up with. And not for a lack of trying.
Everybody also acknowledges that there are issue with DM, it's just that every other known model has bigger issues.
You seem to have a concept that a scientific model is either "true" or "wrong", but that's not case. All models are wrong, but some are more useful than others. It's better to judge models by their ability to describe reality, and that is not a binary property, but a continuous one.
The equations that we already have are sufficient to describe the universe to 99.9999999% but they are a hodge podge of several different theories that all work very very well in their respective regime. QM + GR + Lambda CMD + ...
But this just doesnt look nice to eye and the mind. The laws of nature "must" be shorter, more symmetric. Thats why we invented Superstrings which solves everything, but can never be tested...
QM and GR are mutually exclusive: if one is right, the other must be wrong (of course, it's very much still possible that both are wrong). If you use GR to predict the movement and interactions of photons in a medium, it's just wrong. If you use QM to predict the movement of light around a huge body, especially a black hole, it's just wrong. And what's worse, there is no limit or term you can add such that you could say "QM works only for objects up to size X, GR works for objects larger than size Y".
Also, we're nowhere near explaining 99.99999999% of the behaviors we see in the universe. In fact, we're not even able to explain 6% of the things we see in cosmology - as is often explained, dark matter accounts for 27% of all energy in the universe, and dark energy for 68% - and we have no ideas what these actually are, if they exist at all.
Well, actually we have developed a very good feeling when to apply which theory. From subatomic particles to the large scale structure of the universe the right theories applied give us excellent results. lambda CDM models very well fit to the observed structures and the standard model describes all the particles that have been observed so far. We are desparately looking for effects that are unexplained by the standard model in particle physics so far. That is the reason why it is so difficult to justify yet another particle accelerator at CERN.
Sure, we have a feeling for when to use one and when the other, but a feeling is not a physical theory. Also, the Lambda CDM literally doesn't explain what 95% of the universe is made of and how it behaves, it simply posits that it exists. So even if the Lambda CDM is perfectly correct, it's still extremely far from a complete model.
Not to mention, the reason the tension between GR and QM is not very prominent is that we don't know how to conduct "medium scale" experiments, even though the vast majority of physical objects on Earth are in this medium scale: far too big to count particles, too small to measure observe gravitational bending effects. Basically, both QM and GR are completely useless for telling you what happens in compel scenarios like two billiards balls colliding on a frictionless table. They both have equations that are far too complex to actually solve for anything like this. And QM is even worse - even if you could solve the equation, it doesn't tell you what the balls will do, it only tells what chance they have atof being at some position with some velocity and spin if you were to measure that, whatever "measurement" might mean.
Coming at this from philosophy of science rather than as a physicist,
I feel those quotes around "must".
I think you also recognise how that might be a sort of "fundamentally
wrong assumption".
Imagine your words replayed 50 years in the future, not on physics but
applied tp the problem of general AI/sentience.
"The equations that we already have are sufficient to describe human
thought to 99.9999999% but they are a hodge podge of several
different theories...."
Whereupon a psychologist/neuroscientist in any epoch would say:
"Why on Earth are you looking for a *singular*, unified explanation
of human experience?"
What you can have is a set of "best they can be", internally
self-consistent and well evidenced theories, none of which can ever
fully explain the system - and that is the nature/feature of the
system. Isn't that what Godel and Russell showed us?
It may be - probably is - a feature of the limits of our mental processes so far.
But there's no good reason to assume the system itself just happens to mirror those limits. It would be very strange if the entire universe worked in inconsistent ways that matched the naive reasoning of some not very interesting animals on an ordinary star in the middle of nowhere.
We agree on human incompleteness. One limitation of recent
tree-dwellers on utterly unremarkable small blue-green planets on the
western spiral arm of the galaxy might be how we think about things
like "consistent"? Or "knowable"? What if those things weren't a
component of intelligence at all, but features a singularity which
looks different depending on which side you approach it from? In
maths, that's not even weird? [0]
I would argue that it is falsifiable. What I think you are looking for is verifiable and repeatable. It took almost 30 years to verify Einstien. He said about the collection of a hundred papers trying to prove he is wrong, "you only need one paper to prove it wrong."
