Modern science is full of all sorts of mind-blowing
theories that describe and explain the behavior of the cosmos under conditions that nobody
has directly observed. Perhaps the biggest example is the study of
the Big Bang, which is the beginning of the universe itself. I’ve talked about the Big Bang in other
videos, and some of the comments are what you might call skeptical, including people
who dismiss the idea by saying that, since nobody was there at the time, we can’t possibly
know anything. And, of course, we know a great deal. But I find that a lot of people have some
serious misconceptions about what the Big Bang was. So I thought I would dispel a few and then
tell you what we do know. Perhaps the most common misconception is what
I call the universe began like a giant firecracker. In this model, all of the matter of the universe
was concentrated in a single spot and then it blew up, spreading matter throughout space
and across the cosmos. The location could have been anywhere- perhaps
over there somewhere. This is absolutely not what happened. The reality is much, much, stranger than that. Another misconception is that the Big Bang
explains the exact, precise, moment of creation. It doesn’t. It explains things after the moment of creation,
not the start, and certainly not before. We don’t understand either of those things
in detail. And that’s okay. Scientists admit when they don’t know something. A third misconception is that, prior to the
beginning, the universe was located at a spot, with zero size and infinite density- what
physicists call a singularity. Yeah- that’s not true either. No scientist really believes in a literal
singularity, in spite of what you might have read in some books. The singularity arises from pushing Einstein’s
theory of relativity to the point where it breaks. It’s just a wrong idea. So just what does the theory of the Big Bang
say? And, I want to be clear- this video doesn’t
talk about the data that supports the idea. Just the theory itself. The Big Bang theory says that the universe
was smaller and hotter and has been expanding for billions of years. To best understand the theory, maybe the best
thing is just to start with what we see today, and run the clock backward to the beginning. Let’s start with what we see around us-
a universe that is flat, or nearly so, which means that we can use the simplest math. The part of the universe we can see is 92
billion light years across and the universe that is at least 500 times bigger than what
we see. In a previous video, I said the distance was
at least 250 times bigger, but better measurements have now extended that number. The current universe may actually be infinite
and let’s start with that as a good working hypothesis. If that’s true, our visible universe is
just a bubble in a sea of eternity, extending forever in all directions. Now that’s a little hard to draw, so let’s
just focus on one of the three dimensions, say left-right. If we do that, we can present that direction
as just a number line like you learned in grade school, with a zero point and then two
arrows pointing off to the left and the right. We can make a choice and define Earth as the
center. We can then separate the universe into parts
that are close enough to see and parts that are so far away that we can’t see them. Picking the Earth as the center doesn’t
mean the Earth is the center of the entire universe, rather, it’s simply the center
of the part of the universe we can see. The universe is expanding now, but if we run
the clock backward, we can see that in the past the universe was smaller and smaller. Run it all the way to time equals zero, and
we see that all of the visible universe has collapsed down to what looks like zero size. But we also see that even though we shrunk
the visible universe down to tiny size, the number line is still infinite. That means that, even when the universe began,
it might have already started out to be infinite in size. So that’s kind of a mind-blower. In addition, we see that all the points of
the visible universe all collapsed to the same point. If all points in the universe were once the
same, that’s another reason that we can say that every point in the universe can be
considered the center. So, this dispels the idea of the firecracker
Big Bang. The Big Bang is just the expansion of space,
not the movement of matter through space. Now let’s address the idea that space is
flat. Certainly, the data is consistent with that
conjecture, but it’s not proven. Maybe space is curved, but just super big-
just like parts of the Earth can look flat, when it is (clears throat) very obviously
a giant sphere. How does that work in this expanding space
picture? Well, maybe it’s true. Maybe space didn’t start out as flat, but
rather is some contorted and complicated shape. If that’s so, we need to reconcile the flat
space we see with the possible curved truth. And scientists do that by invoking an addition
to the traditional Big Bang theory. This addition is called inflation. In inflation theory, just a tiny fraction
of a second after the Big Bang, the universe expanded crazily fast. From about ten to the minus 36 seconds until
