Have a look around you. Everything you
see from the skin of your hands to the
screen you're watching this video on is
a different combination of the same
three building blocks of matter.
Protons, neutrons, and electrons.
Now, let's look a little farther. say at
Mars or the Andromeda galaxy or even
halfway across the observable universe
and still there is matter made of
protons, neutrons and electrons as far
as the eye can see. At first, this might
not sound all that surprising. But for
once, the mystery here isn't that we've
seen something we can't explain, but
rather that we haven't seen something we
were expecting. A universe just as full
of antimatter.
I'm Alex Mccoan and you're watching
Astramm. Join me today as we explore the
world of antimatter and learn about its
interactions with other particles and
even with gravity. By the end of this
video, you'll probably agree that
antimatter is a bit weird, but you'll
also see why some physicists are
frustrated that it isn't weird enough.
Let's get one thing out of the way
first. Although it might sound like
something straight out of science
fiction, antimatter is very real. It
forms a critical part of the standard
model of particle physics. And particles
of antimatter have been observed in
experiments going back nearly a century.
The very first detection of antimatter
dates back to a 1932 experiment
conducted by Carl D. Anderson at Caltech
using a cloud chamber immersed in a
magnetic field.
When charged particles from outer space,
broadly called cosmic rays, intercept
the Earth's orbit and fly through this
chamber, the magnetic field curves their
paths according to the charge and mass
of each particle. And the clouds show a
visible imprint of their resulting
trajectories.
Anderson was hoping this experiment
would help determine just what kinds of
particles were streaming into the Earth
from the cosmos. and he may have found
just a little bit more than he bargained
for. What Anderson saw was that these
cosmic rays included both positively and
negatively charged particles. The masses
of the negatively charged particles
lined up exactly with the known mass of
an electron. But some of the positively
charged particles were far too light to
be protons. Instead, they appeared to
have the mass of an electron despite
having the opposite charge. And so,
these neverbeforeseen particles came to
be known as anti-electrons
or later posetrons for short. In 1936,
Anderson would win the Nobel Prize in
physics for this discovery.
Meanwhile, a British physicist who was
also destined to win a Nobel had been
developing a description of electrons
that would fit nicely within the
framework of quantum field theory.
His name was Paul Durk. By 1928, Durk
had realized that in order to describe
electrons as quantum fields in a way
that was physically consistent with
special relativity, they had to be part
of a larger mathematical structure,
later known as a Durac spinner that
inevitably gave rise to both positively
and negatively charged versions of the
same particle.
In this way, Derek had predicted the
existence of posetrons before Anderson
had even built the cloud chamber that
would detect them four years later.
What's even more incredible is that
electrons aren't the only fundamental
particle to come in a two for one direct
spinner package. Other particles of
matter, like the quarks that make up
protons and neutrons, each have their
own anti-quark counterparts.
These anti-quarks can come together to
form anti-rotons and anti- neutrons
which can then bond with positrons to
form anti-atom.
You could make a whole planet out of
antimatter and from the outside it would
look quite similar to an ordinary planet
made of ordinary matter.
But if antimatter were too similar to
matter, if the only difference were the
sign of its charge, then it would be
impossible to explain why our universe
contains so much of one and so little of
the other. This cosmic mystery known as
the barionic asymmetry of the universe
sent physicists on a decadesl long quest
to try and find as many differences as
they could between matter and
antimatter.
That quest lives on today, spearheaded
by particle colliders at CERN that are
capable of producing, trapping, and
studying both posetrons and anti-rotons.
But before we talk about these
experiments, let's try to summarize what
we already know about the properties of
antimatter.
When studying antiparticles in
isolation, experiments have confirmed
with ever greater precision that their
intrinsic properties, namely their
masses, are exactly the same as for
ordinary particles. And when studying
how antiparticles are affected by
electromagnetic forces, experiments have
again found that they behave the same
exact way as ordinary particles, except
with the opposite electric charge, just
as Anderson had observed in his cloud
chamber. But electromagnetism is just
one of the four fundamental forces of
nature alongside gravity and the weak
and strong nuclear forces.
And as physicists began to better
understand the weak force in the 1950s
and60s, they realized that particles and
antiparticles are actually affected by
it quite differently.