It's a therotical paper. Leave it to the expermentaliats to design a test to prove it right. Diffraction gratings prove QED. Right. Feynman's biographer said that.
TL;DR - replace one big singularity with multiple singularities.
As in last sentence there is "The only difference between this work and the standard model is that the temporal singularity occurred only once in the latter, but more than once in the former."
There seems to me to be a real cottage industry in proposing alternatives to Dark Matter and Dark Energy. I wonder why that might be. Why isn't there as much interest in alternatives to, say, Plate Tectonics or the Germ Theory?
There are- things like flat or hollow earth, antivax and HIV deniers, homeopaths etc.
Proponents of all of the above are looking for a grand unifying cosmology. Dark matter and dark energy are confusing and unknown, so people want to just simplify them away. Flat earthers want to simplify away the existence of other planets. Antivaxers want to simplify away medicine.
Because it's complicated astrophysics and largely unknown it is normal to put dark matter and energy theories into a non-fantastical category. The very large majority of physicists think MOND and similar are defunct theories. Dark matter and energy are very different, separate phenomena that just share a name. It does not make sense for them to have a shared explanation. There are also so, so many ways we observe dark matter that make it clear there is matter involved. Every new observation has completely overturned the predictions of every MOND or modified gravity theory, and they just come back with a totally different explanation to fit the new data.
I am not a physicist, buy maybe the nature of dark matter and dark energy? What are their properties aside from their effect on the large scale structure of the universe?
FWIW (which is not much), I was an astrophysicist whose research was Dark Energy, though I arrogate to myself no authority on this topic as frankly a lot of knowledge I used to possess on the topic has been replaced by knowledge of tedious programming APIs (meh...it's a living). My sense was that, perhaps owning to their colorful (colorless?) names, and because they diffused into popular culture more than other equally-important theories in other fields, Dark Matter and Dark Energy were presented to and apprehended by laypeople as "exotic". This invited speculation about these theories and even an urge to overturn them, especially where "exotic" was equated with "tendentious." But, by my lights these theories aren't very exotic. Dark Matter especially seems to me to be very consistent with the history of particle physics. Rutherford first proposed the existence of the neutron in 1920, but it wasn't observed directly until 1932. Pauli first proposed the existence of the neutrino in 1930, but it wasn't observed directly until 1956. Both are forms of "dark matter" and neutrinos were once a candidate for explaining flat rotation curves in galaxies and the "missing mass problem", until they were later excluded by observations of large scale structure. Dark Matter interacts with gravity strongly enough that its effects on astronomical and cosmological scales are pronounced, but it interacts with the other forces weakly enough that so far it hasn't been observed directly. So what?
"the nature of dark matter and dark energy"
I don't know anything about "the nature" of things. That sounds very philosophical to me.
"What are [the properties of Dark Matter and Dark Energy] aside from their effect on the large scale structure of the universe?"
I don't know. I also don't know why you're excluding (in the case of Dark Matter) its gravimetric effect on the large scale structure of the Universe. That seems like a pretty important property to me.
There is an alternate timeline where dark matter receives an extremely boring name like "inferred low luminosity particulate" and 100,000 very repetitive internet conversations never happen.
> The origin of these temporal singularities is unknown—safe to say that the same is true of the moment of the Big Bang itself
If you still want to assume that weird things you mathematically need happen whenever you need them for no reason then why not stick with cosmic inflation?
If you ask me, Inflation is far more problematic than the Dark Stuff. Nevertheless, so far it remains the best explanation we have for observed phenomena ¯\_(ツ)_/¯
> Depending on the Planck energy cutoff and other factors, the quantum vacuum energy contribution to the effective cosmological constant is calculated to be between 50 and as many as 120 orders of magnitude greater than has actually been observed, a state of affairs described by physicists as "the largest discrepancy between theory and experiment in all of science" and "the worst theoretical prediction in the history of physics".
