Hey folks, Phil Plait here, and for the past
few episodes, Iâve been going over what we know about the structure, history, and
evolution of the Universe, and how we know it. Now itâs time to put that into action. We
can use this knowledge of physics, math, and astronomy to figure out what the Universe
was like in the past, going all the way back to literally the first moment after it was
born. So, here you go: a brief history of the Universe: In the beginning there was nothing. Then there
was everything. Oh, you want more? It may seem a little weird to suppose that
we can understand how the Universe got its start. But itâs much like any other field
of science: We have clues, observations, based on what we see going on now. Knowing the rules
of physics we can then run the clock backwards and see what things were like farther and
farther into the past. For example, as Iâve talked about in the
past couple of episodes, the Universe is expanding. That means in the past it was denser, more
crowded, and hotter. At some point it was hotter than the surface of a star, hotter
than the core of a star, hotter than the heart of a supernova. And as we push the timer back
even farther we find temperatures and densities that make a supernova look chilly and positively
rarefied. A lot of what we know about the early Universe
comes from experiments done in giant particle colliders. When the cosmos was very young
and very hot, particles were whizzing around at high speeds and slamming into each other,
creating other subatomic particles in the process. Thatâs exactly what colliders do!
The higher energy we can give our colliders, the faster we can whack particles together,
and the earlier phase of the Universe we can investigate. Thatâs one of the main reasons
why we keep making âem bigger and more powerful, to test our ideas of what the young cosmos
was like. So letâs wind the clock back. Looking around
us, peering into the Universe both near and far, what can we say about the beginning of
everything? When the Universe got its start, it was unfathomably
hot and dense. It was totally different then than it is today, because when you pump more
energy into something, the way it behaves, even its fundamental physical nature, changes. If you take a snowball and heat it up, itâll
melt. We call that a phase change, or a change of state. Heat it more and it vaporizes, changing
into a gas. Itâs still water, still composed of water molecules, but it looks and acts
pretty differently, right? When you heat something up what youâre doing
is giving it more energy. In a solid this means the atoms wiggle around more and more
until they break free of their restrictive bonds with each other, and the solid melts.
The atoms are still bound by other forces, but if you heat them more they break free
of those, too, and the liquid becomes a gas. Heat them more, give them more energy, and
the atoms whiz around faster and faster. Heat them to millions or billions of degrees, and
the atoms themselves fall apart. They collide so violently they can overcome the hugely
strong forces holding their nuclei together, and you get a soup of subatomic particles;
electrons, neutrons, and protons. Heat them more and even protons and neutrons
will collide hard enough to shatter into their constituent subatomic particles, which are
called quarks. And as far as we know, quarks and electrons are basic particles, so they
canât be subdivided any more. Maybe you can see where Iâm going here.
As we wind the clock backwards, the Universe gets denser and hotter. At some point in the
past it was so hot that atoms wouldnât have been able to hold on to their electrons. A
little farther back and it was so hot that nuclei couldnât stay together, and the Universe
was a small, ultra dense ball of energy mixed with neutrons, protons, and electrons. Go a wee bit farther back and even that changes.
Neutrons and protons couldnât form, because the instant they did theyâd whack into each
other hard enough to fall apart. The Universe was a sea of electrons and quarks. The cosmos was a bizarre, unfamiliar place
back then. Even the basic forces we see today â gravity, electromagnetism, and the two
nuclear forces responsible for holding atomic nuclei together as well as letting them disintegrate
in radioactive decay â were all squeezed together into one unified super force. Like the snowball melting and vaporizing,
each of these moments in the history of the Universe was like a phase change. The very
nature of reality was changing, its laws and behavior different.
At some point, we go so far back, so close to that first moment in time, that our laws
of physics⌠well, they donât break down so much as say, âHere Be Dragons.â We
just donât understand the rules well enough to be able to say anything about that first
razor thin slice of time. How far back are we talking here? If we call
the instant of the Big Bang âtime zero,â then our physics cannot describe what happens
in the first 10-43 seconds. Now let me just say, only semi-sarcastically,
that thatâs not so bad. The Universe is 13.82 billion years old, so being able to
go back to that very first one-ten-millionth of a trillionth of a trillionth of a trillionth
of a second is a massive triumph of physics! What happened after that fraction of a second
is better understood. The Universe expanded and cooled, the four forces went their separate
ways, and the first basic subatomic particles were able to hold themselves together. This all happened
in the very first second of the Universeâs existence. Three minutes later â yes, three minutes
â the Universe cooled enough that these subatomic particles could start to stick together.