The first surprise was that ordinary
particles could only feel the weak force
if they were left-handed and
antiparticles could only feel it if they
were right-handed.
The concept of handedness or kirality is
subtle and difficult to conceptualize
for particles with mass. But a loose
analogy can be drawn with a particle's
helicity which describes whether a
particle is spin up or spin down along
its direction of motion. In this
analogy, a spin up particle is called
right-handed while a spin down particle
is called left-handed.
The second and even crazier surprise was
that right-handed antiparticles
experienced a different strength of the
weak force as compared to left-handed
ordinary particles.
In practice, this means that the quantum
probabilities for radioactive decay in
ordinary nuclei are somewhat different
from the probabilities of the analogous
decay processes in anti-uclei.
This fundamental asymmetry between
particles and antiparticles was first
observed in a 1963 experiment run by
James Cronin and Val Fitch of Princeton
University who would be awarded yet
another Nobel Prize for their discovery.
When this asymmetry was discovered,
there was some hope that it would
explain the barionic asymmetry of the
universe. Perhaps these differences in
the weak force were responsible for the
abundance of matter and utter lack of
antimatter around us. But the maths
didn't quite work out. There simply
wasn't enough of a difference between
the strength of the weak force acting on
particles versus antiparticles.
That was when physicists began to turn
their attention to the strong nuclear
force.
Theoretical models predicted that just
like in the weak interaction, there
should be some differences in how
left-handed particles and right-handed
particles feel the strong force.
But antimatter just keeps surprising us.
Every experiment to date suggests that
the strong force treats particles and
antiparticles just the same.
This brings us to the last of the four
fundamental forces and the subject of
today's ongoing experiments at CERN.
Gravity.
To be honest, suggesting that gravity
might treat matter and antimatter
differently is kind of a long shot.
Think back to the popular legend of
Galileo tossing stones of different
sizes of materials from the Tower of
Pisa. They all fell at the same rate
because the gravitational acceleration
on Earth is 9.8 8 m/s squared,
regardless of which object is falling.
Of course, the experiment works even
better in a vacuum chamber where air
resistance is taken out of the equation.
Newton expanded on this idea and showed
in the 17th century that your
gravitational acceleration anywhere in
space depends only on the mass of the
object pulling you and your distance
from it, but not on any of your personal
properties, not even your own mass.
This famous result known as the
equivalence principle is the foundation
of Einstein's theory of general
relativity. Our most accurate and
successful model of gravity to date.
With that in mind, physics is still an
experimental science at its core, and we
can't know for sure whether matter and
antimatter obey the same laws of gravity
unless we check for ourselves. The
physicists at CERN set out to do just
that. Motivated not only by the barionic
asymmetry of the universe, but also by a
few speculative papers suggesting that
the cosmological properties of dark
matter and dark energy could be more
easily explained if antimatter were to
have a negative gravitational charge. Or
to put it simply, if antimatter were to
fall up rather than down.
There are several ongoing experiments at
CERN testing the gravitational
properties of antimatter including Agus
Gbar and Alpha.
Today we will focus specifically on a
key experiment coming out of the alpha
group that was published in the journal
Nature this past September.
After decades of assumptions, this
experiment has brought us realworld data
on the gravitational acceleration of
antimatter on Earth's surface. But
before we show you the results, let's
take a moment to appreciate just how
intricately this experiment was designed
in order to isolate and measure the
effects of gravity.
The first step in the experiment is to
secure a beam of several million
posetrons per second emitted from a
radioactive isotope of sodium.
Most of these posetrons end up colliding
with ordinary matter in the experiment
causing miniature explosions in which
posetrons and electrons annihilate each
other and release a small burst of
energy in the form of light. But a small
fraction of the posetrons survive as
they are guided through the experimental
apparatus where they are cooled by low
pressure gases and trapped by electric
and magnetic fields.
But observing the effects of gravity on
these posetrons would be nearly
impossible. Their masses are so small
that the tiny force of gravity felt by
each particle is overshadowed by even
the smallest fluctuations in the
surrounding electromagnetic fields.
