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Posted: **Sun Jan 15, 2017 5:40 am**

The Matter of the Bullet Cluster

Explanation: What's the matter with the Bullet Cluster? This massive cluster of galaxies (1E 0657-558) creates gravitational lens distortions of background galaxies in a way that has been interpreted as strong evidence for the leading theory: that dark matter exists within. Different recent analyses, though, indicate that a less popular alternative -- modifying gravity-- could explain cluster dynamics without dark matter, and provide a more likely progenitor scenario as well. Currently, the two scientific hypotheses are competing to explain the observations: it's invisible matter versus amended gravity. The duel is dramatic as a clear Bullet-proof example of dark matter would shatter the simplicity of modified gravity theories. For the near future, the battle over the Bullet cluster is likely to continue as new observations, computer simulations, and analyses are completed. The featured image is a Hubble/Chandra/Magellan composite with red depicting the X-rays emitted by hot gas, and blue depicting the suggested separated dark matter distribution.

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Posted: **Sun Jan 15, 2017 6:13 am**

This is hugely interesting! I find the question of the structure and components of the universe absolutely fascinating.

I consider myself a minor expert when it comes to teasing out important facts about stars from their (RGB, UGV, gri) color and luminosity alone, and similarly, I regard myself as a minor expert when it comes to teasing out facts about galaxies from their (B-V and U-B) color, color distribution and structure alone.

But in all other fields of astronomy, I am a complete amateur.

And because I am a complete amateur in all other fields of astronomy, I gain my knowledge from listening to the professionals. I conclude that those that hold the majority opinion among the experts are more likely to be correct than those that hold the minority opinion. That is because I know that science is about making hypotheses and carry out observations, measurements and data simulations to test those hypotheses. If there is a majority of astronomers who believe in one model, when a minority of astronomers believe in another model, then I conclude that "the majority model" fits the observations and data simulations better than the "minority model".

And I'm going to stick to my belief in the majority model until new observations change the playing field.

But I also realize that science is, and must be, "in a state of evolution". Science is simply the best we can do for now. That is why we can't be sure that our current best model will remain the best explanation of the structure and components of the universe for all time. We may very well make new breakthroughs in the future. We will almost certainly make better and better observations and better and better computer simulations, and that may very well - yes, probably - lead to new insights about the nature of the universe.

But for now, today's APOD illustrates our best understanding of the universe, the ΛCDM model of the universe. According to our current best understanding of the universe, most of the matter in the universe is dark, and reveals itself only through its gravity.

Today's APOD gives beautiful support to the ΛCDM model. We can see two galaxy clusters colliding, The pink areas illustrate the hot cluster gas, made of ordinary hydrogen. This hot gas reveals itself through the X-rays it emits. But the blue areas represent dark matter, and the dark matter reveals itself in the way it lenses the light and shape of background galaxies.

The extraordinary thing about the Bullet Cluster is that the violence of the collision between two galaxy clusters made the "ordinary matter" and the dark matter separate. And our Earthly instruments can document this separation.

The ΛCDM model may or may not be the last word on the nature of the universe, but for now it is our best model. And today's APOD goes a long way towards explaining why.

Ann

Posted: **Sun Jan 15, 2017 7:52 am**

if light itself was the dark matter. that would be irony at its finest

Posted: **Sun Jan 15, 2017 10:56 am**

I'm amazed how slow I am on these things! So-called modified gravity has suddenly (I mean months!) become fashionable, whereas before it was "fringe" at best! And the bullet cluster was considered proof of dark matter, and now it is almost the opposite! I was and I guess still am very strongly anti-modified-gravity (because General Relativity is so simple, so beautiful, so tested, so successful, messing with it seemed - and seems - a crime!) Besides, messing with it in a mathematically acceptable way is, as I understand it, not easy to say the least. Oh well, bumps and grinds in science are common. Here we go again?

Posted: **Sun Jan 15, 2017 10:59 am**

Why doesn't the image appear in tweets?

