Is dark matter’s “nightmare scenario” true?

https://bigthink.com/starts-with-a-bang/dark-matter-nightmare-scenario/

by spekcular • 4 years ago

123 177 4 years ago

tablespoon• 4 years ago

Honestly, I hope so. The universe isn't totally amenable to our inspection, and it'd be nice to have an example that just stared us in the face all the time.

aszantu• 4 years ago

I've been wondering for a while if light could be the thing that pushes away the boundaries of the universe and that's the reason for the expansion, if u can push a solar sail... anyways, I don't understand enough about this.

SomewhatLikely• 4 years ago

There are no known boundaries of the universe. The universe expands everywhere continuously. I think of it as though a little bit of extra volume is injected in between everything on a regular basis. Also when we look out as far back in time as we can, everything looks very similar, and there's no "boundary". Each year we can see a little bit farther out into the universe because those photons from billions of years ago finally got to us and so far things keep looking the same the farther out we can see. So as far as we know the universe is far larger than what we can see (what has had enough time to send us photons). I also don't understand enough about this.

https://en.wikipedia.org/wiki/Expansion_of_the_universe https://en.wikipedia.org/wiki/Observable_universe

wkearney99• 4 years ago

But at the same time we're also racing away. There is light from objects that will never get to us because we're moving away from it, due to expansion, faster than it'd ever get to us.

Eventually we'll have moved so far away from everything that we'll start seeing less and less (granted, "we" likely won't be around, our planet having long since been incinerated when Sol dies).

negativelambda3• 4 years ago

We can calculate the force exerted on an object by a photon of light of a given frequency. I'm guessing it is not sufficient to produce the effect you envisage, although I have not done the calculation. Interestingly this photon pressure does put an upper limit on the mass of a star; the Eddington Limit [1]. If a star is too large (and therefore luminous) its photon pressure pushes away other material and stops is from growing larger!

[1] https://en.wikipedia.org/wiki/Eddington_luminosity

stillworks• 4 years ago

Would recommend reading Chapter 3 "From Eternity to Here" by Sean Carroll (or maybe read the entire book) for a "light weight" introduction. If you don't have the time then TL;DR : universe is expanding means literally that, i.e it is almost as if new space is getting created rather than "cosmological scale objects" hurtling away from each other because they "gained momentum (if one can even call it that)" because of that big bang.

woleium• 4 years ago

If the title is a question, the answer is no.

someotherpersom• 4 years ago

That’s likely not the case here, but nice try with the trendy Reddit generalization

miika2• 4 years ago

Nightmare for who? How hard it is to admit that their equations are incorrect? Even amateur like myself can see that the dark matter is adhoc value that was calculated in order to patch the standard model. There are several better alternatives but do we need to wait until these stubborn believers have passed away until the world can move forward?

negativelambda3• 4 years ago

At the moment there are no better alternatives that satisfy all the observations. If you try to replace General Relativity with something like Modified Newtonian Dynamics (MOND) [1], then you can get good fits to galaxy rotation curves. However, there are many sources of astronomical/cosmological data that will not bend to alternative gravity models, e.g the precise fluctuations of the cosmic microwave background put stringent bound on the matter components of the Universe.

[1] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Ou...

rocqua6• 4 years ago

I vaguely recall hearing that there are galaxies "without any dark-matter". Or, more accurately, galaxies whose rotational speed is consistent with general relativity without needing to presume dark matter exists in those galaxies.

If that is correct, does it not immediately invalidate MOND?

dotancohen• 4 years ago

One funeral at a time, as the saying goes.

exmadscientist3• 4 years ago

There are many good reasons to think that dark matter is a good solution to certain cosmological problems. There are also a few good reasons to think that dark matter is total BS. It is a genuine open question. No theory in this area is a good fit for all available evidence.

This is not a case of "progress by funeral" like, say, the Alzheimer's situation also on today's front page. The linked article is a good summary.

sebzim4500• 4 years ago

Anecdotally it seems like, inside the scientific community, the dark matter skeptics are all in the older generation.

whoIsYou• 4 years ago

science advances one death at a time. not the only domain in which the current experts and holding things hostage

heisenzombie• 4 years ago

I think every physicist on the planet would love a nice elegant model of gravity that explains away “dark matter” observations. The next layer after Newton and Einstein. Clean and simple.

… but that’s just proven to be very very difficult. Evidence for dark matter comes from a wide range of observations, and while it’s pretty easy to come up with relativity replacements that could explain things like rotation curves nicely… other observations, from lensing and colliding galaxies like the bullet cluster are, just, well, messy.

mr_mitm8• 4 years ago

An amateur seeing something that almost no respectable cosmologist can see is a red flag. It's not that simple.

Pretty much the only alternative is MoND, and judging by the how well it fits the observations, it's not better.

boxed• 4 years ago

All modified gravity theories are falsified by astronomy as far as I know. There are galaxies out there without dark matter halos for example. Or where the dark matter halo is not centered around the core. This alone shuts down modified gravity.

ephimetheus3• 4 years ago

I guess the nightmare is more that it might be undetectable. There’s a real possibility that DM is real, but we hit the neutrino floor at some point without finding anything (maybe couplings are too weak).

