[lake_vincent]

Correct me if I'm wrong (please), but don't we still lack any kind of fundamental definition of what dark energy/matter is other than..."the cause of the difference between what is calculated, and what is observed"? To the point that we aren't even really sure that there is such a "thing" as dark matter (in that it exists in any conventional sense)?

From Wikipedia: "Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe...The primary evidence for dark matter comes from calculations showing that many galaxies would behave quite differently if they did not contain a large amount of unseen matter. Some galaxies would not have formed at all and others would not move as they currently do."

85% is kind of a lot of "stuff" to be missing...

I find it kind of funny that humans are so confident that our models of reality are correct that we truly think it's more likely there's just hidden "stuff" than there's just something hugely wrong with our idea of what the universe really is, and how it works. Our physics works great in a lot of circumstances, but to be missing 85% of the damn universe might imply we are wildly off base when it comes down to the true nature of things.

Obviously, I don't have an explanation myself, and I understand that we can only work with the evidence we have, but I think it's a sign we need to radically rework our basic assumptions about reality, and not just look for our missing keys...

Perhaps black holes are the right place to look, but not as a cubby hole for our missing stuff - rather, as a path to transforming our assumptions about reality.

[concordDance]

> I find it kind of funny that humans are so confident that our models of reality are correct that we truly think it's more likely there's just hidden "stuff" than there's just something hugely wrong with our idea of what the universe really is, and how it works.

That's not how it works.

Many a physics PhD has spent their career trying to come up with better models, including different sorts models of gravity or indeed "to radically rework our basic assumptions about reality,".

Surprisingly, physics professors aren't idiots and have thought of this. It's just that, so far, invisible matter is still the thing that best fits the data compared to the (non-overfitted) modified gravity models people have been able to come up with so far.

[nostrebored]

Dark matter is overfitted. This isn’t some comparative advantage it has. The number of parameters you set manually in many of these models is insane.

We are fundamentally missing something. Thankfully it’s just not all that important for us right now.

[empyrrhicist]

I mean, we already know about existing "dark" matter particles. The Neutrino comes to mind, though it's not massive enough to explain the gravitational phenomena. LCDM really isn't that weird or unexpected, since it's a lot like what we already observe, and we already think more particles should exist.

[denton-scratch]

Hypothetical "dark matter" doesn't interact with ordinary matter, except gravitationally. Neutrinos do interact with ordinary matter; otherwise we wouldn't be able to build neutrino detectors. Therefore neutrinos are not an example of dark matter.

[consilient]

> "dark matter" doesn't interact with ordinary matter, except gravitationally.

It's possible that this is the case, but particles that also interact with the weak force (hence WIMPs: weakly interacting massive particles) are generally considered better candidates.

[arbitrage]

Neutrinos aren't dark matter.

[mr_mitm]

They are, because they have mass and don't interact electromagnetically. They are just not cold dark matter, because their mass is so low they behave more like radiation than matter, i.e. the scale as a^4 instead of a^3, where a is the scale factor. A sterile neutrino, if it existed, could still be a dark matter candidate.

[MichaelZuo]

Can you give examples of these parameters that must be set manually?

[kelseyfrog]

These are the free parameters of the Standard Model https://en.m.wikipedia.org/wiki/Standard_Model#Construction_...

[btilly]

Every one of those parameters is associated with a field that does symmetry breaking. Every one of those fields has a particle associated. Every one of those particles has been observed, studied, and found to have properties in line with what would be required for the fields to have the values that we measure.

So yes, that's a lot of free parameters. But they are intrinsic to the theory. And we have considerable experimental evidence that they represent something real.

For those who don't know what symmetry breaking is, the Standard Model has a lot more symmetries than the observed universe. For example the theory does not specify that electromagnetism is long range while the strong nuclear force is short range. Or that the muon weighs more than the electron. But for each symmetry in the theory that we don't see in practice, there is a field that specifies the value of the observed asymmetry. Each field is carried by a particle. Each particle has properties that reflect the value of all of the fields. Every particle has been found and almost all have the predicted properties (to within measurement error).