Posted: **Sun Jan 15, 2017 11:17 am**

Ann wrote: And because I am a complete amateur in all other fields of astronomy, I gain my knowledge from listening to the professionals. I conclude that those that hold the majority opinion among the experts are more likely to be correct than those that hold the minority opinion. That is because I know that science is about making hypotheses and carry out observations, measurements and data simulations to test those hypotheses. If there is a majority of astronomers who believe in one model, when a minority of astronomers believe in another model, then I conclude that "the majority model" fits the observations and data simulations better than the "minority model".

Whether something is scientifically correct or not has nothing to do with majorities; there is no democracy in the scientific principle. Indeed, the more complex a particular subject is, the more likely it is that only a minority will be capable of comprehension.

Posted: **Sun Jan 15, 2017 11:26 am**

hamilton1 wrote:
Ann wrote: And because I am a complete amateur in all other fields of astronomy, I gain my knowledge from listening to the professionals. I conclude that those that hold the majority opinion among the experts are more likely to be correct than those that hold the minority opinion. That is because I know that science is about making hypotheses and carry out observations, measurements and data simulations to test those hypotheses. If there is a majority of astronomers who believe in one model, when a minority of astronomers believe in another model, then I conclude that "the majority model" fits the observations and data simulations better than the "minority model".
Whether something is scientifically correct or not has nothing to do with majorities; there is no democracy in the scientific principle. Indeed, the more complex a particular subject is, the more likely it is that only a minority will be capable of comprehension.

Well said! And also, Ann refers to "astronomers." On questions like this one, astronomers (most of us) know exactly nothing about the theory. Even the great majority of PhD physicists have never taken a course in General Relativity! I'm an astronomer, but I forced myself to TEACH a course in GR. A joy! Then I watched over the years as my fellow astronomers mostly rejected the idea of black holes! Gradually, to my amusement, they came around, as the evidence for astrophysical black holes became overwhelming. Now, everyone accepts them routinely, as they should.

Posted: **Sun Jan 15, 2017 2:25 pm**

hamilton1 wrote:
Ann wrote: And because I am a complete amateur in all other fields of astronomy, I gain my knowledge from listening to the professionals. I conclude that those that hold the majority opinion among the experts are more likely to be correct than those that hold the minority opinion. That is because I know that science is about making hypotheses and carry out observations, measurements and data simulations to test those hypotheses. If there is a majority of astronomers who believe in one model, when a minority of astronomers believe in another model, then I conclude that "the majority model" fits the observations and data simulations better than the "minority model".
Whether something is scientifically correct or not has nothing to do with majorities; there is no democracy in the scientific principle. Indeed, the more complex a particular subject is, the more likely it is that only a minority will be capable of comprehension.

I was talking about the majority views among professional astronomers.

Astronomers spend their professional lives trying to understand astronomy. They learn what the astronomical field already knows about the universe, and through what observations, measurements and simulations that knowledge was gained. They learn the general principles about how we can go about to learn more about the universe. And they push against the boundaries, trying to learn more.

We are lucky that there are those who come up with new ideas and new hypotheses, because thanks to them, astronomy sometimes makes new breakthroughs.

But only sometimes. A new idea has to pass many rigorous tests. If it fails at least some of the tests that the old, established theory has passed, then the new idea is not likely to be accepted by the majority of professional astronomers.

My own, amateur belief is that important new breakthroughs often happens because of new, unexpected observations. That was the case with the discovery of dark energy. Two teams, one led by Saul Perlmutter and the other led by Brian Schmidt, discovered, independently of one another, that very distant supernovas appeared to be farther away than they "ought" to be, as if the universe was speeding up.

Many astronomers have tried to challenge the idea of dark energy. They have tried to show that the apparent acceleration doesn't exist and can be explained through other means, or that the observations were faulty or incomplete. It's fine and good that the theory of dark energy is being tested again and again. If it is faulty, we need to know. But so far, a majority of professional astronomers still think that the apparent acceleration of the universe is real.

Personally I think that that is all we can do: listen to the experts, ask them why they reason the way they do, and ask other experts to challenge the majority view to see if the majority view is going to crumble - but not accept the minority view just because it is the minority.

And of course, we can always try to get better observations, better simulations, better data. In the end, I believe that that is what is going to really reveal more of the secrets of the universe.