That's why this collection of posetrons
is merged with a separate container of
anti-roton where they bond and form
neutral anti-hydrogen atoms that are
much less responsive to stray
electromagnetic fields. And where did
the anti-rotons come from? Suffice it to
say that they were produced by firing
ordinary protons into a block of metal
really, really fast. Yes, physics is
awesome like that.
Once the anti-hydrogen atoms are
created, they behave like tiny weak
magnets that can remain trapped by
complicated arrangements of external
magnetic fields.
Now, this magnetic interaction is weak
enough that it no longer overwhelms the
gravitational effects that we are trying
to measure. The chamber containing these
anti-hydrogen atoms is nearly a vacuum.
There are just about 200,000 atoms of
ordinary gas per cm compared to a
typical atmospheric density of 20
quintilion atoms per cm. Under these
conditions, the trapped anti-hydrogen
atoms almost never collide or annihilate
with atoms of ordinary matter. Instead,
they can more or less just float around
the chamber for minutes or longer.
But as the magnetic fields used to
vertically trap the anti-hydrogen atoms
are weakened, this random floating
eventually allows the anti-hydrogen
atoms to escape through either the top
or the bottom of the chamber where they
can collide with a wall of apparatus,
annihilate with some ordinary atoms and
release a small burst of light. In the
alpha experiment, this happens over the
course of about 20 seconds.
The theory behind the experiment is that
if gravity really pulls antimatter
downwards, more of the antihydrogen
atoms escape through the bottom than the
top. The stronger the gravitational
force, the more atoms escape through the
bottom.
The simulations the alpha team ran
showed that under normal gravitational
attraction about 85% of the
anti-hydrogen atoms should escape
through the bottom whereas only 20% of
them would escape through the bottom if
gravity pulled antimatter upwards.
If there were no gravitational force at
all, the simulation showed a more even
distribution of 55% escape through the
bottom, probably only differing from 50%
due to asymmetries in the experimental
apparatus itself.
What did the actual experiment find?
Well, roughly 75% of anti-hydrogen atoms
escape through the bottom of the
chamber, showing a clear preference for
downward pulling gravity.
As any thorough scientist would, the
Alpha team repeated this experiment to
collect a variety of data points that
could tell a more complete story. They
redid the procedure under various levels
of magnetic field bias which applied
external upward or downward magnetic
forces on the anti-hydrogen atoms. On
this graph, a bias of minus1g means that
enough magnetic force is applied to
counteract normal gravity while a bias
of plus 1g means that an extra g of
magnetic force is applied to push the
antihydrogen atoms downward and so on.
The team made predictions through
simulations for each bias and for
various possible gravitational
interactions which produce the orange,
green, and purple curves shown here. As
you can see, the experimental data
points shown in blue best match the
orange curve which represents the normal
simulation where gravity pulls
antimatter downwards. But because the
data falls just a bit below this curve,
the best fit gravitational acceleration
was only 0.75g,
34 of the strength of gravity acting on
ordinary matter. Does this mean that
gravity affects matter and antimatter
particles differently? After all,
not necessarily.
Let's have a look at the error bars.
They indicate that there are two major
forces of uncertainty in the results,
including an uncertainty in the applied
bias, possible errors in alignment, and
other systematic and statistical
uncertainties.
When accounting for these uncertainties,
the best fit gravitational acceleration
is actually reported as 0.75g
plus or minus 0.13g
plus or minus 1.6g.
This means that a full 1g of
gravitational acceleration is still
fairly consistent with the collected
data. Future experiments will be able to
determine more precisely how strongly
gravity acts on antimatter, but we can
already rule out speculative theories
that rely on antimatter falling up
instead of down.
In the end, despite how weird and
backwards the world of antimatter is, it
seems that only the weak force actually
applies differently to particles and
antiparticles.
But explaining the barionic asymmetry of
the universe would require much more
drastic differences between the two. So,
scientists aren't done looking for them.
Could there be new forces and particles
that interact even more weirdly with
antimatter?
or would you be willing to accept that
having so much more matter than
antimatter around us is a mere
coincidence? In any case, let us know if
you've learned something new about
antimatter from watching this video and
whether this is a topic you'd like to
hear more on.
