The author (my former housemate) is an expert on X-ray polarimetry and AGN, so why he's decided to do this I have no idea. It's quite fun though. I had a not inconsiderable hand in correcting the text prior to submission.
Basically he's written a code to try and estimate whether a population could survive in isolation given the effects of inbreeding and overpopulation, here applied to a multi-generational interstellar mission. The clever bit, as I understand it, is tracking the histories of each individual to calculate the degree of inbreeding. The deleterious effects of inbreeding are applied quite crudely, reducing lifespan by some value if above some threshold (neither of which are well-known, but can be adjusted). It can also apply an artificial breeding programme, in the sense of forbidding breeding between individuals who are too closely related.
Here he investigates three possibilities : 1) A small population (150) who are insanely horny and don't care a jot about food supplies, incest, or have any common sense - unsurprisingly, this doesn't work very well; 2) A small population where the number of births is tightly controlled but no other regulations are applied - this works much better but it's not great; 3) A large population (14,000) with the same simple population controls, which succeeds.
Oddly, he doesn't actually use the code's capability of limiting inbreeding. He mentioned something about that needing improvements, but I never found out what exactly. Also the populations of both the small and large (controlled) populations show an initial increase followed by a slow, prolonged decrease of a very similar pattern over time. I'm not sure why that should be, but it's worrying. I also never really understood why the populations are given such restricted initial age ranges (a crew entirely of twentysomethings running a starship ? hmmm...) - that just leaves it more vulnerable to disasters.
What I hope will be tackled with the second paper would be an attempt to find the smallest number that could survive the mission given sensible population controls and natural breeding restrictions (e.g. Jamie and Cersei aside, hardly anyone is going to fool around with their siblings). Of course in a real interstellar mission you'd just take along frozen genetic material and eliminate inbreeding with arbitrarily small populations, but it would be interesting to find the point at which that's not necessary.
I believe the intention is to make the code public (surely this could be applied to the Sims ?) but this isn't the case yet. My guess would be it's not quite in a sufficiently clean state for public distribution.
https://arxiv.org/abs/1708.08649
Wednesday, 30 August 2017
Wednesday, 9 August 2017
Astronomy is too big to fail
Alas the title, "A new astrophysical solution to the Too Big To Fail problem" does not refer to astronomers finding a way to support the banking sector, which would have been more unexpected...
The "Too Big To Fail Problem" is a variant of the "missing dwarf galaxy" problem, where there aren't as many dwarf galaxies detected as expected from simulations. Specifically, the most massive dwarf galaxies should be so large that there's no way - according to the simulations - that their dark matter "halos" can avoid attracting enough gas to start forming stars. It's not just that we're not finding many of the really small dwarfs (that's a problem too), it's that this is a problem even for the bigger guys. It only stops when you get to really massive galaxies.
Supernovae explosions and other processes might be able to explain why the littlest galaxies don't form any stars - a few big blasts could send the remaining gas off into the void, never to be seen again. So they form just a few stars, which eventually die off and thus we can't detect the smallest dwarfs any more. But this explanation shouldn't be possible for the largest dwarfs, which are so massive they ought to be able to hold on to their gas more strongly.
This latest paper uses a bunch of more advanced simulations with better resolution and (they think) treatment of the various processes at work. They make "synthetic observations" of their simulations so they can directly compare their results to real observations. They find that the observations would be underestimating the mass of the dark matter present. Although supernovae can't blast the gas out of the largest dwarfs, it can still cause disruption - the nice thin disc that we see in larger galaxies, and we assume is present in dwarves, gets "puffed up" and becomes substantially thicker. The gas is prevented from collapsing into stars both by its rotation and by this pressure support provided by the various feedback processes from the stars.
This means that the gas discs in the largest dwarves don't extend as far as we expect them to, so their rotation underestimates their true mass. So the dwarf galaxies we detect are actually quite a bit more massive than we've been estimating, if this scenario is correct. They haven't failed, they just aren't quite what we expected from a simpler assumption.