For the next 17 minutes, the Universe did something remarkable: it made atoms. It was still ridiculously hot, like the core
of a star, but itâs at those temperatures that nuclear fusion can occur. For a few minutes
the particles smashed together, forming deuterium, an isotope of hydrogen, helium, and just a
smattering of lithium. A little bit of beryllium was made as well, but it was radioactive,
and rapidly decayed into lithium. Then, at T+20 minutes, the Universe cooled
enough that fusion stopped. When it did, there was three times as much hydrogen as helium
in the Universe. This primordial ratio is still pretty much true today. When we measure
the Sunâs elemental abundance, we see itâs roughly 75% hydrogen, 25% helium. At this point the Universe is still hotter
than a starâs surface, but itâs also still expanding and cooling. As it does, structures
start to form as the gravity of matter can overcome the tremendous heat. These will become
the galaxies we see today. This is important and a bit weird, so Iâll get back to it
in a minute. The next big event happened when the Universe
was at the ripe old age of about 380,000 years. Up to this point, electrons couldnât bond
with the atomic nuclei zipping around; every time they did it was so hot that random photons
would blow them off again. The Universe was ionized. But then, after 380 millennia, it had cooled
enough that electrons could combine with protons and helium nuclei, becoming stable neutral
atoms for the very first time. We call this moment ârecombination.â This was an important event! Free electrons
are really really good at absorbing photons, absorbing light. When the Universe was still
ionized, prior to recombination, it was opaque. A photon couldnât get very far before an
electron sucked it up. But after recombination, the photons were
free to fly. The Universe became transparent! Why is this important? Because the light emitted at this time is
what we see as the cosmic microwave background today! Those neutral atoms emitted light;
they were as hot as a red dwarf star. Those photons have been traveling ever since, fighting
the expansion of the Universe, redshifting into the microwave part of the spectrum, and
seen today all over the sky. That background glow predicted by the Big Bang model has been
on its journey to Earth for almost 13.8 billion years! This light is incredibly important, because
it tells us what the Universe was like not long after it formed. For example, that light looks almost exactly
the same everywhere you look in the sky. It looks âsmooth.â That tells us that matter
was very evenly distributed everywhere in the Universe at that time, and also that all
the matter had the same temperature. If there had been one spot that was denser, lumpier,
then it would have been hotter, and weâd see that in the background radiation as a
patch of bluer light. Thatâs pretty weird. When you look at the
background radiation from opposite sides of the sky, youâre seeing it coming from opposite
ends of the universe! Even back then, those regions of the Universe were separated by
vast distances, and had plenty of time to go their separate ways, change in different ways.
They should look pretty different. But they donât. As telescopes got better, very tiny variations
in the light were found. But they were really teeny, only a factor of 1 in 100,000. In other
words, one part of the sky may look like it has a temperature of 2.72500 Kelvins, but
another spot is at 2.752501. The Universe had lumps, but they were far,
far smaller than expected. Something must have happened in the Universe to force it
to be this smooth even hundreds of thousands of years after the Big Bang. This led theoretical physicist Alan Guth to
propose a dramatic addition to the Big Bang model: At some point in the very early Universe,
the expansion suddenly accelerated vastly. For the tiniest fraction of a second, space
inflated hugely, far faster than the normal expansion, increasing in size by something
like a hundred trillion trillion times! We call this super-expansion âinflation.â
It sounds a little arbitrary, but it actually has quite a bit of physical foundation now;
in a sense itâs like one of the phase changes of the Universe that happened in that first
fraction of second dumped huge amounts of energy into the fabric of space-time, causing
it to swell enormously. Inflation explains why the Universe was so
smooth at the time of recombination: Space expanded so rapidly that any lumps in it were
smoothed out, like pulling on a bedsheet to flatten out the wrinkles. Inflation explains several other problems
in cosmology as well, and although the details are still being hammered out, the basic idea
is almost â pardon the expression â universally agreed upon by astronomers. The fluctuations we see in the background
glow now were actually incredibly small perturbations in the fabric of space at the time of inflation,
which got stretched by inflation to macroscopic size. These denser spots were seeds, eventually
growing even more, their gravity attracting flows of dark matter. Normal matter collected there too, condensing,
eventually forming the first stars about 400 million years after the Big Bang. Eventually,
those teeny little bumps from the beginning of the Universe became galaxies and clusters
of galaxies, now tens of billions of light years away. Our own galaxy, our own piece
of the Universe, started the same way, as a quantum fluctuation in space 13.8 billion
years ago. Now look at us. Howâs that for an origin
story? There are still many unanswered questions
in our understanding of cosmology. Whatâs dark energy? What was the role of dark matter
in the early Universe? Where did the Universe come from in the first place? Are there more
universes out there? Hidden away where we canât see them? If time and space started
in the Big Bang, does it even make sense to ask what came before it, or is that like asking
whatâs north of the north pole? We donât know the answers to these questions,
and trust me, there are thousands more just like them. But hereâs the fun part: we might
yet be able to answer them! After all, even asking if the Universe had a beginning, let
alone what happened between then and now, was nuts just a century or two ago. Now we
have a decent handle on it, and our grip is getting better all the time. Science! Asking â and answering â the
biggest questions of them all. I love this stuff. Today you learned that the timeline of the
Universeâs history can be mapped using modern day physics and astronomical observations.
It started with a Big Bang, when the Universe was incredibly dense and hot. It expanded
and cooled, going through multiple stages where different kinds of matter could form.
It underwent a phenomenally rapid moment of expansion called inflation which smoothed
out much of the lumpiness in the matter. Normal matter formed atoms between 3 and 20 minutes
after the bang, and the lumps left over from inflation formed the galaxies and larger structures
we see today. Crash Course Astronomy is produced in association
with PBS Digital Studios. Head over to their YouTube channel to catch even more awesome
videos. This episode was written by me, Phil Plait. The script was edited by Blake de Pastino,
and our consultant is Dr. Michelle Thaller. It was directed by Nicholas Jenkins, edited
by Nicole Sweeney, the sound designer is Michael Aranda, and the graphics team is Thought CafĂŠ.
Thank you for posting. I love these!