Mystery in the Perseus Cluster | Hacker News Reader

Mystery in the Perseus Cluster

science.nasa.gov

173 pointsuladzislau69 comments

I've worked quite a bit on this object. The Perseus cluster is one of the most interesting galaxy clusters in the sky and is the X-ray brightest (and so well studied). There's star formation, bubbles of relativistic plasma bubbles generated by the central black hole, presumably heating the surrounding gas and preventing catastrophic cooling, sound waves generated by the inflation of the bubbles, a giant nebula producing emission lines. You could give several talks about this object... Some nice pics here of the central galaxy, NGC1275: http://www-xray.ast.cam.ac.uk/papers/ngc1275/

We'll have to wait to see if this result can be replicated by others and we'll look at the results from ASTRO-H when it launches. If you examine at the original paper (http://arxiv.org/abs/1402.2301), it's certainly not cut and dried dark matter emission (yet).

(/me waves at xioxox)

It may be worth noting the significance of ASTRO-H to this kind of science (full disclaimer: it's a long time since I did any astrophysics so I am liable to get things wrong). The attraction of ASTRO-H is not just newer == better. In fact in several ways it's worse than existing X-ray satellites, for example it won't have the angular resolution of Chandra (so no pretty pictures) or the sensitivity of XMM-Newton (so bad for looking at very faint objects). Where it is designed to excel, however, is in spectral resolution, where it is about an order of magnitude better than the other observatories. This means it's going to be great at measuring the precise energy of incoming X-rays, which is essential for studying emission lines like this. If your spectral resolution is too low you'll find it difficult to detect weak lines at all because they get averaged over such a big energy range that they end up just looking like the background emission from the hot plasma in the cluster.

Once you have established that there is a line in the emission, good spectral resolution is also critical to determining which models are compatible with it. There's almost certainly a lot of ideas people can dream up that will fit "a vauge bump around 3.55keV", many of which will be eliminated once you have a better idea of the shape of the line. This is the sort of science that ASTRO-H should be good at.

Hopefully someone will correct me if I made any egregious errors.

(/me waves back at jgraham)

Absolutely right - ASTRO-H brings new capabilities - high spectral resolution in X-rays which we don't have now. It's pretty exciting.

Beyond that, here in Europe, the European Space Agency has now approved our proposal for Athena (http://www.mpe.mpg.de/Athena), which should provide a large collecting area, high spectral resolution and pretty good spatial resolution. It's going to be a large technical challenge, which unfortunately means we'll have to wait until 2028 for launch. It requires development of a new mirror technology (based on stacks of silicon chip wafers!), development of a cryogenic X-IFU (X-ray integral field unit), which is going to provide spatially-resolve high spectral resolution X-ray spectroscopy. There's also a wide field imager, capable of mapping large areas of the sky quickly.

This is exactly what drives real science. First we gather data and make sure that data is valid, then we compare it to our predictions (model), and if not all things are explained by the model, we start refining the model. In the process, our understanding of the world becomes ever more comprehensive. So in short, data not agreeing with the model is the best possible scenario.

Yup. This is awesome. It's been a while and an awful lot of work since somebody found evidence of some new physics. The LHC has been a bit of a disappointment so far - it has yet to surprise us.

higgs boson ?

http://www.theatlantic.com/technology/archive/2012/07/why-th... - and while that probably isn't the best link it links to several other useful things as well. Unfortunately, we pretty much found the Higgs in the general range we expected it to be, which was in some ways the worst possible outcome, and certainly the least surprising.

"It has yet to surprise us."

Looks like it's exactly where everybody thought it would be. Nice to confirm, but it doesn't give us a lead on any new physics.

Anybody care to give a layman/HS explanation of why this might be dark matter?

It's been a long time since High School chemistry class, but as I remember it, spectral emission lines are caused by electrons changing orbital levels and thereby releasing a photon. So dark matter has electrons? Er, would that mean that it's not dark? Why wouldn't this just mean that we've missed an orbital level somewhere?

