If you were on a train moving at 100 km/h, and you threw a ball at 20 km/h in the same
direction as the motion of the train, the ball would move at 20km per hour from your
perspective. But from the perspective of someone standing still outside on the ground, the
ball would be moving at 120 km/h – the speed of the train PLUS speed of the ball. Simple
enough. However, Einstein showed that this concept
does not hold for the speed of light. This means if you are driving a car at 100 km/h
and turn on your headlights, then the light will move at exactly the speed of light from
your perspective, as well as the perspective of someone standing still on the ground. The
speed would NOT be the speed of light plus a 100 km/h. It would move exactly the same
speed for everyone in their frame of reference. This impacts the way that we can understand
the universe, because if light is constant in a vacuum, it means that when we look out
into space, which is effectively a vacuum, that light that we see has come from the past.
And the farther out we look, the further back in time we can see. The night sky is literally
a time machine. This leads us to the question, if we look
as far out as possible, what will we see? Well, we would see a remarkably uniform glow
of low level radiation in all directions. This is called the cosmic microwave background,
or CMB. It is the oldest light in the universe. And it can tell us a lot about its early history.
This may look like a random picture of electromagnetic radiation, but the information that it contains
is actually quite profound. What is that information in this photograph?
And what does it tell us about the nature of the universe in which we live? That is
coming up right now…
I just want to tell you that I was inspired to make this video after watching a documentary
on Magellan TV, today’s sponsor, called “Search for the edge of the universe.”
It takes you on a fascinating journey from the ancient earth-centered static model of
the cosmos to the modern understanding of a dynamic big-bang beginning 13.8 billion
years ago leading to the formation of present day large scale structures of the universe.
You can enjoy many more documentaries like this on MagellanTV.
It’s a new type of streaming documentary service founded by the filmmakers themselves,
who bring you high quality documentary content. Featured subjects include history, nature,
science and technology. You can watch it on any of your devices anytime
without any ads. Magellan has a NEW offer right now for Arvin
Ash viewers. You can get 30% off an annual membership.
And this is valid for prior subscribers too.
I highly recommend Magellan TV! But be sure to click the link in the description
Imagine diving into a sea of milk, and looking all around you. What would you see? Probably
nothing but an opaque, featureless, uniform light all around you. This is analogous to
the way the universe probably looked very early on for thousands of years, shortly after
the big bang. And it was expanding throughout that time.
Then around 380,000 years after the big bang, a series of events took place that resulted
in the universe changing from opaque to transparent. What caused this transition from initial opaqueness
to transparency? Although there was a lot of energy and lots
of photons were around, they were not free to move much. They bounced around and scattered.
The reason is because the universe was so hot at the time that atoms were ionized. This
means that electrons were not bound to protons. Both protons and electrons were moving around feeely in a
plasma state somewhat like the way they are in the center of the sun.
When electrons are free to wander around like this in the plasma, photons can’t travel
through them unimpeded. They interact with the electrons and scatter. This would prevent
the light from one end of the universe to reach your eyes directly. It would be scattered
randomly, so you would not be able to make out any features.
However, after about 380 thousand years, the universe cooled to around 3000 Kelvin or 2700
degrees Celsius. This is still pretty hot, about twice as hot as liquid iron, but the
universe now had a low enough energy such that electrons could combine with protons
to form stable neutral hydrogen and helium atoms.
At this point then, the universe became transparent, because photons were now free to travel from
one end of the universe to the other, without being scattered by electrons.
One thing you might be wondering is why is this light from the very early universe microwave
light, which is a relatively low energy light? Shouldn’t this light be higher energy like
x-rays or gamma rays, since the early universe was extremely hot and energetic?
Well, the answer is that indeed this light was higher energy or shorter wavelength originally.
But at 2700 degrees C, it was not energetic enough to be x-rays or gamma rays. It was
mostly in the infrared range. In fact, part of this light was also in the visible range,
so you would have been able to see it with your eyes if you were there.
However, during the ensuing 13.7 billion years, the universe has expanded. As space expands,
since the speed of light is constant, the wavelength of photons increases with this
expansion. This results in lowering the energy of the photons. So while the original photons
were more energetic, by the time the light reaches us billions of years later, it is
of a lower frequency – it is now in the radio wave or microwave region.
And these microwaves are coming from every point in the universe. It’s like a subtle
background glow. In fact, when measured, it’s intensity to
frequency ratio fits perfectly over the graph for a black body. It is the most perfect black
body every measured. What does this mean? A black body is an object that absorbs all
incoming electromagnetic radiation. In the ideal case, it absorbs light from any direction,
in any frequency and does not reflect anything back. Since it also absorbs all visible light,
at room temperature, it would appear to be perfectly black. That’s where the name black
body comes from. But Black bodies also EMIT photons in the
form of thermal radiation in a continuous spectrum according to their temperature. In
fact, all objects above absolute zero emit some photons. This is called black body radiation.
So black bodies above absolute zero, not only absorb photons, but emit photons as well.
Most objects are not perfect black bodies, so they generally don’t absorb all the radiation.
Objects tend to emit or absorb certain wavelengths of light more than other wavelengths.
As it turns out, however, the CMB is possibly the closest we’ve come to observing a perfect
black body - as you can see from the way its measured light fits on this ideal black body
graph. The spectrum of photons emitted from black
bodies looks different at different temperatures. When we measure the black body radiation from
the CMB, we find that it matches the expected spectrum of an ideal black body for a very
specific temperature. And that temperature is 2.726 Kelvin, which
is 2.7 degrees above absolute zero. So, from examining the CMB we can figure out exactly
what temperature the universe’s background radiation is.