Rapid expansion of the early Universe. We need to have Inflation in order for the Big Bang to work as a theory.
Without it, the Big Bang theory runs into major problems:
1. The Horizon Problem: The cosmic microwave background (CMB) has nearly identical temperature in all directions, suggesting the entire observable universe was once in contact. But without inflation, opposite sides of our universe would never have been able to “communicate” and reach this equilibrium.
2. The Flatness Problem: Our universe is remarkably “flat” (parallel lines stay parallel), which would require impossibly precise initial conditions without inflation.
3. The Origin of Structure: Inflation explains how quantum fluctuations during this expansion became the seeds for galaxies and cosmic structures we see today.
I don't have access to the paper, but it seems to make zero testable predictions and is therefore just mathematical fiction. If it's not falsifiable, it's not science.
EDIT: the last sentence is "Given sufficient redshift (or, equivalently, time) resolution effected by the redshift slicing, one might just find that the Hubble diagram exhibits jumps in the redshift distance relation, which would be very revealing." So they say it's testable. However, we see the effects of "dark matter" (or whatever it really is) today affecting the spin of galaxies, so I don't see how that's compatible with the explanation of these events being "rare".
He wants to look for discontinuities in the Hubble relation. Quoting from the article:
> Looking to the future of his research, Lieu says the next step to validating his model of the cosmos could come through observations using earthbound instruments rather than something like the James Webb Space Telescope.
>"The best way to look for the proposed effect is actually to use a large ground-based telescope—like the Keck Observatory [Waimea, Hawaii], or the Isaac Newton Group of Telescopes in La Palma, Spain—to perform deep field observations, the data of which would be 'sliced' according to redshift," the researcher notes.
>"Given sufficient redshift (or, equivalently, time) resolution effected by the redshift slicing, one might just find that the Hubble diagram exhibits jumps in the redshift distance relation, which would be very revealing."
Not sure how feasible this is though.
All these papers miss the fact that our main evidence for dark matter (or something like it) is the cosmic microwave background. I see no mention on how this very convoluted model can reproduce it.
The evidence is in the oscillations of the primordial plasma seen directly in the CMB. These come from the gravity pulling the plasma and pressure pushing back. Without DM they would be too shallow, DM helps by pulling the plasma gravitationally without opposing the fall with pressure of its own.
“ All these papers miss the fact that our main evidence for dark matter (or something like it) is the cosmic microwave background. ”
There is also the galactic rotation curve evidence. With Newtonian physics, the visible mass in galaxies should rotate faster toward the center and slower toward the edge. This is the rotation curve. We actually observe a very linear curve, where the outer stars and gases rotate at the same speed or even faster than the centers. A dark matter halo would provide the additional gravitational mass for cohesion.
"A dark matter halo"
Could this mass or gravitational impact be outside the expanding bubble of spacetime? If we are in a hole inside a block of swiss cheese, is there a way to determine if we are seeing the effects of the surrounding "cheese"?
Imagine our expanding universe to be inside a black hole. Could the "dark matter halo" be energy/matter dumped into our universe from an external source? Is the expansion of the universe actually our universe growing due to consuming its surroundings?
>All these papers miss the fact that our main evidence for dark matter (or something like it) is the cosmic microwave background. I see no mention on how this very convoluted model can reproduce it.
I am not an astronomer, but IIUC (and I may not), the first evidence for dark matter was posited by Fritz Zwicky[0] in 1933 based on the rotational velocities of galaxies, work by Vera Rubin[1] confirmed Zwicky's hypothesis in more detail decades later.
Since then Rubin's work has repeatedly been confirmed.
And while Penzius and Wilson "discovered" the Cosmic Microwave Background (CMB) a few years before Rubin published her data, (again, IIUC) the CMB was not used as a reliable tool to look for dark matter until better quality data was gathered in the 1980s and 1990s.
