I'm confused. This article says it's about a dark matter map, but it spends most of its time talking about accelerated expansion, the cause of which is labeled dark energy, not dark matter. Dark matter (I am given to understand -- IANACosmologist) acts on a much smaller scale to hold galaxies together which would come apart under the gravitational influence of visible matter alone. What am I missing?
Dark matter and dark energy are both fudge factors to explain gravitational observations that don't make sense. Dark matter corrects for the speed distribution of stars around galaxies and dark energy for cosmological expansion. The observations here collect indirect observations of dark matter via a different method and postulate that the discrepancies from what was predicted may obviate the need for dark energy. (Disclaimer: I am not an astrophysicist)
It's a fudge factor in that the distribution of dark matter is fit to explain any deviations from our predictions about the effects of gravity. Even if we were subtly wrong about how gravity works, we could probably find some dark matter distribution that matches our observations. So long as we are unable to independently observe dark matter, we can't be sure how much of it is actually caused by some different phenomenon.
Fudge factors - lovely. As one physicist put in a public lecture, if scientists use the term "dark" then they are discussing something that they have no clue about.
I don't see why there is such an agitated response to the term "fudge factor" (I see downvoted comments, several knee-jerk defensive replies, etc)
It's important to embrace one's ignorance -- especially when we're at the absolute frontier of human knowledge. If you've seen some of the calculations involving dark matter's effects, calling it a "fudge factor" is a fair characterization, and one that is acceptable among physicists. We postulate a component with certain properties that fudges our calculations in just the right way to match some indirect observations (circumstantial evidence).
We still don't know what is the thing which produces the behavior we expect, and what its properties might be. We're still seeking evidence that would seal the deal, so to speak. It's true that several such fudges historically turned out to be profound stepping stones, but till we get there, "fudge factor" is absolutely honest.
I'll leave some Richard Feynman quotes here:
* "The first principle is that you must not fool yourself — and you are the easiest person to fool."
* "I can live with doubt and uncertainty and not knowing. I think it is much more interesting to live not knowing than to have answers that might be wrong. If we will only allow that, as we progress, we remain unsure, we will leave opportunities for alternatives. We will not become enthusiastic for the fact, the knowledge, the absolute truth of the day, but remain always uncertain … In order to make progress, one must leave the door to the unknown ajar."
> We postulate a component with certain properties
versus
> We still don't know ... what its properties might be.
Which is it?
> stepping stones
Observation of missing momentum-energy in Lithium-6 decays (Pauli, 1930 [1]) until direct detection of the (electron anti-)neutrino (Reines and Cowan, 1956) took 26 years and a number of false starts including failures to detect neutrinos in detectors at nuclear weapon tests and nuclear reactors, where the production of neutrinos was expected. Observation of solar neutrinos took a further thirteen years (Davis, HOMESTAKE, 1969).
Neutrinos are in the strictest sense dark matter: they do not feel or participate in electromagnetism. However, they also have a tiny rest mass, so it is hard to keep them from carrying their momentum-energy away at relativistic speeds. Their fast motion makes them too "hot" to avoid smearing out visible matter during structure formation, so they are not a candidate for the momentum-energy missing in galaxy cluster interactions. On the other hand their fast motion makes it relatively easy to spot atomic recoils (or really the latter's ionization energy) when neutrinos interact with the matter in a detector. That's why detectors were placed at extremely violent neutrino-producing events: the hotter the neutrino, the easier to detect it.
The standard cosmology predicts a relic neutrino field comparable to the relic photon field called the Cosmic Microwave Background. These relic neutrinos were relativistic ("hot") in the early universe, but have cooled down adiabatically to about 2 kelvins, just as the hot photons in the CMB have cooled down to a bit under 3 kelvins. The Cosmic Neutrino Background is vital at high redshift (z > 3000) and there is good indirect evidence for it (BBN element abundance, suppressed early small-scale structure formation, damped acoustic oscillations of the CMB). However, it is extremely difficult to detect relic neutrinos with our current level of technology (although experiments like KATRIN will try: https://www.katrin.kit.edu/ and https://arxiv.org/abs/1304.5632 ), because the recoils from slow-moving neutrinos will be smaller than those from relativistic neutrinos. These relic neutrinos are literally Cold Dark Matter.
