There is an interesting idea circulating around the astronomical community at the moment. This is not to say everyone believes it, but there is some evidence supporting it, so it is getting a fair amount of attention.
The idea is that we might be living in a black hole.
There are lots of black holes in our universe. There are those formed in the death throes of giant stars. Really massive ones, with masses millions or billions of times that of the Sun are sitting in the middle of most galaxies, including ours. There are suggestions there could be lots of tiny ones we have not yet detected.
A black hole is the result of the extreme compression of matter. The usual way we see this happening is in the explosion of giant stars at the ends of their lives. The collapse and subsequent explosion compresses the core of the star to the point where it collapses into a black hole.
Any celestial body, whether an asteroid, a moon, a planet or a star, can become a black hole if compressed enough. There will be a point where its gravitational attraction becomes too large to resist. The body will then collapse indefinitely into something extremely small and unbelievably dense.
During that collapse its gravitational attraction grows rapidly, increasingly distorting the fabric of space-time until it forms a bubble shutting off the shrinking body from the rest of the universe. This bubble is known as an event horizon. Things can find their way in, but nothing, not even light, can get out. The body has become a black hole.
The strength of gravity we experience on the surface of the Earth is determined by two things, the diameter of our world and its mass. If our planet were suddenly compressed to half its current diameter, the pull of gravity would be four times larger and we would weigh four times as much.
It is possible, in theory at least, to compress the Earth down to a diameter of a centimetre. At that point the pull of gravity would be ten billion times stronger than we experience. This would be enough to form an event horizon, turning our planet into a black hole.
If we could compress the Sun, it would become a black hole as soon as its diameter had fallen to 3 kilometres. Such compression sounds utterly impossible, but we need to remember that matter, as we understand it, is almost totally empty space. There is plenty of emptiness to squeeze out.
We can now calculate how much any mass of material has to shrink to become a black hole.
When we look into deep space, we see that distant galaxies are receding from us, and the farther out we look the faster they are going. There is theoretically a point where those galaxies would be receding at the speed of light, and will become unobservable. We call what we can see the "Observable Universe".
Although Einstein's Theory of Relativity states that material objects cannot move through space faster than light, a result that has been confirmed in a number of experiments, space-time itself can expand at any speed, carrying the galaxies with it. Imagine a swimmer who can only swim slowly being carried away by a fast-moving river.
The current diameter of the Observable Universe is about 93 billion light years, and it is estimated to have a total mass of about 6e51 kg (that is six followed by 51 zeroes). This is where things become really weird. According to the relationship between the masses and sizes of black holes, our universe qualifies as a particularly big black hole.
This idea may or may not be true, but it is a testament to the instruments available to us today that we can even consider such questions. For years scientists have wondered what the inside of a black hole may be like. Maybe we already know.
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After sunset, Venus shines low in the southwest with Saturn close by. Jupiter lies high in the southeast, and Mars is rising in the northeast. The Moon will reach Last Quarter on the 21st and be New on the 29th
