>Hajdukovic’s paper on a dark matter alternative is also an attempt to understand cosmological phenomena without assuming the existence of unknown forms of matter and energy, or of unknown mechanisms for inflation and matter-antimatter asymmetry.
so he brings in unknown negative gravitational charge. Basically giving another name to unknown machinery behind the observed phenomena.
I've always cringed at the name "dark matter". There is a discrepancy between what our models predict and what is observed and so we pick one of the possible explanations (there's some matter there that we can't see) and name it that. Then all of the discovery channel scientists try to explain what dark matter is to the general public by invoking this hidden matter theory.
I think it would be much better if we referred to the gravitational pull discrepancy as the problem itself and called "dark matter" just one of the possible explanations.
Basically, there are some knowns:
The universe expands according to the fried,am equation.
Different percentages of matter, energy give rise to different expansion rates.
Expansion history of the universe is known via looking at supernova (which are equally bright at the source).
Knowing matter is mostly hydrogen/helium and energ is mostly photons allows us to calculate the density of matter/energy in the universe. These don't match the terms found from measuring supernova leading the differences to be called dark matter/dark energy.
Additionally, gravitational lensing is seen by objects which can't be seen. Meaning, they have mass, but don't appear to absorb/reflect light. Additional additionlly, similar effects are seen affecting the formation of galaxies: extra matter is needed to give the structure we see, but we apparently can't see that matter.
So it was originally dark because we knew nothing about it, but the name stuck once it appeared we could not see it.
It is definitely matter.
The whole point of the article that we're commenting on is that it might not be matter. The root problem is that the observed doesn't line up with the expected. Is the problem with the observed or with the expected? We don't know. You're assuming the problem is with the observed. This article is saying we can modify our expected values with a tweak to the model.
We should name the phenomenon after the problem, not after one of the not-yet-proved-but-expected-to-be solutions.
It is matter, in that it behaves gravitationally the same way that ordinary matter does. (Unlike dark energy, which does not behave the same way ordinary matter does; dark energy is actually poorly named because it does not behave like ordinary energy.) Basically, the dark matter hypothesis can successfully explain a wide variety of unexplained phenomena if there exists a substantial number of weakly-interacting, but otherwise ordinary particles.
It's not comparable. Dark matter involves hypothesizing something that is being hypothesized solely to explain certain cosmological problems. If we didn't have those cosmological problems that need explaining, no one would be suggesting dark matter.
The possibility of gravity being repulsive between matter and antimatter was hypothesizes long before we ran into those cosmological problems, and has been subject of sporadic ongoing debate at least as far back as the early to mid 20th century.
> If we didn't have those cosmological problems that need explaining, no one would be suggesting dark matter.
The thing is, the dark matter hypothesis explains more than just certain cosmological problems. It also explains the rotation curves of galaxies, gravitational lensing by galaxy clusters, and the velocity dispersions of galaxies, galaxy groups, and galaxy clusters.
Dark matter is not an ad hoc hypothesis to solve a single problem; rather, it's an ad hoc hypothesis (though still more plausible than any alternative) that explains a large number of observed phenomena over many different length scales.
Dark matter involves hypothesizing something that is being hypothesized solely to explain certain cosmological problems
Much as in the 1840s, Urbain Le Verrier predicted the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus?
> Dark matter involves hypothesizing something that is being hypothesized solely to explain certain cosmological problems.
Waiiiiit.... isn't this the way with ALL unknowns? We see some thing we can't explain (a "problem"), and we come up with a hypothesis solely to explain it? Sometimes we're wrong, and we start over with a new hypothesis?
We hypothesized the existence of some hitherto unknown and "dark" agent we called a "germ" to explain infection. We hypothesized the existence of an unknown and invisibile "X-ray" to explain how film got ghostly images on it. Etc.
I'm sure I'm missing your meaning here; can you clarify?
The distinction I'm making is being hypotheses that involve new objects and hypotheses that involve new properties for old objects.
For instance, consider planetary motion before Newton. You could have hypothesized a "dark" force that moved the planets. Or you could hypothesize that a force you already know exists (gravity near the Earth's surface) also exists for each body of the solar system, has unlimited range, and follows an inverse square law.
Ok, I get your distinction, but (and I'm a complete neophyte non-professional here, so bear with me), the layman's explanation for dark matter has been, "We don't know what's causing it, but it acts like gravity, and the only thing we know of now that has gravity is matter, and it appears (hah) to be invisible to all of our known electro-magnetic radiation, so we call it "dark"".
So I apologize if I've misunderstood, but based on that I don't think hypothosizing this unknown phenomenon as some sort of "dark matter" is entirely out of line.
Very interesting to explore, starting with his bold assumption that anti-matter and matter are gravtiationally repulsive. That seems to lead down many inetresting corridors with delightful consequences to attempt to disprove.
They're observable particles. They probably wouldn't fit unless it somehow turns out to explain the entire Standard Model. What I'm getting at is that it might explain some dark matter, but it is unlikely to explain all dark matter even if it turns out to be a correct model.
That whole thing about neutrinos oscillating between flavors still freaks me out though. One of the weirder things in quantum mechanics.
I like the elegance of this theory, if nothing else. Dark matter always seemed painfully ad hoc, and MOND kludgy. Now, we need to know more about antimatter and gravitation...
so he brings in unknown negative gravitational charge. Basically giving another name to unknown machinery behind the observed phenomena.