[0] https://en.wikipedia.org/wiki/Fritz_Zwicky#Dark_matter
[1] https://en.wikipedia.org/wiki/Vera_Rubin#Rotational_curves
Edit: Clarified prose.
They didn't say it was always the main evidence, just that it now is. I'm not personally sure about that either, but it's definitely one of the major lines of evidence.
I’ve never really understood how just adding extra invisible stuff to make the equations work is justified either? It seems more likely there is simply something wrong with the equations.
The neutrino was also a particle that nobody could observe at the time when Pauli proposed it. It was just a manifestation of the energy and spin conservation.
The Higgs is the same. It is not needed, but it solves the mass problem of the weak force. It is the only scalar field so far that we have observed and it was not clear whether it would exist at all.
Quintessence and sterile neutrinos are also just pieces that make the equations of the world look prettier, but they are also candidates for dark energy and dark matter.
Pauli wrote in his famous letter:
"I agree that my remedy could seem incredible because one should have seen those neutrons very earlier if they really exist. But only the one who dare can win and the difficult situation, due to the continuous structure of the beta spectrum, is lighted by a remark of my honoured predecessor, Mr Debye, who told me recently in Bruxelles: “Oh, It’s well better not to think to this at all, like new taxes”. From now on, every solution to the issue must be discussed. Thus, dear radioactive people, look and judge."
https://icecube.wisc.edu/neutrino-history/1931/01/1931-pauli...
"Measure first, discover later" can be a perfectly valid application of the scientific method. It's all about refining the resolution on reality.
The problem with dark matter/energy is that we're not guaranteed to discover anything. It might just be wrong. The neutrinos and Higgs just happened to match their initial theory, so that's a survivorship bias. We can't just assume the same will play out for dark matter. It might just be pure mathematical fiction, reflecting our ignorance and/or limitations to measurement rather than something "real" that we can zoom in on.
Or it might be at such high energies, that conventional particle accelerators will never observe it. Sterile neutrinos, axions and quintessence particles might have masses in the PeV range. We were lucky with the Higgs, and the W and Z Bosons, and the top quark.
Dark matter and dark energy might be a phenomenon in the GUT regime, which we will not observe with particle accelerators. We need more information about the Higgs, because it sticks out like a sore thumb of all the other particles. It is our first glimpse into the underlying fabric of the universe, as the Higgs field is non-zero everywhere and a scalar field. No other (known) particle behaves like that. But a future collider is expensive...
just my 2 cents
good plot for a scifi movie: aliens measure a different vacuum value of the Higgs field and discover sub-space communications ;-)
Well, the point is actually pretty simple. You start with an equation that works for some galaxies. Then you find a galaxy where it doesn't work - you adjust the equation and now it works again. But then you find a new galaxy where it still doesn't work, you adjust again. You try this for 5, 10, 20 galaxies, just adjust the equation - but at some point there is a limit where the equation doesn't bare any more adjustment, and you still have hundreds of other galaxies that don't work.
So, the alternative that starts being simpler is a single simple equation that works for all galaxies, but allow each galaxy to have varying amounts of stuff in it with mass, but that doesn't interact electromagnetically. Right now, this is the simplest solution we have that fits all observations well.
Is it the right answer? We won't be sure unless we can detect particles that fit the necessary characteristics, and a theory that explains the distribution of these particles in different kinds of galaxies. Unfortunately, the models we have allow these particles to be arbitrarily hard to detect, at the level that we can't really rule them out even if we had a particle accelerator the size of the Earth that didn't find them.
Now, in principle a different equation could exist that has the same solutions as the current equations where they work, and different solutions where they don't work, without adding O(number of galaxies) extra parameters. But just like the dark matter particles, unless we stumble upon it, we can't know if it exists or not.
It bears mentioning that the situation is even more constraining than this, because you're not just looking at galactic dynamics - you're looking at galaxy _cluster_ dynamics, and gravitational lensing measurements, and the CMB, and large-scale structure formation, and whatever else.
