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We’ve been failing to detect dark matter for decades. Finally, the latest failure to detect dark matter may have actually proved its existence. One of these is true: either most of the matter in the universe is invisible and formed of something not explained by modern particle physics OR our understanding of gravity is completely broken. The debate over which is true has raged for decades, but may finally have been resolved in an unlikely way: the proof that dark matter exists, and really is an exotic, unknown substance, may have come from the discovery of two galaxies that appear to have no dark matter at all. Today on Space Time Journal Club we’ll look at the papers that reveal this discovery
originally posted by: Woodcarver
a reply to: Arbitrageur
Dark matter is a place holder term. According to our understanding of the size of our universe, the amount of matter that we have calculated to exist, and the speed at which objects are moving away from the center, there should be far more stuff there. Like 95% more stuff in the universe. We don’t know if it is a problem with our calculations, or if there is stuff there that we can't see. We know that we don’t know, so they made up the term to describe that we know we are missing something. Maybe. This has always just been a hypothesis.
originally posted by: Arbitrageur
originally posted by: Woodcarver
a reply to: Arbitrageur
Dark matter is a place holder term. According to our understanding of the size of our universe, the amount of matter that we have calculated to exist, and the speed at which objects are moving away from the center, there should be far more stuff there. Like 95% more stuff in the universe. We don’t know if it is a problem with our calculations, or if there is stuff there that we can't see. We know that we don’t know, so they made up the term to describe that we know we are missing something. Maybe. This has always just been a hypothesis.
The interesting thing about this latest research though, is the discovery of 2 galaxies where unlike most other galaxies, nothing seems to be missing. There doesn't need to be any more stuff there! So some models or ideas of what is going on with observations we can't explain seem to be eliminated by this discovery.
originally posted by: ErosA433
It is an open question. Though with any open question complex stipulation should be done with some caution. It is true that dark matter can be more than one form of matter, it in truth could be many things. It however isn't the truth to say we have no idea about it for example.
From our observations we have the following phenomenological understanding or knowledge, presented with evidence
1) It has electrostatic neutral charge.
Evidence - We do not observe its presence in the EM spectrum
We do not observe any drag effect indicative to a high strength interaction such as with the EM spectrum... - Bullet Cluster)
2) It interacts gravitationally
Evidence
- We observe large lobes of matter in gravitational lensing far beyond the observable stars in galaxies
- We observe universal rapid rotation out to large distances in spiral galaxies
- We have observed Galaxies with very very few stars, but significant mass... so called Dark matter galaxies in which the stars rotate around a central point but there doesn't really appear to be enough stars or gas in that galaxy to support it at all in comparison to say... the milky way
- Clusters of galaxies move too quickly to be bound. This would mean that when we look into the sky, we should not see any observable families of galaxies, and more just a totally random distribution. Again, something is holding it all together in a gravitational well.
3) - Tenable statement - It is probably particulate in nature. This means it could be a particle of some kind rather than a weird field or simply a force.
Evidence
- Objects like the Bullet cluster show how the matter concentration of a cluster can be separated from the particle concentration. This object is two clusters that have passed through each other. The result from gravitational lensing places the centre of mass ahead of the observable mass from optical and X-ray measurements. Optical measures show how hot gas stripped from the clusters drags behind in a shock front structure. What this suggests is that Dark Matter is some kind of particulate matter that experiences gravitation but not electrostatic effects. If it did, it would be dragged back similarly to the hot gas stripped from the clusters.
SO what are the candidates?
Well again we can imagine many, here are a few models...
One problem I see is that observations of our own solar system seem to indicate there's not a significant amount of dark matter in our solar system, since the motions observed locally follow the predictions or relativity well without adding dark matter
Abhay Ashtekar and Javier Olmedo at Pennsylvania State University in University Park and Parampreet Singh at Louisiana State University, Baton Rouge, have taken a step toward answering this question [1]. They have shown that loop quantum gravity—a candidate theory for providing a quantum-mechanical description of gravity—predicts that spacetime continues across the center of the hole into a new region that exists in the future and has the geometry of the interior of a white hole. A white hole is the time-reversed image of a black hole: in it, matter can only move outwards. The passage “across the center” into a future region is counterintuitive; it is possible thanks to the strong distortion of the spacetime geometry inside the hole that is allowed by general relativity. This result supports a hypothesis under investigation by numerous research groups: the future of all black holes may be to convert into a real white hole, from which the matter that has fallen inside can bounce out. However, existing theories have not been able to fully show a way for this bounce to happen. That loop quantum gravity manages to do it is an indication that this theory has ripened enough to tackle real-world situations.
originally posted by: MarioOnTheFly
a reply to: Arbitrageur
bare with me, I have few basic questions, so I can understand it better:
1. Can we detect "dark matter" in any way ? Is it based solely on calculations or is there any kind of observation that lends credence to the idea ?
