New revelations about dark energy

According to a new study, in an expanding universe, the rate of expansion of the universe at the event horizon of each black hole must be a constant in Einstein’s equations. This implies that the only energy present at the event horizon is dark energy.

Origin of black holes and event horizon

Black holes, celestial objects of extreme density and gravitational force so intense that even light cannot escape them, were first predicted by Karl Schwarzschild in 1916. Using Einstein’s equations of general relativity, Schwarzschild described a massive, non-rotating, perfectly spherical object, giving rise to the event horizon concept. Imagine a critical surface surrounding the black hole where any matter or energy inside is irremediably pulled toward the center of the black hole. The event horizon would thus be considered the point of no return. Since then, astronomers have discovered that most galaxies, including the Milky Wayharbor supermassive black holes at their centers.

New discovery

More recently, a study led by Nikodem Popławski of the University of New Haven shed new light on the relationship between black holes and the expansion of the universe. Essentially, the researcher discovered that in an expanding universe, Einstein’s equations dictate that the expansion rate of the universe from space to the event horizon of a black hole is a constant related only to the cosmological constant. It is often interpreted as the energy density of the vacuum of spacetime and is today known as dark energy density. To reach this conclusion, Popławski used the McVittie solution, which was developed in 1933 by the British mathematician and cosmologist George McVittie. He had solved Einstein’s equations to describe the structure of spacetime around a mass in an expanding universe. He had shown that near this mass, spacetime resembles that described by Schwarzschild with an event horizon, while further away from the mass, the universe expands in a way similar to our current universe.

Illustration of a black hole at the center of a galaxy. Credits: ClaudioVentrella/istock

What implications?

Popławski thus discovered that the rate of expansion of space at the event horizon must be a constant. As mentioned earlier, it is related only to the cosmological constant, or dark energy density, which implies that the energy present at the event horizon is only dark energy. The implications of this discovery are significant. First, it suggests that the universe does not expand uniformlybut that different regions expand at different rates, a variation known as the “Hubble tension.” This tension arises from the difference in measurements of the expansion rate of the universe based on older observations versus more recent ones such as the cosmic microwave background. Popławski suggests that this difference can be explained by correctly analyzing spacetime around black holes in an expanding universe using Einstein’s theory of general relativity. In other words, variations in the expansion rate can be attributed to the effects of gravity and spacetime near massive objects such as black holes. Furthermore, his calculations indicate that to allow for these variations in expansion, the cosmological constant, which represents the density of dark energy, must have a positive value. This constant is essential to keep the universe open and constantly expanding. If it were negative or zero, the universe would be closed and unable to support the observed expansion, which would contradict current data. Ultimately, the works Nikodem Popławski’s findings not only enrich our understanding of black holes, but could also transform our view of the expansion of the universe and dark energy. These discoveries raise new questions and hypotheses, propelling cosmology research to previously unexplored horizons.