Our universe is full of secrets that we are chasing and mysteries that we are trying to solve.
This has been the name of the game for centuries. And thousands of scientists have stared into
the night sky trying to figure this out. The information that we have gathered comes
almost exclusively from the analysis of different forms of electromagnetic radiation. These include
not just visible light, but also light in the form of gamma rays, x-rays, UV-light, infrared light,
and microwaves. And we have learned a quite a lot from this analysis, with ever more
sophisticated sensors on advanced satellites. But there is only so much this light can reveal.
In fact, there is an inherent barrier that we can pretty much never cross.
This barrier is the fact that the oldest light that we can ever hope to detect and analyze comes
a full 380,000 years after the Big Bang. And that light is the Cosmic Microwave background, or CMB.
In other words, by analyzing only electromagnetic radiation, we don’t have access to information
about the very young universe any closer to its beginning than 380,000 years ago. But
now in the 21st century we’ve taken a leap in what we can potentially “see”.
You might say that we have an opportunity to see what the universe may have been like at an
earlier age. The reason is that according to the latest models of cosmology, just like there is a
cosmic microwave background from the earliest time that light was able to propagate 380,000 years
after the big bang, there is predicted to also be a gravitational wave background
from an even earlier era of the universe – because this gravitational wave background
would have occurred within the first second of the big bang. This would give us information
that is 380,000 years older than the oldest information we currently have
about the big bang from the microwave background. If we can detect and analyze this
background gravitational wave data, it could reveal the very secrets of creation itself.
What is the nature of the gravitational wave background? Why is it predicted to
be there? And what new secrets could it reveal? That’s coming up right now…
Before we dive into gravitational waves, I want to give a shout out to Magellan TV for sponsoring
this video. It has been a source of inspiration for me, and I think it will be for you too.
For example, one of documentaries that got me thinking about the gravitational
wave background is called “Star Crash – the explosion that transformed astronomy”
– about two rapidly spinning neutron stars that produced huge gravitational
waves and may have formed a black hole. You’ll find thousands of fascinating documentaries
like this on MagellanTV - a new kind of streaming documentary service from the film makers
themselves. Featured subjects include history, nature, science and technology. You can watch
it on any of your devices, anytime without ads. Magellan has a great offer right now for Arvin Ash
viewers. You can get 30% off an annual membership – an entire year for less than $3.50 a month,
and it’s valid for prior subscribers too. I highly recommend Magellan TV! But be
sure to click the link in the description. Imagine yourself on the beach. It’s a sunny
day. You’re enjoying life. The sea is calm, the wind is gentle. Because you are on the beach you
feel the gentle wind, so you surmise that in such calm weather there would not be any choppy waves.
Now if you are sitting in a nearby house, and you don’t feel the wind, but can observe through your
window that the sea is calm, knowing the basic principles of the physics of weather, you can
reasonably conclude that the wind must be gentle. Now, imagine that the waves start to become
bigger and choppier. The observer in the house, knowing about the physics of waves,
would now look at the sea and conclude that somewhere the wind must have picked up.
Remember that a storm far away can form strong waves which can travel very far. So,
the reason for the high waves could be a storm far away, or there could be some strong wind nearby.
He can come to a general conclusion that somewhere at sea the wind has become stronger. So the waves
you observe at sea can help you understand the weather conditions potentially far away.
This is analogous to how it is with gravitational waves. We cannot feel the reason for the
gravitational waves, but knowing the principles of physics, we can get a really good idea
of what may have caused the waves. And in some cases, we can know this with a very high certainty.
Now imagine if there was an earthquake at sea. Some time later, a tsunami
hits the shore. In this case the wave tells us something very concrete. Just by looking at the
fact that this wave is huge and localized, we could conclude that something catastrophic like
an earthquake probably caused the tsunami. If we carefully traced how the wave hit the
shore in different places, then we could also get a good idea where the earthquake happened,
just by considering the tsunami wave. This is similar to what we currently do
with gravitational waves. We are really looking at the equivalent of tsunamis - spectacular events in
space that create a very strong gravitational wave signal. And by analyzing these signals we
learn something about the event. This event could be the collision of two black holes for example,
or two neutron stars forming a black hole. Our current technology only allows us to detect
these very strong – tsunami-equivalent signals. We can’t really detect low
intensity gravitational waves. In principle, any accelerating
mass will create gravitational waves because mass affects spacetime. Our current technology
is such that there is too much noise from mundane earthly phenomena such as moving cars that create
too much background gravitational wave noise. So only very strong waves can be detected.
