Universe's expansion may be understood without dark energy
physorg.com
physorg.com
That's what I was going to say, too. And I got more convinced the further down I got. The weird, convoluted wording was a big tipoff. And then this sealed it:
When a star explodes and its mass is combusted into radiation, conservation requires that the number of quanta stays the same, whether in the form of matter or radiation.
There is no conservation law limiting the number of photons. This is obviously observed any time you turn on a light bulb.
...whether in the form of matter or radiation
Both dark energy and dark matter seem contrived and virtually anything would be less hand-wavy. Just for aesthetic reasons I hope these are found to be the modern equivalents of "ether" as a medium for EM waves.
Dark matter being less elegant in the galaxy rotation problem being a) it hasn't been detected and b) if it does exist, why does it fall into the same halo distribution pattern in different galaxies to account for the same rotational observation of these aforementioned spiral galaxies.
But as I said before, MOND seems to break more physics than it solves, but much smarter people that actually work in physics are trying to put those pieces together which makes it legitimate enough for me to take seriously.
Edit: Actually reading who's responding to what, I learned you're more than well aware of this :) I like to believe that galaxy rotation is solved with a single theory and not a significant combination of explanations (Occam's Razor). In any event I'll leave this comment here as an awareness booster for younger dabbling cosmologists.
First it's hard to call dark energy "contrived" when it hasn't really been contrived, conceived, or created at all. That is, no one has really made any strong, convincing claims as to what it actually is. The term is really just a placeholder for the fact that something is providing a long-scale repulsive force (and so probably has negative energy density). Although it can be "explained" by a cosmological constant, that just punts the question to what the cosmological constant physically is.
As for dark matter, it is the simplest, most elegant way to explain the variety of observations that point to its existence. These observations include small-scale dynamics of dwarf spheroidal galaxies, the rotation curves of galaxies, gravitational lensing due to galaxy clusters, and more. All of them can be explained if there exists some unknown particle which interacts only weakly with other particles. While it's true that the standard model predicts no such particle, it's also true that it's known that the standard model is an incomplete theory.
You might argue that it could just as well be that our theory of gravity is wrong. (Certainly at the quantum level we know for sure that that is the case.) But inaccuracies in our theory of gravity at large scales have difficulty explaining the dark matter problem in an elegant matter. Consider dwarf spheroidals. These are the smallest of galaxies; in terms of luminous mass, they are only slightly more massive than globular clusters. Yet globular clusters are 1000s of times smaller than dwarf spheroidals. How can they possibly remain gravitationally bound on the scale of 1000s of light-years when globular clusters are only bound on scales of a few light-years? Dark matter can explain this by simply saying that dwarf spheroidals have relatively massive dark matter halos, whereas globular clusters have almost no dark matter. (Which one would expect in standard galaxy formation scenarios.) But modified gravity theories have difficulty explaining why two systems with similar masses interact so differently.
We can similarly see that galaxies of all sizes exhibit a range of dark matter fractions. If the problem was merely with our theory of gravity one would expect all galaxies of a given luminous mass to have the same dark matter fraction. Any modified theory of gravity which explained these observations would not be aesthetic. The aesthetic explanation would be some hitherto unknown weakly interacting massive particle.
How is that not contrived?
>You might argue that it could just as well be that our theory of gravity is wrong.
As an armchair physicist, I was rather hoping the Pioneer anomaly would lead us down that road, oh well.
>Any modified theory of gravity which explained these observations would not be aesthetic.
Well aesthetics do invoke subjective qualities. Relativity, modifying Newtonian physics, could have be seen as ugly by some. E = mc^2 is so elegant as to seem improbable.
> How is that not contrived?
All the phrase "dark energy" means is that the expansion of the universe is accelerating. There must be something which provides this repulsive force (maybe some new particle, maybe some unknown property of the vacuum, maybe some new theory of gravity on cosmological distance scales). Dark energy is simply the generic name to describe this something, whatever it is. It makes no claim as to what is causing the repulsion. It's just a name for an unknown. Calling dark energy contrived is like seeing an equation 6x + 4 = 7 and calling "x" contrived.
> Well aesthetics do invoke subjective qualities. Relativity, modifying Newtonian physics, could have be seen as ugly by some. E = mc^2 is so elegant as to seem improbable.
The virtue of relativity is that despite overturning a very elegant theory, it creates new, elegant equations of its own. Any theory of gravity that could explain the varying dark matter fractions would necessarily have to do so in some basically arbitrary way (since dark matter fractions vary in an apparently random manner). So any such new theory of gravity would have to be ugly; there could be no redeeming features.
If a dark energy had been not been allowed by GR and still been observed, then the hand-waving would have started.