Dark matter is not Fermi's elephant, as invoked elsewhere in the thread. It's more like the story of the blind men and the elephant - except that the blind men recognise that their individual observations, taken together, admit a coherent explanation.
It doesn't, and the problems have only become more problematic over time, but it's the least bad hypothesis that's broadly accepted. I suspect a generational succession is required for new paradigms to be contemplated.
There are many researchers proposing simpler, novel, and testable solutions that seem to go unnoticed. For example, I'm a fan of Alexandre Deur's work. He has some simple and elegant solutions that I've never seen discussed even though they appear "obvious". For example, from 21 years ago: https://arxiv.org/pdf/2004.05905
That paper is suggesting that one of the reasons why galaxies are spinning faster than some calculations expect is because they're failing to account for the gravitational lensing of gravity itself, which bends gravity down towards the disk.
That paper focuses on rotation curves, like all DM skeptics. I can only assume because this problem is understandable with high school level math. But that's neither the only nor the best evidence for DM. If your new hypothesis doesn't even mention the CMB power spectrum, it's not really worth listening, sorry. And to be taken seriously, it has to explain at least most of the data. DM does that, everything else does not.
I'm just a layman, but in this[1] paper from 2023 Deur and his collaborators took his model[2] and applied it to the Hubble Tension problem. This paper does mention fitting the CMB well (as I understand it), and the model having no Hubble Tension.
I know his work has been contentious in the past, and that his past work has used multiple models that are not entirely compatible for different problems, weakening his claims.
That said, at least from my armchair it seems like a worthwhile direction to pursue.
[1]: https://arxiv.org/abs/2301.10861
[2]: https://arxiv.org/abs/1709.02481
The phrase "Dark Matter" literally means we don't know and therefore until something testable is postulated and tested (to be fair i believe some candidates have fallen by the wayside over the years as measurement has improved), it's principally equivalent to plugging in a giant X and giving it properties not unlike Fermi's famous elephant curve fitting comment.
Just FYI I have a PhD in cosmology, so no need to explain to me what "Dark Matter" does or doesn't mean, but thanks anyway. It sounds like you saw that video by Angela Collier about how Dark Matter is a set of observations, and while I think it's a good video, it's a bit disingenuous to pretend that working scientists put theories of dark matter and theories of modified gravity in the same category. I know Collier literally says that MOND is a DM theory, but I respectfully disagree, as this does not reflect the reality of the language researchers use. Even if you didn't see that video, my point still stands.
Basically, our equation isn't working, and roughly speaking the equation has gravity on the left hand side and matter content on the right hand side. Matter tells spacetime how to curve and spacetime tells matter how to move, is the old motto. Because the equation isn't working, we have two options: modifying the left hand side or modifying the right hand side (or both). In my perception, researchers refer to the first option as theories of modified gravity, and the other option as theories of dark matter.
Putting both options into one category is over simplifying the situation and isn't helpful.
She's done some interesting stuff, but I really hate that Collier video. It seems to have badly muddied the water.
Hey I'm sure a bunch of Youtube videos from Collier and Sabine makes you an expert in the field of physics because they make you feel smart.
"Just FYI I have a PhD in cosmology" is the credentials for the poster being an expert.
> I suspect a generational succession is required for new paradigms to be contemplated.
There are a constant stream of new paradigms contemplated (including this one!)
The problem is that they’re contemplated, tested and found wanting.
The notion of dark matter (and dark energy, which is a completely different animal) isn’t hanging around because of stubborn professors or a lack of imagination, it’s because nothing better has come along yet.
The good thing about this theory is that it seems easily testable. Maybe it’ll be different.
Change happens in physics one funeral at a time.
Very true - but the CMB has outlived quite some funerals.
This is basically the first and most common objection laypeople have. (In fact, this comes up in every single thread on HN about DM, not exaggerating.) Believe it or not, scientists have thought of that as well. They tried, and nothing fits the data as well as the standard model.
