>Those "prions" in the paper you are referencing are yeast proteins that have distinct conformations that can propagate like human prions.

Thanks, I missed that they were only roughly analogous to prions. That's an important distinction.

And to address my other point myself, it looks like only a few groups of mammals are vulnerable to them, so evolution hasn't had billions of years to work on this, more like tens of millions, I think.

Very interesting paper! Thanks for the link.

I think we're talking past each other because some basic assumptions are left unstated. I think these are:

• Healthy individuals have zero prions
• The body cannot defend against any level of prions. Once they're in, they're bound to replicate until they reach fatal levels.

If those are true, then I agree that transmission is all that matters. If they aren't true, then the prion level would matter, and bioaccumulation (or really biomagnification) could come into the picture. I read something recently about defense mechanisms called "chaperones" that restore prions to the correct shape. It would also be odd for cells to have no defense against a problem that's surely been around for billions of years. So that's the background for my question.

Are prions completely absent in healthy individuals? If they're present at low levels, then cannibalism would allow bioaccumulation of them.

That's all true, but that doesn't mean bears don't hibernate. All these articles call it hibernation.

>Another key fact is that bears don't actually truly hibernate. They go into what is called "torpor".

Your reference doesn't seem to support this? Also, Wikipedia says:

>Historically it was unclear whether or not bears truly hibernate, since they experience only a modest decline in body temperature (3–5 °C) compared with the much larger decreases (often 32 °C or more) seen in other hibernators. Many researchers thought that their deep sleep was not comparable with true, deep hibernation, but this theory was refuted by research in 2011 on captive black bears and again in 2016 in a study on brown bears.

Thanks for these useful links.

(Actually, come to think of it, shouldn't the formula be sqrt(4pi)/(lmax+1)? Alms from 0 to lmax have (lmax+1)² degrees of freedom (sum_0^lmax (2l+1)). This is enough information to split the full sky into (lmax+1)² pixels, which would then have a side length of sqrt(4pi)/(lmax+1). This works out to 1.13*pi/l, so very close to your formula and far from the 2pi/l I had been using.)

Yes, they typically plot out to l=10000, but all the power out there comes from point sources (if one doesn't clean those away). I used 360°/l, but if I use 180°/l the ACT foreground cleaned spectrum becomes too faint to be detectable at 2.9 arcmin.

What's 0.07° (4.2 arcmin) based on? It's somewhat close to Planck's FWHM at its highest frequencies (the main CMB frequencies are more like 5-7 arcmin), but Planck isn't the state of the art at small scales - that's the South Pole Telescope (SPT) and Atacama Cosmology Telescope (ACT) with resolutions of about 1 arcmin.

Though as you say in another comment, there's hardly any CMB left at those scales. ACT has published a foreground-cleaned CMB temperature power spectrum with significant signal detection up to l = 3700 which corresponds to roughly 0.10° = 5.4 arcmin 0.05° = 2.9 arcmin.

I think I read a paper at some point about the feasibility of detecting the time-derivative of the CMB. If I remember correctly, it was actually the largest scales that were considered the most promising there, not the smallest. Those scales change extremely slowly, but the signal is also much brighter there, and from what I remember that won out.

Firstly, 8±3 doesn't mean "it's definitely between 5 and 11". It just means "it's 68% likely that it's in that range", and then it's 95% likely that it's in the range [2:14] and 99.7% likely that it's in the range [-1:17] (really [0:17] in this case, since it can't be negative).

Secondly, why do I say that it's likely to be lower than 8% rather than higher, given that the error bars go both up and down? That's because we're looking at extreme value statistics. Put simply, we're not looking at a random data point, we're looking at the data point with the highest value. That means we're much more likely to see a positive noise fluctuation than a negative one, because a positive noise fluctuation makes a data point more likely to be the highest one while a negative noise fluctuation does the opposite.

Here's a concrete example. Let's say we have two sets of numbers, set A and set B, each with 1000 numbers in them. In set A, each number has a true value of 5 but ±1 in errors. In set B, each number has a true value of 0, but ±10 in errors. So in reality the numbers in set A are much bigger than in set B. But now look at what happens when we take errors into account. In set A, it's unlikely that we will see any numbers higher than around 8, since a +3 error has a likelihood of only 0.15%. Meanwhile, in set B we're almost guaranteed to see numbers higher than 20. So if we're not careful we will incorrectly conclude that B is really bigger than A.

Sorry, that didn't come out as clearly as I had hoped. If you know some simple programming, then I recommend just writing a 5-line program to test it out yourself.

