Recent comments in /f/askscience

Interesting-Month-56 t1_j45nggz wrote

Reply to comment by Aseyhe in How do we know that dark matter isn't just ordinary matter our instruments can't detect? by jmite

I think to restate this simply (with the caveat that simple == less correct) is that we do know it’s not things we can detect easily, like elements (hydrogen, helium, iron, etc). These things tend to clump together and do things like fall into stars where we can detect them by their emission lines. Which how we know things like the hydrogen/deuterium ratios of stars and galaxies. We know the mass is there through observation of galactic rotation. We just can’t see it.

What’s left are largely things that are hard to detect. Like neutrinos. Or theoretical things like subsolar mass black holes or various WIMPs that are also really hard to detect.

As to what dark matter is, there isn’t any definitive proof of one thing over another, though some things are more likely than others.

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hoummousbender t1_j45mll6 wrote

Reply to comment by epanek in Since the world is getting warmer, is the arctic treeline being pushed back? by JanniesStopBanningMe

One example: In Alaska, There is a reduction in permafrost, leading to a boom in trees, and a huge boom in beavers. The ponds they create warm the soil further and create more readily available water, which attracts other animals like muskrats and waterfowl. An entirely new ecosystem is being created. https://www.nationalobserver.com/2022/02/23/news/alaska-warming-tundra-beaver-boom

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Aseyhe t1_j45ksax wrote

Reply to How do we know that dark matter isn't just ordinary matter our instruments can't detect? by jmite

For example, we cannot rule out that the dark matter might be asteroid-mass black holes (e.g. figure 10 of this article). Why couldn't it just be asteroids?

The main lines of evidence against such a possibility are related to the early universe. This is a time when the the universe was very hot. Asteroids could not exist in such an environment; they would dissociate into diffuse plasma like all the rest of the ordinary matter. In this context, all ordinary matter is equally detectable, in the sense that it has an equal impact on what we observe. But what do we observe?

  1. The relative abundances of light elements throughout the universe. We understand nuclear physics and can predict the ratios of hydrogen, deuterium, helium, etc. that should have emerged from the Big Bang. What we find is largely consistent with ordinary matter comprising only 5% of the total energy density today. If the density of ordinary matter were higher, we should find less deuterium and more helium than we do. The first figure of this paper (page 9) illustrates nicely how the primordial element abundances depend on how much ordinary matter there is.

  2. Temperature variations in the cosmic microwave background. In the early universe, the ordinary matter and photons were tightly coupled, which led to such effects as pressure oscillations and sound waves. Dark matter, on the other hand, only interacted via gravity. This causes them to have very different effects on the evolution of temperature and density variations in the early universe, which manifest themselves to us in the cosmic microwave background. Here's an animation of how changing the density of ordinary matter ("baryons") would alter the "power spectrum" of the cosmic microwave background temperature, which is something we have measured extremely precisely, e.g. the top panel of this figure.

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doltishDuke t1_j45jom1 wrote

Reply to How do giraffes breathe? by NimishApte

It's not the same principle but if you're interested in this you might want to check out the breathing system on long-neck dinosaurs. For example brachiosaurus reached up to 23 meters and they were still able to breath.

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mckulty t1_j45hlvp wrote

Reply to When nerve damage occurs, where is the pain perceived? at the site of damage or at the end of the nerve? by menooby

There are no pain receptors in the trunk of the nerve, only at the ends. Pinch the ulnar nerve at the elbow and you get pins and needles along the underside of the arm and last 2 fingers, not at the elbow.

You can open the skull and stimulate the postcentral gyrus directly, and the sensation will be from the foot, not the head.

When the nerve loses its function (or you lose a limb), the brain it was attached to hallucinates and gives you phantom paresthesias that feel like the limb itself, and not the stump.

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