Dark energy and dark matter aren't really theories. They are sort of the default solution to a set of problems that exist in cosmology. In a sense, every physicist wants to ditch dark matter or dark energy (or at least our current understandings of them), but they just don't know what to replace them with.
What annoys many was the pretense of a theory. At some point in the history of physics we stopped calling open problems, puzzles and (yet) unresolved paradoxes as what different but similarly unexplained phenomena were called in the past and pretended we resolved them.
It's simply unnecessary to pretend its a theory, it is possible to name things without pretending they are theories.
> pretended we resolved them
Who did? Dark matter and energy are famously-unsolved problems in physics.
There is an overton window of acceptable dark matter theories which really should be a lot bigger. Many of the current big frameworks agree on too many details that essentially just arise from curve fitting.
And no, I am not referring to the various MOND theories. They still belong on the same edge of that overton window that it currently occupies.
> an overton window of acceptable dark matter theories which really should be a lot bigger
Overton windows pertain to mainstream opinions [1]. Particle physics shouldn’t be subject to mainstream opinion.
Dark matter in particular strikes me as… “yes, obviously”.
There’s about a dozen quantum fields corresponding to particles. These form a graph, which is by no means fully connected; the fields each interact with a subset of each other, and neutrinos in particular only interact with gravity and the weak force.
If the connections are in some sense random, then it should come as no surprise whatsoever that the graph has disconnected subsets. In fact dark matter theory is effectively stating that the subset we’re a part of is one of many, which also agrees with the copernican principle.
That's all essentially true, but it doesn't necessarily mean that there's a quantum field that matches the properties needed to explain the phenomena dark matter is supposed to explain.
It just means that it's plausible, and wouldn't be surprising, if there were such a field.
How are they so beyond direct detection if they similarly permeate the entire universe?
Every quantum field only interacts ("couples with") a subset of other quantum fields. Undetectable quantum fields would be ones that don't couple with any fields we interact with.
We already see something like this with the neutrino: they only interact via the weak force and gravity, so we need massive detectors measured in kilometres and buried underground to detect them.[1,2]
It's estimated that about 100 trillion neutrinos pass through your body every second, traveling close to the speed of light.
Now imagine a particle that doesn't even interact via the weak force. The only way we would be able to detect it is via gravity, but gravity is an extremely weak force at the individual particle level. That's a possible candidate for dark matter. (Even particles that interact only via the weak force and gravity are candidates, but it's generally believed they'd have to be more massive than neutrinos.)
There's also the possibility that, if it turns out that gravity is an emergent property arising from quantum interactions or something along those lines, that there could be fields that don't participate in the gravitational interaction. But that's highly speculative territory.
[1] Ice Cube neutrino detector: https://icecube.wisc.edu/science/icecube/
[2] Super Kamiokande neutrino detector: https://www-sk.icrr.u-tokyo.ac.jp/en/sk/
> Now imagine a particle that doesn't even interact via the weak force. The only way we would be able to detect it is via gravity, but gravity is an extremely weak force at the individual particle level. That's a possible candidate for dark matter.
I think even calling it a particle is presupposing things we don't observe. All we know is that there are places where spacetime curves in ways our theories didn’t predict from the matter we observe.
This could be due to a particle of a field that only interacts via gravity… or it could be that there are things other than particles that bend space-time. (Like maybe there’s an entirely separate dimension of space we can’t see, but its gravity affects ours…) Or it could be that spacetime warpage propagates in different ways than we thought (“gravity is different at large distances” etc), or it could be something else entirely we haven’t even thought of.
Quantum mechanics gives us a mental framework (fields and their corresponding particles) which has been extraordinarily useful for understanding the visible universe so far, but I think it’s important to remember that it’s just a model. It isn’t reality itself.
> I think even calling it a particle is presupposing things we don't observe.
Right, that's why I said it's a possible candidate for dark matter.