Among the general public, people compare it to the aether, phlogiston, or epicycles. Yet almost all astronomers are certain: dark matter and dark energy exist. Here’s why.
The indirect detection method of galaxy rotation curves.
2. If we cant detect it, how do we know these two galaxies dont have any ?
That's a valid question for any conceivable explanation of observations. The state of affairs now is that of the two main lines of thought to explain observations, at least it's possible to say the amount of dark matter can vary but it's much more difficult to explain these observations by saying gravity of ordinary matter has different effects at larger distances than what Einstein's model predicts.
3. If dark matter theory is correct and universal, should there be "exceptions" ?
We have not ruled out the possibility of dark matter in our solar system but even if it's at the expected density it's too small to detect because on a galactic scale, the 93 million mines from Earth to the sun is a tiny distance and cosmically speaking it contains a tiny amount of dark matter. On a human scale the 93 million mile radius around the sun could be expected to contain a "large" amount of dark matter, something like 2.3 billion tons! That sounds like a lot, so how could we not detect that? The reason is because the mass of the sun is roughly a billion times a billion times larger than that and we don't have any measurement methods precise enough to detect something that's a billionth of a billionth of the sun's mass, considering all the other uncertainties and error bars that result when calculating orbits. If we ever can make such precise measurements or calculations to measure a billionth of a billionth in this context which significant above the error bars, that would be a great accomplishment, but we are probably a factor of 100,000 away from being able to do that, so not even close.
You said:
One problem I see is that observations of our own solar system seem to indicate there's not a significant amount of dark matter in our solar system, since the motions observed locally follow the predictions or relativity well without adding dark matter
As a layman, this is something that sticks to my childish mind. Observations of our own solar system should be the default starting point.
I thought the video was good because it went beyond just discussing the latest research and covered the reasons why even before the latest findings, alternatives to dark matter didn't seem to fit observations as well.
Havent had time to check the video yet, but defo will later in the day.
If you are referring to Thornhill and other electric universe guys, they don't have any model so it's impossible to say their model is right if they don't have one.
originally posted by: 727Sky
All I know or have read is every attempt has been unable to detect Dark Matter no matter how many years or how much money on test equipment is spent... So... Maybe the universe is much more dusty than anyone has accounted for or maybe the Plasma universe guys and gals are more right all along..
Thanks, I'm always impressed by your comments as I am with these and as usual I agree. While many generally assume that Hawking's model for black hole evaporation is likely to be true, it's not substantially proven by observation, so it's certainly fair to question the Hawking predictions until they are observationally confirmed. Dr. Copeland does say that PBHs are candidates for dark matter so that is not a question, the question is, how much dark matter could such objects account for? As you suggest the answer to that could depend on whether or not Hawking's model is correct. If Hawking's model is correct, I think PBHs are not the answer:
originally posted by: moebius
a reply to: Arbitrageur
From my understanding there is still a mass window for PBHs of sublunar mass 10^20 - 10^22 g as a possible dark matter candidate. They are too light to be detected by micro lensing but heavy enough to not have evaporated.
If one questions the Hawking radiation prediction, one would get a much broader mass range starting from Plank mass BHs, which would probably not be directly detectable at all.
So apparently they don't rule out that PBHs can explain some portion of dark matter observations, but it's not enough, which is along the lines of what Dr Copeland says in the PBH video in my previous post.
Based on a statistical analysis of 740 of the brightest supernovas discovered as of 2014, and the fact that none of them appear to be magnified or brightened by hidden black hole "gravitational lenses," the researchers concluded that primordial black holes can make up no more than about 40 percent of the dark matter in the universe.
The only pdf link I see is for "Dust Grain alignment and magnetic fields", is that what you're talking about?
originally posted by: 727Sky
I was not referring to Thornhill but to some of the latest papers published. Go to the 2:50 minute mark or on youtube there is a link to a PDF. youtu.be...
All I know or have read is every attempt has been unable to detect Dark Matter no matter how many years or how much money on test equipment is spent...