Wikipedia:Reference desk/Archives/Science/2015 November 24

= November 24 =

What causes certain pathogens to be only transmitted orally or nasally or topically or urogenitally?
Why can't astroviridae spread urogenitally? Why can't trichomonas parasite be transmitted by food contamination or by swimming in contaminated pond water? Does it have to do with finding the right receptors or favorable location in a host body? 140.254.70.25 (talk) 14:49, 24 November 2015 (UTC)


 * The two examples are
 * Urogenital spread of Astrovirus (virus)
 * Trichomonas spread by food/nasal (protist)


 * With viruses, one reason is that they are very small and simple. Humans don't have a gene for an arm and another for a leg, but viruses typically have just a few genes whose products physically make them up (though they may have some extras to mess with the host).  There are exceptions like smallpox, which probably has a fully functional kitchen sink hidden away somewhere in its massive innards, and even larger megaviruses.  But when a typical virus only has a few genes, it can't really do more than a few things.  Finding a receptor which may have limited distribution is one of those things.  If a virus does manage to pick up a new gene for a new receptor, then it's not the same virus any more.


 * For protists, it's a more complex question. Some of it is a matter of ecological niche, I suppose.  Trichomonas vaginalis ought to be clever enough to figure out a way to infect other areas, but the body is a tough opponent and it would face many well-established competitors.  You can see from the broader Trichomonas link though that they don't all work the same way, so it's not impossible. Wnt (talk) 15:32, 24 November 2015 (UTC)
 * Part of it has to do with the fact that your skin itself is pretty tough stuff. The Stratum corneum is a layer of relatively impermeable material on the outside of your skin which stops most pathogens from entering.  However, many surfaces such as mucous membranes of the nose or genitals, the lining of the mouth, etc.  don't have this layer, making them more susceptible to pathogens.  Otherwise, however, there are many infections which are not specific to the genitals, exclusive of other ways of entering the body.  For example, most herpes infections can infect the eyes, mouth, nasal passages, or genitals in equal measure, the actual pathogen is the same regardless of its method of entry.  It just can't get through ordinary skin.  -- Jayron 32 15:39, 24 November 2015 (UTC)


 * Or, putting it another way, different organisms find different environments hospitable. One organism will like a high pH, another a low pH. Some like a relatively warmer temperature. Some "like" a certain humidity or level of oxygen. And so they are only found in environments that permit their growth. And nose, vagina, lungs, skin, and urethra differ in those and other ways. - Nunh-huh 03:18, 25 November 2015 (UTC)

Dark matter "hairs" (gravitational lensing of distant dark matter sources)
NASA points to current/future Astrophysical Journal, which means I look up Arxiv.

The premise is that there are sources of dark matter which are subject to gravitational lensing. Because the dark matter moves through planets, it is focused to different degrees in dense and less dense material, forming "hairs" from a low altitude "root" to a high altitude "tip". They say for the Sun these roots and tips are hidden inside it. For their Earth calculation, the root seems to be 1.1 million km and the tip at perhaps 2.4 million km. For Jupiter, 0.21 million km and the tip at 0.9 million km.

However, all this assumes "weakly  interacting  particles streaming  at  220  km/s  (the  approximate  orbital  velocity  of the solar system around the galactic center (Karachentsev and Makarov 1996)) through a compact body". What I don't get is why. If the dark matter particles are coming from some source, or currently exist as a "fine-grained stream" at a fixed location, doesn't that source or stream have to be revolving around the galaxy itself, perhaps originating somewhere close by? Then the "hairs" might come right down to the Earth's surface, no? And if they are truly a diffuse cloud that has been orbiting the galaxy from the beginning, shouldn't they be spread out homogeneously, with no particular roots or tips in any direction?

Another thing I wonder is if any of the other planets project these "hairs" onto Earth. I suppose Venus, like Jupiter is out - being similar to the Earth, the 2.4 million km just shouldn't reach. But maybe something smaller like Mercury, Mars, or Titan, could focus (weaker) streams to a longer distance? Time for NASA to start up an astrology department, perhaps. :) Wnt (talk) 15:12, 24 November 2015 (UTC)


 * The dark matter streams are something that you have to think of in phase space. They don't necessarily come from a nearby source, they depend more on the merger history of the galaxy and may exist for a very long time.  In the simplest model of a dark matter halo, where there are no streams, you'd expect WIMPs near Earth to have completely random and uncorrelated positions and velocities (including direction).  If there are streams, that gives you dark matter particles that are moving together (same velocity and direction) but not clumped spatially (at least on the scale of the solar system).  When this kind of stream interacts with a mass like Earth, some of the clustering in velocity turns into a spatial concentration.  (Again this is easiest to think about in phase space).  For one planet to focus one of these "hairs" onto Earth, you'd have to have an alignment of the direction of the stream with the direction of the other planet.  This would be very unlikely and also wouldn't last very long. --Amble (talk) 19:18, 24 November 2015 (UTC)


 * I'll admit I don't really have this phase space thing in my head. Especially not where they talked about six-dimensional phase space!  But does this differ from looking at stars?  The light from a star isn't clumped into a stream, but we still look out and see it in one direction, and its gravitationally lensed image is in the other.  Looking at a single particle of dark matter, I'm thinking if it is stationary relative to the galactic center it has to fall straight toward the galactic center from here, while to be coming at Earth from a perpendicular angle it has to have a speed relative to the center of the galaxy that is comparable to Earth's, and I suppose if it has a speed faster than Earth's then it should be bound for some higher apogee.  Dark or not, phase space or whatever, it's still gotta orbit, right?  And if particles have a speed comparable to that of Earth, and if the "hairs" project in all directions, then why isn't there a "hair" from particles overtaking Earth at virtually the same speed as Earth, that focus to a root somewhere close to the surface?  Now I'll admit that it makes sense that most of the particles are in some galactic orbit not similar to that of Earth's and so they're indeed moving really fast relative to it, and so most of the hairs are 185 times the surface of the Earth away, but the few exceptions ought to be a lot easier to study. :) Wnt (talk) 21:01, 24 November 2015 (UTC)