The authors of the study are claiming it as an emission line, but not all emission lines come from transitions of electrons. Some come from changes in quantum rotation states (e.g., some molecular transitions), rotation-vibration states (other molecular transitions), or flips in the spin direction of electrons (the famous hydrogen 21cm line). In this case, the authors have not found known lines from molecules or atoms which would correspond to this energy.

The suggestion of dark energy comes from some theories where postulated dark matter particles can annihilate if they hit another dark matter particle. In these theories, the result is emission of high energy photons with an energy which is related to the mass of the individual dark matter particle. Basically E=mc^2. So it is still an emission line, but not one that arises from an election transition in an atom or molecule.

Yes, if it interacts with photons it means that it's not "dark". But the problem is that atomic physics is awfully well travelled, the chances that we've "missed an orbital" somewhere in a conventional material seems slim (or at least it does to the authors -- I'm not an expert).

So a novel photon interaction in intergalactic space would be huge news even if it doesn't map directly to dark matter.

There is lots of unexplored territory in molecular spectroscopy. The ALMA Telescope[0]—primarily operating at millimeter and sub-millimeter wavelengths, where the bulk of molecular transitions are—is likely to discover many spectral lines which have not been classified. Estimates suggest that 50–80% of the lines seen in some observations will correspond to unknown transitions of known molecules or transitions from previously unknown molecules. But the energies of molecular (rotational) transitions are generally far too low for an unknown molecular species to be a viable explanation for a spectral line in the X-rays.

[0] https://en.wikipedia.org/wiki/Atacama_Large_Millimeter_Array

Not perfectly 'dark' or we wouldn't see this at all. Still makes it dark matter for all practical purposes. Given the absolutely massive amount of dark matter that is out there the fact that there's a tiny emission line hinting at its existence that has taken us decades to find indicates that its still pretty damn dark (assuming this holds up).

The Perseus Cluster is probably one of the most interesting galaxy clusters I have every had the privilege of studying.

Anyone interested in the universe should take a quick gander at this massive, massive scientific wonder.

New physics!

Wait, so there are thousands of galaxies, all of them inside a cloud of relatively dense, incredibly hot gas? And it also contains a very energetic source of energy in the middle?

I'm imagining a boiling ocean, covered in volcanoes, with a nuclear bomb going off in the middle of it.

Well, "relatively dense and incredibly hot gas" in this context means "still indistinguishable from vacuum without sensitive sensors, even if you were immersed in it".

Hm, I thought it said it was in the multi-million degree temperature range. I guess you can still have near-vacuum atoms at a pretty high temperature.

What, no Omicron Persei 8 or Lrrr jokes?

This seems like a better source:

http://science.nasa.gov/science-news/science-at-nasa/2014/24...

Deleted comment

> First of all, it's not necessary to use a word such as "menagerie" in any article, apparently it's being used incorrectly anyways, it means "a collection of animals".

A commonly used secondary definition of menagerie is "a strange or diverse collection of people or things."

Thanks. We changed to that from http://thewatchers.adorraeli.com/2014/07/25/mysterious-signa....

Thanks, the writing alone on that other one was atrocious.

Precisely ! Fe XXV ? That sounds a bit like descendent from a long line of monarchs.

I also thought that FE XXV was a misspelling, or bad use of notation. But it's (apparently) the usual notation in spectroscopy. http://www.nist.gov/data/PDFfiles/jpcrd200.pdf

FE I is not ionized Fe (26 protons, 26 electrons, 0 net charge)

FE II is ionized Fe (26 protons, 25 electrons, +1 net charge)

FE III is not ionized Fe (26 protons, 24 electrons, +2 net charge)

...

FE XXV has (26 protons, 2 electrons, +24 net charge)

FE XXVI has (26 protons, 1 electron, +25 net charge)

This is not the standard chemical notation. In chemistry:

FE II is ionized Fe++ (26 protons, 24 electrons, +2 net charge)

FE III is ionized Fe+++ (26 protons, 23 electrons, +3 net charge)

NASA site spells it the same. Presumably it's how physicists call ions.

I learned to spell ions like that in highschool, though never more than Fe III, I think.