If I tell you to imagine a telescope,
what do you see? Perhaps you picture an
old wooden cylinder with lenses at both
ends, similar to the one that Galileo
Galilee first used to gaze up at the
heavens and see the moons of Jupiter. Or
maybe your mind is constructing a
cathedral-sized dome with mirrors as big
as tennis courts, like the telescopes
high up in the mountains of Chile and
Hawaii. Or you could even be drifting
onto visions of enormous dishes like the
infamous Arosibo telescope that once
captured radio waves from across the
cosmos. What you probably aren't
imagining is a cubic kilometer of
ancient ice under the surface of our
planet south pole. And yet deep below
the frigid landscape of Antarctica,
there lies precisely this. A strange
telescope in one of the most
inhospitable environments on Earth,
designed to observe not visible light,
but instead one of the most unusual and
elusive particles in the universe. I'm
Alex Mccoan and you're watching Astramm.
And today we're joining the Ice Cube
Observatory in its hunt for nutrinos,
also known as ghost particles.
If you stood on this icy surface, you
might not be aware that some of the
rarest events in the universe are being
observed beneath your feet. Located at
the most southerntherly human occupied
base in the world, the Munson Scott
South pole station, Ice Cube makes use
of the very ice in which it is built to
observe high energy nutrino particles.
But why? Why this extreme environment
and weird design?
Because nutrinos are weird. Really
weird. Nutrinos are fundamental
particles like quarks and electrons.
Most of these nutrinos arrive at Earth
from the sun where they are released
from the nuclear fusion reactions raging
inside its core. Other nutrinos will
have come from more cataclysmic origins
such as a super massive black hole,
supernova, or any other cosmic event
with enough energy to rip atoms into
their subatomic particles. Similar to
electrons, nutrinos belong to the
leptton family of particles, meaning
that they do not interact with other
matter through the strong nuclear force
that binds other subatomic particles
like protons and neutrons together.
Unlike electrons, they are neutral and
have almost no mass whatsoever.
This lack of charge and tiny mass means
that they can only interact with other
particles through the weak nuclear
force, giving them their ghostly quality
and making them one of the most
difficult particles for scientists to
detect. So much so that even when we do
measure their presence, it is only
through the impact they have on other
particles.
If that wasn't spooky enough, nutrinos
can also pass through our entire planet
without acknowledging the existence of a
single atom in the crust, mantle, core,
or any other layer of the Earth, flying
straight out of the other side
completely unnoticed.
There are trillions of nutrinos surging
through you every single second. And yet
over your entire lifetime there is only
a 1 in4 chance that even one of those
nutrinos will interact with an atom that
belongs to your body.
Here we find further demonstration of
the nutrino's complete refusal to engage
with matter in the universe. Photons
created in those same nuclear reactions
may take 1 million years between
bursting out of the sun's center to
leaping off the surface into space. They
are constantly absorbed and emitted in
random directions over and over again by
the atoms in our star resulting in a
long meandering path to escape. Nutrinos
instead zoom straight out of the sun in
just 2 seconds. traveling at almost the
speed of light itself.
As far as standard model particles go,
neutrinos are certainly the more koi and
antisocial of the group, but on occasion
they do mingle with other matter and in
the process can create a drama of
energetic particle events cascading out
in their wake. It is this cascade Ice
Cube is trying to observe.
They might seem strange, but the 5,160
detectors or doms suspended down between
1 and a half and 2 1/2 km in the
darkness of the ice shelf are in fact
light detectors. The sun's rays have of
course been completely swallowed up by
the thousands of meters of compressed
ice that lie above, but that doesn't
mean there is no light at all down here.
Nutrinos and cosmic rays can penetrate
into the ice and when they collide with
the atoms of frozen water, they cause
particles like muons to spring into
existence. Muons are the key to
identifying a nutrino interaction. As
the newly created muon hurdles through
the incredibly clear and dense ice, it
can exceed the speed it takes light to
move through that same ice. Now, let's
be clear. Nothing can move faster than
light in a vacuum. But in other
materials such as ice, liquid water, and
glass, light can be outpaced.
