Question, is it possible that our universe could be in a black hole? When you look at the way our existence is structured, everything has a parent. An an electron has an atom has it's parent, atoms have molecules has their parent etc. Germs, bacteria, humans, planets all have parents. Surely our universe has to have a parent.
That logic leads to infinite regress... If everything has a parent, then what's the parent of that thing.
So either there is an infinite chain, or the logic is flawed -- e.g. it's possible to not need a parent.
Another possibility is that our conceptions of causality / structure / parenthood are fundamentally flawed (but still useful for human purposes at this point in time), and the question isn't fully coherent.
Possible? Sure, anything is possible - but apply your argument the other way round. People contain molecules, molecules contain atoms, atoms electrons and so on - does it then seem reasonable to ask whether that goes on forever? And if not, why should it be reasonable if you go upwards in the scale?
But it does seem reasonable. Very much so if going down far enough you find micro black holes at the center of atoms.
But you need some really funky geometry to make it more interesting then this played out trope. Say they are all the same black hole from different times seen through its own gravitational lense.
> atoms have molecules has their parent etc. ... Surely our universe has to have a parent
Well... there's a limit to the scientific reach: when the scientists speak about "the Universe" they typically speak about "everything that is we can expect to reach using our instruments." Whatever is behind that is simply unreachable, and therefore out of the scope of the science.
But the good news is: what is reachable is immensely huge, and these far limits don't affect our lives directly, even if we can observe "what's there" (up to these limits).
In older times, some people thought that the movements of the planets, which are quite close to Earth compared to what we today can observe, affect our day-to-day life (and that was the idea behind "astrology"). Now we know it isn't so, they simply move because the gravity exits. We can observe much farther today: in the last 100 years we discovered that some of the points on the sky aren't stars but the galaxies: each point actually some billions of stars, much, much farther than the single stars we see! And we discovered that the farthest objects we can see are now some 46 billion light years from us, and that we actually see what there happened some 13 billion years ago -- not to mention that the we are actually made from the star dust (anything but our hydrogen atoms) and the rests of the "big bang" (the hydrogen atoms in our bodies)!
That's "the Universe" the scientists talk about. What's behind these limits can't predictably affect our own lives, as no signal "from this universe" can reach us faster than the speed of light: it takes 8 minutes for a light from Sun to reach us. Whatever produced some light that is 13 billion years old is now some 46 billion years away from us and changed through these 13 billion years. But we know that the "Universe" gets bigger so these objects we can never reach not even with the light from us to there, as they will continue to be even farther from us. So that's it. 46 billion light years in every direction.
> Question, is it possible that our universe could be in a black hole?
Not without redefining what a black hole is.
General Relativity is a metric theory of gravitation. Black holes arise in General Relativity, and are defined by the family of metrics they source. There are astrophysical objects that humans observe (with various instruments) that match the predictions for black holes in General Relativity within the limits of measurement error. This article describes further such observations of the black hole in our galaxy's central parsec.
The salient feature is that a black hole metric has a boundary across which there is only one-way travel. Anything inside that boundary inevitably, inexorably, collides with a gravitational singularity inside that boundary. For the sake of quantum mechanics being valid everywhere (including deep inside black holes), there are proposals which would replace the singularity with some final structure of ultradense matter whose internal states resist collapse into a literal singularity, even as one collides arbitrarily large amounts of matter into it. However, in such proposals, the inevitable, inexorable inwards motion from the horizon inwards to quite near the "middle" remains a feature.
The inevitable, inexorable inwards motion is fast according to the wristwatch of an unlucky person crossing an event horizon; you won't need to take a calendar with you in order to count down your final minutes in a black hole like the one at the centre of our galaxy. You would have longer for a much much larger black hole, but as you and everything else would be collapsing inwards quickly, you would not observe the formation of stars or galaxies inside the horizon, even for an arbitrarily large black hole. Your sky would look nothing at all like the skies we have in our solar system; at the very largest scale for a black hole, you would not see the cosmological redshift that we see, and would not detect anything like the same cosmic microwave background.
At the largest scales -- where we take clusters of galaxies as particles of dust -- our universe is well-described by a metric for an expanding spacetime, with lots of supporting evidence from detection of the redshift of emissions and absorption spectra compared with angle on the sky, luminosity, and other features of distant galaxies. The expansion is not like the time-reversal of the inside of a universe-sized black hole, in particular since to a good approximation the distribution of galaxy clusters is isotropic and homogeneous, and groups of clusters nearby one another (on the a tight angle for the line of sight to each, and each at similar distances) are moving slowly and randomly with respect to one another, while such a group taken as a whole is nearly uniformly receding from our own galaxy cluster. This is nothing like the behaviour one would get inside a truly enormous black hole.
> surely our universe has to have a parent
It's not required, but there are cosmological models that have notional "parents". Sean Carroll, a physical cosmologist at Caltech, has a set of slides outlining a few at https://www.slideshare.net/seanmcarroll/what-we-dont-know-ab... -- slide 14, "Reproducing cosmologies" and slide 17, "Reproducing cosmologies don't have an entropy problem", both references in the lower right hand corner that you can follow-up on if you are interested in the technical aspects.
The crucial point, however, is that we do not yet have good observations of the very early hot, dense universe of a bit less than fourteen billion years ago, so we have some freedom to choose among various theories which wholly match the history of the universe that we do have good observations for.
