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Hubble and the VLT Uncover Evidence For Self-Interacting Dark Matter
astroengine writes: A new study carried out by the ESO's Very Large Telescope and the NASA/ESA Hubble Space Telescope has revealed for the first time that dark matter may well interact with itself — a discovery that, at first glance, seems to contradict what we thought we knew about the nature of this invisible mass. "In this study, the researchers observed the four colliding galaxies and found that one dark matter clump appeared to be lagging behind the galaxy it surrounds. The dark matter is currently 5000 light-years (50 000 million million kilometers) behind the galaxy — it would take NASA’s Voyager spacecraft 90 million years to travel that far. A lag between dark matter and its associated galaxy is predicted during collisions if dark matter interacts with itself, even very slightly, through forces other than gravity. Dark matter has never before been observed interacting in any way other than through the force of gravity."
Shed some light on this so called Dark Matter?
We need to cast off the shackles of what we want to be true, and look at the evidence in a cold, anyalytical light.
Yeah, we need to study things we don't like and don't want to be true... like how dark matter was described for the first couple decades. Astronomers didn't like dark matter, especially when easy solutions like MACHOs were questioned or almost eliminated. Yet despite continued analysis and work on both dark matter and several alternative theories, alternatives continue to fall short, and dark matter continues to work well.
At this point, I would think a plea for an analytic mind and considering things you don't like is better directed to people on Slashdot, who spend an awful lot of time dismissing dark matter in a rather un-analytic fashion because they just don't like it.
No, because there would be no easy way for the particles to shed momentum and form a clump.
Even when you have a black hole pulling on normal matter, stuff can't easily just fall in. If you had a black hole and a single piece of something falling toward it, unless that piece directly hits the event horizon or comes close to it, it will just fling back out at the really close to the same speed it went in (if it was massive enough and in a small orbit, the orbit might decay from gravity waves, but that is still a slow process and not applicable to smaller dark matter candidate particles). When you have a bunch of stuff forming an accretion disk, the process of exchanging momentum so that some matter falls further in is a complicated process, often coming down to plasma physics effects like the magneto-rotation instability.
So you end up with a large cloud of dark matter, because the particles pass through each other and just all orbit a center of mass. Very slowly, particles can exchange momentum with other dark matter particles, flinging some out of the system as a way of shedding energy and momentum. But the timescale it would take for that to allow them to cool off significantly is longer than the current age of the universe.
In a rush to tailor the evidence to a flawed theory, dark mentor was invented by humon minds in an attempt to save a beloved theory. We need to cast off the shackles of what we want to be true, and look at the evidence in a cold, anyalytical light. When this is done, I'm quite certain that there will be no need for the magical fairy dust matter that is there but isn't there.
The term dark matter is just the name for a discrepancy. For example, the galaxy rotation speed is 220 km/s at our position in the galaxy (8kpc), and stays the same until 30kpc. But the number of stars, which are the mass we can see, declines exponentially. So some mass (10x more than what we see) must be there to keep the rotation fast (otherwise it would be like the solar system -- Pluto rotating around the sun much slower than Mars).
Then in clusters we see that gravitational light acts as a lens and we can infer the mass that bends the light behind it. And it is much more than we see in stars and gas.
In the cosmic microwave background, which is a relic from the last time electrons and photons interacted very strongly, 380000 years after the "Big Bang", we can estimate the density of the universe there. Also, the fraction of matter interacting with photons, is only a fraction of the total matter there.
All of these *different, independent* probes, and several others, point to the same ratio of total matter to electromagnetically-interacting matter.
Now you can take the state of the Universe at 380000 years age, with its total matter, electromagnetically-interacting matter and photon budget and evolve it following general relativity. And people find that the clustering of galaxies, their total number and sizes can be reproduced quite well. And this is not possible without putting that additional, non-electromagnetically-interacting matter there. And In this experiment you can learn something about how weak the electromagnetic interaction must be (for example, a large population of Neutrinos can be excluded, because they interact to strongly, smoothing out the structures).
