It’s always struck me as indication of something very wrong that current most widespread cosmological theory calls for 90% percent of the universe to be undetectable and unkown (dark matter / dark energy).
I read this in the wiki on alternatives to dark matter:
In a 2004 study at the University of Mainz in Germany,[88] it has been found that if one applies just a standard quantum mechanical approach to Newton’s Gravitational constant at various scales within the astrophysical realm (i.e. scales from solar systems up to galaxies), it can be shown that the Gravitational constant is not so constant anymore and actually starts to grow. The implication of this is that if the Gravitational constant grows at different scales, then dark matter is not needed to explain galactic rotational curves.
So if assuming that gravity works differently at galactic scales explains the observations we see without having to invoke dark matter / dark energy why is dark matter still the most wide spread explanation?
Occams razor would tend to favor an explanation that doesn’t call for an undetectable but incredibly widespread mass wouldn’t it?
I skimmed through the 73 page paper but didn’t read it in depth. I’ll note that there is currently no working theory of quantum gravity, and so while this approach may leverage an existing methodology it doesn’t fit into a framework that is accepted by astrophysicists and physicists working in General Relativity, whereas the application of corrections for missing dark matter (MDM) to the established mechanics of GR is a very simple assumption.
However, I don’t think that anyone believes that slapping the label of “(missing) dark matter” as a correction factor really explains anything, any more than “luminiferous aether” explained the invariance in the speed of light. Until we know what the unseen mass actually is, it is one of the great mysteries of astronomy and cosmology.
Dark energy is something else entirely, and is not needed to explain the rotation behavior of the galaxy.
Dark matter is matter we can’t see; it’s a placeholder name, and though it sounds mysterious and sci-fi it’s actually a very simple idea. There’s stuff there, we just can’t see it. There’s plenty of stuff in the universe that we know about but can’t see. Sure, the galactic rotation may be eventually found to be something else, but dark matter’s a perfectly reasonable and simple hypothesis.
Dark energy is something entirely different. Again, it’s a placeholder name for something we don’t fully understand, but it’s what’s accelerating the expansion of the universe.
Well occasionally, it’s a matter of opinion as to which side Occam is on. I tentatively think that most astrophysicists (but probably not all) would tend to say that widespread ordinary mass that’s non-visible (but not undetectable; we’re seeing its gravity, after all) is a simpler explanation that screwing around with a fundamental and so far well tested component of general relativity.
But, then most astrophysicists would also say “but of course we don’t really have any evidence either way, yet, so it’s pretty much a moot point until we do.”
Except thats not what they are saying. Only 20 percent of dark matter is supposed to be “baryonic” dark matter eg made up of atoms, white dwarfs, massive jupiter like planets, MACHOS. The other 80 percent is supposed to be non baryonic, eg stuff thats not made of atoms (wimps). AFAIK no wimp detection experiments have come up with signifcant results has it?
BTW, is mass thats fallen beyond the event horizon of a black hole considered baryonic or non-baryonic when they estimate that only 20 percent of dark matter is baryonic?
The figures on the baryonic content of the Universe are mostly based on Big Bang nucleosynthesis: By counting up the relative abundances of regular hydrogen, deuterium, helium-3, and various isotopes of lithium (all of which were produced almost entirely in the very early Universe), you can determine what the abundance of protons and neutrons was then. So anything that was baryonic in the early Universe still counts towards that baryonic tally, even if it’s since fallen into a black hole (though primordial black holes, if they exist, would not).
Back to the OP, while the other three coupling constants vary with distance in accordance with quantum field theories, the characteristic distance over which they vary is very, very small. If the gravitational coupling varied over galactic length scales, we’d expect to see some other evidences of the processes which cause it, and in any event, there’s good theoretical reasons to expect that the gravitational coupling wouldn’t vary in the same way as the others (this is due to the lack of negative mass).
About 400 years ago, Galileo found that the Milky Way was actually composed of stars. You can see a few thousand stars with the naked eye, but the remainder is about 99.99999%* of the Milky Way’s mass.
About 100 years ago, we learned that many of the fuzzy splotches seen were actually galaxies on their own, comparable with the Milky Way. Before then, we were utterly ignorant that 99.999999999%* of the universe was not in the Milky Way.
So, given that, why is it hard to believe that a mere 90% is hard for us to detect? As others have said, dark matter is not undetectable, just difficult.
Although these numbers are approximate, they are not “just a bunch of nines”. The numbers are within a digit of the actual figure.
This is semantics, but I would say that neutrinos (for instance) are “ordinary mass”. We know that neutrinos can’t account for a significant part of dark matter, but they’re probably a similar kind of particle (weak interactions, etc.).