Rotation curves: still flat after a million light-years
The article discusses flat rotation curves in galaxies, challenging traditional theories. Vera Rubin's work in the late 1970s revealed extended flat rotation curves, indicating the presence of dark matter or modifications to gravity theories like MOND. Recent data from the KiDS survey supports this phenomenon, questioning existing models and emphasizing the need for more research.
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The article discusses the phenomenon of flat rotation curves in galaxies, a concept established by Vera Rubin in the late 1970s. Despite expectations of declining rotation curves with distance from the center, observations consistently show flat rotation curves extending far beyond the visible matter, necessitating the presence of dark matter or modifications to gravity theories like MOND. Recent gravitational lensing data, particularly from the KiDS survey, confirm that rotation curves remain flat even at distances exceeding a million light-years from the galaxy center. This finding challenges existing models of dark matter halos and raises questions about the nature of galactic mass distribution. While MOND predicts indefinitely extended rotation curves for isolated galaxies, the lack of a clear transition in rotation curves suggests a discrepancy between observations and current theories. The persistence of flat rotation curves poses a significant puzzle in astrophysics, highlighting the need for further research and exploration of alternative explanations.
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12 commentsA galaxy (elliptic, or spiral) is made out of billions of stars, like our sun.
These stars rotate around the center of the galaxy (very slowly, think millions of years for 1 rotation).
A rotation curve measures the velocity of stars as a function of distance from the center of the galaxy.
Newtonian physics (or Einstein's GR) says that the rotation curve should decay with distance, ie. with greater distance the stars' velocity should go down --- assuming the matter in the galaxy is the visible matter that we see, ie. the stars (which shine light).
The problem is, there is a rich set of observational data, from many different experiments, telescopes, and methodologies that show that the rotational curve is in fact flat, it does not decay.
There are 2 big competing theories to explain this discrepancy:
1. Assume that there is a lot of unseen, non-shining, ie. Dark Matter (DM) in the galaxies (also ours). If you put the appropriate amount of dark matter in there, with the right distribution, you can reproduce the observed rotational curve. There are also other places is astrophysics/cosmology where having dark matter (specifically Cold Dark Matter, CDM, where cold just means "slowly moving") is useful. The biggest example is to explain the history of the Universe and the observed Hubble-constant. In fact the standard model of cosmology is called λCDM, CDM for Cold Dark Matter (λ for the cosmological constant, currently modeled as Dark Energy, not relevant for this discussion).
2. Assume that Newton was wrong and gravity is not exactly 1/r^2 --- this is called MOND, Modified Newtonian Dynamics. This way you can also reproduce the observed rotation curves. This is much less popular, because: (i) physicsts don't want to give up the beautiful and geometric simplicity of 1/r^2 (ii) Dark Matter is also useful for solving other discrepancies in astrophysics/cosmology.
What this article is saying is that, even in the first Dark Matter model, per the model DM distributions inside galaxies that also work with all the other places where DM is used to explain something (eg. in cosmology), at some distance from the center, the dark matter bubble has an edge and stops --- and then the velocities should finally break down. However, these latest observations are showing that the velocities remain constant even beyond the modeled/assumed DM bubbles. This is an additional ε argument in favor of MOND, and science proceeds.
No, it doesn't. MOND falls apart in every single theory they put forth. The fact that this happens without fail should lead one to understand the answer probably lies elsewhere than MOND.
Obviously will need additional work and review, it's only one paper. Maybe there are mistakes or factors not fully considered.
There have been a number of other papers recently on measuring wide binaries. Different papers claimed different results on these.
Still, it's certainly something that merits a lot more attention. We may be looking at needing some new theory of gravity (maybe not MOND, but something other than dark matter).
However, what if it holds, but if inertia is quantized, then you get less gravitational effect at 90 degrees to the path of motion at astronomic distances as it recedes into the quantum noise.
Which fits observations with inventing dark matter, or tweaking gravity.
There's quantum fluctuation. Particles appear out of nowhere and disappear again. Hossenfelder talked about negative mass, so allow me to do this as well: What if a pair of two particles, one of negative and one of positive mass can very rarely appear?
They disappear immediately again but for a short moment we have acceleration (more about that in Hossenfelder's video). Could this be enough to explain "dark matter"?
Which is actually a terrifying existential threat to think about: The more an earlier civilization uses FTL, the more space will expand, and eventually it will become all but impossible for younger civilizations to traverse space without becoming dependent on the elder races.
Update: https://en.m.wikipedia.org/wiki/Tully–Fisher_relation But the linear regression looks unconvincing
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The James Webb Space Telescope observed galaxy J1120+0641, revealing a mature quasar with a billion solar mass black hole at cosmic dawn. This challenges theories on black hole growth, suggesting they may start with substantial masses. The study sheds light on early black hole development, showing unexpected normalcy in early quasars, challenging assumptions on their evolution.
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