Physics is weird: you think you’ve learnt all there is to know about a concept, but in reality, you never truly know anything. For instance, we all assume that physics has a definition for gravity, which is that gravity is the curvature of space-time. And here’s the problem! Even after almost 400 years since that apple fell on Newton’s head, we do not understand gravity.
According to the General Theory of Relativity, the gravitational forces in our universe should gradually be pulling on all celestial objects, slowing down the rate of expansion. But when scientists began to measure distances and calculate redshifts for distant galaxies, they found a contradiction: the rate of expansion of the universe is accelerating at an eccentric pace. Thus, the idea of dark energy was established. Do we know what the dark energy is? Again, the answer is no! This is where Professor Claudia de Rham’s theory of massive gravity attempts to provide an alternate explanation for the increased acceleration of the universe.
Before we dive into the rich physics of massive gravity, it is important to discuss a few concepts that I have elaborate on in my previous posts.
The Standard Model of Particle Physics describes the four fundamental forces in the universe, including the gravitational force. Furthermore, the Model also assigns gauge bosons or ‘force carriers’ to each fundamental force. These force carriers are nothing but bundles of energy for a particular kind of field. For example, photons are the force carriers for the electromagnetic field. Well, then, what is the force carrier for our beloved gravitational field? The answer is graviton.
The Theory of Relativity makes a brave assumption that gravitons are massless, just like photons! In fact, gravitons and photons share a lot of common properties: both are massless, both move at the speed of light and both have two types of polarization. The first type of polarization is labeled ‘+’, and one could imagine it as squeezing a circle horizontally and vertically while it moves towards you. The other type of polarization is ‘x’ polarization, and it’s almost the same thing, except for the fact that the circle is squeezed at a 45° angle. Similar to massless photons that are collectively held responsible for light, massless gravitons work together to create the effect of gravity.
On the other hand, De Rham’s theory of massive gravity assumes that gravitons have non-zero masses. By doing so, there is no need for the entire concept of dark energy to exist! Why? Because we just gave gravity the property of mass! Newton’s inverse-square law dictates that the strength of a force decreases with the square of distance between two masses. This means that if I triple the distance between two objects, the force between them will be nine times weaker. By giving gravity a mass, the inverse-square law will not be precise anymore! Instead, depending on the mass of the graviton, the forces will decrease at a much faster rate with increasing distance. This explains the accelerated expansion of the universe and eradicates the use of dark energy by proposing a small yet unimaginably significant change in the General Theory of Relativity.
According to Rham, she expects the graviton’s mass to be below 10-32 electron volts, which (if discovered) would be very small compared to the mass of an electron (500,000 electron volts). On a smaller scale, this tiny mass of a graviton might yield identical results to the ones found by General Theory of Relativity. However, when considering the cosmological distances and time scales in our universe, a graviton’s tiny mass will produce humongous deviations from the General Relativity.
Even though Rham isn’t the first physicist to work on massive gravity, she is being credited for reigniting this age-old debate of whether gravitons are massless or not. The only solution, now, to resolve this conflict about whether gravity has mass or not is to find gravitons. De Rham’s idea of massive gravity also presents a new possible method that could lead to the discovery of these enigmatic particles. In the upcoming articles, I’ll be looking into the various techniques that physicists have been using to find gravitons. The search for Frank-Einstein’s monster (gravity) is gruesomely exhaustive, yet undeniably beautiful. So, keep reading!!! And continue to speculate, innovate, till you constipate!