The trope is so common that there even is an xkcd for it:
https://www.xkcd.com/1758/
A common objection, and for good reason: just because you do not know the right model does not make your bad model somehow true
I mean, when you miss 85% of the stuff, you gotta admit that, perhaps, your stuff is wrong
And yes, it can work even if it is wrong
Nobody said it's "true", whatever that even means for a model. The claim is that it's the best anyone has been able to come up with. And not for a lack of trying.
Everybody also acknowledges that there are issue with DM, it's just that every other known model has bigger issues.
Ha, my bad
When you write "this is the [..] most common objection laypeople have", I understood "In contrast with the experts who know better"
Of course, if as you say, everybody knows that this is a wrong, specifying "laypeople" seems unnecessary
You seem to have a concept that a scientific model is either "true" or "wrong", but that's not case. All models are wrong, but some are more useful than others. It's better to judge models by their ability to describe reality, and that is not a binary property, but a continuous one.
It’s not a good objection. It conveniently ignores the actual data.
I wonder what the motto was before Einstein suggested a new model.
> never really understood how just adding extra invisible stuff to make the equations work is justified
IANAP but here’s my understanding.
At the end of the month you spent $2000, you’re not sure how so you track down your expenses:
That ‘unknown’ is dark matter. It’s a placeholder. It’s there and makes your total but you can’t explain it yet.What if dark matter is actually 10 different items? Or you have rent and gas wrong, or rent and gas inexplicably depend on each other in some way?
Or maybe there's nothing wrong with the equations, they're just the wrong equations. Or based on a fundamentally wrong assumption.
The equations that we already have are sufficient to describe the universe to 99.9999999% but they are a hodge podge of several different theories that all work very very well in their respective regime. QM + GR + Lambda CMD + ...
But this just doesnt look nice to eye and the mind. The laws of nature "must" be shorter, more symmetric. Thats why we invented Superstrings which solves everything, but can never be tested...
my 2 ct
QM and GR are mutually exclusive: if one is right, the other must be wrong (of course, it's very much still possible that both are wrong). If you use GR to predict the movement and interactions of photons in a medium, it's just wrong. If you use QM to predict the movement of light around a huge body, especially a black hole, it's just wrong. And what's worse, there is no limit or term you can add such that you could say "QM works only for objects up to size X, GR works for objects larger than size Y".
Also, we're nowhere near explaining 99.99999999% of the behaviors we see in the universe. In fact, we're not even able to explain 6% of the things we see in cosmology - as is often explained, dark matter accounts for 27% of all energy in the universe, and dark energy for 68% - and we have no ideas what these actually are, if they exist at all.
Well, actually we have developed a very good feeling when to apply which theory. From subatomic particles to the large scale structure of the universe the right theories applied give us excellent results. lambda CDM models very well fit to the observed structures and the standard model describes all the particles that have been observed so far. We are desparately looking for effects that are unexplained by the standard model in particle physics so far. That is the reason why it is so difficult to justify yet another particle accelerator at CERN.
my 2 cent
Sure, we have a feeling for when to use one and when the other, but a feeling is not a physical theory. Also, the Lambda CDM literally doesn't explain what 95% of the universe is made of and how it behaves, it simply posits that it exists. So even if the Lambda CDM is perfectly correct, it's still extremely far from a complete model.
Not to mention, the reason the tension between GR and QM is not very prominent is that we don't know how to conduct "medium scale" experiments, even though the vast majority of physical objects on Earth are in this medium scale: far too big to count particles, too small to measure observe gravitational bending effects. Basically, both QM and GR are completely useless for telling you what happens in compel scenarios like two billiards balls colliding on a frictionless table. They both have equations that are far too complex to actually solve for anything like this. And QM is even worse - even if you could solve the equation, it doesn't tell you what the balls will do, it only tells what chance they have atof being at some position with some velocity and spin if you were to measure that, whatever "measurement" might mean.
> laws of nature "must" be shorter
Coming at this from philosophy of science rather than as a physicist, I feel those quotes around "must".
