I really like your site and how you write about concepts that weren't sufficiently explained in the MCU. Maybe you can help me decide on an issue regarding Heimdall. Does he see everything in the ENTIRE UNIVERSE, or 'just' the stuff that goes on in the Nine Realms linked by Yggdrasill? I'd really prefer it to be the latter, but I can't find anything in the MCU that makes a clear statement in favor of one or the other. What do you think?
There is no absolutely definitive statement to be made here, I think. However, the most interesting line regarding this is ‘from here I can see nine realms and ten trillion souls.’ This appears to indicate he can see nine realms alone. However. (and please correct my maths if I'm wrong)
(around) Ten trillion souls can mean either one million million (1,000,000,000,000) or one million million million (1,000,000,000,000,000,000) depending on whether Asgard is a long or short scale measurer. (The filmmakers probably intended short scale because most English speaking countries do that.) However, if this is divided by 9, that means an average of 111,111,111,111.1111 souls on each realm.
Okay, so far so good. What does he mean by soul, though? Is it just ‘people’? Or is it all organisms?
Let’s look at populations. Earth has over 7,000,000,000 people and uncountable numbers of other organisms. If we assume he means people, this is two whole figures less than that average. Earth is populous compared to most/all realms we have seen so far; Asgard is a dwarf planet that does not appear to have massively high population density. Jotunheim is strongly implied to be that sort of devastated wasteland all acrosss itself. Svartalfheim appears deserted unless a shipful of dark elves (maximum population is in the low thousands, I have estimated; maximum population before the shown battle with Asgard, perhaps tens of thousands) lands there. Vanaheim, well, the bit we see of it has low population density. On very few of the openings onto other realms are there signs of inhabitation.
Basically; ten trillion, even if we assume short scale, even if we remember this is likely to be an estimate, is waaaay too many souls for the people of the Nine. (I think; the first draft of the post argued the exact opposite but I’m pretty sure I have it right now.) Now, it may be that Heimdall identifies other creatures in the Nine as having souls, though I would say that the figure is too low for every living creature. We have no evidence that Asgard deals with peoples who are not the dominant humanoid species on their realm, but it’s possible; however, on Earth, many of the more likely candidates have fairly low populations. It’s possible that the growth of a soul could occur in enough species to push the figure up, though, although it would have to be pushed beyond the average to make up for the fairly sparsely populated and fauna’d worlds elsewhere.
The easier explanation is that he can see beyond the Nine, I’m afraid. However, it’s significant that he still only mentions nine. It’s possible that all realms are somewhere on the boughs of Yggdrasill, and it is Asgardian convention to refer to all of them as the Nine despite only the Nine being subject to the Convergence or Asgard. Or simply that the others aren’t generally dignified with the name of ‘realm.’
Other relevent things; Heimdall did not see Loki. He appears unaware of Thanos. He did not notice the uncloaked dark elf ships before the Convergence. However, Sif and Volstagg could and did visit the Collector, whose base is in Knowhere (which is apparently his line's private feifdom essentially)
Things Heimdall has not spotted in the outer realms can easily be explained by ‘this guy has a universe to watch and he’s doing it on behalf of a polity interested in nine realms in particular, and he still only has two eyes and time moves as fast for him as anyone.’
TL;DR I think the evidence supports him being able to see anything, Anon. HOWEVER. If you need him not to be able to see things for a fic, here’s a post-script to do as you wish with, though remember he can also hear -
We do not know about his viewpoint. The one definite Heimdall-POV shot we get is definitely from above. There is another ambiguous one that is ground-level and indoors, but it may not be properly Heimdall-POV. People have pointed out that he is presumably able to see a planet from all angles to be able to see what goes on (this may link with Yggdrasill, in that a planet needs a certain level of connection with it for him to be able to do that), and perhaps then rather than invariably looking ‘down,’ he can look at an angle and so see things at ground level. This would mean that if you placed walls and a roof to interrupt all tangents along Earth’s surface, or went underground, you could thwart him. It’s striking in this respect that his POV look down into the centre of the SHIELD compound in Thor is into the centre of a square set of open-roofed ‘walls,’ and that his view is then cut off by the canopy, whereas the ambiguous shot of Jane that may be from his point of view is in her glass-walled laboratory.