Ann

Posted: **Sun Jan 15, 2017 3:27 pm**

hamilton1 wrote:Whether something is scientifically correct or not has nothing to do with majorities; there is no democracy in the scientific principle. Indeed, the more complex a particular subject is, the more likely it is that only a minority will be capable of comprehension.

However, consensus is one of the most important elements of science, and more often then not, consensus corresponds with truth (or the more accurate theory).

It is quite unusual in modern science for major theories that have consensus support to be replaced or modified in major ways.

Posted: **Sun Jan 15, 2017 3:44 pm**

https://en.wikipedia.org/wiki/Bullet_Cluster wrote:
<<The major components of the cluster pair—stars, gas and the putative dark matter—behave differently during collision, allowing them to be studied separately. The stars of the galaxies, observable in visible light, were not greatly affected by the collision, and most passed right through, gravitationally slowed but not otherwise altered. The hot gas of the two colliding components, seen in X-rays, represents most of the baryonic, i.e. ordinary, matter in the cluster pair. The gases interact electromagnetically, causing the gases of both clusters to slow much more than the stars. The third component, the dark matter, was detected indirectly by the gravitational lensing of background objects. In theories without dark matter, such as Modified Newtonian Dynamics (MOND), the lensing would be expected to follow the baryonic matter; i.e. the X-ray gas. However, the lensing is strongest in two separated regions near (possibly coincident with) the visible galaxies. This provides support for the idea that most of the mass in the cluster pair is in the form of two regions of dark matter, which bypassed the gas regions during the collision. This accords with predictions of dark matter as only weakly interacting, other than via the gravitational force.

Mordehai Milgrom, the original proposer of MOND (Modified Newtonian Dynamics), has posted an on-line rebuttal of claims that the Bullet Cluster proves the existence of dark matter. Milgrom claims that MOND correctly accounts for the dynamics of galaxies outside of galaxy clusters, and even in clusters such as the Bullet Cluster it removes the need for most dark matter, leaving only a factor of two which Milgrom expects to be simply unseen ordinary matter (non-luminous baryonic matter) rather than cold dark matter. Without MOND, or some similar theory, the matter discrepancy in galaxy clusters is a factor of 10, i.e. MOND reduces this discrepancy five-fold to a factor of 2. Another study in 2006 cautions against "simple interpretations of the analysis of weak lensing in the bullet cluster", leaving it open that even in the non-symmetrical case of the Bullet Cluster, MOND, or rather its relativistic version TeVeS (tensor–vector–scalar gravity), could account for the observed gravitational lensing.

The Bullet Cluster is one of the hottest known clusters of galaxies. It provides an observable constraint for cosmological models, which may diverge at temperatures beyond their predicted critical cluster temperature. Observed from Earth, the subcluster passed through the cluster center 150 million years ago, creating a "bow-shaped shock wave located near the right side of the cluster" formed as "70 million degree Celsius gas in the sub-cluster plowed through 100 million degree Celsius gas in the main cluster at a speed of about 6 million miles per hour". This energy output is equivalent to that of 10 typical quasars.>>

Posted: **Sun Jan 15, 2017 4:16 pm**

Chris Peterson wrote: It is quite unusual in modern science for major theories that have consensus support to be replaced or modified in major ways.

I suggest care! E.g., I took geophysics from J. Tuzo Wilson at Toronto decades ago. Everyone but him believed continental drift was nonsense pseudo-science. Now we measure those drifts every day as they occur. I might add that I think the "group-think" that utterly dominates the climate change question is deplorable. (I'm not interested in climate change, one way or the other, but I AM interested in "science group-think" psychology.)

Posted: **Sun Jan 15, 2017 4:23 pm**

heehaw wrote:
Chris Peterson wrote: It is quite unusual in modern science for major theories that have consensus support to be replaced or modified in major ways.
I suggest care! E.g., I took geophysics from J. Tuzo Wilson at Toronto decades ago. Everyone but him believed continental drift was nonsense pseudo-science. Now we measure those drifts every day as they occur. I might add that I think the "group-think" that utterly dominates the climate change question is deplorable. (I'm not interested in climate change, one way or the other, but I AM interested in "science group-think" psychology.)

