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Space Science

To Explain Away Dark Matter, Gravity Would Have To Be Really Weird (sciencemag.org) 198

Posted by EditorDavid from the gravity-of-the-situation dept.
To discard the theory of dark matter, "you'll need to replace it with something even more bizarre: a force of gravity that, at some distances, pulls massive objects together and, at other distances, pushes them apart." That's how Science magazine describes a new study, adding that "The analysis underscores how hard it is to explain away dark matter" — even though "after decades of trying, physicists haven't spotted particles of dark matter floating around." [T]o do away with dark matter, theorists would also need explain away its effects on much larger, cosmological scales. And that is much harder, argues Kris Pardo, a cosmologist at NASA's Jet Propulsion Laboratory, and David Spergel, a cosmologist at Princeton University. To make their case, they compare the distribution of ordinary matter in the early universe as revealed by measurements of the afterglow of the big bang — the cosmic microwave background (CMB) — with the distribution of the galaxies today....

Pardo and Spergel derived a mathematical function that describes how gravity would have had to work to get from the distribution of ordinary matter revealed by the CMB to the current distribution of the galaxies. They found something striking: That function must swing between positive and negative values, meaning gravity would be attractive at some length scales and repulsive at others, Pardo and Spergel report this week in Physical Review Letters. "And that's superweird," Pardo says...

In a paper posted in June to the preprint server arXiv, theoretical cosmologists Constantinos Skordis and Tom Zlosnik of the Czech Academy of Sciences present a dark matter-less theory of modified gravity they say jibes with CMB data. To do that, researchers add to a theory like general relativity an additional, tunable field called a scalar field. It has energy, and through Einstein's equivalence of mass and energy, it can behave like a form of mass. Set things up just right and at large spatial scales, the scalar field interacts only with itself and acts like dark matter...

Skordis's and Zlosnik's paper is "very exciting," Pardo says. But he notes that in some sense it merely replaces one mysterious thing — dark matter — with another — a carefully tuned scalar field. Given the complications, Pardo says, "dark matter is kind of the easier explanation."
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To Explain Away Dark Matter, Gravity Would Have To Be Really Weird More

To Explain Away Dark Matter, Gravity Would Have To Be Really Weird

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  • It sound's like the "really weird" theory of gravity they're proposing (bounds for) could explain away dark energy as well as dark matter. So it might be a simpler model. And it looks like ANY theory is going to need to be finely tuned to explain the results...including dark matter and dark energy.

    • Re: (Score:2)

      by Z00L00K ( 682162 )

      But have they considered the involvement of other forces? Like electric charges?

      • But have they considered the involvement of other forces? Like electric charges?

        [... insert "your mama" joke here ...]

      • But have they considered the involvement of other forces? Like electric charges?

        Seriously now, do you really think that no one has thought about the other long distance fundamental force?

      • But have they considered the involvement of other forces? Like electric charges?

        Yes. It isn't relevant.

        Electricity has none of the required properties, and doesn't interact with gravity much, other than the very slight amount of mass present in an electrons resting mass

      • Yes. We call it dark matter specifically because we KNOW it doesn't interact electrically. If it did, it would affect (and be affected by) light passing through it.
        Basically, at a human+ scales there's really only two forces, gravity, and electrostatics. And anything that interacts with electric fields is visible.

        Electrostatics includes all physical contact (electrostatic repulsion of the electron clouds around atoms), magnetism (which can supposedly be understood as electrostatics under the transformati

    • I was going to post pretty much exactly this.

      Between dark energy and ordinary gravity, two comoving objects in an otherwise empty universe will inertially drift toward each other if they're below a certain distance apart, and away from each other if they're above that distance apart. (And sit stationary at exactly that distance, with the attractive or repulsive forces ramping up continually as they get further from that exact distance apart).

      That sounds like it could very well be accounted for by a single f

    • At the turn of the nineteench century theorists were trying to explain away the uniformity of light velocity, independent of the observer. Then somebody came up with a really weird explanation called relativity which turned out to be correct.

    • Missing the point (Score:5, Insightful)

      by Roger W Moore ( 538166 ) on Sunday November 22, 2020 @04:08PM (#60754808) Journal

      It sound's like the "really weird" theory of gravity they're proposing...

      They are not proposing it: they are saying that if you get rid of Dark Matter you end up with a really weird theory of gravity. The point is that the theory is so weird that it is hard to believe it is at all possible in which case we need some form of Dark Matter along with a normal theory of gravity.