Your philosophical point about quantum field theory isn't really relevant here. The point is that, as the original commenter pointed out, we observe evidence of the presence of a set of quantum fields that act and interact in very well-defined ways. Given the nature of those interactions in the fields we can observe, it shouldn't be surprising if there are also other fields we can't observe - all it takes is a different set of coupling parameters. And we already see examples of this - we're virtually blind to interactions besides electromagnetism and gravity, and have to build enormous multi-billion dollar machines to explore other such interactions.
No-one is saying that this must be the explanation for dark matter, or that there must be other invisible fields. It's simply a natural prediction that arises from a successful theory.
I agree no one seems to be saying that dark matter must be part of a quantum field, but my point is more to lament the overgeneralizing of “quantum fields” as a concept to explain unobserved things.
I’m absolutely a layman here, so apologies if any of this is misguided or flat out wrong… but its something that’s always gnawed at me about the constant tendency to “quantize” everything including gravity and dark matter… I’ll try my best to articulate:
Fields and particles are a mathematical construct we use to describe our observations. Quantum field theory seems to be a consistent mathematical framework you can use for particle interaction once you’ve observed the properties of the particles and how strongly force is carried, etc. But each field has to have its parameters set via observation; we don’t have a good reason why particle strengths are what they are (the fine structure constant doesn’t have an understood “cause”, for example.)
But my problem with this is that it can be so general that you can postulate a new field every time you don’t understand why something works a certain way. Like, we have the em field with photons, the weak field with neutrinos, the strong field with gluons… so dark matter? Must be another field with some new particle. Gravity? Must be another field with its own graviton or something…
It’s like string theory’s 10 dimensions… I certainly don’t have the expertise to understand string theory or why the extra dimensions are required, but something rubs me the wrong way if you can just postulate another dimension every time the theory is having trouble describing reality.
I’m not saying there aren’t multiple fields or that QFT is wrong, but I will say that QFT is a model we have that usefully describes parts of reality, and we shouldn’t confuse the model with reality itself. Some subset of the phenomena we observe can be described and predicted by it, but there are many phenomena that are not predicted by it (gravity is the big one.) I’m not convinced that the fix is to just narrow down the parameters of a graviton, but instead to accept that we simply don’t know if QFT and the standard model can ever be a truly unified theory of everything or if it’s just a useful tool for a few categories of phenomena. It’s possible that we simply don’t have (and might never have) a better theory.
So when we have a problem like dark matter, to say “maybe it’s a particle in its own field” can feel a bit like physicists going back to the same old well again.
However the connections are not at all random, but they observe certain strict symmetries.
For instance, in each of the 3 "generations" of quarks and leptons, separately for particles and for antiparticles, all the different kinds of charges and spin sum to zero, e.g. for the 8 particles that are the 3 kinds of u quarks + the 3 kinds of d quarks + the electron + the electronic neutrino. Moreover, in a 3-dimensional space of the "color" charges and electric charge, the 8 particles and 8 antiparticles of a "generation" are located in all the corners of 2 cubes, not in arbitrary positions.
So the set of elementary particles that we know is complete, there are no random locations where there could be extra particles.
Any so-called "dark matter", if it would exist, would have to be something completely different and not related in any way with the known elementary particles.
> Dark matter in particular strikes me as… “yes, obviously”.
Why? Historically there have been two "dark matter" theories prior to this one (we speculated non-visible mass in a place to explain motion of celestial bodies). One turned out to be neptune. The other was vulcan, which turned out to be general relativity.
So historically, the inclination to invoke some form of "dark" matter is batting 50/50. im not sure i would put 50/50 into "obvious" territory
I've been doing a deep-dive in the past few weeks of papers and data sets regarding to rotation curves, mass densities etc. (SPARC, papers describing the rotation curves of various dwarf galaxies etc). The impression I get is that most of the authors are not critical of a CDM dark matter halo explaining rotation curve data. The papers I'm reading span from ~1990 to present.
That seems really time consuming.
One question I’ve had for a while but didn’t seem worth the look is if there any consideration for local gravitational interaction between stars exchanging momentum between them and thus flattening the rotation curve.