Until there’s an actual convincing alternative, that indeed is not as apparent as you make it out to be, that would be pretty terrible for a lot of physics.

wruza• 4 years ago

Why isn’t it good for physics? It has found DM and can observe it via gravitational discrepancies, gets finally “solved” in this part. Does physics really need infinite modifications?

taylorius• 4 years ago

This is where I reference Mike McCulloch's theory Quantized Inertia, which predicts Galactic Rotation curves far more accurately than Relativity / Newton, without any fudge factors. (This is also where someone jumps in to inform me that he is "a crackpot", and should be ignored. FWIW QI seems like a promising theory to me, and I think it should be tested experimentally. )

mr_mitm8• 4 years ago

How accurately does it predict the CMB fluctuations? Or weak lensing observations? Rotation curves might have been first, but they're actually not the strongest evidence for DM.

titzer2• 4 years ago

dark matter is adhoc value that was calculated

Well the overall tone of your comment is pretty crummy but this is absolutely not true. You got this exactly backwards. Dark matter is just missing mass that is being observed. It doesn't patch any model, because physicists don't know what it is or what it is made of. But whatever that mass is, it is there, because all the standard ways that we observe mass, it exhibits. So it would instead be a leap to "patch the standard model"--whatever you mean by that--with these other "better alternatives" (I assume you mean MOND? No dice there, doesn't explain gravitational lensing).

furyofantares2• 4 years ago

Even amateur like myself can see that

Appealing to your own ignorance doesn't actually strengthen your argument.

miika2• 4 years ago

Maybe problem is they do consider Space to be empty. Maybe if they would take into account all those quantum fluctuations in there and add that energy maybe they would land at critical mass of the Universe.

To paint this idea with other example, if you were to figure out mass of a world, but the world was just an image reflecting off a surface of water, then they would focus only on the light particles coming off the surface and ignore the water. At some point they would come to realize that something big is missing. They could find out the value for how much it should be. That’s exactly the case with dark matter I’m quite sure. Space is like water they ignore.

But I don’t know anything, really (sorry to hear if that’s a problem for someone). Why there is possibility to comment on these articles anyways?

account-5• 4 years ago

Am I right in thinking no one has been able to detect dark matter despite decades of attempts and research?

My understanding is that based on observations of what can be seen galaxies move as if there's move stuff in them??

If you hypothesis that this extra stuff is dark matter but can't detect it, how do you know it's real?

How have you ruled out all the normal stuff that doesn't give off light or stuff we can't see at the distances we're looking over? Like planets, rocks, dust, etc. Couldn't that stuff, which to my mind there would be vastly more of than stars, make up the extra gravity?

I'm aware I might be talking complete shit because I'm completely ignorant of the detail here. I would love an understandable explanation as to why that's wrong.

negativelambda3• 4 years ago

Am I right in thinking no one has been able to detect dark matter despite decades of attempts and research?

Yup [1]

My understanding is that based on observations of what can be seen galaxies move as if there's move stuff in them??

Yup [2]

If you hypothesis that this extra stuff is dark matter but can't detect it, how do you know it's real?

It produces certain effects in various different enviromennts or domains, which lends more support to that hypothesis. By certain different domains, I mean like Galaxy and cluster dynamics, gravitational lensing around galaxies and clusters and the precise fluctuations seen in the cosmic microwave background, these are all very different regimes but all seem to support the hypothesis of extra mass which is not interacting electromagnetically.

How have you ruled out all the normal stuff that doesn't give off light or stuff we can't see at the distances we're looking over?

There are research efforts to look for exactly this kind of stuff; Massive compact halo object (MACHOS). However they don't see enough of it to account for the observations we see in galaxies and galaxy clusters. [3]

[1] https://lss.fnal.gov/archive/2022/conf/fermilab-conf-22-180-... [2] https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... [3] https://en.wikipedia.org/wiki/Massive_compact_halo_object#De...

exmadscientist3• 4 years ago

I hate to be that guy, but many of those points are exactly the subject of this article.

lkrubner• 4 years ago

Sort of, but gravitational lensing has been seen in places where there is no visible matter. If dark matter doesn't exist, then you'd have to come up with another explanation for the gravitation lensing:

https://kipac.stanford.edu/highlights/using-gravitational-le...

Beldin• 4 years ago

You're absolutely right and this is the one thing about dark mayer that doesn't sound like "just keep sprinkling it on your problem till it works."

I don't know enough about the found examples of gravitational lensing to know whether all possible arrangements of black holes that could have the same effect, have been considered and found lacking. But I presume so.

habibur• 4 years ago

Dark matter and dark energy are like what if there was no Einstein to discover relativity in early 1900s. We later continue to discover that gravity can't explain planetary motions correctly and therefore search for any explanation.

