0:00 – Why there is an “Information barrier”
3:04 – Ocean wave analogy
4:46 – Gravitational waves are cosmic Tsumanis
5:45 – How are gravitational waves detected?
8:37 – What caused the gravitational wave background?
10:19 – What would the gravitational background reveal?
10:54 – Future gravitational detectors
The information that we know about the universe comes almost exclusively from the analysis of electromagnetic radiation. But there is only so much this light can reveal because there is an inherent barrier. The oldest light that we can detect comes 380,000 years after the Big Bang – Cosmic Microwave background, or CMB. We don’t have access to information about the universe any earlier than 380,000 years after the big bang.
But light is not the only thing that can carry information. According to the latest cosmology models, there should be a gravitational wave background. This background would have occurred within the first second of the big bang so it would give us information that is almost as old as the big bang. It could reveal the secrets of creation.
If you were in a house near the beach and the sea was calm, you could conclude that the wind must be gentle. If you saw the waves becoming bigger, you could conclude that somewhere the wind must have picked up, because 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.
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 good idea of what may have caused the waves.
If there was an earthquake at sea, some time later, a tsunami would hit the shore. In this case the wave tells us something very concrete. A huge and localized wave tells us 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 detecting the equivalent of tsunamis. These events 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 currently detect low intensity gravitational waves.
Any accelerating mass will create gravitational waves because mass affects spacetime. Even a moving car will create too much background gravitational wave noise for us to detect very faint gravitational signals. So only very strong waves can be detected. A faint signal equivalent to the calm ocean waves.
We use a laser interferometer to detect gravitational waves. 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 which measures any phase shift. A Gravitational wave stretches and contracts space time, so, the two half beams will get stretched or contracted. So when the beams reunite at the detector, they will be slightly out of phase.
Gravitational waves are believed to have been formed within the first second of the Big Bang. How? One 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. This caused the fundamental particles to gain mass through their interaction with the Higgs potential. This change from massless to massive fundamental particles could cause gravitational waves.
These waves also could have formed during the process of cosmic inflation when the universe expanded exponentially during the first fractions of a second of after the Big Bang, from an infinitesimally small point to about the size of a large orange.
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.