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Dark matter. It’s hard to see, it’s hard to study and it just won’t behave. There’s plenty dark matter around. It’s just that no one knows what it is. It only makes its presence felt through its gravitational pull.
But astronomers have figured out ways to ID dark matter. One tactic is called gravitational lensing. Dark matter’s gravitational pull bends light, so clumps of dark matter distort the appearance of galaxies in the background. Researchers measured those distortions with the Hubble Space Telescope. And they recently mapped out the dark matter in a galaxy cluster called A520. Which is actually several galaxy clusters, all smashing together.
The problem was, the dark matter in A520 wasn’t where it ought to be. Past studies have shown that clouds of dark matter pass cleanly through one another when galaxy clusters merge. That’s because dark matter doesn’t collide much with itself or with ordinary matter. But in this case the dark matter seemed to bunch up in the middle, as if it was sticking together. The study appears in the Astrophysical Journal. [M. James Jee et al., "A study of the dark core in A520 with the Hubble Space Telescope: The mystery deepens"]
Why should dark matter behave so differently in different galaxy clusters? On that front, astronomers are still in the dark.
—John Matson
[The above text is a transcript of this podcast.]
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Add CommentFYI - the free preprint archive version of the referenced research report can be found at: http://arxiv.org/abs/1202.6368
Reply | Report Abuse | Link to thisTo further explain, in the 'established' interpretation of colliding galaxy clusters, exemplified by the Bullet Cluster, the colliding massive intracluster mediums' forward momentum is halted by physical interactions among their gas particles; the momentum of the sparse galaxies is unimpeded, as they do not interact. This commonly produces an image where the collided clusters' galaxies appear to flank the combined, interacting x-ray emitting gasses, usually on opposing sides, since their forward progress continued beyond the point of collision.
The presence of dark matter is generally attributed to what is considered to be excessive weak gravitational lensing effects colocated with the clusters' visible galaxies, based on the estimated mass of each cluster's galaxies. The dark matter is thought to be located with the clusters' galaxies because their forward progress would not be impeded by interactions with other matter: the galaxies because they're so sparse and the dark matter because it is thought to be only very weakly interactive.
The mass attributed to dark matter is identified by weak gravitational lensing, determined through a very complex statistical process of identifying very slight directional optical distortions in usually thousands of background objects. The identified distortions are used to determine location of usually unidentified masses, usually colocated with the clusters' galaxies.
As I understand in the case of A520, several concentrations of mass, identified by weak gravitational lensing, are not coincident with any colliding clusters' galaxies. One of them is located near one of two large regions containing the x-ray emitting gasses of unmerged intracluster mediums.
An alternative explanation for identified weak gravitational lensing effects might help explain all colliding cluster observations. Rather than dark matter, the mass peaks identified weak lensing effects might actually represent spacetime distortions produced by the evacuations of highly accelerated galaxy clusters: the lensing effects might represent the 'wake' of collided clusters. In this case the multiple clusters interacting in the A520 cluster should have been accelerated from the regions identified as mass peaks.
What does it say about your website when the first comment is better written, more informative and useful than the entire article itself? The links are to PAID articles, so I have to thank the commenter and ask you what were you thinking when you bothered to scribble in this one?
Reply | Report Abuse | Link to thisThanks so much for your very kind (to me) remarks)!
Reply | Report Abuse | Link to thisTo be fair, these 'podcasts' are categorized as "60 second [whatever]" and are therefore severely limited in content. However, I also agree that additional information was necessary for most people to reasonably comprehend this subject, which is what I attempted to provide. I don't know why this subject wasn't addressed in a more complete article format...
Great detailed explanation on the DM inferred values. So, why doesn't dark matter form it's own dense structures, including super-massive DM black holes? Why would it need interaction as long as its mass(gravity)Higgs field relationship with spacetime, curve space to concentrate the DM into such structures? Doesn't DM fall into regular black holes? Wouldn't DM falling into a black hole, or even DM black holes emit energy from the event horizon? How about DM black hole Hawking Radiation? Perhaps I need to read some additional articles.
Reply | Report Abuse | Link to thisIt does seem curious - these are among the very few instances where the presence of dark matter has been inferred in specific locations that are not directly associated with the presence of large scale structures of ordinary matter.
Reply | Report Abuse | Link to thisAt the largest scales is the inferred intergalactic filaments of (disperse?) cold dark matter comprising the hypothesized cosmic web.
In all other cases the presence of dark matter seems to be inferred by seemingly excessive gravitational effects produced by ordinary matter that is necessarily directly associated with it. The analyses that successfully established the perceived requirement for dark matter was the galaxy rotation problem, in which it seemed that stars and other masses at the periphery of spiral galaxies rotated faster than was expected for the galaxies' estimated mass, using very general assumptions drawn from Kepler's laws of planetary motion.
For additional comments and references, please see: http://sciencewithoutfiction.com/uploads/JDwyer.PDF
jtdwyer, you seem erudite and cogent in the subject
Reply | Report Abuse | Link to thisof astrophysical evidence re: dark matter, as you
question such evidence's validity.
A physicist well studied on the validity or lack
therof of DM evidence would be good for a naive
layman like me, with a hypothesis about where to
look for dark matter, to ask if my hypothesis is
plausible:
Each isotope of each element has it's own, unvar-
ying vibrational frequency in time. That is what
makes cesium 137 atomic clocks work.