There are more laws of Physics waiting to be discovered. At least two big ones from what we currently observe.

superjan• 4 years ago

There is certainly new physics involved, the issue is more that for dark matter, the evidence points to matter that hardly, or perhaps not at all, interacts* with the stuff we are familiar with (standard model). It might be that the correct theory of dark matter is already found and debated, we just have no way to tell it from the false ones, as for all that we know, dark matter might be a shadow world of spaghetti monsters of which we only know its existence via their gravitation.

*) Except gravity.

throwawaymaths3• 4 years ago

the evidence points to matter that hardly, or perhaps not at all, interacts* with the stuff we are familiar with

I think this is why people get frustrated with the DM story. It's not the evidence that points to that. It's the lack of evidence that points to that. Specifically speaking, we keep hypothesizing limits on interaction in the hopes of finding it below some threshold but keep pushing those limits back. A multimodal observation would be clutch, but we keep confirming that a multimodal observation can't happen.

fsh• 4 years ago

You are cherry-picking one particular example. The neutrino and the planet Neptune were "dark matter" for decades until better detectors became available.

throwawaymaths3• 4 years ago

Ironically Pluto was found the same way that Neptune was, but it turned out to be a calculation error.

akomtu• 4 years ago

Call it stellar matter at least, for its influence on stars. Calling it dark only adds confusion.

febusravenga• 4 years ago

But the very definition of this phenomenon is something interacting with gravity (like matter) and not with EM... And EM in layman terms equals light, thus dark... It totally makes sense to call it "dark matter"

6nf3• 4 years ago

The author is implying that other solutions like Modified Gravity are dead in the water, I don't think they are.

jjgreen2• 4 years ago

The interested reader might find Roscoe's work on MOND worth a browse: https://www.researchgate.net/profile/David-Roscoe-3

6nf3• 4 years ago

Why did my comment get multiple downvotes, I can't work it out?

continuational• 4 years ago

There's a chance dark matter is black holes created in the aftermath of the Big Bang. Some of these then merged into the supermassive black holes we see in the center of galaxies today.

https://news.yale.edu/2021/12/16/black-holes-and-dark-matter...

scotty795• 4 years ago

I'm huge fan of that because in my view it requires least amount of making stuff up. So I probably never be convinced by any other explanation unless this one is solidly falsified for all possible blackhole masses.

titzer2• 4 years ago

Yet black holes do interact with ordinary matter--they can consume it--and they emit Hawking radiation. Dark matter does not appear to be doing either of those things.

choeger2• 4 years ago

My intuitive objection against dark matter is that it's basically an infinite amount of additional parameters to be fitted to the observations. Galaxy behaves weirdly? Add dark matter! It behaves as expected? It obviously has lost its dark matter! It stands to reason that for every possible observation, a particular DM configuration can be calculated.

I fail to see how dark matter could ever be falsified. The best potential replacement would be a theory that's simpler, but even then dark matter would probably explain the same observations.

The second question is: Is dark matter even matter if it only interacts via gravity? Wouldn't it be better named a property of space-time? That is, dark matter is a name for a recursive property of gravity?

namaria• 4 years ago

Unless you have worked extensively with the data and math of modern cosmology I very much doubt your 'intuition' about 'dark matter' is anywhere close to the intended meaning.

AnIdiotOnTheNet6• 4 years ago

Right? After quantum mechanics and relativity there's absolutely no reason to believe that our intuition is worth squat when it comes to how physics works.

doliveira4• 4 years ago

Dark matter is the simplest theory.

TEP_Kim_Il_Sung2• 4 years ago

*hypothesis

Theory implies they found out what it is.

mr_mitm8• 4 years ago

There is only a handful of parameters. They describe the probability distribution of finding one particular matter configuration. We are simply observing one realization of that probability distribution, in which we should be able to calculate the percentage of galaxies with an unusual amount of dark matter for any definition of "unusual". We don't consider the position and momentum of every baryon a parameter of the standard model either, so why should we do so with DM?

DM can be falsified perfectly well. If the rotation curves of all observed galaxies behaved as predicted by Newton, there would be no DM and every reasonable physicist would consider DM falsified. DM simply passes a lot of falsification experiments.

If DM interacts purely gravitationally, then yes, considering it either matter or an artifact of space-time is merely a matter of preference. Simply move the DM part of the energy-stress tensor on the other side of the Einstein equation.

Differentiating it from MoND or similar theories will then be based purely on how well the theory fits.

However, while I'm only a cosmologist and not a particle physicist, I find the implied assertion by proponents of MoND, that the existence of non-interacting particles is impossible, questionable. Why shouldn't there be heavy particles that don't interact electromagnetically, like a heavy neutrino? Or particles that don't even interact weakly? Or at least, why should their abundance be low compared to baryons?

Edit: Regarding falsification of DM, it's not like DM isn't challenged anywhere. Like I mentioned above, we should be able to compute the abundance of galaxies of certain sizes (dark or not), and the dwarf galaxies are coming up short. It's called the dwarf galaxy problem and poses indeed a challenge to DM, albeit not as much as DM alternatives in the mind of most cosmologists. They think it will be solved eventually as our understanding of galaxy formation improves, but who knows, maybe it's the first glimpse at something bigger. https://en.wikipedia.org/wiki/Dwarf_galaxy_problem

rocqua6• 4 years ago

This seems like a very nicely detailed response to OPs question. Thanks! Your response brought up some questions and remarks from myself.