A faint signal equivalent to the calm ocean waves due to gentle winds
are not detectable. Only the tsunamis are. How do we detect gravitational waves anyway?
We use a laser interferometer. It’s a pretty simple device. You take a laser, then you shoot
it through a beam splitter that splits the laser beam into two equal half beams. Then you send each
of these beams away from each other at a 90-degree angle. After having them travel the same length,
a mirror reflects the beam back through the beam splitter. This unites the two half beams,
and sends them into a photodetector. Now how does this detect gravitational
waves? Well, Gravitational waves stretch and contract space time.
So, if a significant wave hits earth, then the two half beams will get stretched
or contracted. The physical length of the beams will change in relation to each other since they
are at 90 degrees. So when the beams reunite at the detector, they will be slightly out of phase.
Depending on how far out of sync the two beams are,
we can tell how intense the gravitational wave is. Now I’ve made it sound simpler than it actually is
because this requires extremely precise measuring devices that can filter out
background noise from moving massive objects such as cars and trucks, and other earthly phenomena.
This is not trivial, but with modern technology it can be done.
Getting back to the analogy of the waves at sea, there are different causes for the waves
we observe. And depending on the cause, we might get big waves, small waves, long waves,
short waves and so on. How the wave looks can tell us something about the physics acting on the sea.
Now in reality the sea is never completely calm. There are always some small waves. This is like a
background noise. But this noise can sometimes reveal that nature of the wind or the sea.
Now, what if something similar was the case for gravitational waves – a kind of faint background
noise. This is what we would need to detect in order to find the gravitational wave background.
The theory is that somewhere in these space time ripples there is a very old signal,
a signal from the big bang. This could be groundbreaking for understanding the
origin and nature of the universe. In the past we have relied largely
on electromagnetic waves to make observations about the universe.
Just like electromagnetic waves, gravitational waves travel at the speed of light.
This means the further away you look, the longer time it takes for any signal to arrive.
The oldest electromagnetic signal we can detect is a microwave signal from the very early universe,
when it was around 380,000 years of age. This is called the CMB, Cosmic microwave background.
The cosmic microwave background is the first light that was able to propagate
as the universe transitioned from being opaque during its first 380 thousand
years of its life, to being transparent. We cannot see anything further back than
380,000 years after the big bang. It is like an information firewall.
But fortunately for us, light is not the only thing that can carry information.
Gravitational waves can also carry information. Gravitational waves are believed to have been
formed within the first second of the Big Bang. So if we can detect the gravitational
waves from this time, which should be present all around us, just like the microwave background is,
it may tell us something about this important era in the evolution of the universe.
How would these primordial gravitational waves have formed?
First, we can’t be certain that this gravitational wave background exists, but it is likely.
One possible way they would have formed is when the combined electroweak force broke apart
into the two separate forces we see today - the electromagnetic force
and weak force. This would have happened within fractions of a second after the big bang.
It’s when the Higgs field transitioned into a non-zero minimum and became massive. It’s the
only field with a non-zero minimum. Coinciding with this, the fundamental particles gained mass
through their interaction with the Higgs potential. This change from massless to
massive fundamental particles could cause gravitational signals. And if we could detect it,
this could reveal new physics to us about the nature of this electroweak transition.
It is also possible that gravitational waves could help us demystify cosmic inflation. Cosmic
inflation is the theory that the universe expanded exponentially during the first
fractions of a second of after the Big Bang, expanding from an infinitesimally small point
to approximately the size of a large orange. It explains a host of observations including
the relative homogeneous and isotropic nature of the universe we observe today.
So detecting the gravitational waves from this era would give us insights
about the early universe that we could never hope to obtain from the CMB. It’s
not certain what we would see, but we could expect to see clues leading to new physics.
So it could be a breakthrough in our efforts to find for a more complete theory of the universe.
Now, even if such a subtle signal exists, it would be no easy feat to find it, since
right now we can only detect the tsunamis. Current detectors are certainly not good
enough but future detector could be! Right now, preparations are in full swing
with research groups in collaboration with the European Space Agency to build and launch
a brand new space laser interferometer. This is the same type of device used today
on earth, but this would be in space. It would allow us to measure weaker gravitations signals
because it would avoid all the extraneous gravitational noise on earth. Unfortunately,
it is many years away, currently estimated not to launch until 2034.
In the meantime thousands of scientists and amateurs will continue to gaze
longingly into the night sky trying to figure out the seemingly magical mechanism of the universe.
And if you have a question on this or any other topic on my channel, please post it
in the comments because I try to answer all of them. I will see you in the next video my friend.