Astronomy

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Originally Posted by YesNo

I admit I don't understand dark matter, prendrelemick. Crawford's summary is interesting. I am also looking at one by Freeman and McNamara ("In Search of Dark Matter") just to make more sense of the topic.

Crawford mentions around page 9 something called "modified gravity", but she says the various modifications to the laws of gravity have not so far accounted for all the observations.

There seems to be two sources where error can enter:

1) Do the current laws of gravity correctly model reality?

2) Are we able to measure the universe accurately enough to answer questions about dark matter?

If both 1 and 2 are true, then we need something called "dark matter", but no one has found any so far.

In all of this there is also "dark energy" which seems to me to be a sort of "anti-gravity" mechanism allowing the universe to accelerate faster than it should based on assumptions about the big bang.

Another thing that keeps coming to mind when I think of gravity (and anti-gravity) are Qigong masters who have been reported to walk on water or people who levitate when they meditate. Given the laws of gravity that should not be possible. Now I know most people would dismiss these as stories, but then we have not found any dark matter yet either.

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I just love all this. All day yesterday (because of your posts above) I was thinking, "suppose Einstien's "rubber sheet" Spacetime continuum had holes in it through to another dimension or universe, or ripples on it caused by the Big bang or something lumpy on the otherside of it. That would affect gravity as we experience it". The advantage of knowing next to nothing about a subject is kind of liberating sometimes. However I think it is probably right to look for answers within the known laws of gravity for the moment.

I think measurement issues are less likely, the sensitivity of instrumentation nowadays is beyond belief. I heard on the radio (In Our Time, BBC radio 4) that they are detecting a distortion in space of one thousandth of the diameter of an electron per metre to prove the existance of Gravity waves!!!!

In the end, dark matter is a theory that fits the observations, the mathematical predictions and the simulations - but..

This is one subject I know next to nothing about as well. I don't even know enough about the laws of gravitation to calculate the orbit of some satellite. However, that doesn't stop me from speculating.

I don't trust the current measurements because I don't know how inaccurate they are. I don't even know what it means if someone gave me a margin of error. I have heard about the detection of gravity waves, but I think they need more precise measurements. Then is the theory correct? Are what they are measuring according to the theory really what they are measuring?

My sense of the margin of error is more intuitive. We don't, for example, know if there is a Planet 9 in our solar system. If our measurements in general are so accurate, I would think that we should know something like that or how can I be so sure they know they measured a gravity wave coming from the collision of black holes in other galaxies? They didn't get a prediction of the alleged black hole (radio source) at the center of our galaxy right. That radio source was supposed to have "eaten" something or other, which would prove it was a black hole, but that lunch did not happen. I do trust the measurements of the cosmic background radiation, but that is only because there is not much there to measure and instruments have been sent into space to get that information a few times. But what does that data mean? Then the theory comes into question.

The problem is made more complex because of all the dwarf planets in the Kuiper Belt, and other objects like meteors going round the sun but a great distance from it. We are starting to catalogue them but there is still a large margin of error.

A paper on the dark matter. I read only the abstract, but it might be of interest. Didn´t so much relish the idea advanced by the Brazilian article that contained the link, that we all may have this dark matter inside us. Oh well, that would explain a lot.:(
http://arxiv.org/pdf/1609.01282v1.pdf

The whole business of dark matter is still too difficult to grasp. I read the paper that Danik refers to and am still not clear.

The item from Popular Astronomy, below, is interesting:

The SOCIETY for POPULAR ASTRONOMY

Electronic News Bulletin No. 429 2016 September 11


NEW EXTREMELY DISTANT SOLAR-SYSTEM OBJECTS
Carnegie Institution for Science

In the race to discover a proposed ninth planet in our Solar System,
astronomers have observed several never-before-seen objects at extreme
distances from the Sun in our Solar System. The more objects that are
found at extreme distances, the better the chance of constraining the
location of the ninth planet that is predicted to exist far beyond
Pluto (itself no longer classified as a planet). The placement and
orbits of small, so-called extreme trans-Neptunian objects, can help
to narrow down the size and distance from the Sun of the predicted
ninth planet, because that planet's gravity influences the movements
of the smaller objects that are far beyond Neptune. The objects are
called trans-Neptunian because their orbits around the Sun are greater
than Neptune's. In 2014, astronomers announced the discovery of 2012
VP113 (nicknamed 'Biden'), which has the most-distant known orbit in
our Solar System. They also noticed that the handful of known extreme
trans-Neptunian objects have similar longitudes of perihelion. That
led them to predict that there is a planet at more than 200 times our
distance from the Sun. Its mass, ranging in possibility from several
Earths to a Neptune equivalent, is shepherding the smaller objects
into similar types of orbits. Some people have called it Planet X or
Planet 9. Further work since 2014 showed that such a massive ninth
planet probably exists, by further constraining its possible
properties. Analysis of 'neighbouring' small-body orbits suggest that
it is several times more massive than the Earth, possibly by as much
as 15 times, and at the closest point of its extremely elongated orbit
it is at least 200 AU from the Sun (over 5 times as distant as Pluto.)