The fact that the black body spectrum is almost perfect, suggests that the temperature is
evenly distributed. And this means that the CMB is in near perfect equilibrium, suggesting
that the universe was very isotropic and homogeneous when the CMB was formed.
This means that the early universe was essentially the same in all locations and in all directions,
when the CMB was formed. This fact not only gives us a clue about what
the early universe looked like shortly after its creation, but has profound implications
for the way it looks today. The isotropy and homogeneity has manifested
in a universe we see today where the average density of galaxies is the same throughout
the universe and does not change much with distance or direction.
You might ask, well, if the universe was the same everywhere at the beginning, then why
are there any clumps at all? Why are there galaxies, solar systems, and even planets? Wouldn’t
the universe be perhaps nothing but a sea of radiation or a sea or atoms?
Well, although the universe was pretty isotropic, it was not perfectly so. It
has a small anisotropy. So this 2.7 Kelvin temperature of the microwave
background is not perfectly uniform, it has a very small variation. The temperature of
the CMB is more precisely 2.7 Kelvin plus or minus 0.00003, or a variation of about
1 on 100,000. And it’s this deviation from being perfectly
the same in every direction, the anisotropy, that led to the seeds which
have grown to the large-scale structure of the universe we see today.
It’s mind boggling if you think about it. But a region that was a fraction of a degree
warmer became a vast galaxy cluster hundreds of millions of light years across containing
trillions of stars. Where did this variation come from? The imperfections
that led to the overall structure of the universe, are believed to stem from quantum fluctuations
in the early universe. The fluctuations led to the fact that some
places had a very slightly higher matter density than other places.
And over time, this led to certain places attracting more mass via gravity than other
places. The structural formation within our universe ultimately comes from this process.
Thus, the quantum fluctuations become the seeds leading to the structure in the universe
as it expands. But its still highly uniform. In fact, some
of our earliest microwave instruments were not sensitive enough to detect such a minute
temperature variation. The current picture of the CMB come from relatively recent advanced
satellite data. But the fact that CMB is almost perfectly
isotropic also gives us a clue about how the universe was probably formed. It is evidence
for the theory of cosmic inflation in the very early universe.
Cosmic inflation is the theory that the early universe from only about 10^-36 seconds after
the big bang to about 10^-33 seconds, expanded exponentially from an infinitesimal point
to about the size of a perhaps large orange. And following this the universe continued
to expand, but at a much slower rate. Many physicists believe that such an inflationary
epoch in the universe explains the isotropy, because it would have stretched out space
very fast. This would even out any significant imperfections, leaving only the small imperfections
due to quantum fluctuations. And these quantum fluctuations magnified to
cosmic scales became the seeds for large scale structures like the galaxies and galaxy cluster
formation of today. The CMB also reveals that the universe is
probably flat. When I say flat, I don’t mean flat like a pancake.
What I mean is flat in four dimensions, meaning there is no overall curvature to our 3D spacetime.
Two parallel beams of light will remain in parallel forever. They will not converge or
diverge. For example, if the universe was like a 4D
sphere, the beams would eventually converge, just like if you make two parallel lines around
the earth, it will eventually converge at the poles.
On the other hand if the shape of the universe was like a 4D saddle, the beams would diverge.
In 4 dimensional flat spacetime, however, the parallel beams remain parallel forever.
How did physicists determine that the universe is flat?
Curvature can be determined by gauging how much the CMB light has been deflected or “gravitationally
lensed” while passing through the universe over the past 13.8 billion years.
The more matter these CMB photons encounter on their journey to Earth, the more lensed
they get, so that their direction no longer crisply reflects their starting point in the
early universe. This shows up in the data as a blurring effect, which smooths out certain
peaks and dips in the spatial pattern of the light.
If the universe was curved in any way, the temperature variations would appear distorted
compared to the actual size that we observe with our telescopes.
This data tells us the density of matter in the universe. If this density is higher than
the critical density, the universe would be closed like a giant sphere.
If it was lower, then it would open like a saddle. Our universe appears to be just at the critical density.
WMAP and Planck satellite observations
of the CMB confirmed that we do indeed live in a flat, critical-density universe.
In addition to flatness, the fact that there is a microwave background at all, is significant
evidence that the Big Bang actually happened. Why? -- because this leftover radiation is
exactly what we would expect if the universe was much denser, hotter and smaller in its
early history. We know the universe is expanding, so it had to be denser and smaller earlier
in its history. And we also know that it had to be hotter because as the universe expands,
it would be expected to cool down as its thermal energy is dissipated into a greater volume.
If the universe was steady or getting smaller, we would either be at the same temperature
or we would be hotter. If the universe was not allowed to expand and cool from the time of the CMB, we would still
be at 3000 kelvin. You might ask, if we can’t see further back
than 380,000 years after the big bang, have we hit some kind of firewall where we can
never analyze the universe when it was any younger than that? In other words, is there
an information barrier that prevents us from knowing anything about the universe’s earliest
beginnings? Well, there is hope because light is not the
only thing that can carry information. You have to remember that we recently discovered
gravitational waves, and we are quickly learning how to detect even weaker gravitational wave
signals. Theoretically there should also be a Cosmic Gravitational Background, in addition
to this microwave background. And this may be able to tell us something
from the very early universe, if we can somehow manage to detect these gravitational signals,
and interpret them. So stay tuned because, what we would learn
would be entirely new information. Who knows what new and profound surprises are waiting
for the future scientist to discover. And if you like this video, please give us
a like. And post any questions in the comments, because I try to answer
all of them. I will see you in the next video my friend!