The last particle found was the Higgs boson. The Higgs field determines the relative masses of different particles.

The "almost all" is the fact that the neutrino has 3 versions and oscillates between them in flight. Also the neutrino is not massless. While the Standard Theory can adapt to match this, this isn't what was originally predicted.

[kelseyfrog]

Right, and some physicists have a nagging feeling that the addition of another symmetry would result in the reduction in free parameters. "We've done it before, so why shouldn't it work again?" is their thought process. So they devise a model which replicates the standard model in observable ranges and to make it testable, doesn't replicate the standard model outside of the observable range. This is an attempt to make the model falsifiable and thus scientific. Then they advocate for expensive accelerators to falsify the model.

[cld8483]

You'd be hard pressed to derive a model for the coarse structure of the universe from the Standard Model, considering the Standard Model can't describe gravity. Maybe this is part of the problem..

[bmitc]

Dark matter is the name for unexplained mass that doesn't interact like typical matter. The existence of such mass is needed because there are galaxies that don't match what is predicted by general relativity. So either general relativity is wrong on the particular predications or there is extra matter.

The universe is expanding. However, not only is it expanding, but the expansion is accelerating. It isn't clear what is driving this, and source of the acceleration was given the name dark energy.

The "dark" in these names are part we don't know what these things are yet and part we can't see them yet with our current observational methods.

> I find it kind of funny that humans are so confident that our models of reality are correct

I don't think this is the case. If you have another model that replaces general relativity, I think people would be very interested in it. People have tried several times to explain the observations that led to dark matter by modifying various forms of gravity. They all work less well than general relativity. So the search continues: keep adjust the models but also don't be afraid that there are new things in the universe that we didn't previously know about. There are several experimental approaches that are searching for ways to directly detect dark matter.

[Maursault]

> People have tried several times to explain the observations that led to dark matter

The observations that led to Dark Matter have been sufficiently explained without the need for Dark Matter.[1] Apparently, subsequent observations that got lumped into Dark Matter have not yet been sufficiently explained, and Science is paradigmatic, so Dark Matter persists.

[1] https://www.youtube.com/watch?v=PL0ewiwqoTw

[slibhb]

> I find it kind of funny that humans are so confident that our models of reality are correct that we truly think it's more likely there's just hidden "stuff" than there's just something hugely wrong with our idea of what the universe really is.

> Obviously, I don't have an explanation myself, and I understand that we can only work with the evidence we have, but I think it's a sign we need to radically rework our basic assumptions about reality, and not just look for our missing keys...

Looking for missing keys is fine. This is how models are validated or invalidated. For example, Neptune's location was predicted in the 19th century before it was observed due to discrepencies in Uranus' observed orbit and what was modeled.

There are alternative theories that could replace dark matter/dark energy (e.g. MOND for dark matter, inhomologous cosmologies for dark energy).

[SkyBelow]

>I find it kind of funny that humans are so confident that our models of reality are correct

The models used aren't used because scientists think they are correct, but because they are the best models that fit existing experimental data. Best normally meaning 'simplest that explain all data', but it isn't quite always that simple. There are scientists who spend the latter parts of their careers trying to find even better models or even proposing worse models that are useful to keep in a folder somewhere in case they ever do become relevant or we come across data the current model can't explain. At the cutting edge of science you even find that the current models aren't matching all the data, but there is no consensus on a better model and scientists are working to either expand or replace the current model to fit the new data and to explore the non-matching data to either see if it is an error (which does happen) or if it gives hints on a better model.

Anyone who thinks that the model is correct is someone who is lacking a foundation in what science is saying. Often scientists don't talk about the distinction because it is too detailed for the public to care about, but that's not the same as not recognizing the distinction. (Much like how programmers talk about computers as if they are intelligent beings making decisions, often to the programmer's detriment, despite any good programmer knowing that isn't the case, newest models of AI tentatively excluded.)

