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".
> 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.
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.
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?
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.
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...
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.
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 ;-)
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.
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 though. For instance, the latest in a litany of such failures is that rotation curves are flat past a million light years [1]. There is no plausible DM distribution that could reproduce such rotation curves while being consistent with other observations.
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.
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.
MOND successfully predicted the first peak of the power spectrum. I wonder why everyone focuses so much on LCDM predicting the second peak.
> DM does that, everything else does not.
Whenever someone says DM "does that", it's often after its initial prediction was falsified and the calculation was modified in some way to account for the new observations [1,2]. This has been going on for decades, so that's hardly a ringing endorsement.
I'm not surprised mind you, this is the hallmark of the confirmation bias that's been characteristic of LCDM for decades now.
[1] From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331
[2] Not that MOND is a suitable replacement because it too has its problems. My only point is that this tendency to sweep these inconveniences under the rug as if DM is a compelling and successful theory and saying "nothing else does the job" is disingenuous at best. What you should say is that "nothing does, period, not even our best DM theory", because that's the truth.
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.
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.
The trope is so common that there even is an xkcd for it:
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
Everybody also acknowledges that there are issue with DM, it's just that every other known model has bigger issues.
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
That model would be “true” no? Assuming of course that it’s possible to come up with a list of observations that covers “it all” and we won’t one day see something weird and new that contradicts our theory.
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:
- rent $500
- groceries $120
- gas $80
- …
- unknown: $123
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
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.
my 2 cent
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...."
"Why on Earth are you looking for a *singular*, unified explanation
of human experience?"
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.
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."
tldr
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.
If a theory doesn’t generate at least one falsifiable prediction it’s not a scientific theory.
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?
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 ;)
At astronomical scales, however, things are different. At those scales, gravity wins out, and is one of the dominant things we observe in astronomy. Dark Matter may not feel the electromagnetic and nuclear forces the way normal matter does, but it feels gravity the same as every other particle does. Nobody gets a pass from gravity, not even Dark Matter.
Consequently, it's relatively easy to observe the gravitational effects of Dark Matter at the astronomical scales of the rotation of galaxies and the dynamics of galaxy clusters, even while it's difficult or impossible to observe the non-gravitational effects of Dark Matter at laboratory scales.
Basic statistics. If you have lots of stuff, the probability of detection should be higher, all else being equal.
Maybe you mean that this is a sense in which how hard it is to detect has something to do with how much of it there is, which is fine, but the original question is suggesting some much more constraining relationship, where the fact that dark matter is hard to detect and the fact that there is a lot of it poses some kind of apparent contradiction, or at least a puzzle. I don't know of a reason to think that any such contradiction exists.
It's of course true that the continued non-detection restricts the available parameter space. But there is no physical principle that says that if something is as abundant as dark matter it really ought to have been detected by now - it's not as though if something is as abundant as dark matter, then it really needs to have some minimal coupling to baryonic matter that the existing experiments are now ruling out. Dark matter can just be really very hard to detect, there's no issue of how that "can be". Things that can change the rotation of galaxies are not obliged to be detectable by 2025.
If your weighting of the relative probabilities is such that you feel you should be going to bat for MOND at this point, that's your prerogative. But it's not related to the question that was asked.
"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.
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.
Ironically, you are positing a grand unifying theory here by stating that detractors from mainstream opinions just want simpler answers. But isn't it simpler to just go with the mainstream opinion and go with the flow? Alternative models of the earth are typically rooted in Biblical or ancient belief systems. Any specific argument based on physics come long after a person's acceptance of, e.g., the Bible. And I can't even imagine how you might think anti-vax beliefs are simpler so I can't speak to that. Homeopathy sounds pretty dumb to me, sure, but also there are tons of reasons to distrust the medical system in the USA, I'd be surprised if anyone seriously argued against that. I can understand why people suspicious of America's medical culture of prescribing 10 medications to someone instead of lifestyle changes might see a bottle of sugar pills at the store labeled with "natural remedy" and think it could be a better alternative.
> 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.
That's how science works. Scientists are not prophets, they do not have the luxury of starting with all the answers. It's really disheartening to see you make these theories into an "us vs them" ego contest.
I'm guess I'm just a dyed-in-the-wool agnostic, but when I hear someone suggest an alternative theory I don't immediately feel the need to group them with everything I think is wrong with the world. Instead I think, "That's really cool they came up with a different model of the universe. I wonder how they account for everything we observe and how it will be refined over time and whether it will predict new discoveries." Even if your current theory never gets overturned, I think your attitude is a perfect example of why science is said to advance one funeral at a time. Just because they got it wrong in the past doesn't mean they can never get it right, and it doesn't mean there's a 100% certainty that you got it right.
"I think your attitude is a perfect example of why science is said to advance one funeral at a time"
Hey now, don't you think that's a little uncalled-for? It's difficult to determine the attitude of a stranger over the internet via text.
"just because they got it wrong in the past doesn't mean they can never get it right, and it doesn't mean there's a 100% certainty that you got it right."
Nobody said either of these things, but people are entitled to extend to or withhold trust from purported experts based on the successful or not-so-successful application of their claimed expertise. If the MOND people and the MOG people keep getting things wrong, they're still free to persevere just as we're free to ignore them.