So there have been stepping stones on the road towards direct detection of a form of cold dark matter. It is not the cold dark matter working cosmologists or galaxy astrophysicists are looking for to explain galaxy-and-higher-scale momentum-energy anomalies (notably because its rest mass is too low), but it's suggestive of something more than "fudge".
There are options other than a heavy neutrino-like particle or even particle dark matter for galaxy-scale and even cluster-scale problems; indeed, a relativistic "left hand side" term in the Einstein Field Equations is plausible -- one would tend to treat that as a modification of gravity. However cold dark matter is needed to explain several fine features in the cosmic microwave background, and those features are ever sharper in the results of each generation of CMB observatory since BOOMERanG.
Finally, returning to the ~ 40 years between observation of anomalous results to direct detection for neutrinos: Vera Rubin et al.'s galaxy rotation curve paper was published in The Astrophysical Journal in 1980. So the long gap between the apparent need for dark matter and today does not seem so long compared to the long gap between the apparent need for a light spin-1/2 particle [1] and direct detection.
> "fudge factor" is absolutely honest
How exactly does this honesty scale work?
Apropos honesty and fudging things, who exactly is in each of the six instances of "we" (excluding the ones in the Feynman quotes) in your comment?
IMHO, we seek models explaining certain observed cosmological behavior.
As long as plausible models for dark matter range from WIMPs to primordial black holes or axions and what not, I consider it unreasonable to claim that we know what dark matter is, and all it’s properties.
Eg: if all dark matter experiments (direct and indirect detection) returned null results over the next several decades, then dark matter would still be an open puzzle.
With regards to “we”, I think it applies equally well to any group as small as physicists working on DM, to all scientists, or everyone discussing the subject. I expect most particle/astro physicists I’ve interacted with to find those statements very reasonable.
I can understand why some people might want to down vote the above comment. However, the comment by the physicist was made in a public lecture and as far as I can tell he was/is a supporter of the "dark" matter concept.
In thinking about the follow-up comment I made, I was pondering why people and scientists in particular get upset about others who object or show problems with their preferred models and theories.
In real terms, our understanding of the universe around us is extremely limited, as are our theories and models. Yet, it seems to me that we get so caught up in wanting to be right that we forget that we are on a path or voyage of discovery. There are so many instances in relatively recent history over various ideas, theories and models becoming anathema to the "consensus view" that we seem to have lost the excitement for discovery.
Over the decades, I have come across people who started out being excited about science but as the years passed, they grew disillusioned with it as they saw various goings on within their fields.
Instead of saying that it is okay for our theories and models to wrong and that anomalies should be investigated, it seems to be (at least with my experience) that the "party line" must be adhered to.
As I have said in other places, science and its methodologies are agnostic to your own personal belief systems. You can do science irrespective of whether you are and atheist, pagan, deist, communist, capitalist, moslem, buddhist, hindu, christian, jew, humanist or anything other specific bent.
The theories and models you derive may be based on your perspective, but the processes you use to develop them should be the same. How you view and resolve the data collected in your experiments will depend heavily on your basic viewpoint and often you will not challenge those assumptions and viewpoints. Others might though.
Science is a wonderful tool to discover more about our universe, but it is a tool that has its limits and limitations. Let's get back to making science exciting for everyone and that there is so much more to discover.
True that the history of physics has had many "fudge factors" but truths - mmmhhh.
I think what we have seen is that our explanations of how the universe works has changed and sometimes those changes have meant a radical change in our perspectives. However, truths are much more the domain of philosophy and religion.
Dark MATTER. We are reasonably sure it is matter because is has mass. It isnt totally unobserved. It is a thing causing an observable effect on other matter. In that sence it is no less real than the black holes that we only observe via thier effects on nearby matter.
A correction: based on the model that gravity is the only significant force in operation at the galactic and intergalactic level and that the stars within a galaxy aren't moving in accordance with the model, it is being assumed that there is some amount of unseen and unobservable something that is supposed to have mass and is causing the anomalous movement by the stars within the galaxy.
That is the correct presentation. No test yet has occurred to demonstrate that any such "dark" matter exists. That is, all experimental tests have come up nil.
If gravity (and it may be a very big if) is not the only significant force operating at galactic and intergalactic distances then "dark" matter is no longer required.
The assumption that gravity is the only significant force at these distances is just that, an assumption. It may be correct, but on the basis of the observable anomalous behaviours of stars, there is a good likelihood that the assumption is wrong.
There is nothing wrong with the assumption being wrong, it just means that we should be investigating other possible reasons for what is going on.