If one of the engineers responsible for developing the technology had written this headline: "Possible Widespread Bug In X-ray Telescopes Uncovered"

That was indeed the original assumption when the Cosmic Microwave Background was detected. The problem with the CMB, from an observational standpoint, is that it's enormously uniform. It differs by only a few parts per million across the sky. Which means that when you perform observations with less sensitive equipment the signal tends to look like a uniform noise source. But it really does come from the sky, as we eventually figured out.

I think Nobel committee did Ralph Alpher deep disservice. He had not only conjectured the existence of CMB radiation but also computed properties the radiation would have. The Nobel instead went to two radio engineers who happened to chance upon CMB purely by accident.

George Gamow, his advisor, thought it would be funny to include Hans Bethe as a coauthor for Alpher's original research, just for the author list "Alpher, Bethe, Gamow". Bethe was not involved in anyway in that piece of research.

More here http://en.wikipedia.org/wiki/Ralph_Asher_Alpher#Big_Bang_nuc...

If you have not read Gamow's popular science writings.... highly recommended.

Experimentalists often tend to get more credit than theoreticians. People (apparently along with the Nobel committee) prefer results over potential theories.

That appears to be so, but the committee's application of that preference is rather self-contradictory. Without an explanation, the result would not even have been a contender for the prize, and it was Alpher who first provided the explanation.

As Alpher also showed that the universal ratio of hydrogen and helium isotopes can be explained by nucleosynthesis in the big bang, he seems to have been seriously overlooked.

Just to be clear, I'm not endorsing the committee's views, just stating them. I agree that Alpher deserves more attention and credit for his work.

Yes. As the article states, theoreticians have already suggested 60 potential dark matter explanations. Teasing those apart requires careful experimental and theoretical work, and that's what the Nobel committee is (supposed to be) looking for.

I agree, but as one astronomy professor put it, "Dem's the breaks"

I had conveniently forgotten about the origins of the CMB when jotting down my witty riposte. Well put!

Buzzfeed style headlines. Article includes this gem:

'The menagerie of dark matter candidates that might produce this kind of line include axions, sterile neutrinos, and "moduli dark matter" that may result from the curling up of extra dimensions in string theory.'

1) Assumption of dark matter 2) String theory

It's never hard to find a group of theoretical physicists willing to tag anything currently, poorly misunderstood as most likely an effect of dark matter and string theory.

And, of course, this will require a new observatory.

The headline perfectly summarizes that this observation is unexaplained by modern physics. You are right that associating this with dark matter and string theory is unfounded speculation, but there's nothing wrong with the headline.

Also, a new observatory is a good thing. Bring it on.

Which is why they changed the headline. "Bring em on." - George W. Bush

We changed the headline along with the url.

This observation notwithstanding, if dark matter cannot be observed, isolated or detected, then why should we treat it as anything more than an arbitrary hypothesis? Why not dismiss it similar to how we dismiss the idea that God created the universe, or indeed that God is holding the cosmos together?

It can be both observed and detected, it can be mapped even. The detection is a pretty indirect one right now though.

There's the usual stuff about galaxy rotation curves, there's the http://en.wikipedia.org/wiki/Bullet_Cluster which people consider to be more of a smoking gun, but then there's also evidence from http://en.wikipedia.org/wiki/Baryon_acoustic_oscillations visible in the CMB. That last one's slightly over my head, but apparently cosmologists find it extremely convincing, especially when combined with the other evidence. The wiki page talks about dark energy, but the subject has significance for dark matter too. Let me see if I can find the relevant plot...

At the end of the day it's just another weakly interacting particle. Last time we discovered one of those was the neutrino in the 50s, it's not an outlandish thing to think about.

EDIT: here's the BAO stuff: http://ned.ipac.caltech.edu/level5/Sept09/Einasto/Einasto6.h...

Well, the thing is that "dark matter" doesn't necessarily have to all be super-exotic unknown particles. It's just that the behavior we see in the universe suggests that galaxies, clusters of galaxies, clusters of clusters, etc. contain more matter than we are able to detect, since if they contained only the matter visible to us their gravitational behavior would be different.

