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.
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.
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.
They shouldn't have named them then. What they really are is an approximation of what they think they do not know.
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?
Maybe that's the evidence that dark energy and dark matter are the reality which our dark side refuses to admit.
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.
> 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.
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.
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...
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?
> 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.
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
> 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.
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.
"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.
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."
Is there any experimental evidence for this model, does it actually lead to any verifiable predictions?
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!
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)
Sabine's video version of the same stuff https://youtu.be/frJy-sSriHM
> 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
From https://news.ycombinator.com/item?id=36222625#36265001 :
> Further notes regarding Superfluid Quantum Gravity (instead of dark energy)
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.
It's really just a stopgap.
This one is about dark energy.
Why don't they want to ditch dark matter? It is more plausible candidate to ditch :)
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.
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.
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."