I’m assuming that’s one of the first things looked at but couldn’t find a paper on the subject. Remember any references on the topic?
The problem isn't so much the flatness of the rotation curve, but its continued high value: as you go farther and farther out in distance, it should drop rapidly because most of the visible matter is concentrated toward the center of the galaxy, but it doesn't. This implies that there is more matter, less centrally concentrated than the visible matter.
Note that most "rotation curves" are actually measured from gas, not stars, and also that strong gravitational interactions between individual stars are extremely rare except in very dense star clusters and galactic nuclei, due to the increasingly large distances between stars as you go out from galactic centers. The time required for individual stellar interactions in the main or outer parts of galaxies to significantly affect their motions is much larger than the age of the universe (see, e.g., https://en.wikipedia.org/wiki/Stellar_dynamics).
Finally, this wouldn't address other evidence for dark matter, like the halos of hot (millions or tens of millions of K) intergalactic gas in galaxy clusters. The pressure of the gas should have driven the gas to expand way billions of years ago, if you assume that only the gravity of the individual galaxies and the gas itself is restraining it.
Edited.
> strong gravitational interactions between individual stars are extremely rare except in very dense star clusters
We’re talking 225 million years for the sun to orbit the galaxy, rare events become commonplace on those timescales. Anyway, I’m sure someone has actually done this kind of simulation I’m just curious about what the result is and how they did it.
Have you seen any work from Stacy McGaugh? https://tritonstation.com/new-blog-page/
No; checking it out. Ty!
not saying MOND is correct, but the list of predictions it made without fine-tuning are stunning:
- keplerian rotation curves ("no dark matter") in elliptical and lenticular galaxies
- EFE (a group tried to find evidence against EFE and found it instead)
- early galaxies in the universe
- renzo's rule
- keplerian descent in the milky way (due to the effects of the magellanic clouds)
There's also explaining the tully-fisher relation, which is a post-hoc rationalization, so not really a prediction, but the model wasn't fine tuned to obtain the exact numerical solution.
The typical CDM response to this is "What about the Bullet Cluster?" (Gravitational lensing not correlated with visible mass). What is your thought on that? I think a convincing answer there would go a long way towards convincing people. I believe Milgrom's speculation is undetected matter.
I don't know enough about the statisical image processing to measure weak gravitational lensing to have a cogent answer, but also it's totally ok to have one or two things that don't quite match up.
Some of the "there's no DM" or "there's almost entirely DM" diffuse galaxies are likely due to measurement error in the distance to the diffuse galaxy, for example.
Maybe that's the evidence that dark energy and dark matter are the reality which our dark side refuses to admit.
Yeah, the best analogy for people that know a little physics is aether - it’s obviously ridiculous now, but there was a time when it filled in some unknowns and people took it seriously. It would be nice if it weren’t presented as fact in tv and planetarium shows, but what can you do?
They shouldn't have named them then. What they really are is an approximation of what they think they do not know.
> just don't know what to replace them with.
Which is fine, but every physicist seems to just assume that it must be a "force", a "particle", or a "field".
It can be other things, including errors in the math, errors in the models, errors in the observations, invalid assumptions, etc, etc...
It's rather irritating to see the n-th experiment "searching" for some previously unseen particle while literally only one team thought of revisiting the maths to see if something was missed.
Dark matter is and always has been curve-fitting to residuals between theory and data. There is no there there, every map you see is nothing more than subtracting theory from data and having residuals left over. Dark energy is similar except much more coarse, in that the "model" is just a single parameter with a very simplistic interpretation.
Neither are theories, but good luck coming away unscathed when mentioning this in the presence of ΛCDM dogmatists.
This is nonsense. Dark energy is a theory. Dark matter is a theory.
As soon as you have at least two observations that you put together into a batch, you are at a minimum suggesting "these observations are causally connected". You have theorized that suggestion. You did not arbitrarily group those observations. (it's not "my dog pooped this morning" and "the car started when I hit the gas this afternoon") That makes it a theory.