      It's like Schrodinger's cat. That was a thought experiment designed to show that the Copenhagen interpretation of Quantum Mechanics was seriously flawed. No physicist believes that the cat is in some weird superposition of alive+dead: the intent was that it was such an absurd idea that it would make people look for better explanations. Sadly, it seems to have failed and now those who do not know any better hold it up as a supposed example of how QM works!

      • Re: (Score:2)

        by MrL0G1C ( 867445 )

        IDK about dark energy but dark matter is to a degree proven, at least with regards to behaviour.

        " Dark matter and normal matter have been wrenched apart by the tremendous collision of two large clusters of galaxies. The discovery, using NASA's Chandra X-ray Observatory and other telescopes, gives direct evidence for the existence of dark matter. "

        https://www.nasa.gov/home/hqne... [nasa.gov]

        So, any weird gravity theory will have to explain this event.

        Maybe dark matter causes gravity as an emergent phenomenon from the c

      • Re: (Score:2)

        by vix86 ( 592763 )

        They are not proposing it: they are saying that if you get rid of Dark Matter you end up with a really weird theory of gravity.

        I don't know, I think I'd find "gravity gets weird at large distances" to be more realistically probable and worth exploring, than trying to find a ghost of a particle in my data that I invented because nothing else makes sense.

        It seems just as feasible to me that the gravitational field (in a quantum sense), does weird stuff at galactic distances. I sometimes joke with my more science minded friends that "dark matter" could just be a "rounding error" in the universe. Maybe this bonkers idea that gravity/th

        • The Big Bang theory says the universe expanded. When we look at distant objects/stuff we are looking into the past. Maybe that's why we get an expanding universe, due to looking back in time at an expanding universe.
  • Once the God denying materialistic evolution believing scientists assume there is some thing called gravity that pulls masses together. You can call it postulate instead of assumption to sound sciency. They can also call their echo chamber with enforcers peer-reviewed journals and editors, and the echo-chamber confobs as conferences and seminars. But, despite all this they are not even able to make their own absurd theories to agree with observations.

    But, after all these assumptions, sorry postulates produce absurd results like dark matter and dark energy, just make even more weird assumptions and be done with it. Based on logical reductio ad absurdum they should reject their original postulate that led to this absurd conclusion and accept the theory of intelligent falling. But they won't. They won't accept the tide goes in and out and the planets, the Sun and the Moon go around the Earth because God said so.

    • So far, most really weird assumptions like quantum mechanics and relativity have held up pretty well under close scrutiny, they match observed results to an ever closer degree, and have useful real-world applications. In contrast, the assumption that "god said so" has led to exactly zero useful results.
      • Look, science encourages original thinking, supposedly and allows anyone to question the orthodoxy.

        All we are asking for is equal access to the minds of small children at impressionable age to present our theory to too. Theory of gravity, theory of intelligent falling, both should be in the science text book and let the third grader can make an informed decision about which one to believe. At a minimum there should be disclaimers and warnings about the Theory of Gravity, The theory of gravitation is only

      • Re: (Score:2)

        by gtall ( 79522 )

        Uh, maybe you could give your satire detectors a tune up? They appear all tired and warn out.

      • Actually, most really weird assumptions turn out to be somewhere between wrong and stupid, heavily weighted toward the stupid end. You're just looking at the very, very few that actually work.

    • Re: (Score:2)

      by nagora ( 177841 )

      the Sun and the Moon go around the Earth because God said so.

      Which one?

  • Considering that we know very little about the most massive objects in the universe, it is very useful to posit some alternatives that can be experimentally explored. That said, there are a number of other points of evidence of dark matter that they will need to explain. In particular, we can see some galaxies where the dark matter is not distributed in the same way as ordinary matter.

  • This is probably going to sound like the rantings of someone who had too much pot (though I swear, I've never touched the stuff), but what if there's actually another force at play? Let's say in addition to gravity, all matter also emits a weak repulsive force that extends to much greater distances than gravity.

    Imagine a toy drone with a strong neodymium magnet glued to the bottom. At close enough range, the magnet will overpower the thrust from the propellers and stick it to a metal surface, but as long

    • Yes, people have been looking for such a thing for a while now especially after dark energy was detected.