It's a rather technical paper, and personally I think they could have explained their results in a simpler way. I don't think they explain their various scaling relations in a very clear way, making it hard to check if their interpretation of their simulations is sensible. Also (although this isn't crucial), their sample is currently rather small. It would be useful to include more galaxies at different masses to see how well the gas traces the true rotation for other objects.
http://adsabs.harvard.edu/abs/2017arXiv170303810V
The "Too Big To Fail Problem" is a variant of the "missing dwarf galaxy" problem, where there aren't as many dwarf galaxies detected as expected from simulations. Specifically, the most massive dwarf galaxies should be so large that there's no way - according to the simulations - that their dark matter "halos" can avoid attracting enough gas to start forming stars. It's not just that we're not finding many of the really small dwarfs (that's a problem too), it's that this is a problem even for the bigger guys. It only stops when you get to really massive galaxies.
Supernovae explosions and other processes might be able to explain why the littlest galaxies don't form any stars - a few big blasts could send the remaining gas off into the void, never to be seen again. So they form just a few stars, which eventually die off and thus we can't detect the smallest dwarfs any more. But this explanation shouldn't be possible for the largest dwarfs, which are so massive they ought to be able to hold on to their gas more strongly.
This latest paper uses a bunch of more advanced simulations with better resolution and (they think) treatment of the various processes at work. They make "synthetic observations" of their simulations so they can directly compare their results to real observations. They find that the observations would be underestimating the mass of the dark matter present. Although supernovae can't blast the gas out of the largest dwarfs, it can still cause disruption - the nice thin disc that we see in larger galaxies, and we assume is present in dwarves, gets "puffed up" and becomes substantially thicker. The gas is prevented from collapsing into stars both by its rotation and by this pressure support provided by the various feedback processes from the stars.
This means that the gas discs in the largest dwarves don't extend as far as we expect them to, so their rotation underestimates their true mass. So the dwarf galaxies we detect are actually quite a bit more massive than we've been estimating, if this scenario is correct. They haven't failed, they just aren't quite what we expected from a simpler assumption.
It's a rather technical paper, and personally I think they could have explained their results in a simpler way. I don't think they explain their various scaling relations in a very clear way, making it hard to check if their interpretation of their simulations is sensible. Also (although this isn't crucial), their sample is currently rather small. It would be useful to include more galaxies at different masses to see how well the gas traces the true rotation for other objects.
http://adsabs.harvard.edu/abs/2017arXiv170303810V
Friday, 4 August 2017
I won a thing !!!
I have an email, so I guess it's official now...
Dear Rhys,
We have received an information from the Head Office of the Czech Academy of Sciences saying that you are selected for the prize for young researchers, which our Institute proposed some time ago.
There will be a ceremony in Villa Lanna on 4th October 2017. Please save the date!
An official letter will follow.
Congratulations,
It comes with the glamorous title : "The Award of the CAS for young scientific employees for outstanding results of scientific work, achieved with the financial support of the CAS before reaching the age of 35."
I should get that on a badge. This is a general CAS award, not even limited to astronomy...
As far as I can tell, I think I might be the first non-Czech/Slovak winner...
http://www.avcr.cz/en/about-us/awards/prizes-of-the-cas/
Villa Lana is also super-shiny :
http://www.vila-lanna.cz/
Excuse me, I'm off to drink enough tea to shrink my head back down to its usual size...
http://www.avcr.cz/en/about-us/awards/prizes-of-the-cas/
Dear Rhys,
We have received an information from the Head Office of the Czech Academy of Sciences saying that you are selected for the prize for young researchers, which our Institute proposed some time ago.
There will be a ceremony in Villa Lanna on 4th October 2017. Please save the date!
An official letter will follow.
Congratulations,
It comes with the glamorous title : "The Award of the CAS for young scientific employees for outstanding results of scientific work, achieved with the financial support of the CAS before reaching the age of 35."
