A flash in the sky. With such high energy that our eyes cannot detect it, as it is outside of their electromagnetic range. But gamma-ray telescopes in orbit can. That was what the Vela satellites did in 1967. They caught a flash of gamma rays in the sky. And since they were meant to track nuclear tests, the questions began – is it us, is it the enemies? Soon it became clear the explosion they observed came from far, far away. In fact, farther than most objects we can observe. The flashes didn’t stop and soon, it was discovered they came from everywhere. They were named gamma-ray bursts (GRB). Amazingly, one of them was detected by the Venera 11 and 12 missions on their way to Venus in 1979. This detection marked a milestone for the studies of gamma-ray bursts and led to an incredible journey that made them one of the most observed objects in the sky. The fascination with GRBs came from the realisation that their emission offers insights into the ways the stars end, the evolution of the Universe, and even what the eventual theory of quantum gravity could be.
GRBs are not just highly energetic, they are the most powerful explosions that we have observed. The energy emitted from a GRB is about the same as if we would turn the whole mass of the Sun into energy instantaneously. Or, if we forget about special relativity, it would be enough to propel the Sun to the speed of light in seconds. It is so high that it is comparable to the highest energy scale we know of – the Planck scale. The Planck scale is where the four fundamental forces of nature – gravity, electromagnetism, the strong nuclear force, and the weak nuclear force – are hypothesized to unify. It is where we need a quantum theory of gravity to describe physics. It is much larger than the energy we have achieved in our particle accelerators. Yet in Nature, apart from the very early stages of the Universe, it can be produced during the death of massive stars. And it turns out, this is exactly how GRBs are born.
The stars live quite long and happy lives. But the bigger the star, the sooner its life ends. And that end is violent. When the balance between the attractive gravity and the expansive pressure of radiation is broken, the end of the star is near. Depending on how heavy it is, there are different paths to its destruction, but for GRB’s production only two are important. If the star is very massive, its core can collapse directly to a black hole while its envelope explodes as a supernova. What we see on Earth is what we call a supernova Type Ic preceded by a GRB. These GRBs are usually long and very energetic.
AI impression of the death of a massive star. Credit: Leonardo AI non-exclusive license
If the star is not massive enough to become a black hole straight away, but its core is too heavy to turn into a white dwarf (the so-called Chandrasekhar limit), the star will collapse to a neutron star. Neutron stars are incredibly interesting objects in which the pressure is so high that the matter stops behaving normally. When they happen to have another compact massive companion (such as a black hole or a neutron star) – they will orbit around each other in a binary inspiral, until they get into the final stage of their relationship – a merger to a black hole. In the final fireworks of that merger, a short GRB is born. And one that comes accompanied by gravitational waves, which the LIGO and VIRGO gravitational waves detectors confirmed in 2017 when they observed a short GRB born from the merger of two neutron stars. These are the two general classes of GRBs we distinguish – long and short. As they are born in different processes, they have different characteristics. Yet, they have in common the extreme energy output and the high distances at which they can be seen. The furthest long GRB is so far away, that the Universe has been only about 500 million years old at that time. In order for the light of an object with the mass of only a few Suns to reach us from this distance, the explosion has to be incredibly powerful.
AI impression of the merger of a binary system of a black hole and a neutron star. Credit: Leonardo AI non-exclusive license
This leads us to the key to their importance for cosmologists. Distances in space are complicated to measure. To do so, astronomers make a series of measurements starting from the closest objects and extending it to the furthest. This is the so-called distance ladder. Eventually the GRBs could be the next step (rung) of the distance ladder, allowing us to measure distance much further. And this could be crucial when working on the Hubble tension. The Hubble constant measures the expansion of the Universe. We expect it to be the same for objects very close to us and for such that are very far away from us – meaning the same during the late stages of the Universe and the early ones. Yet, the two measurements do not match and we call this “The Hubble tension”. Having information of objects coming from different times/distances, such as the supernovae, baryonic acoustic oscillations, quasars, GRBs, combined with the cosmic microwave background, has allowed cosmologists to look for possible explanation of the Hubble tension in different models for the evolution of the Universe. While so far this approach hasn’t been able to resolve the Hubble tension, it has led to active work on new extended models and new theories of modified gravity. For example, in my work I’ve found that adding GRBs increase the tension in some cosmological parameters, asking the question whether this is a problem of the data or a sign of aggravating the Hubble tension.
But this is only part of the story – GRB can be also a perfect test for quantum gravity theories. Such theories predict specific effects that can be observed only at these extreme energy scales. We know that in special relativity, the speed of light is a constant. Yet some quantum gravity theories predict an energy-dependent speed of light. This means that when high energy photons and low energy photons are emitted at the same time, they should arrive at different times in our telescope. This effect is called quantum gravity time-delay (to distinguish it from the classical time-delay) and it is expected to be extremely small. It however can be amplified by the distance the photon travels to reach us and by its energy. And GRBs are among the most distant objects in the sky, with emissions observed at very high energies. Spotting these quantum time-delays in the plenitude of observations of GRBs can open the door to understanding quantum gravity and selecting a suitable theory.
Since the effect is so tiny and the precision of time-delay datasets is still not high enough, for now, the evidence of quantum gravity remains elusive. However, if such quantum time-delay exist, these datasets can be used also in cosmology. Including GRB time-delay data in cosmological models would allow us to peek at the secrets of the universe from completely different angles. It could give us important information about the Hubble tension and how it is affected by the inclusion of quantum gravity effects. The challenges in front of such work include developing better GRB models, accounting for the propagation of the high energy photons through the interstellar space and precise measurement of the time-delay in our data. Yet, the first steps in this direction have been made and there is concentrated effort by many groups. And the advances in machine learning, which have brought such amazing improvements to astronomy and astrophysics, may revolutionize the field soon. In my recent studies, I’ve found that allowing even for small quantum time-delay changes the cosmological parameters in a way that increases the Hubble tension. The eventual detection above certain threshold of this effect can dominate over our local universe measurements, again challenging our preferred cosmological model. In conclusion, studying GRBs offer incredible opportunities to astrophysics. They can help us understand the nature of the Hubble tension by telling us new stories of the past and the future of the Universe. They can gives us clues about quantum gravity provided we are able to measure the tiny Planckian effects. In this way, in the intersection of observational and theoretical astrophysics, along with data analysis and numerical modelling, lies the unravelling of the mystery of GRBs – Nature’s most powerful fireworks.