DM can be falsified perfectly well.

I recall hearing that DM was attempted to be falsified based on gravitational lensing. Is that correct?

If DM interacts purely gravitationally, then yes, considering it either matter or an artifact of space-time is merely a matter of preference. Simply move the DM part of the energy-stress tensor on the other side of the Einstein equation.

Would DM as matter not have inertia, whereas treating it like an artifact of space-time would leave it without inertia?

Differentiating it from MoND or similar theories will then be based purely on how well the theory fits.

A simple question here would be: do all galaxies of similar size have the same amount of dark matter. If so, then the uniformity of dark-matter would need explanation. If not, then I think something like MoND would have a hard time explaining why these similar galaxies rotate at different speeds.

naasking13• 4 years ago

However, while I'm only a cosmologist and not a particle physicist, I find the implied assertion by proponents of MoND, that the existence of non-interacting particles is impossible, questionable.

I think this is a strawman. MOND proponents don't discount that some observations are better suited to a particle, they just assert that the particle dark matter sledgehammer is vastly overused, and that modified gravity has been unusually successful in cases where it had no right to be if gravity acts as it does under GR.

For instance, this review [1] of the astrophysical evidence by a MOND proponent makes a good case for MOND + sterile neutrinos. This review also covers numerous falsifications of DM, so if you're correct that DM can be falsified, why did these falsifications fail to convince the majority?

[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/htm

treis4• 4 years ago

If DM interacts purely gravitationally

If it's purely gravitational then that implies infinite density and infinite force for any matter that occupies the exact same space.

antonvs• 4 years ago

Dark matter is not a “recursive property of gravity” because it’s not consistent like that, as your own initial objection suggests.

The concept of a particle that only interacts via gravity is straightforward. Consider neutrinos, which only interact via gravity and the weak force. Trillions of them pass through your body every second. Take away the weak force interaction and add some mass, and you have a dark matter candidate particle.

I fail to see how dark matter could ever be falsified.

You’d have to explain what should be done about that. It could nevertheless be a correct theory, and we might one day verify it more directly.

Naive falsificationism is an almost useless principle at the frontiers of modern physics. We’re long past the days when you could run some simple experiments and come up with a clear yea or nay. That’s living in the 19th century, basically - which admittedly, many people still seem to want to do.

choeger2• 4 years ago

You’d have to explain what should be done about that.

Well, there should be something that follows from the assumption that there are huge amounts of particles that only interact via gravity. What's the prediction that this model makes? If it doesn't make any useful prediction, it's pretty much worthless.

That’s living in the 19th century, basically - which admittedly, many people still seem to want to do.

I think this attitude is inappropriate. The scientific progress made with 19th century methods, and that includes most of the stuff discovered until the 1950s, is massive. Maybe scientists should focus more on a clear nay or yea? I mean, the LHC is certainly a significant achievement, but compared to the detection of gravitational waves or even just exoplanets it doesn't seem to deliver as much bang for the buck.

onli• 4 years ago

If it is not falsifiable, it is not science nor physics. It is meta-physics, ersatz-religion. Which is exactly the core of the complaint against concepts like this.

Of course same thinking can't easily be applied to things that are not always true, like social behaviour. Part of the reason the replication crisis hit so hard in the social sciences.

And of course this is not really as absolute as I wrote above. No field works like this all the time. But it is a good guideline. If falsifiability is completely ignored, then a concept really is nothing more than a religious belief and should be considered as such.

BTW, Popper did not publish in the 19th century.

donatj• 4 years ago

Naive falsificationism is an almost useless principle at the frontiers of modern physics. We’re long past the days when you could run some simple experiments and come up with a clear yea or nay. That’s living in the 19th century, basically - which admittedly, many people still seem to want to do.

To a layman like myself this implies both that the things being studied then are both unprovable and not potentially of practical use or significance. I certainly am not saying that is the case, but rather am looking for more informed opinions to the contrary.

The cynical side of me feels like study of something unfalsifiable is as the study of religion. A poor investment for public funds.

As I mentioned however, I am seeking opinions here to the contrary of my gut reaction as my gut is often dumb and reactionary.

oneoneonetwo2• 4 years ago

which admittedly, many people still seem to want to do.

This occurs because most science is not the "the frontiers of modern physics". But even then, falsification in the frontiers of modern physics is still a useful tool.

nonrandomstring• 4 years ago

"Dark Matter" defies parsimony. Occam's Razor suggests rejecting the assumption that spacetime is uniform. The search for "Laws" may be a parochialism built into human epistemology. Advanced aliens may be comfortable with the idea of "space weather" and local variations. :)

rocqua6• 4 years ago

I very much agree with you, and have raised these questions before. I got some interesting responses that half-mollified my worries.