I should get that on a badge. This is a general CAS award, not even limited to astronomy...
As far as I can tell, I think I might be the first non-Czech/Slovak winner...
http://www.avcr.cz/en/about-us/awards/prizes-of-the-cas/
Villa Lana is also super-shiny :
http://www.vila-lanna.cz/
Excuse me, I'm off to drink enough tea to shrink my head back down to its usual size...
http://www.avcr.cz/en/about-us/awards/prizes-of-the-cas/
A lot of knowledge is a dangerous thing
Yesterday I removed from a certain community's spam folder a post consisting of 94 slides about some sterotypical pseudoscience theory. The usual stuff : "Let us begin with E = mc^2..."
No. Let's not do that, because it's obvious that you haven't got a clue what you're talking about. Lo and behold, it got rapidly worse, with some boiler-plate text praising God at the top of each slide. Aaaarrgggh. If you want to praise God, then fine, but do it in your own time, mate. Not in an internet-based slideshow you apparently expect people to read.
Total ignorance is indeed never damaging, providing one understands that one is totally ignorant and not put in a position where expertise might be required. A little knowledge is a dangerous thing as well. But a lot of knowledge without proper understanding is much, much worse.
Thursday, 3 August 2017
The most useless law in nature ?
In which I summarise the current debate over whether this apparent "new law of nature" means anything or not, and conclude (spoiler !) that it probably doesn't.
It's been known for many years that there's a correlation between how fast matter in a galaxy rotates and how much gas and stars it contains. The problem is that there shouldn't be a nice relation, because galaxies seem to be dominated by dark matter. This strange relationship between normal and dark matter, which should be independent of each other, has been shown in different ways over the years. Recently it was claimed that all of these are just manifestations of a deeper underlying "law" : the Mass Discrepancy Acceleration Relation (or technically the Radial Acceleration Relation, but whatever).
The problem is that the expected acceleration of matter (based on its mass and standard Newtonian gravity) correlates very, very well with its actual acceleration. If there is, as we think for many other reasons, actually a huge amount of unseen "dark matter" present, then it shouldn't do that. Could it be that the dark matter theory is wrong ?
YES ! But it probably isn't. Although modified theories of gravity do predict this relation - indeed, did predict it 30 years ago - it seems that it also occurs in standard dark matter simulations without any difficulties. You can see a comparison of observations (left) and the simulations (right) in the example image below. Since both modifying gravity and using dark matter give the same result, this discovery is probably useless. Weird, but useless.
Read on for tales of academics behaving badly, pretty pictures of galaxies, lots of and lots of graphs, and a surprised dog...
Placeholder post intended to be replaced with a slightly better summary.
It's been known for many years that there's a correlation between how fast matter in a galaxy rotates and how much gas and stars it contains. The problem is that there shouldn't be a nice relation, because galaxies seem to be dominated by dark matter. This strange relationship between normal and dark matter, which should be independent of each other, has been shown in different ways over the years. Recently it was claimed that all of these are just manifestations of a deeper underlying "law" : the Mass Discrepancy Acceleration Relation (or technically the Radial Acceleration Relation, but whatever).
The problem is that the expected acceleration of matter (based on its mass and standard Newtonian gravity) correlates very, very well with its actual acceleration. If there is, as we think for many other reasons, actually a huge amount of unseen "dark matter" present, then it shouldn't do that. Could it be that the dark matter theory is wrong ?
YES ! But it probably isn't. Although modified theories of gravity do predict this relation - indeed, did predict it 30 years ago - it seems that it also occurs in standard dark matter simulations without any difficulties. You can see a comparison of observations (left) and the simulations (right) in the example image below. Since both modifying gravity and using dark matter give the same result, this discovery is probably useless. Weird, but useless.
Read on for tales of academics behaving badly, pretty pictures of galaxies, lots of and lots of graphs, and a surprised dog...
Placeholder post intended to be replaced with a slightly better summary.
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