(Click for larger view.)
This lovely composite image of Abell 520 that includes an inferred distribution of dark matter (blue) in a cluster of galaxies) illustrates a bit of a puzzle. It is not clear why the dark matter is separated out so much from the visible (including the hay red dust cloud). Learn more from the Astronomy Picture of the Day site (where this was posted on Monday*) here. You can learn more about the image credits there too. For a reminder of how people go about locating clusters of dark matter (even though it cannot be directly seen, and we don’t know what it is), see an earlier post here. For more on learning about dark matter by its interactions with clusters, see my earlier post on the Bullet cluster event.
-cvj
(Thanks Nick W!)
This reminds me of the recent puzzle in gravity waves. The E&M astronomers saw a gamma ray burst that they concluded was near enough that gravity waves from it should be detected. The gravity wave groups looked, but no detection. They concluded that the cause of the burst had to be farther away, and its apparent location nearby was just a coincidence. The same effect could happen from the speed of gravity being different from c.
Similarly here. If dark matter acts relativistically different from observable matter, for example, if it had a maximum speed different from c, then one can imagine a situation where in a steady state dark matter acts as expected, but in a collision, its behavior is outside expectations.
An example of this sort of split behavior is Newtonian gravity as compared to Einstein’s gravity. Newton’s gravity has a speed of infinity, Einstein gave it a speed of c. And yet both theories predict the same orbits in low velocity regimes far from the gravitating body.
“A puzzle” That’s all it is?
You know there is the Tully Fisher relation that states that a galaxy’s luminosity is proportional to its highest orbital velocity raised to a power like 3 or 4.
With the Tully Fisher relation we have galaxy’s luminosity linked to its gravitational acceleration. With Abel 520 group of clusters we have the hot luminous gas that is in between the clusters linked to the most amount of the gravitational lensing. We could not have the hot gas in between the clusters being actually gravitationally attractive could we? No more than we could assume that the Tully Fisher relation means what it says it means?
The fact that I can get as much as a 11% change of weight with the “radial spreading of infrared luminosity” got to be due to some fault in my experimental methodology? Shouldn’t it?
Here I shine 3500 W infrared radiation from twelve 250 watt heating lamps on a large hollow, hanging aluminum hemisphere and get a 3% change of weight?
I have been trying for years to get someone to replicate my experiments so that they might be taken seriously. Now, with much more expensive equipment than I can afford, a team of scientist, in a well publicized way, have inadvertently found a means to confirm my experimental results.
It is not the dark matter we are having trouble seeing.
It is what is starring right in the face face that we are having trouble seeing.
Peter Fred
By the way the greatest amount of the dark matter is observed in the red around peak (3).