Originally dark matter was hypothesized because of galaxy rotation mismatches. This allows for mapping where 'dark matter' should be. One falsification of dark matter that was tried is gravitational lensing. If there is indeed extra curvature of space-time in these galaxies, then they should bend light-rays, Potentially showing the same galaxy twice.

This was tested, and confirmed. Hypothesized dark matter, as detected by galaxy rotation, causes the expected gravitational lensing.

I suppose another potential falsification is by 'moving' dark matter through a gravitational tractor (very heavy mass with rocket engines) and then detect the resultant change in gravity. You could attempt that detection by gravitational lensing, orbit velocity, or perhaps something else.

Despite all this, dark matter still feels like our theory of gravity was falsified, and instead of concluding the theory was wrong, it was attempted to add random parameters to the theory. Its not like there is a better theory though (I believe MONS has trouble with explaining gravitational lensing), so perhaps this is a problem of PR much more than a problem of science.

As for the second question, for dark matter to be matter, I think it suffices if it has mass and inertia. My gravity tractor detection idea tries to work with that.

Edit: also see this: https://medium.com/starts-with-a-bang/7-independent-pieces-o... from a different reply in this HN discussion.

scott_w• 4 years ago

My intuitive objection to your comment is that it ignores the basis of the article: that it’s entirely about learning more about dark matter. This is the complete antithesis to what your comment claims.

sega_sai2• 4 years ago

You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy. Instead you 'just' need to add a weakly interacting particle with the right density (30% of critical). That particle needs to be 'slow enough', i.e. non-relativistic. After that it turns out that the evolution of the universe basically reproduces the vast array of observations.

* The right primordial abundance of different elements

* The amount of structure in the cosmic microwave background.

* The rotation speeds/velocity dispersions in large number of galaxies

* Large scale structure/The amount of clustering of galaxies.

* Gravitational lensing observations.

And much much more.

It is certainly true that there are individual cases that look challenging for Lambda CDM paradigm (CDM stands for Cold Dark Matter), but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM. (there are currently some tensions with the Hubble constant measurements and some other cosmological parameters between different techniques, and it remains to to be seen whether those are systematic errors or point towards some modification in the theory).

Comment on the article itself. I think at some point I would be fine with dark matter being essentially a computing trick, i.e. if the behaviour of the universe is well reproduced when we add gravitationally interacting but not interacting otherwise matter. It would be up to theorists to see if somehow that can be written as some sort of extra-term in Einsten's Equations and show we don't really need an actual particle. I think there is a possibility of that, but personally I still think it's more likely there is a particle responsible for dark matter.

naasking13• 4 years ago

You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy.

No, he's correct that each galaxy does need tuning. Specifically, the amount of DM and it's specific distribution is tuned to match each observation, which is what the OP said.

but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM.

I think this "strong case" for LCDM is often oversold. See this review of the evidence for LCDM compared to modified gravity:

From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm

_a_a_a_• 4 years ago

Where does "infinite amount of additional parameters" come from? It's just one parameter - something's (we call dark matter) is either there or it isn't.

I fail to see how dark matter could ever be falsified

I don't think science cares. Frankly if you could back up your opinion with some actual reasoned argument, it would be a better post.

mhh__• 4 years ago

infinite amount of additional parameters

Not in any technical sense I can think of.

doliveira4• 4 years ago

Why do people in the comments always ignore that dark matter has way more evidence for it than just galaxy rotation curves (something very well explained in the article)? It's quite tiresome to read the same objections as if physicists somehow hadn't thought of them.

scotty795• 4 years ago

Do you know of any good derivation of existence of dark matter a layman could follow through and be convinced?

ctchocula• 4 years ago

This is a good write-up that justifies dark matter using 7 independent pieces of evidence: https://medium.com/starts-with-a-bang/7-independent-pieces-o...

pja• 4 years ago

You could read through the relevant Wikipedia pages for an overview: https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...

kryptiskt• 4 years ago

"The Road to Precision Cosmology" is a good summary of how we ended up with the current theories: https://arxiv.org/abs/2201.04741

mr_mitm8• 4 years ago

Probably because it's the only piece of evidence you can understand with just high school physics. Most laypeople have trouble understanding that the universe is not expanding from a single point, so how would they be able to understand what the cosmic background radiation is, much less why its angular power spectrum has peaks at particular positions?

It's a common pattern with crackpots as well, that they focus on "big" but easy to understand things like special relativity, which doesn't involve more math than you learn in high school. No crackpot ever revolutionizes condensed matter physics or statistical mechanics.

Although the bullet cluster should be a clear indicator as well, the rest of the evidence of DM appears much more subtle to a layperson.

Anyway, cosmology is a weird topic which not only HN reveals tons of arrogance over. Lots of misplaced skepticism. (Probably similar to the Covid topic.)

MrScruff• 4 years ago

Yes exactly. We've all experienced reading an account on our particular domain of expertise by a journalist and realising they have absolutely no clue what they're talking about. I find it amazing how presumably otherwise intelligent people reading Hacker News don't see the issue with offering lay opinions on Cosmology of all topics.