This is generally not how scientists, if they are being careful, use the word "theory".
What you describe, the idea that two things are connected, is not a theory. That's a hypothesis. A hypothesis is a claim about the world, and it might not (yet) be equipped with an acceptable explanation for why it should be believed.
A theory would be a formula or equation or perhaps a process which is consistent with a set of information and allows scientists or mathematicians to calculate more information. It's a system whose consequences you can work out on paper, or in a computer, or in your head. But notably, a theory need not have any bearing on reality. You can develop a robust theory in all its mathematical glory and never find or expect to find anything like it out in the universe. It is a theory nonetheless, because you can work with it and explore it for it's sake.
Now, certainly, we have developed some theories of dark matter over the years and hypothesized that they are candidates for explaining the real world. There are many such theories. And, for each one, some scientists hypothesize that that one might be an accurate description of the world.
But, no, the idea of dark matter is not a theory.
as you stated:
a hypothesis is the model (alone) for how things might be
a theory is the hypothesis + corroborating observation
if you have a set of observations and put forth a hypothesis from it (versus a hypothesis that has no observations backing it), it is automatically a theory. it may not be a good one but it is one nonetheless.
One of the better nautilus articles I’ve read, usually they’re unreadable and boring, despite an interesting title. Unsurprising that it was written by Sabine Hossenfelder. Good science communication is a real skill.
Every science documentary I've watched tells me most physicists want to ditch "dark energy" because it's a placeholder term for something we still don't understand yet. Map-makers didn't actually believe there were dragons after all.
I've heard the same about Dark Matter, and that was previously my amateur mental model. This was naive; in practice it confidently refers to CDM (cold dark matter), which is matter that interacts gravitationally, but not electromagnetically, with normal matter.
That's literally what a placeholder is: it's a description of a suite of observed properties.
Hot dark matter would be a black body emitter - we would see it.
If it interacted electromagnetically then effects like the bullet cluster's gravitational lensing shouldn't happen.
And if it didn't interact gravitationally then we should see normal galactic rotation curves.
>Hot dark matter would be a black body emitter - we would see it.
Hot dark matter would be stuff like neutrinos and axions which don't interact electromagnetically.
I think we are on the same page. I was sloppy in my wording; my point is, "dark matter" is not a placeholder for discrepancies in Newtonian simulations of cosmological features with observation, but a placemholder for a type of gravitational-interacting bodies.
No, hot dark matter would not magically start interacting with the EM field to make black body radiation. I believe the hot/cold dark matter question is entirely about the velocities of the particles.
> it's a description of a suite of observed properties
"Looks like matter" is not a property, this interpretation already presupposes a theory that excludes many others that are heretofore still plausible. Physicists tend to ignore those other options, I believe, because of a combination of natural human linguistic biases and the desire for certainty in explanatory models that drives many of us into studying physics. Some get too confident too early and start making dogma their entire academic personality.
Although i'm not a physicist and i have an average understanding of these things, dark energy and dark matter always felt to me like physicists had created mystical entities with magical properties because they couldn't explain some experimental results.
And my average brain always thought and still thinks that instead of chasing these unicorn entities that still can't be found, maybe we should reconsider some things, as it seems to be the case presented here in this article.
Except that sometimes chasing these unicorn entitles leads to... finding the unicorn entities.
That's basically what happened with the neutrino. Neutrinos were originally proposed in 1930 by Wolfgang Pauli to solve apparent violations of energy and momentum conservation in beta decay. He suggested that the missing energy and momentum were being carried off by some additional -- undetected and mostly undetectable -- particle. For a while, it looked like these proposed ghost particles might never be detectable, but Fred Reines finally managed it... in 1956, 26 years later.
So don't write off unicorn particles. Sometimes they're real, even if you have trouble detecting them.
It feels somewhat like the Aether theory [0], which fell out of place after special relativity got introduced. It was interesting to me that the Aether was proposed by Isaac Newton.