> 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.
I think that's pretty uncalled for. To me, positing and exploring alternative views of the universe is a noble and interesting goal, even if the currently accepted answers don't get overturned. The generated ideas might still have other value. Maybe there is a piece of it that's salvageable and can be integrated with other concepts. Or it can inspire future ideas. I think it's pretty uncalled for to lump them in with "flat or hollow earth, antivax and HIV deniers, homeopaths". Do MOND and MOG people make conspiracy YouTube videos about how governments of the world just want you to believe in dark matter and energy to shutter your mind from realizing the truth of their theories? Are they telling people to stop taking chemotherapy and be healed through faith alone instead? Perhaps I'm simply unaware of some nasty behavior, but if not it's pretty rude to lump them in with those other groups.
From my perspective they're engaging in a theoretical pursuit and you're just taking a dump on them.
> If the MOND people and the MOG people keep getting things wrong, they're still free to persevere just as we're free to ignore them.
Are you ignoring them? There are a lot of things I don't pay attention to, MOND/MOG included, but I also don't feel the need to demonize them for getting out of bed in the morning and trying out a new idea. Couldn't you just say, "Interesting concept. I doubt it works out for cosmic background radiation, but would make for a cool Star Trek episode". Or just say nothing?
Why do you have to accuse them of being small-minded, simplistic thinkers who are really just motivated by being too dumb to understand the current meta and not unlike harmful idiots that earnestly believe space is just government propaganda and viruses aren't real?
> people are entitled to
> they're still free to persevere just as we're free to ignore them.
I don't think we were talking about what people should be penalized for by the government, in which case rights and freedoms are primary considerations. We're talking about what things people can do to deserve to be considered unreasonable and be ridiculed, looked down upon, and likely considered harmful to society. I don't think MOND/MOG people qualify.
I don't share your thoughts on the passage you're specifically referring to. It seems pretty anodyne to me.
"I think it's pretty uncalled for to lump them in with "flat or hollow earth, antivax and HIV deniers, homeopaths"
I don't share that view, either, but I also don't think they were lumping things together. They were giving examples to answer a specific question I had asked.
"From my perspective they're engaging in a theoretical pursuit and you're just taking a dump on them."
You're entitled to your perspective, just as I'm entitled to my perspective that yours is overblown.
"Are you ignoring them?"
Like many things, that's a matter of degree. I'm mostly ignoring, though not so much that I won't occasionally engage the subject on HN.
"I also don't feel the need to demonize them."
I'm not aware of anyone here who's doing that
"Couldn't you just say, "Interesting concept. I doubt it works out for cosmic background radiation, but would make for a cool Star Trek episode". Or just say nothing?"
I could, but I won't, because I don't consider MOND and MOG to be interesting concepts. Why do you want me to say that I do?
"Why do you have to accuse them of being small-minded, simplistic thinkers who are really just motivated by being too dumb to understand the current meta and not unlike harmful idiots that earnestly believe space is just government propaganda and viruses aren't real?"
I'm not.
"I don't think we were talking about what people should be penalized for by the government."
We're not talking about that at all. That's a non-sequitur. I was just emphasizing that what people can do and still be considered reasonable is a matter of personal judgement. It's a matter of personal judgement if I want to scorn the MOND and MOG people. I don't, but I could if I wanted to.
I DON'T scorn them, but I do largely ignore them. I'm ignoring them even now.
Modified theories of gravity are not new and there is nothing immediate about my feelings on them. I understand homeopathy, and I am very confident that the idea that water has a special memory for particular chemicals is very silly. In the same way I understand MOND theories, and they are mostly silly.
> That's how science works. Scientists are not prophets, they do not have the luxury of starting with all the answers.
You are unfamiliar with the history of these theories if you think that their changes have been in any way scientific. MOND originates when the first observations of dark matter were very simple: galactic rotation curves. It was a reasonable attempt to explain why stars at galactic edges rotate faster than you would expect.
Later, we got much more detailed observations which revealed that there was actual, complex structure to those orbital perturbations. Structure that is totally independent of the visible matter. We found the Bullet cluster, where 2 galaxy clusters are orbiting a third invisible cluster.
Rather than admit some of this structure could not be explained, MOND theories grew wildly more complicated, and starting adding in more and more terms with arbitrary constants attempting to fit each new piece of data. That is not scientific. It's not how we got from Newton to Einstein. It's Aether theory.
There are certainly physicists who do respectable work on MOND. They put that work out as a proposal, and recognize how good the evidence for dark matter is. They are a minority of the proponents, and I really mean that. There are a huge number of people publishing fringe time-cube types of theories about it. I am not lightly comparing it to homeopathy.
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.
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".
I was being somewhat tongue in cheek responding to OP, who was not specific about what they find problematic. The vacuum catastrophe is the area in physics where our explanations explain the smallest amount of our observations. We're very confident in our measurements and understanding at both the large and small scale, and there is essentially no explanation that anyone has any confidence in.
By comparison, observations of dark energy/matter are practically perfectly accurate. It's a single order of magnitude difference from expected observations. We have some half-decent candidates to explain them. Unlike magic, inflation is not a theory in search of an observation. The fact that we don't see inflation is what is so desperately confusing.
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.