This isn't a violation of physics, but
rather a consequence of light slowing
down more than other particles within
the ice. As the muon races beyond its
photon counterparts, it generates a
shock wave of blue light, expanding in a
coneshaped pattern out from its path. An
eerie phenomenon known as Tranov
radiation.
Tranov radiation is most commonly
associated with nuclear reactors, eerily
emanating through the water that
surrounds their cores. This blue hue in
nuclear cores and strangely also down in
the ice of the South Pole can look
peaceful and atmospheric, but it is in
fact the quantum equivalent of the sonic
boom that roars out from a supersonic
jet plane flying overhead.
In this case, the plane moves through
the air and pressure or sound waves are
pushed out in all directions. With the
jet's speed building up towards the
speed of sound, the waves in front start
to bunch up, forming a shock front of
high pressure. When the plane exceeds
the speed of sound, it smashes through
the shock front, causing a rapid change
of air pressure. This is the sonic boom.
The shock front then travels out in a
cone, trailing behind the aircraft,
which observers will hear as a
thunderous clap. In the case of the
muon, the particle is not pushing air,
but instead disturbing the electric
fields of atoms in the ice with its
electromagnetic charge, polarizing the
water molecules. As the molecules
depolarize back to their usual state,
they release the energy that forced them
into polarization as photons of blue
light traveling out in all directions.
This is happening all along the path the
muon takes through the ice. The
traveling muon is triggering emission of
electromagnetic waves from the water ice
molecules which radiate out into all
directions producing a cone of emission
in the direction of the muon's motion.
If the muon was traveling slower than
light in the ice there would be no
wavefront and all the expanding light
spheres would destructively interfere
with each other caning out to complete
darkness. But since the muon is
traveling faster than light, the shock
front of constructive waves in the form
of blue light is created. The thousands
of digital optical modules of ice cube
are primed to detect this signature blue
light flashing through the ice. The
minuscule differences between when one
DOM registers the flash and when the
neighboring DOMs detect it allow
scientists to triangulate the nutrino
and trace its path back through the ice
and ultimately through the universe to
its origin.
These orblike detectors are marvelous
intricate devices designed to detect the
faintest luminal hints in the icy mass.
Each dome is encased in a 13in spherical
glass pressure case which protects the
electronics inside from the extreme cold
and high pressure of the Antarctic ice.
Inside there is a photo multiplier tube
sensitive enough to detect even a single
photon along with various components
that amplify the signal and convert it
into digital data which is then
transmitted up the cables to the
surface. And with so many detectors
making up this three-dimensional array,
there is a lot of data. A whole terabyte
of measurements are captured every day
with hundreds of gigabytes of that being
beamed by satellite to scientists across
the planet for them to comb through in
search of their ghostly quantum target.
Within this trove of data lie the
imprints of many particles arriving from
beyond our atmosphere. The vast majority
are cosmic rays. Ice Cube detects around
275
million of them daily. Cosmic rays are
interesting in their own right, but
serve as an obscuring curtain of noise
to those seeking nutrino interactions,
which occur a million times less
frequently with only around 275 detected
daily.
But even with the 100,000 nutrino
detections that Ice Cube makes each
year, almost all of them are generated
in our own atmosphere as debris from
collisions of cosmic rays with atoms in
the gas that surrounds our planet and
not from cosmic origins themselves. The
key word here being almost.
On the 22nd of September 2017, an
exceptionally bright burst of Sharenov
radiation was registered by Ice Cub's
array of detectors. Working together,
the doms used the tiny time differences
between when they each detected the
flash to trace the path of the muon and
the exceptionally energetic nutrino that
created it, all the way back to a
powerful and mysterious cosmic source
out in the depths of our universe. a
blazer, a super massive black hole at
the center of a distant galaxy, hurling
jets of particles towards Earth.
This discovery was monumental. It was
not the first time that Ice Cube had
detected nutrinos from deep, deep space.
But this detection was the first to
trigger a realtime alert to telescopes
across the globe, indicating that there
was something significant to look at in
that direction.
Ice Cube and Nutrinos had entered the
world of multi- messenger astronomy
where the cosmos is studied through not
just light like it has been for most of
the history of astronomy but also
gravitational waves, cosmic rays and now
nutrinos.