However their amplitudes vary with temperature,
ie. the spacial aspect of isotope's vibrations.
The hotter the atom, the more widely it shakes.
'Vibrational' however, means 'rotational' in
this context, because isotopic vibration occurs
in all three known spatial dimensions, unless
some law restricts atoms' vibrations to one or
two spacial dimensions. Even if restricted to
two, isotopic 'vibration' would mean 'orbit'.
I know of no reason the isotopic frequency vibr-
ations of atoms would be restricted to less than
three spacial dimensions. If there is none, atoms
orbit points in space (apart from the whole atom's
other motions).
Then what ranges of size do atom's orbits have,
relative to the size of the elements?
When helium is created in the Sun, what is the
size of it's most common isotope's vibration?
How does the size of He's vibration compare with
the size of the atom? Could it be that atoms
can orbit points in space at distances greater
than the size of their nucleus or even the size
of the whole atom?
If so, that atoms have isotopic frequencies
implies their nuclei orbit a second, massive
nucleus, of the same or different particles
that may even have it's own electrons or ana-
logs orbiting somewhere in that atom's kinetic
system.
If the weighing of atomic elements' masses
and relations of those values to other physical
values have been worked out to practically
everyone's satisfaction, yet we do not know a
lot of things about atoms, or particle colliders
would not be built to find out more.
If 'how much mass do atoms have' is long seen a
settled question, i suggest value corrections
can be hidden by unoticed factors on scales
from macrocosmic to quantum. What hidden or
unoticed factor can change atomic mass values
i can't say, but if atoms can follow orbits
greater than their nucleus' diameters or whole
diameters, by their isotopic vibrations, they
must be orbiting an additional mass inside
the atom: a second atomic nucleus.
Thanks for your kind remarks, but I must make clear that I have no formal education or experience in any field of physics - in fact I'm a retired information systems analyst. I do, however, reference published works of professional physicist to support many of my assertions.
Reply | Report Abuse | Link to thisI tend to focus on the perceived requirement for galactic dark matter that improperly established its presumed existence among many physicists, as its fundamental errors are much simpler for an information analyst to explain. Observational evidence for dark matter derived from discrepancies between statistically identified slight distortions that conflict with gross estimations of galaxy cluster mass, for example, are far too complex for me to analyze. Likewise with CMB derivations. It think it's fair to say that neither of these examples on their own could have convinced the physics community of the existence of dark matter without the falsely compelling argument earlier presented for the galaxy rotation case.
That said, I cannot rule out the existence of dark matter; I only assert that no new form of matter is necessary to explain the rotational characteristics of galaxies.
At any rate, it seems your idea would increase the mass of ordinary matter, which does not meet requirements.
To explain the galaxy rotation problem most simply, spiral galaxies were considered to be 'standard' orbital systems that should comply Kepler's laws of planetary motion which, while they had been empirically derived solely from observations of the Solar system, were erroneously thought to be universal laws.
Standard Keplerian rotation charts derived from the laws of planetary motion specified that the further away an object was from the center of mass, the slower its rotational (orbital) velocity should be.
Spiral galaxies didn't comply with these expectations: the rotational velocity of galactic disk objects remained relatively constant regardless of the distance from any center of mass or axis of rotation.
In order to explain this deviation within the context of the laws of planetary motion, it would be necessary to make the galaxy periphery essentially part of the center of mass by significantly increasing the total diameter of galactic mass. This requires that some separate, undetected mass be located far from the ordinary, visible galactic mass.
Spiral galaxies are not simple orbital systems in which a few planets independently orbit a Sun that contains 99.86% of total system mass. Billions of discrete galactic disk masses primarily interact with each other.
Reply | Report Abuse | Link to thisIt appears one third more of the masses of 3
local-universe-typical galaxies closest to the
Milky Way can be accounted for by overlooked,
additional, HI hydrogen, according to radioas-
tronomer Robert Braun of CSIRO. Not most of
the typically perceived galactic DM mass dif-
ference, but part of it.
http://www.spaceref.com/news/viewpr.html?pid=37219
Thanks for the link to the very interesting report!
Reply | Report Abuse | Link to thisRecent observations also seem to confirm that there are unexpected variations in the initial mass function of early galaxies, which should also mass to light (M/L) relations and dark matter requirements. How these factors affect astronomers' interpretations of observations is beyond my comprehension, but I think my yield some important new insights. It seems to me,anyway, that if astronomers' estimation of distant galaxies' ordinary mass requires correction then their estimations of dark matter requirements might be affected... Please see:
"Galaxies Defy Astronomers' Expectations", http://news.sciencemag.org/sciencenow/2012/04/galaxies-defy-astronomers-expectations.html?ref=hp
Cappellari et al., (2012), "Systematic variation of the stellar initial mass function in early-type galaxies", http://www.nature.com/nature/journal/v484/n7395/full/nature10972.html, http://arxiv.org/abs/1202.3308
In addition to the invalid assumption about galaxy dynamics, to the extent that astronomers may have been undersestimating (ordinary) galactic mass there is likely no need for any compensatory galactic dark matter...