A new theory proposes that black holes can exist in balanced pairs, appearing as a single entity due to a force known as the "cosmological constant."This force, combined with gravity, keeps the black holes at a fixed distance even as the universe continues to expand. Scientists speculate that because of the "cosmological constant" that balances the expansion and gravity of the universe, balanced pairs of black holes can exist and appear just like a single black hole.
Researchers from the University of Southampton, working with colleagues from the Universities of Cambridge and Barcelona, have shown that it is theoretically possible for black holes to exist in pairs in a perfectly balanced manner - held in balance by cosmic forces - mimicking a single black hole.
A black hole is a huge celestial body with such strong gravity that even light cannot escape. Their density is astonishing. A black hole can pack the mass of the Earth into a space the size of a pea.
The effect of cosmic motion on black holes
Traditional black hole theory is based on Einstein's general theory of relativity, which typically explains how stationary or rotating black holes can exist independently in space. Pairs of black holes are eventually blocked by gravity, which draws them together until they eventually collide.
However, this would be the case if the universe were assumed to be stationary. What about the universe that is constantly in motion? In an expanding universe, could pairs of black holes exist harmoniously, perhaps masquerading as a single black hole?
Two black holes can remain a fixed distance apart when their gravitational pull (red arrow) is canceled out by the expansion of the universe (blue arrow) related to the cosmological constant. To a distant observer, this appears to be a single black hole. Image source: APS/AlanStonebraker
Professor Oscar Dias from the University of Southampton said: "The standard model of cosmology assumes that the universe was born with the Big Bang. About 9.8 billion years ago, the universe was dominated by a mysterious force (called 'dark energy'), which accelerated the expansion of the universe at a constant rate.
The influence of the cosmological constant
Scientists call this mysterious force the 'cosmological constant'. In a universe with a cosmological constant explained by Einstein's theory, black holes are immersed in a background of cosmic acceleration. This changes the theoretical standards for how black holes interact and coexist.
Through sophisticated numerical methods, the team behind this latest study showed that two static (non-rotating) black holes can exist in equilibrium - their gravitational attraction is offset by the expansion associated with the cosmological constant. Even as the universe's expansion accelerates, the distance between black holes remains constant. Although the expansion of the universe tries to pull them apart, the gravitational pull compensates.
"From a distance, the attraction of a pair of black holes is offset by the expansion of the universe, and it looks like a single black hole. It may be difficult to detect whether it is a single black hole or a pair of black holes," commented Professor Dias.
Professor Jorge Santos from the University of Cambridge added: "Our theory was proposed for a pair of static black holes, but we believe it can also be applied to rotating black holes. Furthermore, our solution also appears to be applicable to three or even four black holes, which opens up a range of possibilities."