BrandoElFollito• 4 years ago

No crackpot ever revolutionizes condensed matter physics or statistical mechanics

Well, the memory of water still has a lot of support, as the scientific explanation for homeopathy. Lots and lots of crackpots here.

I would not call this crackpot, but it took an awful amount of time for some to let go cold fusion.

As for statistics - my personal hell is when I have to read some biology or biology-like document and there are numbers that are tortured to make them sing the way they are expected to. I do not even talk about economy, psychology, sociology and other similar ologies.

mabbo• 4 years ago

Anyway, cosmology is a weird topic which not only HN reveals tons of arrogance over. Lots of misplaced skepticism. (Probably similar to the Covid topic.)

It's the curse of being clever. People believe that since they're clever and were able to be successful at what they do, it must be that all other topics are similarly easy. And with easy access (via the internet) to the most basic introductions for lay people to any topic, the Dunning Kruger effect really kicks in.

The more clever you are, the easier it is to fall into this trap. example: "Buy twitter and solve all its problems myself".

CuriouslyC• 4 years ago

Crackpots emerge when a conceptual model is weak. They don't care how strong a mathematical model is, which is why they ignore SM and other areas where "theory" literally means a mathematical model, and nothing else. Having a lot of crackpots proposing bad theories is a sign that your conceptual model is garbage, just a bad attempt to hand wave a bunch of math that happens to predict reality, and you should stop pretending.

There are infinitely many mathematical systems that could describe "reality" all with divergent behavior in the parts of the system we haven't observed yet. Trying to make authoritative claims by analogy to coincidentally correspondent mathematical systems is foolhardy and vain.

Ygg2• 4 years ago

Because dark matter is too convenient, and it's not like LCDM prediction are 100% on the money.

http://backreaction.blogspot.com/2017/01/the-bullet-cluster-...

Another big problem is that all LCDM predictions (MACHO/WIMP) were disproven but it still remains a theory. Now we need energy too high to use in universe.

It's eerily similar to Phlogiston theory that at one point was considered to have negative mass.

pfdietz2• 4 years ago

MACHO/WIMP was one way to implement LCDM, but not a requirement for LCDM.

rocqua6• 4 years ago

Because it is the only evidence that is presented, and lay-people can easily come up with objections. These people then express their frustration or confusion why some of these objections seem completely unaddressed.

These people are effectively asking about these objections. Perhaps many of them are phrasing it differently. But, with exceptions, I think people tend not to be angry or vitriolic. At best they are frustrated. Just treat these remarks as questions.

It might be annoying to get the same questions repeatedly, but that is a fault of wider communication about dark-matter.

hsn915• 4 years ago

Maybe you could have bothered to list some of this "evidence" instead of venting that no one has heard of it?

furyofantares2• 4 years ago

It's listed in the article.

lamontcg• 4 years ago

It's quite tiresome to read the same objections as if physicists somehow hadn't thought of them.

Okay now do climate change and climatologists.

And most people just don't bother to read the article at all it seems. No better than reddit.

Changing gears, though, and trying to be more useful, if you want more coverage on this topic there's a note in Peter Woit's blog:

https://www.math.columbia.edu/~woit/wordpress/?p=13232

Points to another blog which attacks the "nightmare scenario":

https://tritonstation.com/2022/11/29/the-angel-particle/

I don't particularly buy that a gravity-only interacting particle "is not physics". If that is what exists, that is what exists. And we've already detected it via dark matter observations. So it is physics. What we can't do is ever really prove it other than by exclusion, but if we racked up a thousand years of failure, we'd have to accept that it was real (hopefully by then not dogmatically and we'd let any kid who wanted to tilt at windmill to go for it).

And Woit links to a paper on dark matter being only a right handed majorana neutrino, requiring no real physics beyond the standard model (plus three right handed non-interacting neutrinos): https://arxiv.org/abs/1803.08930

And links to a comment on that in John Baez's blog:

https://golem.ph.utexas.edu/category/2022/12/neutrino_dark_m...

credit_guy4• 4 years ago

Here. I'll come with a different objection.

Everything we see in the Universe outside our own galaxy is static. We don't have enough parallax to actually perceive objects (stars) moving outside our galaxy. Ok, there's an exception, people were able to perceive the movement of one star in one of our satellite galaxies in 2012. That's about it. So, when people talk about galaxy rotation curves, that's based on a sample of one, our own galaxy. Actually, we can plot galaxy rotation curves for other galaxies, but based on indirect evidence, such as the redshift. How accurate is that? Come to think of it, we can't estimate directly the distance to any galaxy beyond and including the Andromeda galaxy. But again, we can estimate indirectly. Only that estimation is based on a stack of estimations each with its own uncertainties.

Now, if you work in the area, stop me right here and tell me you went through that stack of estimates, and check all the uncertainties, and are confident there's no potential error anywhere.

