- 4509
- GRAVITY - how
galaxies rotate? - Rotation curves of galaxies stay flat
indefinitely. For over a century now,
Einstein’s theory of relativity and gravity has been the existing framework.
However, cracks are starting to show
when they failed to find decreasing rotational energy in galaxies even
millions of light years away from the galaxy’s center.
-
------------------------------------------ 4509
- GRAVITY - how
galaxies rotate?
-
- Galaxies are known to rotate, even our solar system travels in a circle
around the center of the Milky Way galaxy at around 200 kilometers per second,
though we can’t perceive any motion on human time scales. According to
Newtonian dynamics, this rotational speed should slow down the farther away a
star is from the center of a galaxy. However, observations didn’t support this,
showing that the speed kept up no matter how far away the star is.
-
- That led scientists to create another force
impacting the speed of rotation of the farthest-out stars. Today, we commonly
call it “dark matter”. However, scientists have also spent decades trying to
puzzle out what exactly dark matter is made of and have yet to come up with a
coherent theory.
-
- But in some cases, even the existence of
dark matter as we know it doesn’t match the observational data. The rotational speed of galaxies doesn’t
drop off, no matter how far out they are and no matter how long they’ve been
doing so.
-
- This data flies in the face of a
traditional understanding of dark matter, where its gravitational influence is
felt by a “halo” surrounding the dark matter itself. Even these dark matter
halos have an effective area. The evidence of maintaining rotational speed that
should be well outside the sphere of influence of any dark matter halo existing
in these galaxies.
-
- To collect this data, the authors used a
favorite tool of cosmologists – “gravitational lensing”. They collected data on
galaxies that were far away and had their light amplified by a galaxy cluster
or similarly massive object that was nearer.
The speed of rotation of the stars in a galaxy plotted against the distance of those stars from the
galaxy’s center, known as a “Tully-Fisher” relation in cosmology.
-
- The result was an almost perfectly straight
line. The rotational speed of stars in
a galaxy did not seem to diminish with distance from the galaxy’s center, as
both traditional Newtonian dynamics and relativity via dark matter predicted it
would. So, what alternative explanations are there?
-
- Why do galaxy rotation curves matter? One theory in physics accurately predicted
the data collected. It is called the “modified Newtonian
Dynamics” (or MOND) theory. Designed explicitly to account for things like
galaxy rotations, MOND was developed in 1983 and remains controversial to this
day.
-
- That disconnect points to the need for a
deeper understanding of gravity which many cosmologists already believe is
afflicting the discipline. While there is no current consensus on what might resolve
that crisis, the evidence is mounting for the need for resolution. If we’re
truly going to understand our place in the universe, we will eventually need to
figure out a solution – it just might take a while.
-
-
June 22, 2024 GRAVITY - how galaxies rotate? 4509
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--------------------- --- Sunday, June 23, 2024
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