Nutrinos are a particularly useful
window for us to look through because
like gravitational waves, they can
arrive at us before the light from a
major astronomical event does. Because
of their ghostly nature that I talked
about before, nutrinos take a very
direct path to our detector. Whereas
light faces many obstacles like dust and
magnetic fields that interfere with its
journey and slow it down. Ice Cube is
now fully integrated into a planetary
network of observatories, constantly
scouring the skies for signs of high
energy events, ready to inform radio
dishes, mirrors, and lenses precisely
where to point to see rare and transient
phenomena that we would otherwise miss
completely.
Ice Cube is the noble watchman, sounding
the alarm to rally an army of telescopic
troops across the globe, commanding them
where to aim to catch sight of raging
cosmic fires. And even further to that,
Ice Cube can give us a unique view into
the heart of these phenomena. Because
again, unlike photons, nutrinos can
escape relatively unscathed from high
energy environments such as the core
collapse of supernova explosions or
cataclysmic merges of unfathomably dense
neutron stars or even the searing fury
of super massive black holes. Ice Cube
can deliver new and different
information. It can see things
light-based telescopes are blind to, so
to speak.
The nutrinos it detects can hugely
broaden our understanding of all these
events and of the most fundamental
nature of the universe. During these
events, physics is pushed to its limits.
Particles are accelerated to speeds and
energies thousands of times greater than
what we can achieve in our particle
accelerators on Earth like that of CERN.
This allows physicists to test their
theories and use the arrival of nutrinos
at ice cube to make new discoveries not
only about the behavior and properties
of nutrinos but also about dark matter
and even about ice itself.
This is why so much effort was put into
building such an extreme telescope as
the ice cube nutrino observatory. An
effort that took seven years to complete
with work pausing during the Antarctic
winter when planes are shut out from
Earth's most southerntherly continent by
the brutal weather. Only able to briefly
resume for the summer months of November
to February. Over those seven years, the
Ice Cube team drilled 86 holes 2 and a
half km deep, using 18,000 L of fuel per
pole and melting 750,000
L of ice in the process. All to have a
few hundred scientists live and work in
one of the most inhospitable
environments on our planet in attempt to
understand the most extreme events in
our universe and to capture a glimpse of
the most elusive particle in the
universe.
Nutrinos are the phantoms of the
particle kingdom. It takes bold and
ingenious ideas from ambitious
scientists to witness even a single one
nutrino. We know so little about them
compared to other particles, but they
are just as important in furthering our
understanding of the fundamental laws of
nature. With each nutrino detected, we
are moving closer to comprehending the
universe and the powerful forces that
shaped it.
Few questions hold greater importance in
science than how it all began. What did
the beginning of the universe look like?
Why did it happen in the first place?
And although we are a long way off from
having a complete answer, scientists
have learned enough that a theory has
been conceived, the Big Bang. By
examining stars and the spectrographic
data given off by matter when light
passes through it, we have assembled the
jigsaw pieces of the Big Bang and have
begun to form a picture of those first
explosive moments.
But here's the thing about jigsaws.
Sometimes you find a piece that just
doesn't fit. At that moment, you realize
with a sinking heart, you've made a
mistake somewhere along the way, and you
have no idea where. Is this what's
happening to the Big Bang Theory, or are
we looking at our puzzle piece the wrong
way round? It all comes down to an
element we use every day in medicines,
alloys, and most importantly, the
batteries in our phones. one that is
strangely missing from the universe, at
least in the amounts we would expect.
So, here's the big question. Where is
all the lithium?
I'm Alex Mccoan and you're watching
Astramm. Join me today as we examine the
lithium discrepancy. A mystery that
continues to confound scientists to this
day and might force us to re-evaluate
our models if we ever hope to answer the
question of where we came from.
To begin with, let's cover some of the
basics about lithium. Lithium is a
relatively simple element. It's one of
the alkaline metals and for a metal is
surprisingly soft, capable of being cut
with just an ordinary steel knife. You
might have seen it used in science
experiments in school as it reacts quite
satisfyingly when mixed with water,
producing lithium hydroxide and hydrogen
gas. More commonly is used in phone
batteries, laptop batteries, and in
other electronic devices due to its high
electrochemical potential. But the
feature of lithium that makes it so
important for our discussion is its
internal structure. It's made with just
three protons in the nucleus, meaning
it's one of only a handful of very
lightweight elements that could be
created in the early moments of the
universe. But it's the expected presence
of lithium in the universe's beginning
that causes our problem with the big
bang model. So what do we think was
going on back then? How exactly does
lithium prove it's wrong?