Also not physicist, but they always felt like smudge factors. Put in this stuff we don't know what it is and only interacts in certain ways to right places and finally math works out. Well it is obvious that they can work if you can mostly freely tune parameters until they do. But then you need two different things...
Somewhat reminds me of aether...
Former physicist here! I used to work on this stuff. I don't think anyone has ever been a "true believer" in dark matter or dark energy.
There was a pattern in late-19th- through late-20th-century physics wherein someone noticed something weird, said "hey, if X were true, it'd be kind of weird, but it would explain it all", then people went out and looked for evidence of X. In many, many cases, people did indeed find X, some closely related X', or at least got strong evidence in favor of something else Y, which might have disproved X but at least settled the matter.
This has not happened with dark matter or dark energy. The questions still remain open, with no good evidence for any explanations.
So... what I'm saying is, don't think less of people for trying to explain things, just because they tried and it didn't work out yet. You're seeing the scientific method play out right now: there is a whole lot of wrong. There was a lot of wrong in the old days, too, but we weren't around to see it; only the successes got passed down.
That’s… kinda how it always worked. They invented something that fit existing observations. Remember when ether was a thing?
The other problem is that nothing else really fit… until now. Science works, turns out.
So when you find yourself in the situation in which the vast majority of professional physicists, especially those who dedicated a good chunk of their life to study this one phenomenon, believe one thing, and you, as a self-proclaimed layperson, believe another thing, don't you wonder whether you would actually start believing the first thing as well if you knew what the aforementioned professional physicists know?
I mean obviously not, I just don't understand the thought process behind coming to the conclusion that it's the professional physicists who must be wrong or have failed to reconsider something and not the layperson whose knowledge about the thing is dwarfed in comparison.
You realize this is how basically every single scientific discovery ever has worked right?
The truncation of the title to remove "These" does severe violence to its meaning.
The anti-clickbait truncation HN applies to headlines works well in the singular: "This politician wants to ban bicycles" --> "Politician wants to ban bicycles."
In the plural it should probably convert "These" to "Some" rather than implying that "all physicists want to ditch dark energy."
@dang this title needs fixing.
> He called it the “timescape.” This is because, in Einstein’s theory, time runs at different speeds depending on the amount of matter that a region contains.
I wonder if he called it "timescape" as a reference to the star trek the next generation episode where small bubbles of space experience time moving at different speeds.
From the article,
> discovering that the expansion of the universe was accelerating. They came to this conclusion by observing faraway exploding stars. These distant supernovae showed that the cosmos was getting bigger faster because the farther away the supernovae, the faster it appeared to be moving away from us.
This explanation always bothers me. After a long time, things that move faster WILL be farther away than things that move slower. Thats just the definition of speed. It does not, by itself, demonstrate acceleration.
Oh please. Do you really think the people who won the nobel prize for this discovery have a grade school level understanding of speed? You seem to be talking about peculiar velocities, which are absolutely negligible on cosmological scales and only the velocity caused by the expansion of spacetime is relevant.
What they really did was measure the distances and redshifts of dozens of supernovae and looked which energy contents of the universe could explain the observed relationship assuming that the Friedman-Lemaitre-Robertson-Walker metric accurately describes the universe. Turns out you need 70% of something whose density does not change with the expansion.
This is the original paper if Hossenfelder's summary of the explanation compressed in one sentence does not convince you:
https://arxiv.org/abs/astro-ph/9812133
For a derivation of the relationship between redshift and distance see any introdctory text on cosmology, e.g. chapter 8 of https://arxiv.org/abs/gr-qc/9712019 (he arrives at the equation on the last page).
Edit: This goes into more detail of matter species in sec. 1.5 and touches on the SN observations in sec. 1.7: https://www.thphys.uni-heidelberg.de/~amendola/teaching/adv-... (on an unrelated note: he was my PhD advisor)
> Oh please. Do you really think the people who won the nobel prize for this discovery have a grade school level understanding of speed?
Of course not. My comment is a quibble on the one sentence explanation. You seem to agree with me it's insufficient. Thank you for the additional resource.