I think you also recognise how that might be a sort of "fundamentally wrong assumption".
Imagine your words replayed 50 years in the future, not on physics but applied tp the problem of general AI/sentience.
Whereupon a psychologist/neuroscientist in any epoch would say: What you can have is a set of "best they can be", internally self-consistent and well evidenced theories, none of which can ever fully explain the system - and that is the nature/feature of the system. Isn't that what Godel and Russell showed us?It may be - probably is - a feature of the limits of our mental processes so far.
But there's no good reason to assume the system itself just happens to mirror those limits. It would be very strange if the entire universe worked in inconsistent ways that matched the naive reasoning of some not very interesting animals on an ordinary star in the middle of nowhere.
We agree on human incompleteness. One limitation of recent tree-dwellers on utterly unremarkable small blue-green planets on the western spiral arm of the galaxy might be how we think about things like "consistent"? Or "knowable"? What if those things weren't a component of intelligence at all, but features a singularity which looks different depending on which side you approach it from? In maths, that's not even weird? [0]
[0] https://cmsa.fas.harvard.edu/event/yip-2025/
I would argue that it is falsifiable. What I think you are looking for is verifiable and repeatable. It took almost 30 years to verify Einstien. He said about the collection of a hundred papers trying to prove he is wrong, "you only need one paper to prove it wrong."
It's a therotical paper. Leave it to the expermentaliats to design a test to prove it right. Diffraction gratings prove QED. Right. Feynman's biographer said that.
Paper here: https://arxiv.org/pdf/2503.08733
TL;DR - replace one big singularity with multiple singularities.
As in last sentence there is "The only difference between this work and the standard model is that the temporal singularity occurred only once in the latter, but more than once in the former."
tldr
> I don't have access to the paper
https://arxiv.org/abs/2503.08733
> it seems to make zero testable predictions and is therefore just mathematical fiction
I'll have a look when I get the time, but the reference to his previous paper really doesn't bode well.
Not sure why you’re being downvoted.
If a theory doesn’t generate at least one falsifiable prediction it’s not a scientific theory.
welcome to modern physics, enjoy your stay.
phys.org used to be less of a gosh wow outlet.
phys.org as a domain feels reliable, I guess that's how they make money doing "non-profit" (by having negative effects on humanity) stuff.
Really? Since I first noticed links to pyhs.org on HN, I have always wondered why it is not permanently banned as a link target.
The amount of ads I see there in Chrome is stunning.
Related paper: https://iopscience.iop.org/article/10.1088/1361-6382/adbed1
Here: https://arxiv.org/pdf/2503.08733
Only managed a first read, but it seems there's no explanation for the CMB.
Let alone any explanation for the CMB power spectrum peaks...
...therefore I won't bother further ;)
There seems to me to be a real cottage industry in proposing alternatives to Dark Matter and Dark Energy. I wonder why that might be. Why isn't there as much interest in alternatives to, say, Plate Tectonics or the Germ Theory?
There are- things like flat or hollow earth, antivax and HIV deniers, homeopaths etc.
Proponents of all of the above are looking for a grand unifying cosmology. Dark matter and dark energy are confusing and unknown, so people want to just simplify them away. Flat earthers want to simplify away the existence of other planets. Antivaxers want to simplify away medicine.
Because it's complicated astrophysics and largely unknown it is normal to put dark matter and energy theories into a non-fantastical category. The very large majority of physicists think MOND and similar are defunct theories. Dark matter and energy are very different, separate phenomena that just share a name. It does not make sense for them to have a shared explanation. There are also so, so many ways we observe dark matter that make it clear there is matter involved. Every new observation has completely overturned the predictions of every MOND or modified gravity theory, and they just come back with a totally different explanation to fit the new data.
"There are- things like flat or hollow earth, antivax and HIV deniers, homeopaths etc."
That's a fair point. It's just my perception that those things don't tend to be taken very seriously by people commenting on HN.