Well, the universe's origin is murky to
us. After the discovery in the 1920s
that our universe was expanding, a
Belgian cosmologist Gor Lmetra theorized
that if the universe was getting larger
and cooler, it used to be smaller and
hotter with all that energy being
squeezed into a smaller and smaller
area. The logical conclusion to this was
that if you rewound the universe back to
its very beginning, all of the
universe's energy would exist in a
single point in space. This singularity
would be infinitely hot and dense, which
makes sense. Based on that heat, this
idea is known as the hot big bang model.
But it's this infinity that causes the
murkiness to creep in. Einstein's theory
of relativity can be used to predict the
motions of objects under the effects of
gravity. But no equation tends to
function when you start adding infinity
into the mix. Hence, at the very first
moment of the universe's origin, we
don't have the equations to describe
very well what was going on or what
caused the universe to start expanding.
And with an infinitely hot and dense
singularity, there's no way of knowing
what might have gone on before it. Any
evidence or signals from before that
time would have been met with the
hottest incinerator the universe has
ever created. It happened somehow.
That's all we can say for sure.
At this point, there would be no
lithium. The very start of the universe
would have been simply too hot. After
that though, even just 1 second after
the universe begins to expand, the
picture becomes much clearer.
1 second after the universe's expansion
began, the temperature of the universe
dropped down to a mere 10 billion° C,
which is actually only 1,000 times the
temperature at the center of our sun.
This was still hot enough that matter
couldn't form yet. The fundamental force
that holds together the nuclei of atoms
would have been overwhelmed by this
intense energy. So everything would have
existed in the form of radiation
instead. The remnants of which we can
still see today in the cosmic background
radiation.
Discovering this became one of the
proofs that scientists are on to
something with this model.
The universe didn't remain as just
radiation for long. Just 99 seconds
later, it would have cooled to 1
billion° C and the strong force would
have kicked in. The universe's first
protons by then had started forming and
began binding with neutrons, creating
hydrogen and the one proton, one neutron
isotope of hydrogen known as dutyium.
Hydrogen and its isotopes, or versions
of hydrogen that had the same number of
protons, but more or less neutrons, were
the most common element in the universe.
But hydrogen wasn't the only one.
Everything was still filled with so much
energy that the hydrogen protons
frequently smashed into each other,
underwent nuclear fusion, and soon
created the larger, more complex element
helium with two protons. Once you run
the equations, in the primeval universe,
matter was nearly 76% hydrogen and
nearly 24% helium. But sometimes, very
rarely, another proton would merge with
the other two and would form isotopes of
lithium, bringing the number of protons
up to three in these atoms. By mass,
this made up only 0.00 0 0 07%
of matter in the early universe. But
here is where the mystery comes in. By
examining the spectroscopic data from
stars or by evaluating that same light
as it passes through clouds of matter,
we can evaluate the distributions of
elements in the universe today. And what
we see roughly matches this prediction.
Our own sun is by mass 71% hydrogen, 27%
helium, and 2% other heavy elements,
which makes sense as the sun would have
had time to undergo more nuclear fusion
in its lifetime, slightly altering the
ratios of these elements. These ratios
are mirrored throughout the universe as
a whole. Another strong proof that the
hot big bang model and our equations are
right and that the universe did evolve
in this way. And yet our predictions
about lithium are wrong.
Although the prediction for the amount
of lithium in the early universe is
really low, the prediction is actually
still too high compared to what we see.
Three times too high. Or to put it
another way, as we study the
spectroscopy of primordial stars, the
further away the star, the earlier in
time the light would have left it. We
don't see anywhere near the amount of
lithium we ought to be seeing if our
prediction was correct. There are
further mysteries surrounding lithium.