    • That is how we explained what keeps atomic nuclei together. Electromagnetism means that protons repel each other and it drops of as an inverse as the square of the distance. The strong-nuclear force is much stronger but drops off much faster with distance. Force the protons close enough and the SN force dominates and they stick together.

      If a new force is responsible for the weird behavior of dark matter, then it would need to be a repulsive force that drops off faster than 1/r^2. So at large galaxy-scal

    • When I read "To discard the theory of dark matter, "you'll need to replace it with something even more bizarre: a force of gravity that, at some distances, pulls massive objects together and, at other distances, pushes them apart." I also immediately thought "why not two forces"? We've found formerly undetected forces before, why not again?

      I'm sure others have had the same thought, though, since they think about this a lot more than I do.

  • As there's not much direct evidence for dark matter, I don't understand this part: "[it is hard to] explain away dark matter". After all, dark matter is already the explaining away of observed discrepancies. So any alternative "explaining away" is equally legit, assuming it's no worse than dark matter, eg. doesn't require a lot of additional assumptions.

    • So any alternative "explaining away" is equally legit

      The problem is that, so far, all the alternatives fail to explain observed phenomena.

      For instance, MACHOs and/or micro primordial black holes would produce gravitational lensing that we don't see. Massive gas or dust clouds would be opaque.

      Every hypothesis other than WIMPs fails. The only problem with WIMPs is that we haven't detected them (yet).

    • They're trying to find an easier explanation than the current Dark Matter / Dark Energy theory. The observations of course remain the same. The idea only is that there may perhaps be a simpler and more intuitive explanation for what we observe. I don't dismiss such ideas as horrible or stupid, because it's always interesting to ponder new theories just for sport.

    • As there's not much direct evidence for dark matter,....

      Whoa pardner, stop right there. You are making a stunningly false statement to introduce the rest of your typing.

      The direct evidence for dark matter is overwhelming at this point. It consists of the observed behavior of matter under gravitational influence at cosmic, down to sub-galactic scales, and its seen everywhere in the Universe we look. There is no doubt that the observations are valid. The purpose of the TFA is to show that attempting to explain dark matter away by trying to propose some new gravity

      • The direct evidence for dark matter is overwhelming at this point. It consists of the observed behavior of matter under gravitational influence at cosmic, down to sub-galactic scales, and its seen everywhere in the Universe we look.

        That is indirect evidence for dark matter, by definition.

        Find some dark matter and we'll talk about direct evidence.

        Dark matter is real. We just don't know what it is made from.

        Capture some in a known volume, and then we'll know it's real.

  • There is no gravity (Score:3)

    by PPH ( 736903 ) on Sunday November 22, 2020 @03:13PM (#60754620)

    It's all just curved space. And we are just missing a few terms in the equation to describe that curvature.

    The classical (and overly simplistic) graphical representation of gravity in two dimensions is a membrane distorted by a mass. Objects 'fall into' the 'gravity wells' represented by this distortion. Well, what if space is like a water bed? Put a heavy object in the middle of it and the surrounding surface pushes up further away. And that upward rise represents an area of gravitational repulsion.

    Now, imaging that someone dropped a bowling ball on to the far end of the water bed. A really big waterbed. Waves would propagate (in the case of space, at the speed of light). Someone might have dropped a bowling ball onto space outside of our light cone (more than 13.8 billion light years away). And now we are riding on that very long wavelength slosh as the local universe rebounds. Resulting in weird gradients the cause of which are beyond our ability to sense.

    • "It's all just curved space. And we are just missing a few terms in the equation to describe that curvature."

      Umm ... ok stuff like that is really easy to say. Why should we believe it? I mean, it could be invisible pink unicorns instead.

    • That's identical to dark matter hypothesis, which also more or less "fixes" the problem by adding an extra term or few. Problem is, no matter what label you give your extra terms, if fudge factors are all you have then you haven't actually gained any extra insight to how reality works. And thus the problem remains unsolved.

      • Re: (Score:3)

        by robi5 ( 1261542 )

        At least in name, "dark matter" assumes more: something that's matter, and is also dark. Meanwhile the fractured, rippled, imperfect etc. spacetime just assumes that some kind of fabric is not even, which sounds like assuming _less_, because we removed the assumption of perfectly homogeneous spacetime. And all of us have seen wrinkled clothes, well, they don't iron themselves.

