July 28, 2011 | 30
On March 28, 2011, the Swift Burst Alert Telescope detected a gamma-ray event that, in contrast with any previously observed gamma-ray burst, remained bright and highly variable for 48 hours. The gamma-ray emission was accompanied by bright x-ray emission that continued for two weeks. Astrophysicists attributed this event to the tidal disruption of a star by a black hole in the center of a distant galaxy. I would argue, however, that it would have been more accurate to describe this event as the tidal disruption of a star by a compact object. This distinction is important because the black-hole model has serious problems. The March event lends support to a heretical idea: that black holes do not exist.
The brightness of the gamma-ray and x-ray emissions suggests they are coming from a jet of charged particles moving at nearly the speed of light, but there is no obvious reason why the tidal disruption of star by a black hole should give rise to such a jet. In fact, the astrophysical community has been struggling to explain the observed ubiquity of jets. A leading idea is that, in the presence of a external magnetic field, electromagnetic energy is extracted from a rotating black hole and used to accelerate charged particles. The source of the field could be the disk of material swirling around the black hole. Yet disks do not generate magnetic fields with the right shape to produce well-collimated beams of particles.
More deeply, there are fundamental reasons why no compact object can be a black hole. The problem is that solutions of Einstein’s general-relativity equations that contain event horizons are inconsistent with quantum mechanics. For example, these spacetimes do not possess a universal time, which is required for quantum mechanics to make sense. Astrophysicists came to accept the idea of black hole because the gravitational collapse of sufficiently large masses cannot be stopped by ordinary means. But Pawel Mazur and I realized some time ago that quantum gravitational effects modify the collapse process.
Ordinary matter will be converted into vacuum energy when it is compacted to the point where general relativity predicts that an event horizon would begin to form. In contrast with ordinary mass-energy, vacuum energy is gravitationally repulsive, so it would act to stop the collapse and stabilize the object. At the surface of such objects, there is a transition layer between the large vacuum energy of the interior and the very small cosmological vacuum energy. In 2000 my colleagues and I suggested that this transition layer represents a continuous quantum phase transition of the vacuum. In 2003 George Musser wrote in Scientific American about the concept and suggested the name "crystal stars". But I prefer the name "dark energy stars".
Low-energy particles entering a dark energy star do not disappear, but follow a curved trajectory and emerge from the surface in much the same way that light does in a defocussing lens. On the other hand, the surface is opaque to elementary particles having energies exceeding a certain threshold. This is a result of the fact that near to a continuous phase transition, there are large fluctuations in the energy density, which in the case of a dark energy star means in the vicinity of the surface. Because the quarks inside protons and neutrons have energies exceeding the threshold for opaqueness, protons and neutrons falling onto the surface of a dark energy star will decay into positrons, electrons, and gamma-rays. In fact, one can make use of quantum chromodynamics (QCD) to predict the energy spectrum of these decay products . The result is that for both the leptons and gamma-rays the spectrum extends up to energies of several MeV. Thus the model predictes that matter falling onto the surface of a dark energy star will result in the production of high-speed electrons and positrons and gamma-rays. The March 28 Swift event is perhaps the clearest evidence to date of this process.
Dark energy stars can readily explain jets. Their angular momentum is carried by spacetime vortices concentrated near the axis of rotation (arXiv.org/abs/gr-qc/0407033). As a result an external magnetic field will be wrapped around this vortex core in a barber-pole pattern. Injecting nucleon decay electrons and positrons into a rotating dark energy star will result in a highly collimated lepton jet. Such a jet has a structure very similar to that seen in the jets emerging from the centers of many distant galaxies. What is unique about the March 28 Swift event is that we can see for the first time that the formation of this kind of jet is completely in accordance with what would be expected in a dark energy star.
We arrive at the following picture. When matter from a nearby star hits the surface of a dark energy star, it is instantaneously converted into gamma-rays, electrons and positrons, the majority of which have energies in the 100 keV to few MeV range. It takes about a minute for these particles to fill the interior of the compact object and form a jet. Because the gamma-rays can scatter off the magnetically guided positrons and electrons, a burst of gamma-rays directed along the axis of rotation will initially accompany the jet. After the supply of gamma-rays is exhausted, a beamed emission of x-rays will persist as long as the supply of electrons and positrons lasts.
I doubt that this event alone will dislodge black holes as the astrophysical community’s standard model for compact objects. On the other hand, the unique properties of the March 28 event, together with other ways that the dark energy star theory might be tested in the near future, such as direct millimeter VLBI observations of the massive compact objects at the center of own and nearby galaxies, may soon allow the astrophysical community to see that black holes are really crystal stars.
About the Author: George Chapline is a theoretical physicist at the Lawrence Livermore National Laboratory. He led the team that demonstrated the first working x-ray laser, developed the concept of a "gossamer metal," and has contributed to string theory.
The views expressed are those of the author and are not necessarily those of Scientific American.