Lithium 7 is the isotope that we tend to
see in the early universe. This means
that a lithium atom has three protons
and four neutrons in its nucleus. But
there is another isotope of lithium
known as lithium 6 with one fewer
neutron that is meant to be much less
stable and much less common than its
counterpart. Supposedly making up only
two out of every 100,000 lithium nuclei.
While the amount of lithium in the
universe is too low, the ratio of
lithium 6 to lithium 7 is much too high.
Lithium 6 is a staggering 1,000 times
more prevalent in the early universe
than it should be. So, what does this
mean for the hot big bang model? Is it
really back to the drawing board for our
most trusted theory about the universe's
origins? Not necessarily. Two
possibilities exist. One that involves
rewriting the Big Bang theory as we know
it. But there is another explanation
that might just hold the answer to it
all.
When faced with something like this, one
of two things must be true. Either our
observations are wrong or our models are
wrong. Let's consider each possibility.
It's difficult to throw out the hot big
bang model completely. Nothing else does
as good a job at explaining the
existence of the cosmic background
radiation or the amounts of hydrogen and
helium that we see in our universe. The
quantities of those elements check out
almost exactly with the hot big bangs
predictions which seems to be too
perfect to be a coincidence. That said,
scientists are becoming less certain
about the model as it stands. Steven
Hawking, who was once one of the leading
advocates of the idea that our universe
began with a singularity, later went off
the idea. In his book, A Brief History
of Time, he stated, "Roger Penrose's and
my work became generally accepted, and
nowadays nearly everyone assumes that
the universe started with a big bang
singularity. It is perhaps ironic that
having changed my mind, I am now trying
to convince other physicists that there
was in fact no singularity at the
beginning of the universe. As we shall
see later, it can disappear once quantum
effects are taken into account.
Those quantum effects are too
complicated to go into for this video,
and this doesn't necessarily solve the
lithium problem, even if true. But it
does highlight that the origin of the
universe is an evolving branch of
physics and cosmology where the full
explanation of it has not yet been
found. Perhaps our models will one day
adjust in such a way that the lithium
problem falls away even as the broad
brush strokes of the theory remain in
place, letting the jigsaw piece of the
lithium numbers fit nicely with hydrogen
and helium.
The other option is that there is
something wrong with our observations
and there may well be some grounds to
this. Because of the small amounts of
lithium being discussed, it is quite
difficult to see the exact amounts of it
that exist in the interstellar medium
once you look far back enough. The only
way to probe the interstellar medium's
makeup is to shine a light through it
and then see which lines in your
spectrograph get blocked out. This can
tell scientists what elements might make
up a particular nebula or dust cloud.
But this gets harder and harder to do
the further away the target is.
Eventually, it becomes completely
impossible to get an exact estimate.
Instead, scientists are left looking at
the spectrographs of primordial stars
which are not necessarily representative
of the universe as a whole at that time.
A star is a special case. Something
going on inside there might be altering
the amount of lithium present within,
converting the lithium into something
else while boosting the amount of
lithium 6. Although it's difficult to
guess what that process might be. If our
observations could improve and we could
better measure the amount of lithium in
the early universe as a whole, perhaps
the numbers would come back into line on
their own. Maybe the lithium problem
never existed in the first place.
Ultimately, no answer to how the
universe came to be will ever be
complete until all the available data
matches the theory's predictions.
Whether that's lithium or other
problematic elements in the universe
like the overabundance of gold out
there, which I talk about in this video
here.
All these numbers have to be brought
into line. There is still so much to
learn about the universe's origins. I do
find it incredible just how much we are
able to learn and deduce about events
that happened billions of years ago back
before matter itself had properly formed
using just our telescopes and our minds.
That picture, the grand picture of how
it all began is waiting to be assembled.
However the universe came to be, it is
built on a foundation of an underlying
order that allows us to unlock its
principles, tease out its mysteries, and
reconstruct the configuration of its
individual pieces. One day, I am
confident we will solve the lithium
problem, too. How can we resist when the
answer to our ultimate origins awaits?
Some questions are just too important to
leave unanswered. And I for one can't
wait for the day this jigsaw piece is
ultimately completed.
Thanks for watching and thanks to our
the link to this playlist for more
Astramm content. I'll see you next