Of course, quite a bit of it might turn out to be exotic new types of particles, but that's a question for theory and experiment.

The precedent here is quite strong, incidentally, on a few counts.

We have high confidence in our understanding of gravity on the macro scale, and that understanding has survived some crises. For example, the orbit of the planet Uranus, as originally calculated, was "wrong" -- it did not appear to be conforming to the understood behavior of gravity. One theory which was developed to explain this, and which could at the time have been criticized in the style of your comment, was that there must be another planet further out, whose gravity was perturbing Uranus' orbit. Calculating backwards from Uranus' behavior, this theory predicted the location in the sky of the additional planet... which was then promptly observed via telescope (this was the discovery of Neptune).

Similarly, we have high confidence that weakly-interacting particles -- undiscovered variants of which are a popular candidate for examples of dark matter -- exist. The neutrino is the classic example: a particle which was posited by theory as "this is the only thing that makes sense given what we see", but which would necessarily be incredibly difficult to detect on account of barely if ever interacting with anything (and thus, again, would be open to your criticism). Of course, the neutrino was eventually detected and its existence confirmed.

So there is nothing wrong or arbitrary or unusual, in terms of the history of physics, in positing something like dark matter.

>> It's just that the behavior we see in the universe suggests that galaxies, clusters of galaxies, clusters of clusters, etc. contain more matter than we are able to detect, since if they contained only the matter visible to us their gravitational behavior would be different.

I've often wondered about this(and don't know any physicists so would really love some insight on this), why do we assume that there's additional matter rather than question whether our models are correct at that scale?

>, why do we assume that there's additional matter rather than question whether our models are correct at that scale?

There are also physicists that question the models. Some scientists follow a path of inquiry based on assumptions that Einstein's theory is mostly correct -- hence a postulation of "dark matter" which leads to searching for the existence of it(1). But there are other physicists that assume there's something missing in the accepted equations that would explain the galaxy rotation speeds without any need for "dark matter".

Each competing theory tries to accumulate more and more evidence for their case until one "wins" (scientific consensus)(2). Right now, the "dark matter" line of inquiry has more scientists, more papers, and more press coverage.

(1)direct instead of indirect observation

(2)Some might say Dark Matter theory has already "won" in the marketplace of ideas

Some do: http://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

The reason that we are talking about dark matter is that it is detectable by its gravitational effects on ordinary matter. Our ways of looking at it are still incredibly blunt but they're much better than what we had 20 years ago. We can make hypotheses about dark matter and conceive observations to test these hypotheses.

As for dismissing God, I don't see why, we just need to come up with some signature that would differ between a designed universe and an accidental one and apply for observation time on a suitable instrument.

Dark matter has a different cognitive status than God, because there is some evidence, albeit inconclusive, that suggests dark matter (from what I understand; IANAP). In contrast, there is no evidence that suggests God, so a claim that God exists would be arbitrary and is therefore rightfully dismissed.

I would say that God is exactly what you are studying when you are looking at dark matter. God is a metaphor for the unknowable ground of being, the mystery that holds us all. The wisdom traditions have been about understanding the patterns of the Universe, in all senses and to provide a mechanism and context to dissolve the idea that something exists as separate from anything else. Theists and Atheists both seem to get it wrong when they argue over the "existence" of God, as Joe Campbell said, it's about understanding how to read a metaphor and experiencing the direct internal subjective experience of reconnection.

Isn't it the other way about? The reason we are hypothesising dark matter is because we can't explain some gravitational measurements. Did someone postulate dark matter particles, then later say 'well if these exist, they must cause some strange gravitational patterns.' ?

Yes, dark matter is of course an explanation invented because of observations. But it would just be a just-so story if you couldn't probe further consequences of that explanation by making new targeted observations.

Dark matter was proposed as an explanation for observations, e.g. the galaxy rotation problem. [1] Assuming the hypothesis is correct, then we have therefore already observed it (albeit indirectly, but all observations are indirect).