In an expanding universe things like distance and speed become ambiguous at large distances, with several sensible definitions that all give the same results under normal circumstances suddenly disagreeing. When it comes to distance, this is due to the expansion of space changing the scale of the universe while light is traveling towards us, so effectively changing things in the middle of our measurement. When it comes to speed, it is due to the difference between things moving apart because of their own motion, or things moving apart because new space appeared between them.

As a rough analogy for the former, imagine two ants separated by a piece of string they can walk along, but they're currently standing still. Now someone cuts the string and splices in a much longer piece of string between the ants. The ants didn't move, but now the distance between them (along the string = through space, in this analogy) is much longer. Does that mean the ants had a huge relative velocity when the splicing took place?

It's up to you, really, but I think most of us would prefer to factor out the expansion part and only include the moving part in the definition of velocity. In cosmology, this definition of speed is called peculiar velocity, and would not be particularly lage for two objects on opposite sides of the observable universe.

All of these complications go away if you only look at nearby objects. It's relative speed between two objects close to each other that's limited to 299792 km/s.

• Individual particles reach speeds extremely close to the speed of light, but I guess those don't count as celestial objects
• Streams of plasma ejected as beams by quasars, or as shells of matter ejected by supernova explosions, or plasma spinning around a black hole as part of an accretion disk also move close to the speed of light, but these aren't really objects either.
• Black holes or neutron stars that orbit each other in a pair gradually lose energy, causing the orbit to gradually shrink while speeding up. Just before they hit each other they reach speeds very close to the speed of light. With current technology we can observe these minutes to seconds before merger in gravitational wave detectors like LIGO, when they're at their fastest but most transient. But they will be moving very fast for years before the merger - we just haven't found any at this stage in their life yet. The famous Hulse-Taylor binary is 320 million years away from merger, but is already moving at 0.15% the speed of light and will only speed up from here.
• Stars orbiting supermassive black holes, such as those orbiting the one in the center of the Milky Way, can reach very high speeds. The fastest of these with a robust speed measurement is S14, which reaches 3.83±0.06% of the speed of light.[*] Of course, there could easily be even faster stars orbiting this (or other) supermassive black holes that have not been discovered yet
• Simulations show that collisions between rapidly spinning black holes result in asymmetric emission of gravitational waves. These carry away huge amounts of momentum, and by conservation of momentum, the merged black hole gets a substantial kick in the opposite direction. In the most extreme cases, this may result in the black hole reaching 1.3% the speed of light.
• The fastest known free-flying star, S5-HVS1 according to Wikipedia, moves at 0.59% the speed of light compared to the galaxy.

[*]: The stars S4714 and S175 are nominally faster, at 8±3% and 4.27±0.47% of the speed of light, but given their large uncertainty they are probably slower than S14. S62 with 7.03±0.04% looks like it's the fastest one with a good measurement by a good margin, but this one turned out to be an error (which reminds me that I should get around to updating the wikipedia articles mentioning this star).

What do you mean with "not technically a telescope"? Is there a technical definition of a telescope that this doesn't fulfill?

>these are mostly in highly bottlenecked populations with low genetic diversity

Don't humans have remarkably low genetic diversity though? Especially outside of Africa.

That layer is called the tapetum lucidum, and helps with sensitivity, but comes at the cost of resolution, making their vision blurrier than it otherwise would be.

Are you sure you're not confusing it with the old "brought along with feet and feathers"-hypothesis? The abstract of this recent paper (2020) says the gut hypothesis was only suggested recently:

>Fish have somehow colonized isolated water bodies all over the world without human assistance. It has long been speculated that these colonization events are assisted by waterbirds, transporting fish eggs attached to their feet and feathers, yet empirical support for this is lacking. Recently, it was suggested that endozoochory (i.e., internal transport within the gut) might play a more important role, but only highly resistant diapause eggs of killifish have been found to survive passage through waterbird guts. Here, we performed a controlled feeding experiment, where developing eggs of two cosmopolitan, invasive cyprinids (common carp, Prussian carp) were fed to captive mallards. Live embryos of both species were retrieved from fresh feces and survived beyond hatching. Our study identifies an overlooked dispersal mechanism in fish, providing evidence for bird-mediated dispersal ability of soft-membraned eggs undergoing active development. Only 0.2% of ingested eggs survived gut passage, yet, given the abundance, diet, and movements of ducks in nature, our results have major implications for biodiversity conservation and invasion dynamics in freshwater ecosystems.

This was found for desert ants, but I haven't heard of it being shown for other ants. Can any experts here say if it's thought that step counting is an important mechanisms for ants in general?