Okay depends whether by "us" you mean, us individually, or humans as a species. For humans individually, many things can of course. For instance, anything that kills you within the reaction time, of about a fifth of a second, so you don't have time to register that anything has happened.
So whether you can be aware of anything then depends on what you think happens after you die. Certainly as a living human being you have no opportunity to be aware of what happened. And after you die, if awareness continues in some form (I think it does myself though in what form I think is a big mystery that may have different equally valid answers), again you might well have no memory of the event itself.
But for humans as a species, about the only likely thing I can think of is a Primordial black hole if they exist.
A large black hole, large enough so that it would take almost no time for Earth to collapse into it, would be visible from some distance as it approaches our solar system from the matter it picks up swirling into it.
These could be very small, however, micro black holes, so hard to spot.
Even then, first, if it travels at any speed relative to the Earth, it would be most likely to go right through the Earth and out the other side and continue without doing any harm to anything.
If it did manage to get trapped inside the Earth would take some time to collapse into the black hole because its event horizon is tiny and all the matter has to pass through its event horizon for Earth to collapse. Maybe even take many years if the black hole is tiny. Even millions of years if it is really tiny, because it has a small event horizon, for instance if its event horizon is atom sized, it can only swallow up the Earth one atom at a time which would take a long time.
That's an unlikely scenario though, because the sun is a far larger target and so more likely target than the Earth, which is tiny.
So if there were any micro black holes in our stellar neighbourhood and one of them drifted our way - it would be far more likely for the sun to suddenly stop shining (over some period of time as it collapsed into the growing black hole) than for the Earth to disappear into the black hole - so then not instantaneous as many people would survive for a long time, and in principle you could have a few people at least survive indefinitely using geothermal heat sources after a vanishing sun, so not instantaneous .
Anyway, if these micro black holes exist, we'd see stars suddenly "winking out" in the galaxy. Since that's never happened in observations of huge numbers of stars - then I think we can safely assume that these mini black holes are either vanishingly rare - or else - that for some reason they are unable to swallow up planets and stars . In either case, surely our sun and Earth are both safe from being swallowed by mini black holes.
If Earth was hit by a primordial micro black hole, because of its high speed and small cross section, it would go right through the Earth and do no noticeable damage.
Primordial black hole
An approaching asteroid wouldn't be able to catch all of humanity unawares in this way either. An asteroid large enough to make all humans extinct would be far larger than Mars's moons Phobos and Deimos and be visible the other side of the solar system, easily, even in the larger amateur telescopes.
And - it would be almost certain to make many close flybys of Earth before it hits us, because the Earth is such a small target in the solar system. So during those flybys we'd have an opportunity to deflect it.
And any comet from the outer solar system would be very unlikely to be in the plane of the orbit of the Earth initially. So although an "Earth crosser" it would be like Halley's comet and pass either a long way above or a long way below Earth's orbit when it passes Earth, at least until it has had many flybys and gravity assists, probably of Jupiter, to flatten its orbit out and turn it into a short period comet.
It would also be likely to be ripped apart by Jupiter first, or hit the sun, or another planet - and - it turns out - that our solar system doesn't have any really large craters on any of the inner planets of the solar system since about 3.8 billion years ago.
There are no craters big enough to make humans extinct on Earth on Mercury, Mars, the Moon or Earth itself, in the last over three billion years. Plenty that could make dinosaurs extinct - but they didn't have boats, or planes or submarines, or ability to build fireproof shelters or stockpile food and oxygen for the moment of impact (never mind also ability to deflect asteroids which we could do with enough warning).
So - I think that shows an large asteroid impact is unlikely. Probably because we are protected by Jupiter.
These could be devastating if you live on another solar system that doesn't have the protection of Jupiter - or may be devastating also in some future version of our own solar system, e.g. if some other star gets so close that large numbers of comets from its Oort cloud start coming into our own solar system. But not for present day solar system it seems. And we'd have millions of years of warning of an event such as an approaching star hugely increasing the numbers of large comets entering the inner solar system.
As for supernova and gamma ray burst - that would be sudden and impossible to predict for the side of Earth facing the blast.
