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Our universe started with the Big Bang.
But only for the right definition of our universe
and
"started" for that matter. In fact, the Big Bang is probably nothing like what you were taught.
A hundred years ago, we discovered the beginning of the Universe.
Observations of the retreating galaxies by Edwin Hubble and Vesto Slipher,
combined with Einstein's - then - brand-new general theory of relativity,
revealed that our universe is expanding and if we reverse that expansion far enough -
mathematically - purely according to Einstein's equations,
it seems inevitable that all space and mass and energy
should once have been compacted into an infinitesimally small point -
a singularity. It's often said that the universe started with this singularity
and the Big Bang is thought of as the explosive expansion that followed.
And before the Big Bang singularity, well, they say that there was no 'before' because time and space simply didn't exist.
Now, if you think you've managed to get your head around this bizarre notion,
then I have some bad news: that picture is wrong.
And at least according to pretty much every serious physicist who studies the subject.
The good news is that the truth is way cooler, at least as far as we understand it
Now, before a certain crowd starts
with "all the scientists keep changing their minds - they don't know anything", or "the Big Bang Theory is just a theory"
Let me be very clear, the evidence for a hot dense early universe is practically incontrovertible.
The Cosmic Microwave Background is a direct line of sight to the universe as it was
Only a few hundred thousand years after the hypothetical beginning of time.
We can see pretty much directly that all space and matter in the universe was once crunched at least a thousand times closer together
There's also the relative abundance of simple elements hydrogen and helium in particular
Whose ratio is exactly what we expect if the entire universe was a dense
billions of degrees nuclear furnace for the first several minutes of its existence
In fact,
There's powerful evidence that we should not rewind Einstein's equations that far, at least without introducing some very new physics
For one thing there's also convincing observational evidence that the time before around 10 to the power of negative 32 seconds
Included a period of extremely rapid expansion called cosmic inflation
We've talked about the reasons we need inflation in previous episodes and I'll come back to it in a bit
adding that initial growth spurt solves a couple of the big problems with the Big Bang Theory, but it doesn't change the fact that
Rewinding the expansion of the universe even at different speeds still leads us towards the T equals zero
singularity. I'm going to come back to why we need to forget the idea of this singularity
Doing so will change the way we think about cosmic inflation and about the beginning of the universe
But before we kill the whole idea of the Big Bang singularity, we need to understand what we're killing
What does it really mean for all of space to be compacted into a single point?
This idea is especially weird if the universe is infinite
Now the universe may or may not be infinite
but if we can understand this for the infinite case
Then getting all of this for the finite case is baby stuff at least by comparison
It's tricky to talk about the size of an infinite universe
Instead of the overall volume or radius we talk about the size of an expanding infinite universe in terms of the scale factor
That's the distance between any two points in space at some moment in time
Relative to their distance at some other reference moments that reference moment is typically taken to be right now
So the scale factor of the universe is currently one
Several billion years ago, the scale factor was half, all points in the universe were half as far apart as they are today.
So when I talk about rewinding the expansion,
I mean running the clock backwards to track a shrinking scale factor.
One way to do that is to keep halving the scale factor.
Do that enough times and any two points, no matter how far apart they were, will end up
as close together as you'd like.
Do it enough times and the universe could end up as hot and dense as you like
But it'll still be infinite, spatially, the scale factor is incredibly small
But an incredibly small number times infinity is still infinity
Rewinding the universe this way doesn't leave us with a singularity
The singularity is when all points are not just next to each other
but literally in the same spot
at which point temperature and density are infinite.
That last tiny step is a doozy
The scale factor goes from incredibly small to zero.
So the infinite universe becomes
infinitesimal all points become the same point and
three-dimensional space becomes zero dimensional
That's the singularity
We say that it didn't happen in any one place because a point is zero dimensional there weren't
spatial dimensions for it to happen in
At the same time we say the Big Bang happened
Everywhere at once because even the tiniest fraction of a second later
The universe has infinite size and everywhere is expanding equally
Even if the universe is not infinite then whatever space there is
Comes into being at the same time from that singularity. But what happens to time at the Big Bang singularity?
