Event horizons, a consequence of general relativity, separate black holes from the rest of the universe. If you create a singularity, an event horizon will appear. But what if the supermassive objects at the center of galaxies are not black holes?

This suggestion has been studied in several alternative theories of gravity, so researchers from the University of Texas at Austin and Harvard decided to put them to the test. They looked at what properties a massive, not-collapsed object would have and reached the conclusion that it would have a hard exterior.

In a paper published in the Monthly Notices of the Royal Astronomical Society, the researchers determined what would happen if a star were to slam into this hard surface. They decided that the star would be destroyed and stellar material would spread across the surface.

"Our motive is not so much to establish that there is a hard surface, but to push the boundary of knowledge and find concrete evidence that really, there is an event horizon around black holes," senior author Professor Pawan Kumar said in a statement. 

He added: "Our whole point here is to turn this idea of an event horizon into an experimental science, and find out if event horizons really do exist or not."

This theoretical background allowed for a clear observational prediction. If general relativity is wrong and there are hard surfaces where event horizons should be, then we should be able to see the stellar signal. The team used the Pan-STARRS telescope in Hawaii over a period of 3.5 years, looking for these objects.

"We estimated the rate of stars falling onto supermassive black holes," lead author Wenbin Lu added. "Nearly every galaxy has one. We only considered the most massive ones, which weigh about 100 million solar masses or more. There are about a million of them within a few billion light-years of Earth."

The team estimated that they should have had at least 10 detections, but they saw none. Event horizons do seem to exist and general relativity appears to have survived another test.

"Our work implies that some, and perhaps all, black holes have event horizons and that material really does disappear from the observable universe when pulled into these exotic objects, as we’ve expected for decades," co-author Ramesh Narayan of Harvard concluded.

There is still plenty that we can’t explain about gravity, so general relativity will have to be amended in time, but for now the theory is safe.