One of the oldest mysteries, mulled by generations
of deep thinkers, is the question of how the universe began. And we know a great deal about what happened. Science offers a compelling and solid narrative
for a great deal of the story, but it's not complete. Let me recap what we know for sure. We know that the visible universe was once
smaller and hotter and it's expanding. We know that there was a moment when the expansion
began. We know that that moment was about 13.8 billion
years ago. For over fifty years, we’ve known of the
cosmic microwave background radiation, which also called the CMB. The CMB is the earliest thing we can see in
the history of the universe. In fact, it’s the remnant afterglow of when
the universe began. The CMB presents a picture of when the universe
was about four hundred thousand years old, which is when the universe was about 0.003
percent its current age. If we equate my current age to the age of
the universe, seeing the CMB is like seeing a baby picture of me when I was 14 hours old. Man, I was a cute baby. However, while the oldest thing we can literally
see existed 400,000 years after the universe began, we actually know about the conditions
in the universe much, much, before that time. For instance, at the moment which we can see
the CMB, the universe had a temperature of about 2,700 degrees centigrade. But the universe is expanding and cooling
off. At even earlier times, the universe was hotter
than that. While we can’t see the universe when it
was much hotter, we routinely recreate the conditions of the early universe in huge particle
accelerators like the Large Hadron Collider in Europe. When we smash nuclei of lead together, the
temperatures are as high as 7 trillion degrees centigrade. One has to be a little more careful about
the definition of temperature when talking about the most energetic collisions ever made,
but when the LHC is colliding beams of protons, researchers create temperatures last common
in the universe when it was a tenth of a trillionth of a second old. That’s ten to the minus thirteen seconds
for the amateur scientist crowd. So these are all facts. If we define the moment when the expansion
of the universe began to be time equals zero, we have hard data for times after ten to the
minus thirteen seconds. That should impress the heck out of you. It still blows my mind, and it’s what I
do every day. But for all we know, there are still things
we don’t. For instance, there is that time period between
time equals zero and ten to the minus thirteen seconds. Science doesn’t know exactly what happened
during that time. And we don’t know about time equals zero
and even less about what happened before that. On the other hand, not knowing everything
is quite different from not anything. We have some very informed thoughts. The most popular idea explaining both the
expansion of the universe and uniformity of matter and energy is an idea which says that
at a time of about ten to the minus thirty six seconds the universe began expanding at
speeds faster than light. Now you might think that this is impossible,
because you’ve learned that nothing can move faster than light, but that’s not quite
true. It’s true that nothing can move through
space faster than light, but there are no restrictions on how fast space can expand. Thus, this period, which is called the inflation
period by the way, doesn’t break any laws of physics. What would cause space to begin expanding
so quickly? Well, on this, we have only an informed guess. In our current universe, we have four known
forces, called electromagnetism, gravity and the strong and weak nuclear forces. Judging from what we know about the behavior
of those forces, it looks as if at high enough energies, they merge to be a single force. This is kind of like how Isaac Newton realized
that there is a single explanation for why things fall and the march of the planets across
the sky, and that single thing is now what we call gravity. If all forces were once the same and they
act differently now, then there must have been a time at which they became different. And one idea is that the thing that gave the
energy to cause the universe to expand is when the strong nuclear force became different
from the others. That’s called a phase transition, and it’s
a perfectly reasonable idea. In case that idea is confusing to you, here’s
an analogy. Suppose you had a container that contained
both air and water. If the temperature of the container is above
the boiling point of water, the container will contain both air and water vapor. However, as the temperature drops to below
one hundred degrees centigrade, the water turns to liquid, while the air remains gaseous. At that temperature, two things that looked
the same suddenly look different. Now, you shouldn’t believe in either inflation,
nor the idea that a single force starts looking like four forces is what caused inflation
to exist. But they both are quite reasonable conjectures
and they don’t require physics beyond what we know to be true from observation. Indeed, both ideas are exactly consistent
with known data. Okay, we haven’t gotten to the moment where
the universe began, but we’re almost there. Let’s recap what we do know. Assuming that no new physical principles arise
at higher energies, we have time equals zero, followed by about ten to the minus forty three
seconds. Before ten to the minus forty three seconds,
the energies and temperatures are all so high that all known physics fails. None of our intuition from what we know can
apply during that time. From ten to the minus forty three seconds
seconds to about ten to the minus thirty six seconds, the universe was expanding and cooling
relatively slowly. At about ten to the minus thirty six seconds,
the strong force became different from the others, which caused the visible universe
to inflate from much smaller than an atom to something about the size of a grapefruit. The inflation period only lasted to about
ten to the minus thirty two seconds. From that time to ten to the minus thirteen
seconds, the universe continued to expand, but now it was coasting. After ten to the minus thirteen seconds, the
expansion continued to coast, and that’s the time when we finally have hard data. Everything after ten to the minus thirteen
seconds is solidly known. Everything before that is speculation, albeit
sensible speculation. Okay, so now what was time equal to zero like? Well, since before ten to the minus forty
