The mechanism by which massive stars explode is still poorly understood. Satellite-based observation of gamma-rays offers a particularly promising new source of data with which to constrain models, but for this to be useful the rates of certain nuclear reactions must be measured.
Alex Murphy of the Edinburgh Nuclear Physics group is spokesperson for an experiment on ISOLDE at CERN which aims to determine one of the most important such reactions. The article below explains how this experiment could allow a new theory of scalar gravity to be tested.
Star experiment on ISOLDE
This story was first published in the newsletter 'UK news from CERN'.
Scalar field theories are just like buses. You wait 50 years for a theory to be proved correct and then another revolutionary theory comes along, right behind it.
Results from an experiment about to start in the ISOLDE facility could be the first step towards proving whether an audacious new theory is correct. The new theory goes beyond Einstein’s Theory of Relativity, and beyond the Standard Models of Cosmology, Astrophysics AND Particle Physics. If the theorist is right, there’s another particle waiting to be discovered...
Supernovae are some of the brightest objects in the Universe. These dying stars are responsible for producing many of the heavy elements that are required to support life. But the processes that take place within the star and lead to its catastrophic explosion are less well understood. Astrophysicists have a theory but due to its complexity, it is impossible to model it successfully on computers. Experimental observations have added to the confusion.
Stars are massive balls of burning gas, with their energy supplied by a series of nuclear fusion reactions. Stars bigger than about eight times the mass of our own Sun have cores that burn atoms all the way from hydrogen to iron. Fusion of elements lighter than iron releases energy, but fusion of heavier elements requires energy.
Once the burning has reached iron the nuclear fires go out and the core of the star collapses. Such is the weight of the star that atoms are crushed together, and it is only when the tiny nuclei of the atoms begin to overlap that the collapse is suddenly halted.
Following this ‘core bounce’, the star explodes, but not immediately. Astrophysicists know that the supernova gets hotter and that it generates neutrinos, with some current theories suggesting that neutrinos are the key to successful explosions. These are supported by the observation of about 20 neutrinos from supernova 1987a. However, when modelling this on a computer, only very weak explosions occur. This could be down to insufficient computer processing power, our understanding of neutrino physics, a fundamentally incorrect model, or something to do with gravity.
New theoretical framework
Charles Wang, a theoretical physicist at the University of Aberdeen, and a Visiting Fellow at the STFC Centre for Fundamental Physics, favours the gravity option. As supernovae implode, they release lots of gravitational energy and Charles has proposed a theoretical framework in which a new scalar particle leads to increased energy transfer, giving the extra boost needed to make core collapse supernovae explode. This is radical thinking, and if Charles is correct, his idea could eventually lead to a Grand Unified Theory.
But a theory remains just that until it can be proved.
Alex Murphy, a nuclear astrophysicist at the University of Edinburgh, read Charles’ theory and thought that his planned experiment on ISOLDE might help.
The experiment will use a beam of Titanium 44 (44Ti), an isotope collected from recycled radioactive waste at the Paul Scherrer Institute in Switzerland. Alex’s team will fire the beam of 44Ti at a helium gas target and look for reactions which generate a proton and Vanadium 47.
Titanium 44 is generated right in the core of the supernova as it cools. What’s more, it emits a very specific energy gamma ray, which could be seen by satellites such as Nasa’s NuStar and ESA’s Integral missions. By comparing the amount predicted by the model to what is actually observed, you can do a sensitive test of what’s happening at the core of a supernova.
“So far, the observations made by the satellites and other measurements such as at the T2K neutrino experiment in Japan are not robust enough for us to draw a firm conclusion about the validity of the computer model. There are just too many variables. One of the biggest is the nuclear physics of how the titanium is made. With this tied down we should be able to finally make clear predictions of the amounts of 44Ti produced. Comparing this to the observations might be able to confirm if Charles’ idea is right!” Alex Murphy, Nuclear Physics group, University of Edinburgh.
Whilst many more experiments would be required to prove the existence of the ‘Wang’ particle, this is the first step and Charles Wang is at CERN to see the experiment happen, “As a theoretical physicist, I have the great privilege of working with experimental physicists – I find it very stimulating.”
Two PhD students - Vincent Margerin and Dave Mountford - are members of the Edinburgh experimental team.
Vincent Margerin is in the first year of his PhD. “I didn’t know about Charles' theory when I started my PhD and it wasn’t until much later that I learned that my project was linked to such a challenging theory," he says. "My PhD consists of several experiments, all aiming towards a better understanding of the explosive star phenomena. Our current experiment is wonderful, though very challenging. It’s hard to see where it will carry my PhD; but the fact that its potential results could help shake our understanding of the Universe is fascinating. Whatever happens there are some exciting times ahead!"
“The results of this experiment will be very exciting. They could lead to a fundamental change in the way we think the Universe works. If Charles is right, I will be able to look back and say that I played a small part in a massive scientific discovery!” Dave Mountford, final year PhD student who is at CERN to provide an extra pair of hands during the experiment.