> This theory has it that all types of energy—including matter, radiation, and pressure—curve space, and the curvature in return influences how the energy-types move. The authors of the new paper, led by Antonia Seifert, don’t question this. They question instead how we use Einstein’s math.
This is so fascinating. I think the principle applies to so much of the natural sciences. GR describes a set of rules (differential equations using tensors) that describe how matter moves in spacetime, and how it curves it. But outside of certain specific conditions (Schwarzschild etc), we can't (yet) use it to build useful models! We can use it to an extent to validate parts of models, but it leaves so much to the imagination. We are still using Newtonian models in cosmology, then applying GR effects like GEM piecemeal, and the time dilation effects in the article, where complexity and understanding allow.
We have these rules, but don't know how to use them to model! See also: Quantum mechanics and ab-initio chemistry. It's as if the universe is written in differential equations, but we are novices at how to use them.
Sabine's video version of the same stuff https://youtu.be/frJy-sSriHM
Why are these anti-dark matter articles upvoted on hacker news.
As I understand it, most cosmologists still thing dark matter is the most likely candidate as it explains multiple different daya points unlike Mond.
This one is about dark energy.
It's really just a stopgap.
Not sure what that even means. It's our best theory at the moment and helps explain multiple different cosmological phenomena.
There are also hypothesized particles that fit within the Standard Model that researchers are searching for experimentally.
Honestly, these articles are upvoted by people who know very little physics and think that dark matter is somehow a very mysterious idea.
Ah yes, the “cosmological principle” that everything averages out to be basically the same everywhere over large distances. It makes sense that this principle would need to be violated in order to eliminate dark energy.
One of my favorite sci fi concepts is a universe where the cosmological principle was false.
> One of my favorite sci fi concepts is a universe where the cosmological principle was false.
As it stands now, the cosmological principle seems false at every scale we look (with confidence decreasing in falshood slightly decreasing as scale gets larger). So that doesn't seem to be a "sci fi" concept, just a "sci" concept.
Note that this can also be distances in time, rather than just space.
Good news, the cosmological principle is almost certainly false since it leads to absurd results like dark matter and dark energy!
Or if it was true once (CMB-era) it sure as shit ain't true now.
Alas, the top hits for "timescape" refer to a sci-fi book.
But timescape wiltshire leads to a nice presentation:
https://www.nottingham.ac.uk/physics/documents/talesoflambda...
And without math, https://en.m.wikipedia.org/wiki/Inhomogeneous_cosmology
Why don't they want to ditch dark matter? It is more plausible candidate to ditch :)
When you run the Schwarzschild radius calculation for the universe (relating mass of a black hole to event horizon radius) you get a prediction that's close enough to the size of the universe and it's mass, so to me that's pretty good evidence our universe is an event horizon.
This means all 3D points in our space are on the horizon itself, and the time dimension is the normal vector to that "surface" (3D manifold). It explains why space is expanding, because Event Horizons always only expand (excluding considering Hawking evaporation of course, which happens too slowly to affect things)
Sean Carroll discusses this in "The Universe is not a Black Hole": https://www.preposterousuniverse.com/blog/2010/04/28/the-uni...
Relevant quote:
> "Still, some folks will stubbornly insist, there has to be something deep and interesting about the fact that the radius of the observable universe is comparable to the Schwarzschild radius of an equally-sized black hole. And there is! It means the universe is spatially flat."
Sean Carroll is not really an unbiased party. He has his entire reputation staked on him being right about his own theory, so he's going to simply disagree with anything else, and he's going to disagree with anything that's not a "standard" theory.
He likely believes in the Big Bang too which is an absurd, and disproven theory. There are far too many mature galaxies that are far too old for the Big Bang theory to be correct.
> He has his entire reputation staked on him being right about his own theory
You may be thinking of someone else. I'm not aware of Carroll having "his own theory" that he's "staked his entire reputation on."
In any case, he's explained his position, even in the quote I provided. He's simply describing what general relativity says about this. If you have some issue with it, you can respond on technical grounds, without needing to resort to ad hominem.
