What is the universe expanding into? - Sajan Saini
The universe began its
cosmic life in a big bang
nearly fourteen billion
years ago,
and has been expanding
ever since.
But what is it expanding into?
That's a complicated question.
Here's why:
Einstein's equations of
general relativity
describe space and time
as a kind of inter-connected
fabric for the universe.
This means that what we know of
as space and time
exist only as part of the universe
and not beyond it.
Now, when everyday objects
expand, they move out
into more space.
But if there is no such thing
as space to expand into,
what does expanding
even mean?
In 1929 Edwin Hubble's
astronomy observations
gave us a definitive answer.
His survey of the night sky
found all faraway galaxies
recede, or move away,
from the Earth.
Moreover, the further the galaxy,
the faster it recedes.
How can we interpret this?
Consider a loaf of raisin bread
rising in the oven.
The batter rises by the same
amount in between each
and every raisin.
If we think of raisins as a
stand-in for galaxies,
and batter as the space
between them,
we can imagine that the
stretching or expansion
of intergalactic space will make
the galaxies recede from each other,
and for any galaxy, its faraway
neighbors will recede a larger
distance than the nearby ones
in the same amount of time.
Sure enough, the equations of
general relativity predict a cosmic
tug-of-war between gravity
and expansion.
It's only in the dark void between
galaxies where expansion wins out,
and space stretches.
So there's our answer.
The universe is expanding
unto itself.
That said, cosmologists are pushing
the limits of mathematical models
to speculate on what, if anything,
exists beyond our spacetime.
These aren't wild guesses,
but hypotheses that tackle
kinks in the scientific theory
of the Big Bang.
The Big Bang predicts matter to be
distributed evenly across the universe,
as a sparse gas --but then, how
did galaxies and stars come to be?
The inflationary model
describes a brief era
of incredibly rapid expansion
that relates quantum fluctuations
in the energy of the early universe,
to the formation of clumps of gas
that eventually led to galaxies.
If we accept this paradigm, it may
also imply our universe represents
one region in a greater cosmic reality
that undergoes endless, eternal inflation.
We know nothing of this
speculative inflating reality,
save for the mathematical prediction
that its endless expansion
may be driven by an unstable
quantum energy state.
In many local regions, however,
the energy may settle by random
chance into a stable state, stopping
inflation and forming bubble universes.
Each bubble universe
—ours being one of them
—would be described by its own
Big Bang and laws of physics.
Our universe would be part
of a greater multiverse,
in which the fantastic rate of eternal
inflation makes it impossible
for us to encounter a neighbor universe.
The Big Bang also predicts that in the early,
hot universe, our fundamental forces
may unify into one super-force.
Mathematical string theories suggest
descriptions of this unification,
in addition to a fundamental structure
for sub-atomic quarks and electrons.
In these proposed models, vibrating strings
are the building blocks of the universe.
Competing models for strings have now
been consolidated into a unified description,
and suggest these structures may interact
with massive, higher dimensional surfaces called branes.
Our universe may be contained
within one such brane,
floating in an unknown higher dimensional
place, playfully named “the bulk,” or hyperspace.
Other branes—containing other types of
universes—may co-exist in hyperspace,
and neighboring branes may even share
certain fundamental forces like gravity.
Both eternal inflation and branes
describe a multiverse,
but while universes in eternal inflation are
isolated, brane universes could bump into each other.
An echo of such a collision may appear
in the cosmic microwave background
—a soup of radiation throughout our universe,
that’s a relic from an early Big Bang era.
So far, though, we’ve found
no such cosmic echo.
Some suspect these differing multiverse hypotheses
may eventually coalesce into a common description,
or be replaced by something else.
As it stands now, they’re speculative
explorations of mathematical models.
While these models are inspired and
guided by many scientific experiments,
there are very few objective experiments
to directly test them, yet.
Until the next Edwin Hubble comes along,
scientists will likely be left to argue about
the elegance of their competing models…
and continue to dream about what,
if anything, lies beyond our universe.