[credit_guy]

From what I understand, this has nothing to do with dark matter. It's only about dark energy, which is an unrelated concept.

[codethief]

+1

In particular, dark energy is not

> "the cause of the difference between what is calculated, and what is observed"

Dark energy / the cosmological constant Λ is a completely natural parameter to the field equations, not a corrective term (even though it might feel that way if you follow the history of cosmology). It's not that we calculated Λ to be zero but observations forced us to set it to something non-zero. We had no clue what it's value should be and simply deduced from observations that it happens to be non-zero. End of story.

Now, particle physicists / field theorists have been looking for a particle physics-based explanation for Λ and preliminarily called it "dark energy". But there's no guarantee there is one. Maybe Λ is indeed just a boring parameter.

[tlholaday]

> correct me if I’m wrong …

I think you’re oversimplifying, like seeing a forest and overlooking that it’s Prototaxites, not Malus domestica.

Here’s Sabine Hossenfelder’s curious-undergraduate-level history of Dark Matter theories:

https://youtu.be/4_qJptwikRc

Proper nouns include Renzo’s Rule and Baryonic Tully Fisher Relation.

[Vvector]

"...humans are so confident that our models of reality are correct..."

Dark Matter is just a hypotheses to explain our current observations. Future discoveries may back up dark matter, or point towards something different. This is how Science works.

[mr_mitm]

> Correct me if I'm wrong (please), but don't we still lack any kind of fundamental definition of what dark energy/matter is other than..."the cause of the difference between what is calculated, and what is observed"?

Sure. You seem to take issue with this notion. However, this very strategy lead to the discovery of elementary particles (neutrino) and planets (Neptune) in the past. It's how science works.

> Obviously, I don't have an explanation myself, and I understand that we can only work with the evidence we have, but I think it's a sign we need to radically rework our basic assumptions about reality, and not just look for our missing keys...

This is an extremely common opinion among laymen (relevant: https://xkcd.com/1758/). I find it a bit patronizing, because it implies that scientists haven't considered everything under the sun to explain the observations. Please take it from me, who got a PhD in cosmology, that every single person I encountered during my graduate studies was extremely bright, in particular the professors. They have thought of everything a layman can think of a billion times over. The knowledge gap in a field as unintuitive as cosmology between a professor and a layman is basically that of an adult and a toddler.

That's not saying that dark matter or dark energy is beyond any doubt, just that "we need something radically different" is not a very helpful take. You're basically saying "We just need another Einstein", except better data is harder and harder to come by. It's not sufficient anymore to just observe the perihelion shift of mercury. We now need to do things like measure the shapes of billions of galaxies to make any progress, and hope that the billions of galaxies we can observe (there is an upper bound) will yield sufficient precision to even tell two theories apart. We need to build gigantic particle accelerators and build humongous particle detectors in the antarctic to even hope to make some progress.

[bena]

It is very much a "if this, then that" situation.

We know how gravity works locally (for various definitions of local). We can calculate and predict with a lot of accuracy how objects in our immediate vicinity will behave.

All of the evidence is pointing to confirmation of our understanding of the situation.

Then this bullshit star three dozen light years away says "fuck that noise" and does something completely wild. Either we're wrong about all of our math or we missed something.

The "we missed something" crowd is the "dark energy"/"dark matter" people. And by "dark", they just mean "undetectable by current instruments". They believe our math is right. I believe the other crowd is the "Grand Unified Theory" people. That the math itself changes at scale.

And why is "85% of matter is undetectable at distance" not transformative to our assumptions? Do you realize the implications of that? That if 85% of what we should be able to perceive is imperceptible, then the "great silence" may be here sooner than we expect. Maybe that 85% is stuff that is just beyond the speed at which we'd be able to perceive it.

[sgregnt]

Dark matter and dark energy are different notions. The article is about the later.