> He likely believes in the Big Bang too which is an absurd, and disproven theory.
Big Bang is not "disproven" - the Lambda-CDM model of the Big Bang is still known as the "Standard Model of Cosmology," and nothing has replaced it as a consensus among mainstream scientists.
> There are far too many mature galaxies that are far too old for the Big Bang theory to be correct.
The issue of galaxy ages has a number of reasonable explanations in the context of Lambda-CDM. One of them could simply have to do with age estimates, which is an area that suffers from limited observational data that's difficult to interpret. Recent work on the Hubble tension may help resolve that, e.g.: https://news.uchicago.edu/story/new-webb-telescope-data-sugg...
The idea that galaxy ages "disproves" Big Bang theory is at best, a misunderstanding of pop science hype, and at worst pseudoscience. That's not how science works, especially in cosmology where most evidence is very indirect and subject to interpretation.
The Big Bang theory makes more assumptions and contradicts more evidence than just assuming our universe is an event horizon embedded in a higher dimensional space. The Event Horizon explains lots of things the Big Bang doesn't also, like why time is a special dimension, and why space always only expands. The Event Horizon theory explains everything that the Big Bang theory cannot explain and contradicts NONE of the evidence.
The Event Horizon theory doesn't postulate a "beginning" of everything, in the comical "it popped into existence" nonsense of the Big Bang. The EH theory doesn't try to establish a first-mover or uncaused-cause. It just describes how the universe is without trying to describe any "magical" beginnings. The Big Bang is a theory about magic, not a theory of science.
Really curious, how do you quantify "close" in such numeric ranges?
Quick calculations say that ratio is 1.71:1 (https://rentry.co/k85wy696). I guess given the scale of the numbers having such a low ratio is interesting.
But my intuition says that in physics constants are scattered in a sort of logarithmic way, i.e. the orders of magnitude are uniformly scattered in some range. So small ratios between such constants not impossibly rare.
I may be full of shit though!
I admit I haven't run the calculation myself, nor even asked OpenAI (or other AI, which are almost certainly capable of doing it), but I heard a presentation say the number is off by a factor of 3. To me '3' is best described as "no where near an order of magnitude". Since there's no way we're measuring the radius of the universe nor the mass in it accurately, I think being off by only a factor of 3 is astoundingly "accurate". When you're dealing with many many orders of magnitude like these astronomically large numbers, and you end up being only off by 3x that's actually pretty close. Too close to be an accident. If the theory was "wrong" it would be off by many orders of magnitude.
Is there any experimental evidence for this model, does it actually lead to any verifiable predictions?
There's more evidence for this Black Hole theory than there is for the Big Bang, but not it's not like there's any laboratory experiment that can be done afaik. It's all about measuring things in astronomy. Big Bang theory at this point is hilariously wrong, and doesn't fit much evidence nowadays. Big Bang is disproven.
Ok, by that comment you told me everything I wanted to know: you're pulling this out of thin air. Please provide sources whenever you make insane claims like "Big Bang is disproven". Because it's not [1]. Also you told me there's "more evidence for this Black Hole theory than there is for the Big Bang", so please link to it, that I might examine it and maybe change my mind.
[1] https://en.m.wikipedia.org/wiki/Big_Bang#Observational_evide...
The Big Bang is disproven because we see fully formed galaxies when we look out at the edge of the universe using James Webb Telescope. That's conclusive proof it's wrong.
I'm not a physicist but both dark energy and dark matter have a definite whiff of 'luminiferous aether' imho.
Of course they're just placeholders for things we don't understand, but my guess is that they are not any form of energy or matter at all, but a misunderstanding of the geometry of space or something similar.
Totally! All these ideas are just a placeholder to make a note of something we can observe until we have a better theory.
Luminiferous aether was invoked to explain the ability of the apparently wave-based light to propagate through empty space. [1] Eventually we found evidence that contradicted that idea.
Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be observed. [2] We are searching for direct evidence, but haven’t found any yet.