As you say, another path is to modify the theories of GR, and every week there are papers explaining Dark Matter with alternative theories, sometimes in combination with Dark Energy. This is a path that many people are working on. If you see the term "Dark Matter" as the *name of the problem*, namely the discrepancy between observations and normal matter + GR, then there is no conflict, it does not say how to solve it. Dark matter is real, because the discrepancy exists. And the search for particles is also not concluded yet: Larger, cold objects have been proposed (e.g. brown dwarfes, Jupiter-size planets), as well as new fundamental particles (Neutrinos, as well as as-of-yet unobserved particles, like the sterile Neutrino, or totally new particles from some theories of supersymmetry). Some of them have been excluded -- for example it can not be stellar-size black holes, because of the number of binary star systems we observe in the outer parts of the Milky Way; those would be destroyed by frequent interactions with a large population of masses. The upgraded LHC will try to produce more particles, and there is a real chance it will produce (or exclude) a specific candidate dark matter particle predicted (proposed) by supersymmetry.
Believe me, Astronomers really do not like the idea of Dark Matter, and have been fighting it for decades. But the evidence from many different experiments is there. We still don't know what it is, whether the laws have to be changed or additional particles have to be put there (and which ones). But the range of possibilities is getting smaller and smaller. And putting particles there that do not interact except for gravity has been very successful in explaining various observations. I used to be cautious because in principle you could just arbitrarily put mass where you need it -- but if you start from the Big Bang and only use general relativity, then the created galaxies with dark matte
NB: The message above might reflect my opinion right now, but not necessarily tomorrow or next year.
The dark matter is currently 5000 light-years (50 000 million million kilometers) behind the galaxy —
it would take NASA’s Voyager spacecraft 90 million years to travel that far.
Right. Would it? Okay. How is that supposed to help me imagine 5000 light years? I already know it's a bloody long way. You might as well have told me it was the length of x football pitches or y times the length of the Amazon river.
A comparison with the diameter of the galaxy in question would have been more useful.
systemd is Roko's Basilisk.
I realize the term Dark Matter is something of a placeholder for the cause of some as-yet unexplained observations but many people (including physicists) are taking the term quite literally There are three possibilities for what it could possibly be.
1) There is some form of exotic matter (or other phenomena) whose properties have yet to be discovered but which has a gravitational effect
2) There is an error in the measurements of the matter we can see
3) There is an error in the models we are using to describe the matter we can see
What I don't understand is why so many scientists are favoring 1 when 2 and/or 3 seem to be just as likely. 1 is the most exciting possibility but we have nothing more than indirect evidence for it. I'm waiting for someone to explain why so many seem so sure that it actually is some form of exotic matter. We've been down this road before. We couldn't explain phenomena like the orbit of Mercury until Einstein showed that Newtonian mechanics was merely an approximation of the more accurate relativistic models. People were trying to use the observations as evidence that there might be some undiscovered matter when really it was an inaccurate model. We also make measurement errors all the time. I just don't get how we've ruled out a measurement error or a modeling error.
Contrary to the summary, this is one of the expected properties of Dark Matter. The leading candidate that answers the dark matter observation problem (which is already well-described by buchner.johannes above) is a new kind of particle, known as a WIMP, for Weakly Interacting Massive Particle. "Weakly" doesn't just mean "not strongly", it means "through the weak force". It's postulated that this new kind of particle, predicted by various extensions to the Standard Model of particle physics, interacts with itself through the weak nuclear force.
What we don't know very well is how efficiently this interaction takes place. Ways to measure this (and hence detect WIMP dark matter) include:
1) Direct detection: Wait for a stray WIMP to hit a block of stuff and detect a flash/vibration/decay product/whatever. Many experiments. Status: ongoing.
2) Production: Make some WIMPs in a particle collider. Status: check with LHC in a few months.
3) Indirect: The weak nuclear interaction produces some by-products, like neutrinos and gamma rays. Thus if you look at a spot where there ought to be lots of dark matter (like the center of the galaxy), you might see some extra gamma rays. The Fermi-LAT satellite is doing exactly this. Status: ongoing.