Actually, the matter of continental drift is an interesting one, and supports my point. At the time, there was no consensus theory of geophysics. Continental drift was a theory that essentially replaced nothing. Most certainly, when new theories come along, they can take time to be accepted (which is generally a good thing). But that's quite different from the point I was making that modern theories that have supported substantial consensus acceptance have rarely been replaced.

I can't evaluate your "group-think" comment without knowing what the "climate change question" is.

Posted: **Sun Jan 15, 2017 4:45 pm**

ygmarchi wrote:Why doesn't the image appear in tweets?

http://asterisk.apod.com/viewtopic.php?f=28&t=36753

Posted: **Sun Jan 15, 2017 8:52 pm**

When professional astronomers accept the term 'light years away' rather than 'years ago' when referring to entities in the universe, they are not accounting for time and events since. Thus, significant amounts of forces that might be a source of dark energy are being ignored. For example, such a seemingly inconsequential matter as the transference of light energy from the Earth to the moon and back during moonshine (and the reverse) are left out of calculations. Multiply this energy reservoir throughout even one galaxy and then apply that number to all known galaxies and then extend that calculation to all the events in the time since and, voila, something missing becomes apparent.

Posted: **Sun Jan 15, 2017 9:04 pm**

DL MARTIN wrote:Multiply this energy reservoir throughout even one galaxy and then apply that number to all known galaxies and then extend that calculation to all the events in the time since and, voila, something missing becomes apparent.

Well, no, it doesn't.

We use both comoving distance and light travel time, as is appropriate for what's being studied. It's a matter of convenience, however, not physics, and the difference carries with it no implications for "energy reservoirs".

Posted: **Mon Jan 16, 2017 6:02 am**

heehaw wrote:
Chris Peterson wrote: It is quite unusual in modern science for major theories that have consensus support to be replaced or modified in major ways.
I suggest care! E.g., I took geophysics from J. Tuzo Wilson at Toronto decades ago. Everyone but him believed continental drift was nonsense pseudo-science. Now we measure those drifts every day as they occur. I might add that I think the "group-think" that utterly dominates the climate change question is deplorable. (I'm not interested in climate change, one way or the other, but I AM interested in "science group-think" psychology.)

Interesting point, heehaw.

I remember looking at an Earth globe back in high school in the early 1970s and thinking that the east coast of South America looked like it would fit pretty well along the west coast of Africa, almost like two pieces of a jigsaw puzzle. And I remember thinking, in an idle sort of way, that it was funny that no one thought that it was funny.

Two decades later, I had a sorely missed amazingly learned generalist as a colleague at the school where I worked. My colleague, who was a trained geologist, told me that when he had studied geology at the University of Lund, Sweden, perhaps in the late seventies or early eighties, there was a grumpy old professor who refused to accept the newfangled nonsense of plate tectonics. Only when this professor had retired did the topic of plate tectonics stir a lot of interest and receive a lot of funds at Lund University!

Why was there so much resistance to the idea of plate tectonics? I think it had to do with a reluctance to shake old established ideas about the most fundamental nature of our own dear old Earth. Basically, the idea of plate tectonics was too radical to those who had grown up thinking of the Earth as, in some respects, eternal.

Also I think that the very idea of plate tectonics was so new that experts on the crust of the Earth didn't know what to make of it, and didn't have many ideas of how to study it, or how to get money to fund their studies.

But you must also remember that this was before the advent of the internet. Back then, there was no quick and easy way to get in contact with large numbers of people and discuss whatever topic you were interested in. Very few people were interested in plate tectonics because hardly anyone had heard about plate tectonics! There certainly wasn't a great popular demand that scientist fess up and admit that dear old Earth's continents are adrift - even though it's obvious to anyone who takes a look at the coastlines of South America and Africa on either side of the Atlantic that they are!

My point is that back then, at least some scientists in some fields could just reject an idea if they didn't like it, because hardly anyone would take them to task for it. But things are very, very different now. Imagine that astronomers or cosmologists of today would just flat out reject an idea because they didn't like it! Do you think they would get away with it? No, they sure as heck wouldn't.

Any professional astronomer who dared to be a vocal critic of a new and promising idea about the universe even though (s)he couldn't name any definite specific flaws or shortcomings associated with this new idea would look like an idiot in the eyes of the world!