maybe ten to the minus 32 or 33 seconds, the universe expanded at speeds faster than light. According to some calculations, during this
time, the universe expanded by a whopping ten to the 26 times, with our visible universe
going from a size smaller than a proton to about the size of a grapefruit. Other calculations give different answers. The precise numbers aren’t crucial. But the big idea is that this superluminal
expansion can take any weird shape for the infant universe and make it look flat, which
is what we see today. And, after the inflation stopped, the universe
continued to expand in the way we see today. This expansion after inflation is what most
scientists would call the big Bang. Okay, so what about the singularity- the idea
that all of the matter of the visible universe was collapsed into a single point with infinite
density? Well it’s true that the visible universe
was collapsed into a tiny volume, but the volume probably wasn’t zero size. Remember that the zero-size thing was something
that came from Einstein’s theory of general relativity and we know that his theory doesn’t
work at tiny size scales. For that, we need a new and improved theory,
that we call quantum gravity. Since we don’t have a theory of quantum
gravity, we don’t know the details, but what we believe is, because of quantum mechanics,
the size of the visible universe was just ridiculously small, just not mathematically
zero. Finally, what happened before the Big Bang,
when the universe was perhaps still infinite in size, but with the matter of the visible
universe crushed into a tiny volume? The simple fact is that we don’t know. There are lots of ideas, including our universe
budding off from another universe like a blob in a lava lamp, or multiple universes floating
around in a big space called a multiverse, crashing into one another, or- and this is
the least speculative- that there is just one universe- the one we live in- and it existed
essentially forever in the crushed state, waiting to expand, like a bowstring just before
it is released. And, in this scenario, it’s familiar quantum
mechanics that released the string. So those are the key points of the Big Bang. So, let’s pull it all together by starting
at the beginning and running the clock forward. We’ll start at time equals zero and ignore
what came before it started. Maybe I’ll make a video on the various ideas
people have on that. But not today. To simplify the explanation, we’ll show
just one dimension. The matter and energy of the visible universe
was crushed into an unimaginably tiny volume, represented by this dot here on the number
line. The volume was crazy small, but it wasn’t
zero size. In fact, truth be known, we don’t know much
about the conditions then. While the conditions of the instance of creation
are unknown, it could well have been some sort of quantum foam, with particles winking
in and out of existence. Something caused the universe to begin expanding. We don’t know what. And the first thing that happened was inflation,
which was an extremely rapid expansion of space, where the visible universe grew from
much smaller than a proton to maybe the size of a grapefruit. Remember that for our purposes, the exact
numbers don’t matter. Then, at a time of about ten to the minus
32 seconds or thereabouts, the energy that drove inflation dissipated and turned off. The expansion of the universe was then coasting,
and this is the period that scientists call the classic Big Bang and it’s still going
on today. From that point onward, we know a great deal
about what happened. The universe was hot. Particle physics and then nuclear physics
dominated the landscape. The universe expanded and cooled leading to
atoms, stars, galaxies, and, eventually, us. The key message about the Big Bang is that
it wasn’t an explosion. It is simply the expansion and stretching
of space. And another important point is that no place
in our universe is the location where the universe began or, equivalently, every place
in our universe could be considered the center. No spot is special. It’s also true that no reputable scientist
will claim that we understand in detail what happened at the exact moment when the universe
began. We just don’t. But we’re looking at it and we know a great
deal more than we did a century ago. Imagine what we’ll know a century from now. Hopefully my medical colleagues will come
through with a breakthrough that will let me still be around to see it. Because- man- I sure want to know. Don’t you? In spite of the fact that we don’t know
everything about how the universe began, I’m constantly staggered by the fact that we know
so much. Hopefully this video gave you a better idea
of what we know and some of the common misconceptions. If you enjoyed the video, please be sure to
like it and share it on social media. And be sure to subscribe to the channel, including
hitting the little bell icon so you get notified the next time I make a video about the Big
Bang, which I certainly will. After all, the Big Bang is physics and, of
course, physics is everything.