 * Yes, any particular orbit will be in some orbit, but not necessarily a circular orbit or one in the galactic plane. So it can be traveling in any direction relative to our solar system.  There won't be hairs projecting in all directions.  A hair only exists if there's a stream of dark matter coming from some particular direction and making up a large fraction of the local dark matter density.  So you could have a stream that's in a similar galactic orbit to our Sun, such that the particles would pass by Earth at much lower speeds than 220 km/s and the hair's "root" would be close to the ground.  This is what you're suggesting, right?  I believe that's possible, but not a great detection target for a few reasons:
 * There's a lot less phase space available for the stream to match the solar system's orbit than for the stream to have a very different orbit, making it unlikely for such a stream to exist. It would have to be an extreme "lucky shot", while the case considered in the paper is a typical sort of dark matter stream (assuming that such things exist at all).
 * Detecting dark matter depends on the number of particles passing through per unit time. If the relative velocity is small, then the number of WIMPs per unit time going through your detector is also small (proportional to v).
 * Detecting dark matter may depend on detecting the energy from a dark matter particle scattering in a detector. (Certainly for WIMP detection this is the case).  If the velocity is small then the energy available is small and it will be very hard for the signal from a single interaction to be larger than the energy threshold for a realistic detector.
 * I don't see a discussion in the paper of possible low-velocity streams (unless I've missed it), so the considerations above are the ones that I think are likely relevant. If you want to know the author's reasons for this choice, you could try emailing him directly.  --Amble (talk) 22:47, 24 November 2015 (UTC)


 * These are good points, and of course I can't really argue against an explicit assumption. I suppose what I was really wondering is if the Gravity Field and Steady-State Ocean Circulation Explorer or something similar might have picked up one or more fixed anomalies on east-west ley lines under the sign of Scorpius (sorry, I couldn't help myself!), but I suppose that's probably too much to hope for. Wnt (talk) 14:41, 25 November 2015 (UTC)
 * The local dark matter density is something like 5×10-25 g/cm3, so even with a huge enhancement you won't see it as a gravitational anomaly. The idea with the "hairs" is that it will boost the rates enough to help with direct detection. --Amble (talk) 17:06, 25 November 2015 (UTC)
 * Wow. I'm always getting tripped up on some paradox with dark matter ... I was thinking since it's invisible but has mass and makes up most of the matter in the universe (excluding dark energy), you could detect it gravitationally more easily than by directly imaging it.  I guess not!  The paper says 10E+7 enrichment, so that's 5x10E-18 g/cc = 5x10E-12 g/m3 ... yeah, that's not going to stand out much against the ocean currents!  Dang how can something be so prevalent and so hard to find! Wnt (talk) 20:26, 25 November 2015 (UTC)
 * Annoying, isn't it? ;-) One way to think about this that I find somewhat helpful is to turn the vastness of empty space (as described by Douglas Adams) on its head.  The remarkable thing is not that there are great empty distances between things, but instead, the remarkable thing is that a small fraction of the baryonic matter has somehow made itself incredibly densely concentrated -- by a factor of something like ×1030!.  A lot of open questions in astrophysics and cosmology have to do with how such a thing could have happened at all.  The "default" behavior would be to stay spread out and nearly undetectable (and this is true not only of the nonbaryonic dark matter, but most of the hydrogen too).  So what we think of as ordinary matter is actually very special stuff that has somehow figured out a way to make itself mind-bogglingly concentrated in a few places here and there.
 * If you do ask the paper's author about nearer hairs from low-velocity streams, I'd be curious to hear his response as well. --Amble (talk) 20:45, 25 November 2015 (UTC)
 * As a side note to the refdesk / more importantly for the encyclopedia, we really ought to have an article on dark matter direct detection. Started a draft Draft:Direct detection of dark matter split from dark matter; feel free to edit.  --Amble (talk) 21:06, 25 November 2015 (UTC)

Fossil crinoids of Ireland
Hello! I would like to ask for some link to a website with a key to Irish and English fossil crinoids right down to the species level. I searched before and could not find anything!:( And if anyone suspects that this is for homework,... It is not. Megaraptor12345 (talk) 17:31, 24 November 2015 (UTC)
 * Not a key, but there is a review article here: Let me know if you can't find a copy of that article. Graeme Bartlett (talk) 20:32, 24 November 2015 (UTC)
 * Another review is here: Graeme Bartlett (talk) 20:57, 24 November 2015 (UTC)
 * Yet another really old one is here: It has a nice diagram summarising all the species, but since it is so old is sure to miss many. Graeme Bartlett (talk) 21:04, 24 November 2015 (UTC)
 * Are not Cystoids different from Crinoids? I am not ungrateful, but I do want the right thing;) Megaraptor12345 (talk) 14:30, 25 November 2015 (UTC)
 * They look to be different groups, but they are hard to distinguish, so it depends on exactly why you want this information. See Cystoid. Graeme Bartlett (talk) 19:42, 25 November 2015 (UTC)
 * It's a Gothic horror set in England, but any fan of krynoids needs to see The Seeds of Doom. μηδείς (talk) 22:11, 25 November 2015 (UTC)