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According
to our best understanding of the Universe, if you travel back in time as far as
you can, around 13.8 billion years or so, you'll eventually reach a singularity -
a super-dense, hot, and energetic point, where the laws that govern space-time
breakdown.

Despite
our best attempts, we can't peer past that singularity to see what triggered
the birth of our Universe - but we do know of only one other instance in the
history of our Universe where a singularity exists, and that's inside a black
hole. And the two events might have more in common than you've ever considered.

It
might sound a little crazy, but, as physicist
Ethan Siegel explains over at Forbes, from a mathematical perspective, at
least, there's no reason that our own Big Bang couldn't have been the result of
a star collapsing into a black hole in an alternate, four-dimensional universe.

In
fact, the idea was first proposed by theoretical physicists at the Perimeter
Institute and University of Waterloo in Canada back
in 2014, and despite physicists' best attempts, no
one has been able to rule it out. So let's step back for a second here.
What we know about the Big Bang is that, immediately after the singularity, our
Universe began expanding. Within a few fractions of a second, it underwent a
rapid period of inflation, increasing in size
by around 10

^{26}, before slowing down again and expanding more gradually.
What
we know about black holes, is that, in our three-dimensional Universe, black
holes spawn a two-dimensional event horizons - which basically means that
they're wrapped in a two-dimensional boundary that marks the 'point of no
return' for matter.

Below
is an artist's impression of what that might look like:

What
black holes and the Big Bang have in common is that they're the only two
instances of a singularity that we know of in the Universe. (A singularity
basically just means a point where the rules that govern our Universe no longer
work.) To the best of our knowledge, our Universe is dictated by two
sets of rules: quantum
mechanics for all the small stuff like particles; and general
relativity for all the bigger stuff, like stars, planets, and you and
me. If you crunch the numbers, black holes defy these rules, because
their event horizons are bigger than can be explained by the behaviour of the
particles inside it.

"The
fact that black holes in our Universe are much more massive than this isn’t a
problem, it simply means that the laws of physics that we know break down at
the singularity we calculate at the centre. If we ever want to describe it
accurately, it’s going to take a unification of quantum theory with general relativity.
It’s going to take a quantum theory of gravity." explains
Siegel.

For
now, though, we don't have that 'theory
of everything', so our understanding of black holes ends at the singularity
- just as our understanding of the Universe does. Knowing that, three
physicists from the Perimeter Institute and University of Waterloo suggested
two years ago that the two singularities could be one and the same - maybe our
Universe was born out of the singularity of a much larger black hole. Or, to
put that another way, maybe our Universe is the three-dimensional packaging
around

*another*universe's event horizon.
"In
this scenario, our Universe burst into being when a star in a four-dimensional
universe collapsed into a black hole," a Perimeter Institute press
release explained back in 2014.

Mathematically
speaking, this holds up. While we can't calculate what happens with a black
hole's singularity, what we can calculate is what happens on the boundary of
the event horizon - and it matches up pretty well with what happened at the
birth of our Universe, as Siegel
explains.

"As
the black hole first formed, from a star’s core imploding and collapsing, the
event horizon first came to be, then rapidly expanded and continued to
grow in area as more and more matter continued to fall in.

If
you were to put a coordinate grid down on this two-dimensional wrapping, you
would find that it originated where the gridlines were very close together,
then expanded rapidly as the black hole formed, and then expanded more and more
slowly as matter fell in at a much lower rate. This matches, at least
conceptually, what we observe for the expansion rate of our three-dimensional
Universe."

Of
course, this whole idea remains an hypothesis until we have some measurable way
of merging the laws of quantum mechanics and general relativity, and peering
past a singularity. But, until then, the coolest thing to consider is that,
based on this concept, there's no reason that our own Universe couldn't be spawning
brand new two-dimensional universes every time black holes are born. "As
crazy as it sounds, the answer appears to be

*maybe,*" writes Siegel. Oof.
We
can't wait to eventually get that theory of everything so we can begin to test
some of these big ideas out. You can hear more about the black hole birthing
our Universe hypothesis in the video below, and read their full paper on arXiv.org.

This post was written by Usman Abrar. To contact the
writer write to iamusamn93@gmail.com.
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