Researchers are conducting a special survey for objects beyond Neptune
and the Kuiper Belt, and have covered nearly 10% of the sky to date.
As they find and confirm extremely distant objects, they analyze
whether their discoveries fit into the larger theories about how
interactions with a massive distant planet could have shaped the outer
Solar System. We are now in a situation similar to that in the
mid-19th century when Alexis Bouvard noticed Uranus' orbital motion
was peculiar, which eventually led to the discovery of Neptune. The
new objects submitted to the Minor Planet Center for designation
include 2014 SR349, which adds to the class of the rare extreme trans-
Neptunian objects. Its orbital characteristics are similar to those
of the previously known extreme bodies whose positions and movements
initially led astronomers to propose the influence of Planet X.
Another new-found extreme object, 2013 FT28, has some characteristics
similar to those of other extreme objects but also some differences.
2014 FE72 is the first distant Oort-Cloud object found with an orbit
entirely beyond Neptune. It has an orbit that takes the object so far
away from the Sun (3000 AU) that it is probably being influenced by
forces of gravity from beyond our Solar System such as those of other
stars and the Galactic tide. It is the first Solar-System object
observed at such a large distance.

This item in EarthSky explains pulsars very clearly, even to someone like me who is not trained in astrophysics:

http://earthsky.org/space/impossibly...onster-pulsars

That the ULXs could be coming from neutron stars rather than black holes was interesting although I don't see why black holes could not be pulsars.

Can you explain how that could be?

This post has one reply that relates to this issue:http://earthsky.org/space/impossibly...onster-pulsars, that monster pulsars are perhaps huge "black holes", the Japanese authors suggest. Is this to do with dark matter?

Dreamwoven: Dark matter is a theory that is based on universe wide observations, rather that solar system wide. Is that where you are having difficulty? The kuiper belt, dwarf planets, 9th ,10th, 11th planets beyond Neptune are negligible in their affects on this scale. In fact Newtonian laws are seen to be working perfectly well within the solar system - there are no discrepencies observed on such a small scale.

It's when you get to galaxy and galaxy cluster scale, and apply Newton's law to them, a huge discrepency is seen - there is extra gravity there - they are moving at a speed consistant with something of 10 times the mass they are seen to have.

If you say The law of gravity is consistant and known then that mass must be there. (There are many 'ifs')

Over the years many candidates for this mass (called Dark Matter) have been eliminated.

Scientists are now looking for sub atomic particles because they seem to 'work' when they run computer simulations, and there are theories that allow for their creation in the Big Bang.

This search is extremly difficult and so far the only 'success' has been eliminating possibilities.

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Originally Posted by prendrelemick

Dreamwoven: Dark matter is a theory that is based on universe wide observations, rather that solar system wide. Is that where you are having difficulty? The kuiper belt, dwarf planets, 9th ,10th, 11th planets beyond Neptune are negligible in their affects on this scale. In fact Newtonian laws are seen to be working perfectly well within the solar system - there are no discrepencies observed on such a small scale.

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No, I get all that, about the universe, not the star-system. And that, as you say,

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Originally Posted by prendrelemick

"This search is extremely difficult and so far the only 'success' has been eliminating possibilities."

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We are at the very edge of research and know little about dark matter and black holes.

From what I understand a neutron star is the result of a massive star collapsing so much that the protons and electrons merge into neutrons while a black hole is not the result of a star collapse but just a region of space where the gravity is high. I might have that wrong. Based on that, I suspect a black hole, being just a region of space, should not be "rotating" so that a pulsar effect can be formed.

Regarding dark matter, Freeman and McNamara ("In Search of Dark Matter", 2006) claimed (page 18) that the current thinking is that there is no dark matter within the disk of the Milky Way and the original suspicion that there was in the early 20th century was incorrect do to inaccurate measurements of the amount of gravitational mass. What that would mean in our solar system is that any gravitation discrepancy between the observed movements of the planets would be caused by some missing real, not dark, matter. The current dark matter is supposed to be in halos around galaxies rather that within the galaxies or clusters of galaxies, but it makes me wonder whether there aren't more measurement issues that will be resolved in the future.

The Triangulum II galaxy is supposed to contain a lot of dark matter: http://earthsky.org/space/a-nearby-d...-triangulum-ii This might be a good one to keep in mind. Since this is closer to us, we could expect better measurements.

YesNo: "Based on that, I suspect a black hole, being just a region of space, should not be "rotating" so that a pulsar effect can be formed. "

I have this romantic notion that the massive black holes in the center of spiral galaxies are the engines that provide galactic spin and that the lines of unimaginable gravitational force that is exerted upon the multitudes of celestial bodies are analogous to the arms of a mobile - being the black matter sought for. Probably not scientific in any respect but I thought I'd share.

Ta ! (short for tarradiddle),
tailor STATELY

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Originally Posted by Dreamwoven

No, I get all that, about the universe, not the star-system. And that, as you say, We are at the very edge of research and know little about dark matter and black holes.

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I wrote that for myself to be honest, to clear up my understanding of the subject.

Here is yet another article I read, about the distribution of Dark Matter. As yes/no said, most is in a halo around Galaxies. Within our Galaxy there is thought to be about 100 earth masses/cubic light year of dark matter, which is a teeny tiny amount. Nevertheless they are hoping to detect the odd particle deep down in an old gold mine in Sth Dakota.

http://arstechnica.com/features/2014...ere-is-it-all/

I'm trying to make sense out of how the cosmic microwave background data suggests the existence of dark matter. It has something to do with gravity pulling matter together and photons pushing it apart. https://medium.com/starts-with-a-ban...ba8#.vgjlfi7yh

That article also mentioned the failure of the LUX (Large Underground Xenon) experiment to find direct evidence of dark matter. But maybe they have found it by now.