> 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 g

> 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-about-the-beginning-of-the-universe 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.