To get that you can't think about the universe as having one big clock that
Rewinds and then winks out of existence of the Big Bang or into existence if you're going forward
No, you have to think about time in the way Einstein
Intended there is no universal clock time is relative
Clocks are attached to each observer each moving frame of reference to see what time does at the Big Bang
We have to trace a path through space and time
back to the singularity
We trace a path called a geodesic which in general relativity is the shortest path between two space-time coordinates
These are the grids we use to map space-time
Remember that in our rewind all points in the universe get arbitrarily close together before merging at T equals zero
Well, that's the same as saying that all geodesics in the universe converge at the Big Bang singularity
In the same way all lines of longitude converge at the North Pole
so each
Geodesic tracks earlier and earlier times as it approaches the Big Bang
infinite clocks rewinding toward zero and then they all converge and
Then what? well, then nothing.
All geodesics end at the Big Bang singularity and their timelines end with them
Or they start depending on how you want to think about it
The point is that in the pure Einsteinian picture
There is no before the Big Bang because no time line in this universe can be traced there. This is called
geodesic in completeness and it also happens at the singularity in the center of a black hole all
timelines end this time in the forward direction
The analogy with the North Pole is a good one and Einstein himself used it.
Lines of longitude end at the North Pole and it's meaningless to ask what is north of the North Pole?
from the pure Einsteinian point of view
It's meaningless to ask what happened before the Big Bang or after reaching the black hole Center?
Okay, so I'm taking my time to explain something
I already told you is wrong
But it's important because the extreme weirdness of the Big Bang singularity is part of what tells us. It's wrong
Any time you encounter a singularity in the mathematics of a physical theory you have good reason for skepticism
It's probably telling you that your physical theory is incomplete and that you push that theory too far
That's what's happening here
We used general relativity to rewind the universe, but we already know that despite its incredible successes
GR Is an incomplete theory?
At the crazy densities and temperatures of the Big Bang singularity, and just after, GR comes into terrible conflict with quantum mechanics
We've talked about that conflict and its possible resolutions before
But the upshot is that we just don't know how the universe behaves in those conditions
But we do know that pure general relativity is not a good description
and so he probably shouldn't believe its prediction that all space was compacted into a single point and that this is where
Time started. Ok. So what are the alternatives?
Can we really track Geodesics?
and the timelines they embody through the Big Bang and out the other side
If so, what do we find there?
There are several possibilities and they deserve their own episodes and we'll actually get to those soon
But to whet your appetite, first up, cosmic inflation can offer a temporary reprieve from the singularity.
eternal inflation suggests that our universe appeared as a
regularly expanding bubble in an
unimaginably larger continuously inflating space-time in that case before the Big Bang was a period of
exponential expansion that could have lasted
indefinitely
We'll get to the nitty-gritty of that with its inflow tongs and bubble universes real soon
There are also various cyclic universe options
the first cyclic universe idea was the Big Bounce in which the
Gravitational attraction of all matter in the universe was enough to cause it to re-collapse and then presumably bounce outwards again
We now know that there isn't anywhere near enough matter to do that
unless we bring in string theory the
Steinhardt-Turok model suggests that our universe floats in a higher dimensional space
living on geometric objects called brains
collisions between those brains initiate cycles of expansion of contraction
Then there's Roger Penrose
Conformal cyclic cosmology it's even weirder because it postulates the infinite future
boundary of an eternally expanding universe
Looks like the Big Bang of a new universe
Mathematically so our heat death is someone else's Big Bang?
There are some less abstract ways to get a new universe out of an old one
for example an extreme quantum fluctuation could initiate a new Big Bang given infinite time or
The same amount of time could lead to all particles randomly converging back to the same spot
Or maybe black holes birth new universes as in Lee Smolin's "Fecund Universe" hypothesis.
There's a poetry to that last one. The geodesics approaching the black hole singularity
Become the geodesics emerging from the new Big Bang singularity
people love cyclic and regenerating universes
They appeal to our sense of narrative which might be a reason to be wary of these hypotheses
Now they also appeal to our intuition for causality
Things happen because prior events caused them many of our ideas
Just push back the uncomfortable something from nothing moments
physicists have a thing or two to say about that from quantum fluctuations from nothing - Stephen Hawking's
timeless interpretation of internal inflation that draws on the holographic principle
all things we'll discuss in the future as we travel beyond the beginning of Space-Time.
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