4) Behavior: The interaction will "slow down" the movement of WIMPs by introducing a little bit of drag. This would be a much much weaker version of what happens to normal matter when clouds of gas run into each other. Using gravitational lensing we can probe the mass distribution and look for such drag effects. That's what this article is addressing.
Whoever is the first to confirm the existence of dark matter (whether WIMP or otherwise) is pretty much guaranteed a Nobel, so the race is on.
If we still don't find anything in ~10 years, then we probably need to go back to the whiteboard and figure out something else.
Shameless self-plug: I'm going to discuss this more in an upcoming episode of my podcast.
ad 3: Plenty of people are working on modified models, such as alternatives to general relativity. There are papers coming out every week. https://en.wikipedia.org/wiki/...
ad 2: Errors in measurements can be somewhat excluded as a possibility because many different measurements looking at different aspects and scales find the same result. Wikipedia lists 3.1 Galaxy rotation curves, 3.2 Velocity dispersions of galaxies, 3.3 Galaxy clusters and gravitational lensing, 3.4 Cosmic microwave background, 3.5 Sky surveys and baryon acoustic oscillations, 3.6 Type Ia supernovae distance measurements, 3.7 Lyman-alpha forest and 3.8 Structure formation . See also my other post.
NB: The message above might reflect my opinion right now, but not necessarily tomorrow or next year.
The scientific establishment nurtures contrarians just fine, but they have to play by the same rules as the rest of the community when it comes to backing up their claims and fitting them to the rest of existing data, which is what the armchairs and the crackpots do not wish to do.
Electromagnetic force created chemical bonds and the illusion of substance in normal matter. Even though normal matter is 99.9999% "empty", EM chemical bonds keeps solids and liquids from interpenetrating each other. Since dark matter doesnt seem to have EM chemical bonds, it just difuses through the general emptiness of normal matter. It just may make us feel a little heavier than were really are from just normal matter.
Dark matter IS a modeling error. Every time they talk about 'dark matter' they are talking about modeling error.
Having worked in physics, I can tell you that hunting for modeling and measurement errors is a great way to get funding. Finding a specific modeling error is a HUGE career boost, and papers are written all the time exploring possible errors and how they can be (or were) tested. Same with measurement errors, there was even a story on this very site a few months back about a paper being published going over an error in measurement once new techniques were able to correct for it and the impact it had on previously published results.
But to claim they are never talk about modeling errors represents a fundamental misunderstanding of research into dark matter or even the very concept of it. The whole point is that dark matter is an unattributed correction to current models that demonstrates that they are not correct, but the exact error has not been ferreted out yet. Once it is, 'dark matter' goes away.
Umm No. General Relativity's whole "mass distorts space time" thing kind of does explain what makes things move. As opposed to Newtonian physics that only mathematically (and inaccurately at stellar mass scales) predicted how things should move. That's not to say that through the Scientific Method we will not continue to learn new and interesting things. But GR does offer an explanation of what makes things move.
The USA is only 4X older than me...perspective
Corrections to gravity were discussed at great length when they were still a reasonable alternative. You CAN explain some galactic rotation, and the movement of some clusters. The problem is, in order to explain all galaxies, all clusters, or all galaxies and clusters, modified gravity theories need lots of dark matter anyway.
Dark matter isn't really all that revolutionary of an idea. Neutrinos are "dark" in that they don't interact electromagnetically, and they were mysterious mathematical figments (very prominent physicists literally thought they were fudge factors in nuclear equations) until we observed them.
One only needs to define the photon as a thermodynamic reexpansion of spacetime that was compressed by nearby matter.
Unfortunately that is not a meaningful statement. I have no idea what a "thermodynamic reexpansion" is versus a "non-thermodynamic reexpansion", for example. Nor is it clear how this would be expressed mathematically as a generalization of Maxwell's equations. Nor does your paper do anything more than repeat this meaningless statement.
There may be something meaningful and interesting to say about the thermodynamics of electromagnetism and space-time, but until you give us a mathematical statement of the physical principles you are trying to enunciate it is going to be very difficult for anyone to understand what, if anything, you are talking about.
Blasphemy is a human right. Blasphemophobia kills.