The Need for Dark Matter
The need for dark matter is apparent when considering just our Sun.
Can we accurately predict the Sun's motion?
from Wikipedia for the Sun:
The Sun orbits the center of the Milky Way, and it is presently moving in the direction of the constellation of Cygnus.
The Sun's orbit around the Milky Way is perturbed due to the non-uniform mass distribution in Milky Way, such as that in and between the galactic spiral arms. It takes the Solar System about 225–250 million years to complete one orbit through the Milky Way (a galactic year), so it is thought to have completed 20–25 orbits during the lifetime of the Sun. The orbital speed of the Solar System about the center of the Milky Way is approximately 251 km/s (156 mi/s).
The Sun's orbit is 'pertubed.' I saw another online comment say its motion is 'non-keplerian.' How can we predict its future with no rules?
continue the excerpt:
Within 32.6 ly of the Sun there are 315 known stars in 227 systems, as of 2000, including 163 single stars. It is estimated that a further 130 systems within this range have not yet been identified. Out to 81.5 ly, there may be up to 7,500 stars, of which around 2,600 are known. The number of substellar objects in that volume are expected to be comparable to the number of stars. Of the 50 nearest stellar systems within 17 light-years from Earth (the closest being the red dwarf Proxima Centauri at approximately 4.2 light-years), the Sun ranks fourth in mass.
There are defined quantities of unknowns here; or known unknowns.
This is certainly more than a three-body problem.
In physics and classical mechanics, the three-body problem is the problem of taking the initial positions and velocities (or momenta) of three point masses and solving for their subsequent motion according to Newton's laws of motion and Newton's law of universal gravitation. The three-body problem is a special case of the n-body problem. Unlike two-body problems, no closed-form solution exists for all sets of initial conditions, and numerical methods are generally required.
With complex software models I expect we can approximate an expected future motion of the Sun even with missing data.
One obvious problem is we don't have a long history of observations to check the model against.
I had expected we had a very well grounded understanding of our Sun's motion in the galaxy. We do not.
That would be the foundation when looking at stars in other galaxies.
The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. A significant discrepancy exists between the experimental curves observed, and a curve derived from theory. The theory of dark matter is currently postulated to account for the variance.
One of the variances is the motion in the outermost arms. These stars out there are at one end of their elliptical orbit. If their observed velocity is not as expected I would expect the first step to be taken is fix the model of orbits to conform to observation. Apparently that was not the step taken but instead cosmologists proceeded to claim there is dark matter causing the orbital disturbance beyond what was already expected (in the known unknowns).
There were recent news stories of galaxies discovered to have no dark matter. That just means the velocity curve has no variance from the model for orbits of the stars.
Dark Matter is the solution when cosmologists create a model to include known unknowns and it is an apparent surprise when all galaxies do not conform.
Does the term hubris apply here?
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