A new paper from cosmologist Nikodem Poplawski examines the relationship between the expansion of our universe and a hypothetical rebound force that might counter gravity inside a black hole. Among the implications, it could mean we're all living inside a black hole.
Well, early on in the 20th century we figured out the universe is expanding. Here's a concise history:
We know that light acts as a wave. There are higher frequencies and lower frequencies, which correspond to the different kinds of light (from longest to shortest wavelength: radio, microwave, infrared, visible light, ultraviolet, x-ray and gamma ray). Due to the Doppler Effect, whenever a source of light is moving away from us at a sufficiently high speed, we can discern a redshift, meaning that its light becomes slightly redder as a result of the light waves being stretched out. This is much like the Doppler Effect we hear when an ambulance speeds by: as the ambulance recedes from us, the pitch of the siren is appreciably lower to our ears than when it was coming towards us, as the sound waves stretch out behind the vehicle. By looking at the redshift of distant galaxies, astronomers could see that almost all the other galaxies were rushing away from us (this is not to imply that we are at the center of the universe... observers in every galaxy would see essentially the same thing). From this discovery, the theory of the Big Bang was born.
Then the next big question: will the universe expand forever, or will it come to a halt and start collapsing, resulting in a Big Crunch? The answer to this question would hinge on whether there was sufficient matter in the universe to pull it all back together. In either case, this means the universe would eventually die -- either from becoming so diffuse that matter can no longer condense into stars and galaxies, effectively ending the energy output of the universe and becoming a cold soup of elementary particles -- or by being consumed in one inconceivably violent collapse of everything in the universe... a real day of reckoning. Neither one of these eventualities are very appealing, but we can take some comfort in knowing that they would be many billions of years in the future.
Then in 1998, it was discovered that not only is the universe expanding, but it's actually accelerating! This creates a problem, because if the universe is accelerating you need some sort of mechanism driving it... the initial energy of the Big Bang is not going to be enough. Cosmologists have been puzzling over the problem ever since this discovery, and usually point to a hypothetical form of energy called dark energy as the culprit. This is where it gets complex, so I'll leave the details to Wikipedia. But the idea of a kind of cosmological constant energy source is not new: Einstein actually came up with the idea, but in his case he introduced the variable to counteract gravity and create a static universe (neither expanding nor contracting), an idea he found less unsettling.
That basically brings us up to the present. Cosmologists are still trying to figure out what's driving the accelerated expansion of the universe, which is where Poplawski's new paper comes into play. Cosmologists have long speculated over whether our universe is really just the inside of a black hole, but Poplawski's work, rejecting the notion of a singularity at the heart of a black hole and using the hypothetical rebound force as an analog for the universe's expansion, seems to be a novel approach to the idea.