three seconds all of our known physical laws fail, we don’t know. Indeed, without a huge advance in physics,
we can't know. So, there’s an admission for you. Science can tell you nothing about this time
for sure. But that’s okay. It’s not a sin to not know something. It’s only a sin to think you do, when you
clearly don't. However, what are some ideas? It turns out that there are a few. Certainly, all of the visible universe was
much smaller than it is now. But the entire universe– including the parts
so distant that we’ll never see them– had to have been much bigger than the visible
universe– at least 500 times bigger. Drawing a three dimensional universe is hard,
so I’ll try to present the key ideas using a one dimensional stand in. Suppose that when the universe began, we represent
the visible universe as this one dimensional line, with us as the center. If that’s the visible universe, the smallest
the rest of the universe can be is represented as the circumference of this circle. That’s the smallest the universe could be
compared to the visible universe. Now the universe could be much larger than
that. Indeed, the circle could be infinitely large,
which means the universe effectively has no curvature and it's infinite. On the other hand, the universe could have
been some squiggly shape. Science doesn’t know and may never know
the answer to that. So let’s concentrate on the visible universe. It was very tiny. All the matter and energy of the universe
was squashed down to a size that is super, incredibly, microscopic. It wasn’t zero size- that’s a common misconception–
but it was very, very, small. What did the universe look like when it was
so small? Remember that we know that the known laws
of physics don’t work back then, so nobody knows. We imagine that perhaps it looked like spacetime
does now, with matter and energy constantly appearing and disappearing. And that word spacetime is important. It’s also often misused when people talk
about the Big Bang. That’s because people hear that time slows
down when gravity gets strong, which is certainly the situation when matter is compressed to
such a tiny volume. But what really happens is that when a person
not in a region of strong gravity looks at a person standing in a region of strong gravity,
time seems to slow for the strong gravity person, at least from the weak gravity person’s
point of view. But for someone in strong gravity, time seems
to continue in the usual way. Furthermore, it’s often said that the Big
Bang created space and time and there is some truth to that. Certainly the Big Bang expanded space and
time. But remember that the theory of general relativity–
along with all other known physics– doesn’t apply before time equals ten to the minus
forty three seconds. So that means that statements about time not
existing at time equals zero or before should be considered as suspicious. So what are considered to be reasonable ideas? There are a few, but always remember that
these are really speculations. There are three. The first is that the universe always existed
in a super-compact state, like a taut bowstring. Then, using the same ideas that govern quantum
mechanics, the universe transitioned into an expanding state, much like when an arrow
is released. This is basically very similar to how nuclear
decay works. That idea is a static one and requires that
the universe existed forever, whatever time meant under those conditions. The other ideas require a less static environment,
but are far more speculative. Suppose that there exist additional dimensions
beyond the usual three of space and one of time. If that’s the case, there may be existing
different universes in those higher dimensions that we can’t interact with. That’s not so different from birds soaring
in three dimensions, when we must walk in two. If that’s the case, then perhaps the universe
we exist in now always existed, floating around in higher dimensions. Perhaps our universe crashed into another
universe and the impact energy is what caused our universe to heat up and expand. It’s hard to imagine that we’ll find an
experimental signature that will confirm this conjecture, although scientists have had a
few testable ideas. Then there’s a third idea, which is called
eternal inflation. Perhaps universes move like the blobs in a
lava lamp, with blobs breaking apart and combining. In this idea, a universe existed, and we budded
off from it. Similarly, universes have budded off from
our universe. In this idea, there is a constant creation
of universe after universe, each from a parent universe. Again, it's hard to see how we would confirm
this idea. Of course you should remember that the bottom
line is that we really don’t know what happened at time equals zero, and certainly not before
then. Furthermore, since we have no data about the
nature of the universe before ten to the minus thirteen seconds and we think there is a time
where known physics absolutely must break down, it's premature to even sound like we
have an educated guess. After all, as my colleagues and I explore
higher and higher energies, we may discover something that radically changes our current
ideas. In that case, the data may point our thinking
in a very different direction. Indeed, I’d be shocked if the final answer
was what you might have read about in popular science literature. I wish I could have told you that science
knew what the answer will be, but that would be a lie, and I’ll never lie to you. We know a lot– I mean we know a ton about
how the universe got from a hot and dense state to where we are now, but we don’t
know everything. And it will take a long time and the effort
of thousands of scientists to get us closer to understand how the universe came to be. I mean- exploring the unknown and pushing
back the frontiers of our current ignorance is what my colleagues and I do. In fact, if you’ll excuse me, I think I
hear the lab calling. Talk to you later. Okay, so that was a heady video. There’s a lot in it, both what we know and
what we don’t. I hope you learned something, including the
point that there’s still a lot to learn. If you liked the video, please be sure to
like it and subscribe to the channel, including clicking on the little bell icon. And, of course, share it on social media,
so all of your friends can learn a little more about physics. After all, and after this video, I’m sure
you’ll agree- physics is everything.