But - first - this is unlikely. Since they are visible over a large part of the galaxy, then statistically you'd expect to see many more distant supernovae and gamma ray bursts first before you see a nearby one. So the next one we see is far more likely to be a long distance away and harmless. And so far we haven't seen anything at all - lots in other galaxies, but no supernovae in our galaxy since the seventeenth century.
And there are no nearby stars that are obvious candidates to go supernova in the next few thousand years - not close enough to harm us beneath the protective layer of the atmosphere - bear in mind that our atmosphere provides as much shielding against radiation as ten meters thickness of water.
But on remote chance it did happen, anyone deep underground, or in a submarine, or indeed, everyone on the other side of the Earth would survive.
So again wouldn't make all humans extinct. The Earth's atmosphere and ecosystem would be severely disrupted surely. Many creatures would go extinct. Ozone layer damaged. But the atmosphere would remain breathable and Earth would still have its oceans etc, and its plants, would remain by far the best place for humans in our solar system.
That leaves the false vacuum. There - yes - that could work like that. Not with a bang, but with a Big Rip: how the world will end
But the hypothesis of the false vacuum generally postulates that this happens some time in the distant future - in that article - twenty billion years into the future. So, we don't need to worry about it quite yet. And - it is just one of many possible scenarios for the far distant future, and new ideas there keep being suggested.
For a long time one popular hypothesis was that the universe collapses back again as a "big crunch" but now that's generally thought unlikely. But I wouldn't be surprised if that changed again :). My own favourite is Roger Penrose's "Conformal mapping" where the universe keeps expanding and decaying until there is no matter left, only light. At that point, because there's only light it loses its sense of scale, and then turns into a new big bang and the universe starts again.
Anyway - if it happens twenty billion years in the future, I think probably doesn't count for the purposes of this question.
So, I can't think of anything that we know of that would make all of humanity go extinct instantaneously, or indeed extinct at all in the next few hundred million years.
In theory a big comet or asteroid could do it, but we'd have at least many months of warning, but much more likely it would do many flybys of the inner solar system first.
And anyway it is a pretty sure thing that any asteroid or comet this large would get broken up by Jupiter or impact into the sun or Jupiter first, since an impact this large has never happened to any of the planets in the inner solar system for the last well over 3 billion years.
So whether you can be aware of anything then depends on what you think happens after you die. Certainly as a living human being you have no opportunity to be aware of what happened. And after you die, if awareness continues in some form (I think it does myself though in what form I think is a big mystery that may have different equally valid answers), again you might well have no memory of the event itself.
BLACK HOLES
But for humans as a species, about the only likely thing I can think of is a Primordial black hole if they exist.
A large black hole, large enough so that it would take almost no time for Earth to collapse into it, would be visible from some distance as it approaches our solar system from the matter it picks up swirling into it.
These could be very small, however, micro black holes, so hard to spot.
Even then, first, if it travels at any speed relative to the Earth, it would be most likely to go right through the Earth and out the other side and continue without doing any harm to anything.
If it did manage to get trapped inside the Earth would take some time to collapse into the black hole because its event horizon is tiny and all the matter has to pass through its event horizon for Earth to collapse. Maybe even take many years if the black hole is tiny. Even millions of years if it is really tiny, because it has a small event horizon, for instance if its event horizon is atom sized, it can only swallow up the Earth one atom at a time which would take a long time.
That's an unlikely scenario though, because the sun is a far larger target and so more likely target than the Earth, which is tiny.
So if there were any micro black holes in our stellar neighbourhood and one of them drifted our way - it would be far more likely for the sun to suddenly stop shining (over some period of time as it collapsed into the growing black hole) than for the Earth to disappear into the black hole - so then not instantaneous as many people would survive for a long time, and in principle you could have a few people at least survive indefinitely using geothermal heat sources after a vanishing sun, so not instantaneous .
Anyway, if these micro black holes exist, we'd see stars suddenly "winking out" in the galaxy. Since that's never happened in observations of huge numbers of stars - then I think we can safely assume that these mini black holes are either vanishingly rare - or else - that for some reason they are unable to swallow up planets and stars . In either case, surely our sun and Earth are both safe from being swallowed by mini black holes.
If Earth was hit by a primordial micro black hole, because of its high speed and small cross section, it would go right through the Earth and do no noticeable damage.