As best as I could tell (IANAC), the paper is looking at the distribution of dark matter halos and how it compares to predictions from our models of the growth and evolution of the large scale structure of the universe (the LCDM model). This model also encodes our limited understanding of expansion.
They found less dark matter halos than predicted (with large error bars, though), which means the LCDM model may need some work.
Suppose the gravitational attraction of particles, like spin, was signed, such that like attracts like, and unlike repels unlike? Because of the weakness of gravity, every other force would dominate at the local scale. But the prevalence of one or the other sign in a region (at the galactic scale) would vary - as there would have been a (very gentle, as gravity is weak) "sorting" in the early universe, as like signs attracted and unlike signs repelled. "Dark matter" would be variation in the prevalence of one sign or the other.
On the galactic scale, my hunch is that the sorting dynamic would be very strong, and you would basically have g+ and g- galaxies. Which would be very interesting, but doesn't solve the dark matter issue.
I think that this would also create a lot of other complex behavior that we don't see. For example, I think we would expect to see many cases of two galaxies or even two galaxy clusters pushing away from each other, because one is g+ and the other g-.
Expansion does have the appearance of "pushing away". But, as noted, to explain "dark matter", one would observe "anti-gravitational" separations in inert gases at local scale, which has never been reported.
Ah yeah nice idea. However, the expansion we see is stronger when things are farther away. Gravity gets weaker at a quadratic rate as distance increases, so I don't think opposite gravity signs could be contributing significantly to the observed expansion.
Excellent. "ordinary" matter would be a 70%/30% mix of gravitational types, with dark matter being a 99%/1% mix (figures obviously pulled from nowhere) - but then one would see effects at the local scale, where an observable percentage of helium atoms would exhibit "anti-gravitational" effects. Surely, that would have been previously noticed, thus the model is refuted.
The key constraints come from General Relativity; the successes of the Schwarzschild and Kerr(-Newman) metrics for physically plausible arrangements of matter need to be preserved if you try to add in a symmetry.
If we use perturbation theory to capture deviations from the metric of a reasonable exact solution to the Einstein Field Equations, the structures can be represented as classical waves in the metric tensor. Quantizing these waves ("perturbatively quantized gravity") inevitably results in a massless spin-2 particle. Spin-2 interactions have a feature you wrote yourself:
> that like attracts like, and unlike repels unlike
(maybe more clearly put as: like charges mutually attract and unlike charges mutally repel). Note that this is backwards from the spin-1/2 of fermions.
> "Dark matter" would be variation in the prevalence of one sign or the other.
In the standard cosmology, dark matter is attractive. Ordinary matter falls into areas where dark matter is dense, as discussed in the "Unprecedently wide and sharp dark matter" phys.org article, or if you prefer, http://www.esa.int/Our_Activities/Space_Science/Planck/Histo...
If you want a particle which interacts gravitationally with ordinary matter but which can carry a gravitational charge of opposite sign, you probably shouldn't call it "dark matter". However, you can certainly define such a thing with some rigour and test it.
> How would one test such a model?
Mathematically? After all, you need to generate reasoned (rather than guessed) observables that you can then look for.
You would probably do like Noether and Hilbert and solve the equations of motion of your matter theory producing the canonical stress-energy tensor, and then improve it using a metric variational procedure. Forger & Römer is a good place to look: https://arxiv.org/abs/hep-th/0307199
You can see how Hossenfelder does this in her anti-gravitation paper (direct link: https://arxiv.org/abs/gr-qc/0508013) in which her matter theory doubles the number of particles in the Standard Model, with the second set identical to the first except for the opposite gravitational charge); Forger & Römer is her reference [20].
Once you have a self-consistent and reasonably complete model you can consider areas where it generates identical observables to the standard cosmology where those are already being tested (the fine detail in the cosmic microwave background; gross structures like galaxies, clusters, and filaments; large-scale homogeneity and isotropy; the Hubble flow) and where your model makes predictions that differ from the standard cosmology.
This is funny, as I assume that spin is an effect of the force of gravity. Swinging and pendulum spin is just what happens in nonlinear dynamical systems. It's called ringing in sound engineering. You only need three bodies to simulate that. A binary pulsar will rotate. Crazy idea ... what if that's not a two star system of many neutrons, but just two neutrons moving really really fast, thus becoming heavy, bending space and thus appears to glow, because it act's as a lens for light.