Because those are coherent theories, that don’t have massive gaps in them like dark matter.
What massive gaps?
I am not a physicist, buy maybe the nature of dark matter and dark energy? What are their properties aside from their effect on the large scale structure of the universe?
FWIW (which is not much), I was an astrophysicist whose research was Dark Energy, though I arrogate to myself no authority on this topic as frankly a lot of knowledge I used to possess on the topic has been replaced by knowledge of tedious programming APIs (meh...it's a living). My sense was that, perhaps owning to their colorful (colorless?) names, and because they diffused into popular culture more than other equally-important theories in other fields, Dark Matter and Dark Energy were presented to and apprehended by laypeople as "exotic". This invited speculation about these theories and even an urge to overturn them, especially where "exotic" was equated with "tendentious." But, by my lights these theories aren't very exotic. Dark Matter especially seems to me to be very consistent with the history of particle physics. Rutherford first proposed the existence of the neutron in 1920, but it wasn't observed directly until 1932. Pauli first proposed the existence of the neutrino in 1930, but it wasn't observed directly until 1956. Both are forms of "dark matter" and neutrinos were once a candidate for explaining flat rotation curves in galaxies and the "missing mass problem", until they were later excluded by observations of large scale structure. Dark Matter interacts with gravity strongly enough that its effects on astronomical and cosmological scales are pronounced, but it interacts with the other forces weakly enough that so far it hasn't been observed directly. So what?
"the nature of dark matter and dark energy"
I don't know anything about "the nature" of things. That sounds very philosophical to me.
"What are [the properties of Dark Matter and Dark Energy] aside from their effect on the large scale structure of the universe?"
I don't know. I also don't know why you're excluding (in the case of Dark Matter) its gravimetric effect on the large scale structure of the Universe. That seems like a pretty important property to me.
There is an alternate timeline where dark matter receives an extremely boring name like "inferred low luminosity particulate" and 100,000 very repetitive internet conversations never happen.
How do I get out of this chickensh*t outfit and onto that other timeline?
correction: "OWING to their colorful...names" not "owning to their colorful...names"
> The origin of these temporal singularities is unknown—safe to say that the same is true of the moment of the Big Bang itself
If you still want to assume that weird things you mathematically need happen whenever you need them for no reason then why not stick with cosmic inflation?
If you ask me, Inflation is far more problematic than the Dark Stuff. Nevertheless, so far it remains the best explanation we have for observed phenomena ¯\_(ツ)_/¯
Definitely more problematic. And Webb only made it more confusing :(
https://en.wikipedia.org/wiki/Cosmological_constant_problem
> Depending on the Planck energy cutoff and other factors, the quantum vacuum energy contribution to the effective cosmological constant is calculated to be between 50 and as many as 120 orders of magnitude greater than has actually been observed, a state of affairs described by physicists as "the largest discrepancy between theory and experiment in all of science" and "the worst theoretical prediction in the history of physics".
What does inflation explain though? It was proposed as an explanation for CMB spectrum. Anything else?
Rapid expansion of the early Universe. We need to have Inflation in order for the Big Bang to work as a theory.
Without it, the Big Bang theory runs into major problems:
1. The Horizon Problem: The cosmic microwave background (CMB) has nearly identical temperature in all directions, suggesting the entire observable universe was once in contact. But without inflation, opposite sides of our universe would never have been able to “communicate” and reach this equilibrium.
2. The Flatness Problem: Our universe is remarkably “flat” (parallel lines stay parallel), which would require impossibly precise initial conditions without inflation.
3. The Origin of Structure: Inflation explains how quantum fluctuations during this expansion became the seeds for galaxies and cosmic structures we see today.
It's obviously tied pretty closely to any theory of large scale structure formation too
Help me understand how can something so hard to detect exist in such vast quantity to change the rotation of galaxies.
Why do you think that how hard it is to detect should have anything to do with how much of it there is?
Basically all our observations are from detecting photons. If something in the universe is not emitting photons, we are in troubles