Primordial black hole
ASTEROIDS
An approaching asteroid wouldn't be able to catch all of humanity unawares in this way either. An asteroid large enough to make all humans extinct would be far larger than Mars's moons Phobos and Deimos and be visible the other side of the solar system, easily, even in the larger amateur telescopes.
And - it would be almost certain to make many close flybys of Earth before it hits us, because the Earth is such a small target in the solar system. So during those flybys we'd have an opportunity to deflect it.
And any comet from the outer solar system would be very unlikely to be in the plane of the orbit of the Earth initially. So although an "Earth crosser" it would be like Halley's comet and pass either a long way above or a long way below Earth's orbit when it passes Earth, at least until it has had many flybys and gravity assists, probably of Jupiter, to flatten its orbit out and turn it into a short period comet.
It would also be likely to be ripped apart by Jupiter first, or hit the sun, or another planet - and - it turns out - that our solar system doesn't have any really large craters on any of the inner planets of the solar system since about 3.8 billion years ago.
There are no craters big enough to make humans extinct on Earth on Mercury, Mars, the Moon or Earth itself, in the last over three billion years. Plenty that could make dinosaurs extinct - but they didn't have boats, or planes or submarines, or ability to build fireproof shelters or stockpile food and oxygen for the moment of impact (never mind also ability to deflect asteroids which we could do with enough warning).
So - I think that shows an large asteroid impact is unlikely. Probably because we are protected by Jupiter.
These could be devastating if you live on another solar system that doesn't have the protection of Jupiter - or may be devastating also in some future version of our own solar system, e.g. if some other star gets so close that large numbers of comets from its Oort cloud start coming into our own solar system. But not for present day solar system it seems. And we'd have millions of years of warning of an event such as an approaching star hugely increasing the numbers of large comets entering the inner solar system.
BURSTS OF RADIATION
As for supernova and gamma ray burst - that would be sudden and impossible to predict for the side of Earth facing the blast.
But - first - this is unlikely. Since they are visible over a large part of the galaxy, then statistically you'd expect to see many more distant supernovae and gamma ray bursts first before you see a nearby one. So the next one we see is far more likely to be a long distance away and harmless. And so far we haven't seen anything at all - lots in other galaxies, but no supernovae in our galaxy since the seventeenth century.
And there are no nearby stars that are obvious candidates to go supernova in the next few thousand years - not close enough to harm us beneath the protective layer of the atmosphere - bear in mind that our atmosphere provides as much shielding against radiation as ten meters thickness of water.
But on remote chance it did happen, anyone deep underground, or in a submarine, or indeed, everyone on the other side of the Earth would survive.
So again wouldn't make all humans extinct. The Earth's atmosphere and ecosystem would be severely disrupted surely. Many creatures would go extinct. Ozone layer damaged. But the atmosphere would remain breathable and Earth would still have its oceans etc, and its plants, would remain by far the best place for humans in our solar system.
FALSE VACUUM
That leaves the false vacuum. There - yes - that could work like that. Not with a bang, but with a Big Rip: how the world will end
But the hypothesis of the false vacuum generally postulates that this happens some time in the distant future - in that article - twenty billion years into the future. So, we don't need to worry about it quite yet. And - it is just one of many possible scenarios for the far distant future, and new ideas there keep being suggested.
For a long time one popular hypothesis was that the universe collapses back again as a "big crunch" but now that's generally thought unlikely. But I wouldn't be surprised if that changed again :). My own favourite is Roger Penrose's "Conformal mapping" where the universe keeps expanding and decaying until there is no matter left, only light. At that point, because there's only light it loses its sense of scale, and then turns into a new big bang and the universe starts again.
Anyway - if it happens twenty billion years in the future, I think probably doesn't count for the purposes of this question.
CONCLUSION
So, I can't think of anything that we know of that would make all of humanity go extinct instantaneously, or indeed extinct at all in the next few hundred million years.
In theory a big comet or asteroid could do it, but we'd have at least many months of warning, but much more likely it would do many flybys of the inner solar system first.
And anyway it is a pretty sure thing that any asteroid or comet this large would get broken up by Jupiter or impact into the sun or Jupiter first, since an impact this large has never happened to any of the planets in the inner solar system for the last well over 3 billion years.
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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