This applies here too: See my comment above; most cosmologists are confident (And/or focus most work on) Cold Dark Matter, e.g. in an ellipsoidal halo around galaxies. I am not making a qualitative judgement, but dark matter is not used in cosmology papers as a placeholder.
It's a placeholder in the sense that we don't know specifically what it is though, right? No cosmologist uses "matter" when they mean "protons" for example.
>No cosmologist uses "matter" when they mean "protons" for example.
Don't astronomers call elements like neon metals? Maybe cosmologists don't?
> No cosmologist uses "matter" when they mean "protons" for example.
They will call that "baryonic matter".
No, they'd call them protons; baryons are a class of particles, of which protons are a member.
This is what a particle physicist use the term for. But for cosmologist on the question of matter composition they will use the term in different way. I was replying to exactly the quote from your reply.
Yes; it's still reasonably broad. (MACHOs? WIMPs?) But this is still, in terms of all possible explanations for observations re rotation curves, and the Bullet Cluster etc, well defined. It includes a lot of possibilities, but also excludes.
Unreasonably narrow, rather! We have not convinced anyone, much less done the proper science, to stick to our guns so long that it is truly matter we are hunting to explain this phenomenon.
The point is, it's already a theory to even suggest it's "matter" and not any other explanation.
It certainly doesn't matter yet.
I'll see myself out.
"Dark matter" is just a description that refers to matter which interacts gravitationally but not, or at least very weakly, in other ways. It doesn't imply anything about the nature of such matter. We know that there are several types of dark matter, specifically neutrinos and brown dwarfs, although it is now established that they don't make up anything near the density required by the cosmological models. Primordial black holes, which may exist, would be a dark matter. Some particle physics models, like supersymmetry, also naturally predict that there will be a massive particle that would behave like dark matter. So I don't think the analogy with the aether is very good, because that really was pulled out of thin air (pun intended). Given how successful general relativity is, it's perfectly rational to interpret the observations as probing mass distributions while assuming GR will continue to hold. There are of course people also assuming that GR is wrong and attempting to explain the observations without dark matter, but my impression is that they struggle to come up with self-consistent theories that fit all the data, although I'm not very familiar with that work.
What other people have been doing over the past 40 years is attempting to devise tests for these various dark matter candidates. We know, for example, through lensing observations, that MACHOs/brown dwarfs don't exist in the required numbers and that the neutrino mass seems too low. The problem, of course, is that there are only so many ways to try to observe matter that is truly dark.
I agree, though, that, in the end, it may be that dark matter will be an untestable hypothesis, just like quantum gravity or whatever.
I've been saying this for at least 6 years [0]. Got downvoted to oblivion back then, glad to see opinions are changing.
> In this timescape model, what we observe in our vicinity, in our own patch, is governed by different laws than what happens on average at larger distances. It is much like how what you observe in your home city may be a poor description for what happens in the world on average.
If a dark matter alternative ends up being an accepted theory, then RelMOND stans are gonna be beyond smug, and well rightfully so I suppose.
No, the laws are the same everywhere. The timescape model just takes into account gravitational time dilation in large voids (where time passes faster). They say this can explain the observation that the universe is expanding faster (in other words, it's not, it just looks like it).
Also, this is about dark energy, not dark matter
Ah right yeah, had them mixed up for some reason.
The submission title is edited in a questionable way. Specially that the original one is not clickbait and does fit in the title letters limit.
There is a world of difference between " These Physicists Want to Ditch Dark Energy " and " Physicists Want to Ditch Dark Energy". One is about new model from some physicists and the other implying a conciseness around ditching dark energy.
edit: I didn't know that there is automatic re-write rules for HN. However the fact that the edited title is clickbait now regardless the reason. Just clarifying in case of this is considered an attack on the submitter.
It‘ almost certainly Hacker News‘ automatic title rewriting, and not the submitter‘s fault. A missing word at the start is a sure sign.
That's interesting, I didn't know before that there are re-write rules applied. It seems that this at least a case of when this failed.