[1] http://en.wikipedia.org/wiki/Galaxy_rotation_curve

Isn't that putting the cart before the horse a bit? You need to explain galaxy rotation, fine, but I could invent any arbitrary hypothesis to say why the galaxies are rotating too fast, then cite the rotation as evidence for the hypothesis. This doesn't strike me as very good science. I would want to capture some dark matter in a lab and examine its properties (or something equivalent) and then conclude that this might explain our astronomical observations.

Many scientific theories have a history of the following sort:

1. Huh, those are some weird observations. What could explain it? 2. Maybe it's X? That could explain it. 3. So could Y? 4. Here's an experiment we could do to rule out Y! 5. Hmm, actually the result of that experiment was consistent with Y, but constrains some of its free parameters. And Y could still be false.

And so on.

Science doesn't require that you have something in a lab, just that you make hypotheses which are testable by some sort of observation. Otherwise we'd have to throw out nearly all of cosmology.

There are competing hypotheses still with respect to dark matter (e.g. that the force of gravity works differently than we thought over long distances), precisely because we haven't yet figured out how to do the experiments to rule out all but one hypothesis. But that's why it's called a hypothesis. We haven't yet figured out the answer! That's the beauty of science.

Sometimes you need a lab so big that you need to observe astronomical phenomena before your time runs out.

Dark matter explains two things that we currently don't know how else to explain: how the galaxies are rotating so fast without flying apart and why we see gravitational lensing from seemingly otherwise empty space.

I have never heard about detecting gravitational lensing coming from empty space. Do you have any info/references on that?

When two galaxy clusters collide, the ordinary matter can get slowed down by non-gravitational interactions, leaving the dark matter to continue on inertia and gravity alone.

Closest references I could find: http://phys.org/news/2013-02-dark-galaxies-ways-collision-he...

"In the Bullet Cluster, a collision between two galaxy clusters appears to have caused a separation of dark matter and baryonic matter." - https://en.wikipedia.org/wiki/Dark_Matter

Here are the first links that came up when Googling it:

http://www.lsst.org/lsst/science/scientist_dark_matter

http://scienceblogs.com/startswithabang/2011/04/20/how-gravi...

http://www.ifa.hawaii.edu/~ger/ASTRO-110_sp08/Lecture28_Dark...

Because there are numerous calculations and observations indicating that it is there. In other words, the hypothesis is supported by evidence. That cannot be said for certain supernatural beings.

Because dark matter is the only theory that fits all of the observational evidence. There have been many theories put forward to explain the phenomenon which has been labeled "dark matter" and the only one that has withstood the test of time is "cold dark matter" in the form of "weakly interactive massive particles" (or WIMPs). Perhaps that theory is wrong, but so far it's held up remarkably well while other theories have crumbled.

Is it? Cosmologists constantly keep adding Dark Matter, Dark Energy and yet they can't explain things like the results of the experiment in this article. Could it be that the solution is much simpler? Could it be that we're simply misinterpreting or miscalculating some of the data?

Could it be that your understanding of the issues is simplistic, and scientists are actually asking all the relevant questions and dark matter is still the best answer they can come up with?

I'm going to second what gnaritas has said and suggest that perhaps you just have an insufficient understanding of the evidence and the current state of cosmological theory.

Also, despite the name "dark energy" has no relation to dark matter and the evidence for either does not overlap. Moreover, the inability of one theory to explain everything else in the universe does not invalidate that theory. The theory of biological evolution fails to explain the existence of neutron stars, yet it is still a valid theory.

Very unlikely. There have been a huge number of observations and a lot of eyes on the data. The evidence for something that behaves as if dark matter exists is overwhelming.

There are observations which we can't explain with current scientific theories, and Dark Matter is a theory that seems to explain many of these observations. Either we'll find irrefutable evidence for it, or we'll ditch it, and find another theory to explain the observations. Most science is formed by testing theories, even if those theories are wrong and you end up making different theories to explain the observation.

IMHO our current conception of dark matter/energy is just a modern application of 'aether'[1]. It basically means we're not sure what it is yet, but it's there.

[1] http://en.wikipedia.org/wiki/Aether_(classical_element)

I'm a layman, but my guess is that as physicists know that there isn't enough matter and energy in the universe to account for its mass, they really want to know why.

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