        Also, would we call black holes fudge factors too? After all, they don't emit much light. Aren't most observational tests that confir

        • Re: (Score:2)

          by dryeo ( 100693 )

          Dark as in the dark side of the Moon or darkest Africa? Dark has two meanings, lack of light and unknown. So unknown matter.

  • Cosmologists are in a pretty weird place with this. Their current models don't explain reality, so they need to find this mysterious, supposedly invisible matter to validate their theory, or come up with a theory that explains the model, or find some other missing piece that allows everything to fit together neatly.

    I've always thought the "dark matter" explanation was a bit too convenient, like an unknown constant added to an equation just to make it balance out. I've got a feeling there is some fundament

    • Re:Looking forward to an answer (Score:5, Insightful)

      by crunchygranola ( 1954152 ) on Sunday November 22, 2020 @04:49PM (#60754910)

      I have noticed that every single "dark matter skeptic" on /. shows no evidence of actually having done even minimal reading on the subject to find out what the observational evidence shows.

      • Re: (Score:3)

        by vix86 ( 592763 )

        I'd consider my self a dark matter skeptic. I do know that the body of evidence showing that "something is going on" is huge and I'm pretty sure I've heard astrophysicists on a couple of occasions state, in counter to naysayers on the topic, that "if it doesn't exist, then our fundamental understanding of physics is fucked." (I'm embellishing the paraphrasing a bit)

        But this story, while its meant to kind of counter the naysayers, hits exactly on the kind of thinking we really need more of in my opinion. Ins

    • Re: (Score:2)

      by SEE ( 7681 )

      supposedly invisible matter

      The only property that matter needs to be "invisible" in this case is to not interact with electromagnetic forces. And we already know of a type of matter -- neutrinos -- that has that very property (even if the known neutrinos for other reasons have to be ruled out as constituting "dark matter").

      Further, now we know for several reasons that the known neutrinos have mass, and under established particle physics that implies the existence of counterpart "sterile neutrinos". Under physics formulated prior to t

      • supposedly invisible matter

        The only property that matter needs to be "invisible" in this case is to not interact with electromagnetic forces. And we already know of a type of matter -- neutrinos -- that has that very property (even if the known neutrinos for other reasons have to be ruled out as constituting "dark matter").

        The existence of a kind of matter which does not interact with electromagnetic forces but can't be dark matter is a strike against the theory, though, since the only kind of matter we know of which could fit the bill... doesn't.

        What all that means is that you're living in a world where the existence of one type of "invisible" matter is well-established, and where the simplest explanation of the known properties of that type of "invisible" matter is a second species of "invisible" matter.

        It would be if not for neutrinos' behavior. Since the only kind of matter we know of which doesn't interact electromagnetically doesn't behave the way it would have to in order to be what we're looking for, all we really know is that we don't know how likely it is. If we find another

    • Re: (Score:2)

      by dryeo ( 100693 )

      You think that's bad, you should see the fudge factor they introduced to explain why the Sun shines. They came up with this stupid theory that elements could be transformed into a different element by fusing atoms together, releasing energy, basic alchemy. And to make their equations work, they had to invent a little tiny particle that was so small and non-interacting that one of these imaginary particles could travel through the Earth without hitting anything.
      Stupid cosmologists, always adding fudge factor

  • Oh, horse shit (Score:4, Insightful)

    by Aighearach ( 97333 ) on Sunday November 22, 2020 @03:16PM (#60754628)

    All you need is a tiny effect that we don't know about that is too small to detect locally, and since we don't have any sensors a few light years away we'd never even notice the phase error.

    We don't know what we don't know, there is no reason to think the thing we don't know about is actually related to gravity.

    When the prediction is wrong, it means the hypothesis is wrong. It does not mean the hypothesis is correct, but physics was different in the past, or that maybe a bunch of other variables not in our equation exist, and we can ignore them because we made a separate hypothesis that accounts for them (but only in the frequency domain, we don't even have time domain measurements).

    A bunch of morons trying really hard to fit their incorrect hypotheses to experiments, not because it is good science, but because they know that nobody in their lifetime is going to install sensors far enough apart to actually test any of this. And they really, really want to participate in this science anyway, even if means masturbating over a manuscript and calling it sciencey.