Because, as an outsider, if I do a bit of Bayesian estimation, and I have to choose between the dark matter and the possibility that everything that can be explained with dark matter is actually a result of estimation errors, I'm somewhat on the fence. Every year that the hunt for dark matter brings nothing I'm more inclined to think there's a different explanation out there. Like the frame dragging explanation discussed in [1].

[1] https://physics.stackexchange.com/questions/618981/has-frame...

doliveira4• 4 years ago

I'm sorry, but misplaced confidence like yours is the tiresome part. I think you should also apply Bayesian thinking to the whole debate as well.

I'm not a cosmologist, but I have a degree in Physics. I won't be able to fill in the details, but all your objections sound pretty basic and unfounded even to my semi-amateur ears. About the cosmic distance ladder (as if parallax were the only way. I guess we don't see Earth's curvature either), about how to propagate errors (literally learnt in the 1st week of Physics. Those bars around the scatter points are literally the error bars, and we propagate them from start to finish). And even about the scientific method itself. No one can claim "there's no potential error anywhere" in anything.

And most importantly, which was my original main point, galaxy rotation curves are not our only evidence.

BTW, I would encourage you to look at how fossils are dated. To me it sounds kind of similar in spirit to how distances are measured in astronomical scales.

raattgift3• 4 years ago

I'm encouraged to reply by your reference to a (undergraduate cosmology) textbook in your more recent comment in this thread. I'd suggest borrowing (or taking advantage of a free-as-in-beer Internet copy of) the most recent edition of Binney & Tremaine's _Galactic Dynamics_, a standard textbook for graduates (and a good reference for researchers) that explores in detail how galaxy mass distributions are calculated. FWIW Binney has from time to time explored and even embraced MONDian ideas, and this is captured in his publication record.

In their textbook you'll find that galaxy rotation curve studies are spectroscopic. That is they are keenly interested in the relative redshift in the 21 cm neutral atomic hydrogen lines (among others like H2 and CO molecular lines) at the limbs of edge-on disc galaxies, and adapting that to spirals and other disc galaxies that are tilted away from edge on. [Binney 2e sec 6.1]. Practically invariably the relevant lines are relatively redshifted and relatively blueshifted at the limbs, leading to the interpretation that generically there is equatorial spin in disc galaxies.

In elliptical galaxies there is practically no equatorial spin to speak of; instead the spread of relative redshift across the face of the elliptical is interpreted as blobs of hydrogen gas moving radially, that is sinking deeper into the galaxy or rising out of the galaxy's depths. It is also useful to study a wide range of absorption lines given that the clouds are backlit by a galaxy's worth of starlight (and sometimes a quasar), in a process which grinds out surface densities.

These relative redshifts do not depend on cosmological redshift (the whole galaxy, or its whole cluster, is cosmologically redshifted identically for all practical purposes, so the opposite limbs in discs are affected similarly). It is also not sensitive to an isolated galaxy's peculiar motion within a cluster. It may matter for merging galaxies.

We can also look deeper than disc limbs and ellptical surfaces. Optical interferometry is highly sensitive in this application, and provides direct evidence of the motion of the various sources of emission and absorption lines from various gas clouds, dusts, and even starlight. The Large Binocular Telescope does some work in this area. Radio interferometry is useful for looking into the bulk motions within the more central regions of galaxies and clusters; dust obscures optical signals but millimetre signals cut through.

The investigated starlight is bulk and while the interpretation depends on assumptions about the bulk stellar chemistry of an observational target (maybe metallicity varies slightly in different parts of an elliptical which might have a history of galaxy mergers) there is in no way a dependence upon any single star and its meanderings through its galaxy. We're interested in the light generated by ~billions of stars, not the positions or momenta of single stars, mostly because we just cannot resolve the latter with current technology.

as an outsider

You seem interested in the topic. Learning how observations are made (and the history of them) is probably not inaccessible to you given your comments here. Whether that leads you into any sort of conclusions about the structure and evolution of galaxies is up to you, but I think textbooks will be better for you than whatever ultimately led you to the stackexchange link in your comment. (I did notice however that the Ciotti preprint discussed later in your link cites Binney & Tremaine multiple times, as does the Ludwig paper in the stackexchange question).

scotty795• 4 years ago

How small a black hole would have to be to be completely undetectable by us even when it flies through the solar system?

LatteLazy2• 4 years ago

The minimum mass for a black hole is about 3 times the size of the sun. So we would definitely detect such an object. In fact it would have pretty catastrophic consequences if it entered the solar system...

mr_mitm8• 4 years ago

You mean stellar black holes, right? Primordial black holes could be much smaller, if they exist.

I just looked up what would happen in that case, and it sounds really bizarre:

Nevertheless, a PBH would have a notable local impact, Rahvar notes. “The passage of a black hole could melt a cylinder along the interior of the Earth with a radius of almost 10 cm. After a short time, this tunnel would freeze,” he explains. In theory, this would produce a distinct metamorphic fingerprint of the passage in the rock record. But given that we might expect only eight of these to have been produced in the Earth’s lifetime (one on each side of the planet per collision episode), “I would expect that such a geological trace is difficult to detect,” Rahvar concedes.

https://physicsworld.com/a/concerning-primordial-black-holes...

scotty795• 4 years ago

I did not mean stellar black holes.

atemerev• 4 years ago

“Gravitationally only interacting dark matter” is sort of synonymous with “non-homogeneous spacetime”, or am I missing something?