Ann

Posted: **Mon Jan 16, 2017 10:49 am**

I like such intellectual Clashes of the Titans! Thanks to all!

Posted: **Mon Jan 16, 2017 1:48 pm**

Ann, I thought I must be misremembering what I learned about plate tectonics when I was quite young, based on what you say above, so I checked with Wikipedia. I have to conclude that you were accidentally in a bit of a backwater on that subject. Here's a quote from the first paragraph of the plate tectonics article at Wikipedia...

The theoretical model [of Plate Tectonics] builds on the concept of continental drift developed during the first few decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.

I probably read about plate tectonics while following the International Geophysical Year* articles, back when I was an annoyingly precocious sprout. (The precocious part of me didn't survive school; the annoying part did.)

Rob

The International Geophysical Year ran from July 1, 1957, to December 31, 1958, according to its Wiki article.

Posted: **Mon Jan 16, 2017 3:32 pm**

Ann wrote:I remember looking at an Earth globe back in high school in the early 1970s and thinking that the east coast of South America looked like it would fit pretty well along the west coast of Africa, almost like two pieces of a jigsaw puzzle. And I remember thinking, in an idle sort of way, that it was funny that no one thought that it was funny.

That was an observation that goes back a few hundred years, and quite a few people through the years considered it interesting and worthy of comment.

Why was there so much resistance to the idea of plate tectonics?

Because it was a bad theory. Just because Wegener's ideas from the early 20th century turned out to be right doesn't mean they represented very good science. It was all fine and dandy to talk about continental drift. But for a long time there was still a great paucity of evidence, and worse, any real testable mechanism consistent with the geophysical theory of the time. In a sense, his ideas were before their time, and other science had to catch up. It wasn't until the 1950s that the evidence really began to support the observation of continental drift, and it wasn't until the 1960s that solid physical theory began to offer plausible, testable mechanisms for it. Once the evidence and the theory began coming together, resistance (which is better described as healthy skepticism) rapidly declined. A geophysicist of the 1970s who disputed it was already a fossil in his own field.

My point is that back then, at least some scientists in some fields could just reject an idea if they didn't like it, because hardly anyone would take them to task for it. But things are very, very different now. Imagine that astronomers or cosmologists of today would just flat out reject an idea because they didn't like it! Do you think they would get away with it? No, they sure as heck wouldn't.

Any professional astronomer who dared to be a vocal critic of a new and promising idea about the universe even though (s)he couldn't name any definite specific flaws or shortcomings associated with this new idea would look like an idiot in the eyes of the world!

Let us hope not! We venture into a possibly dangerous time for science. We have simultaneously elite and flaky operations like the Perimeter Institute, which thrive on extremely speculative, but often very unscientific explanations (think String Theory). It isn't the job of scientists to point out the flaws in "promising ideas", it is the job of those proposing those ideas to point out- very critically- why those ideas are better than what we already have. All too often in recent years that isn't happening, and people (who usually lack complete understanding) are eager to hop onto these not-very-scientific hypotheses (which sometimes don't even rise to that level), often it seems for no other reason than an eagerness to throw a wrench into anything that most scientists accept as a good, if incomplete explanation for something.

Posted: **Mon Jan 16, 2017 6:48 pm**

It was confusing (well it was to me

) that the image was released in 2006 yet the 'Magellan' link in the explanation brought up information about the Giant Magellan Telescope whose commissioning is scheduled to begin in 2022!

Posted: **Tue Jan 17, 2017 1:29 pm**

I don't think it has do with HTTPS, it never worked even before the switch

bystander wrote:
ygmarchi wrote:Why doesn't the image appear in tweets?
http://asterisk.apod.com/viewtopic.php?f=28&t=36753

Starship Asterisk*

APOD: The Matter of the Bullet Cluster (2017 Jan 15)

APOD: The Matter of the Bullet Cluster (2017 Jan 15)

Re: APOD: The Matter of the Bullet Cluster (2017 Jan 15)

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Re: APOD: The Matter of the Bullet Cluster (2017 Jan 15)

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Re: APOD: The Matter of the Bullet Cluster (2017 Jan 15)

Re: APOD: The Matter of the Bullet Cluster (2017 Jan 15)