  • If it fits the data and makes useful predictions, why not?
  • A house-sized boulder, stuck in a steep hillside, doesn't move, where a 2 meter boulder rolls to the bottom of the valley. An ant, sitting atop either boulder, is only aware of his own locality, but not the hill or the valley. We know that gravity (space) is shaped by matter, but we assume that it is uniform (flat) everywhere and everywhen. If the shape of space, (the slope, if you will) differs from Newton even by one part in a million, how can we know what the effect on the shape of space of a super-m

    • Re: (Score:2)

      by ytene ( 4376651 )
      Not qualified to comment on this: just interested...

      Picking up on your observation, "we assume that it is uniform (flat) everywhere and everywhen."

      When we first began to look into distant space, our best understanding was that space was (forgive the simplification), "dark, empty and cold". Later, we discovered microwaves, which then in turn led to us to detect "interference" from them, which then in turn led us to discover the CMWB, which we've since figured out is basically the dying echoes of the Bi

  • Mike McCulloch's MiHsC proposes an interesting new formulation for intertia and gravitational attraction. His theory seems to predict galaxy rotations very well.

    http://physicsfromtheedge.blogspot.com/

    • However he has to explain all observational evidence at all scales as well or better than existing theory. Let us know when he does that, he will get a Nobel Prize then.

  • There is a famous quote from John Von Neumann , "With four parameters I can fit an elephant, and with five I can make him "wiggle his trunk [mpi-cbg.de] .
    Fermi gave a young Freeman Dyson a hard lesson by dismissing his theory out of hand with this reasoning. This model looks pretty much the same to me. Of course the context is a bit different, they don't hope that they found something, rather the opposite.

  • The Universe is not only weirder than we imagine, its weirder than we can imagine.

    (I know the original quote (from Haldane I think) used the word queerer instead but these days that would upset some of the 'differently gendered' people and their SJW supporters.)

    • Don't mangle the quote. It has meaning, and that meaning has not changed even if some people have adopted the term and others are offended by it.

  • "you'll need to replace it with something even more bizarre: a force of gravity that, at some distances, pulls massive objects together and, at other distances, pushes them apart." so a magnetic field, basically?

  • And look, Quantum Mechanics cannot deal with Gravity! Now if that is not a colossal hint, I do not know what is.

  • If acknowledging something we thought was a constant could actually be a variable explains our observations, that seems like a simpler explanation - even if it is "weird".

  • ! OK, you can mod me down as a troll, but...

    "would be attractive at some length scales and repulsive at others"

    Maybe call it the Trump Field Affect?

  • Changing direction of gravitational force depending on the distance is not strange. Strong force also changes direction depending on the distance and nobody is horrified by it. If their model is simpler than dark matter/energy then why not. It is just a model. The simplest model which fits the observations and keeps predictive power is the best.

  • Rmember that Occam's razor is actually a logical fallacy. Just because it is easier or simpler, doesn't mean it's more right.
    Its very badly expressed underlying kernel of truth is, that you should check for the simpler soutions first. Because that, on a statistical average, gets you the answer faster. (Though nature has no need to actually obey a certain statistical distribution in your lifetime.)

    In say that as somebody who currently favors dark matter over this scalar field explanation, mind you. So keep y

  • There is another experimental sign of gravitational repulsion: the Dipole Repeller [wikipedia.org].

  • Here's how I do it: Gravity doesn't exist. Gravity and Dark Energy are identical forces. Gravity is just the space pushing us against the planet, in a Casimir effect sort of way. I still can't explain dark matter, though...

  • Due to all the big bangs and collisions, the Space-Time Continuum is crumpled.

  • What if magnetism is what's pushing things apart? If the magnetic force is stronger than gravity (since like magnetic poles repel each other when they are facing each other), then massive objects will repel each other. When massive objects are closer together such that gravity is stronger than the repelling force of magnetism, they will crash into each other.

  • All these "gravity must be weird" or "there must be a lot of dark energy around" are based on slightly flimsy evendence of measurements on the expanding universe. Wild extrapolations and assumptions to come at the results. For "small" distances inside our galaxy we can use parallax measurements by taking pictures 0.5 years apart. The "viewpoint" has shifted by 200 million miles, so you can calculate how far things are.

    But beyond that there are assumptions that certain supernovae are always the same intensit

  • That idea of gravity both attracting and repelling is only weird because we haven't observed other forces behaving similarly. But we have no real reason to think that gravity should behave the same as the other forces.

    Let's not let our biases of what we consider weird be what discounts a theory. The evidence should be what rules it out.

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