Written by Lotte ter Haar **
On August 17th, 2017, we made the extraordinary observation of two colliding neutron stars. The event was of huge scientific importance and would become one of the most studied events in astronomy, with 70 observatories—on all 7 continents and in space—moving their telescopes in their direction. Results were presented in numerous publications in prestigious journals, with one paper being co-signed by over 4000 astronomers. To give you an idea of the huge collective effort behind this observation: this is about one-third of the worldwide astronomical community. Although these numbers are impressive, they do not explain why we care so much about these objects. Before we dive into that question, let’s take a step back and discuss a bit more of what neutron stars actually are.
Neutron stars are the smallest and densest stars that exist, typically only a few kilometres in diameter, with a mass between one or two times that of our Sun. They owe their existence to the death of massive supergiant stars, that usually end their lives violently with something we call a supernova explosion. Throughout their lives, neutron stars continue to be characterised by extremity. They have a surface temperature of around 600 000 K, provide the strongest magnetic fields of any known object, and tend to spin with extreme velocities (up to one-fifth of the speed of light). But what gives the interest to explore these objects more is gravity around neutron stars is in the extreme regime.
For about 100 years now, gravity has been successfully described by Einstein’s theory of General Relativity. This theory has made several predictions, such as the existence of black holes and gravitational waves, which both have been observed directly in the last years and continue to pass tests in many other ways. As of now, we do not have many answers in cosmology, another field of research where the past, present and future of the Universe is studied. One of the big mysteries in cosmology is why does everything move away from each other, in an accelerated way; in trying to answer this question, the existence of some new component was proposed, appropriately named dark energy. There is one problem though: even after many years of research, the nature of dark energy remains unknown to this day. Trying to approach this problem from a different angle, physicists have been coming up with so-called modified gravity theories in which they do not postulate the existence of dark energy, but rather slightly change the laws of gravity to explain the accelerated expansion of the Universe.
Although these theories work well on very large (cosmological) distances and are useful in addressing the problem of dark energy, they should also make sense on shorter (astrophysical) scales. In fact, we know that here gravity behaves exactly as Einstein predicted, and therefore General Relativity and modified gravity theories should be identical in astrophysical systems. Whether this is true or not, it crucially depends on whether the new theory presents something we call the screening mechanism. The basic idea behind this mechanism is that it can mask any modifications with respect to General Relativity on short distances, but still allow deviations on larger distances.
The screening mechanism has been studied in stars that are more similar to our Sun, but not yet in extreme stars such as neutron stars. This is where our work comes in, where we focus on one theory in specific, that goes by the name of k-essence. To study a theory, one usually has to solve a very complex set of equations, simplifying a task by using numerical methods. By doing so, we have shown that the screening mechanism is also present in neutron stars.
Although this is good news, it is not quite enough. Since we know that neutron stars exist and are stable (meaning they do not transform into something else if they are disturbed slightly), we want the same to be true for the stars in the new theory. We can study the stability of stars by using another numerical code that can see their evolution in time. It seems that in many cases the stars are indeed stable, but in some cases, they are not. Our current research is focused on understanding the origin of this instability. Finding an answer to that question will bring us closer to either falsifying the theory of k-essence and confirming General Relativity or understanding the origin of the accelerated expansion of the Universe.
This short outreach article is in reference to the latest paper on https://arxiv.org/abs/2009.03354