Ah, yes, we also need to have some sort of bound energy, to balance Einstein’s equations. So, geons! https://en.wikipedia.org/wiki/Geon_(physics)

Doorstep2077• 4 years ago

Always been interested in physics since I was a kid, so seeing this kind of stuff with dark matter and being able to understand how it'd be used in practical applications is fascinating to me.

eisvogel• 4 years ago

The only real nightmare is that a couple of Jesuits, the same guys who imprisoned Galileo for daring to suggest that the Earth revolves around the sun, came up with the Big Bang theory to explain cosmological redshift, thereby carving out another century of confusion and creationist psychosis under the psychological rule of their lawful-evil overlords in the vatican.

hilbert42• 4 years ago

"There are more things in Heaven and Earth, Horatio, than are dreamt of in your philosophy."

Hamlet

William Shakespeare

tlogan• 4 years ago

If mass of dark matter is created by Higg effect, does it mean dark matter particle needs to interact via weak force?

AnIdiotOnTheNet6• 4 years ago

I find it very interesting that every time Dark Matter, a theory that actual physicists largely find very convincing, comes up a bunch of non-physicist HNers come out to explain why it doesn't make sense.

plasticchris• 4 years ago

It feels like this is implying that questioning is unscientific? I mean, I get being annoyed with laypeople but if you didn’t question the scientific consensus the historic record and the philosophy of science are against you.

AnIdiotOnTheNet6• 4 years ago

It's more the level of confidence they seem to have in their take, given their relative lack of understanding of the subject.

TillE• 4 years ago

HN is big into contrarianism across a wide variety of topics.

sylware2• 4 years ago

Isn't there a modified MOND which fits all the observation data?

pfdietz2• 4 years ago

I don't believe so. If it did exist, it would be more complex than LCDM.

6nf3• 4 years ago

Some people like Sabine Hossenfelder disagree. They think MOND could be a better fit for observations than Dark Matter.

puffoflogic• 4 years ago

This isn't evidence against dark matter's existence,

If the repeated failure to find tangible evidence of something in the physical universe isn't evidence against its existence, then you're no longer doing science, but rather playing some religious game

graderjs• 4 years ago

Ugh..this writing style, so unreadable.

I think the first paragraph could be rewritten better as:

"Dark matter puzzles us. Einstein's theory of gravity, plus known matter and radiation in the Universe, including particles and antiparticles described by the Standard Model, cannot explain cosmic observations. We need an additional gravity source - dark matter."

api2• 4 years ago

I’m sure physicists have already thought about this but…

Has anyone considered modeling the effect currently explained via dark matter as a new kind of force or field? Like a distinct force generated by mass and its relationship to space time?

This wouldn’t be modified gravity but a proposal for an entirely new force whose effect is what we currently explain with the “fudge factor” of dark matter.

From the rough picture I do have of dark matter this force would be one whose relationship to distance is different from the usual inverse square law that governs forces. That would be weird but maybe no weirder than particles that can never be detected or MOND.

mr_mitm8• 4 years ago

Someone has: https://en.wikipedia.org/wiki/Self-interacting_dark_matter

They even think about dark chemistry: https://iopscience.iop.org/article/10.3847/1538-4357/ac75ef

Didn't have much support last time I checked.

igiveup• 4 years ago

If the unmodified gravity and this new force always oocur together, why not just call them one force? What is it that makes them "distinct"?

api2• 4 years ago

Do we know they always occur together?

SAI_Peregrinus• 4 years ago

"Dark matter" implies a new field. Or rather, a new field or force that affects gravity implies some sort of interaction exactly like a massive particle that interacts only gravitationally. Particles are just localized disturbances in fields (in quantum field theory), and new fields can have such disturbances and thus have particles. In this (trivial) sense, MOND is a type of "dark matter" theory that posits a particle with only gravitational interactions.

Of course we don't have a quantum field theory for gravity, so the above is hand-waving away a HUGE issue of what that new field would be like, how its particles would act, etc, since we don't even know what the quantum gravitational field is like and what its particles are like (assuming such a thing even makes sense).

thewatcher23• 4 years ago

there's no such thing as dark matter

rapjr9• 4 years ago

Reading this I realized that dark matter is starting to sound somewhat like the idea of the "ether". It's everywhere, it affects everything, but we can't see it or interact with it easily. It's somewhat different than the idea of the ether in that it is not thought to be a homogenous, pervasive field; instead it clumps around gravity. But it has some very ether-like aspects, maybe some we have not discovered yet. I always thought we abandoned the idea of an ether too soon. Why does it have to be evenly spread? Why does it have to respond to things like direction or EM waves? Maybe there are some simple experiments that could explore the idea of an ether/dark matter further. Pure speculation but that is my intuition.

Is dark matter’s “nightmare scenario” true?

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