Black holes and neutron stars are currently the only ones representing experimentally confirmed compact objects in the catalog of cosmic objects. However, others of these objects – called exotic compact objects – like fuzzballs, gravastars, or Planck stars, have been suggested by physicists over time. And they could signal their presence via the gravitational waves they would emit. Although current detectors like LIGO and Virgo are still too insensitive to detect such signals, updates to their performance could lead to the detection of these hypothetical objects in the future.
A new study determines that in the years to come, gravitational wave observatories on Earth could help find hypothetical exotic compact objects (OCEs). LIGO (Laser Interferometer Gravitational-Wave Observatory), based in the United States, and its European counterpart Virgo, were designed to capture ripples in the fabric of space-time radiating from massive objects such as black holes and neutron stars. Yet, there is always a chance that scientists will encounter something unexpected.
Exotic compact objects
Researchers have been speculating on the possibility of the existence of exotic compact objects for many years and are trying to determine what they would look like for a gravitational wave detector. The term “exotic compact object” encompasses a variety of different theoretical entities. Among these objects are gravastars (similar to ordinary black holes, but filled with dark energy, a mysterious substance causing the expansion of the Universe to accelerate).
Other compact objects that could be lurking in the Universe are fuzzballs. A fuzzball a cluster of fundamental strings proposed in string theory and supposed to replace the classic black holes of general relativity. The factor that connects exotic compact objects is that, unlike a black hole, they shouldn’t have a region known as the event horizon. According to Albert Einstein’s theory of relativity, the event horizon is a sphere surrounding a black hole beyond which it is impossible to return.
Objects can pass through the event horizon, but nothing can come out – not even light. But physicists know that Einstein’s theory of relativity will one day have to be replaced. While the theory is extraordinarily successful in describing gravity and massive cosmic entities, it says nothing about the behavior of subatomic particles. For this, physicists are turning to quantum mechanics.
The hope is to finally have a theory of quantum gravity that unifies relativity and quantum mechanics. Exotic compact objects, which would resemble a black hole, but without an event horizon, could help provide the information needed to build this future theory.
Identify OCEs using gravitational waves
When two black holes collide and merge, they revolve around each other, distorting space-time and generating gravitational waves, which can alert LIGO detectors on Earth. After they meet, the event horizon prevents additional gravitational waves from escaping outward.
But since exotic compact objects would not have an event horizon, some gravitational waves could fall towards the object’s center and then bounce back, creating gravitational echoes that escape outward. These echoes are too weak to be detected by LIGO and Virgo at the moment, but the facilities are currently being upgraded for increased sensitivity, and they have been joined by the Kamioka Gravitational Wave Detector (KAGRA) in Japan. , which became operational last year.
Luís Longo (Universidade Federal do ABC in São Paulo) and his colleagues calculated that during the next gravitational wave detector observation race, which is due to start in the summer of 2022, LIGO and his counterparts could be sensitive enough to pick up the signal from one or two exotic compact objects, if they exist. However, even if LIGO were to detect any echoes, it would probably still be a long time before the scientific community could confirm that they really came from these hypothetical objects.