Could the birth of a four-dimensional black hole have created our three-dimensional universe?
That’s the idea put forth by the authors of a new paper on the arXiv preprint server1. Traditional Big Bang cosmology aligns well with many of today’s precision astrophysical measurements, they write, but it still leaves some important questions unanswered: In particular, what happened at the infinitely dense point, or singularity, from which the Big Bang sprung?
As Niayesh Afshordi, an astrophysicist at the Perimeter Institute for Theoretical Physics and one of the paper’s authors, told Zeeya Merali for Nature News, “For all physicists know, dragons could have come flying out of the singularity.”
“In the current best theories that we have, we know that we don’t know,” says Sean Carroll, a theoretical physicist at Caltech who was not part of the team that published the new paper. “We have theories of the universe that work really, really well, but they just don’t say anything about the Big Bang. They fail to give an opinion.” And when the equations of general relativity are applied to the Big Bang singularity, they pop out infinite answers. “What that really means is that the equations are breaking down,” explains Carroll.
Many physicists can stomach that breakdown as long as the singularity is quarantined behind an event horizon, an invisible boundary beyond which no information can pass to an outside observer. “If they [singularities] are ‘hidden’ behind event horizons, they do not affect our predictions, and so we can still use laws of standard physics,” says Afshordi. But the Big Bang singularity is not shielded in this way; instead it is what physicists call “naked.”
“’Naked’ singularities are not hidden, and thus anything to the causal future of ‘naked’ singularities will be affected by laws beyond standard physics,” says Afshordi.
Searching for a way to avoid the naked singularity at the Big Bang—and perhaps explain other vexing properties of our universe in the process—the authors of the new paper turned to a model of the cosmos called the “braneworld.” In the braneworld, our observable, three-dimensional universe actually lives inside another universe which has extra spatial dimensions. To use a two-dimensional analogy, our universe is like the skimmable membrane (“brane”) of fat on top of the pea soup of the universe.
We can’t detect this soup, called “the bulk,” directly, but it could explain some bizarre quirks of physics, like why gravity is so much weaker than the other fundamental forces. Yet physicists have not had much to say about what kinds of objects might live in the bulk and how they might affect us here on the brane.
The new paper analyzes what would happen if a black hole formed within the bulk. Unlike a regular old three-dimensional black hole, which is surrounded by a two-dimensional event horizon, a four-dimensional black hole would have a three-dimensional event horizon. And that event horizon would be constantly expanding. Sound like any universe you know?
The paper’s authors argue that this picture could address other mysteries of Big Bang physics, like how the universe settled down to such a uniform temperature so quickly. Physicists typically explain this problem using a phenomenon called cosmic inflation, which is believed to have caused the universe to swell up rapidly soon after the Big Bang. This swift, early expansion means that parts of the universe that seem disconnected today—that is, they are so far apart that they can’t exchange photons—could have “touched” in the distant past.
Inflation has passed nearly every test we’ve put it to. It even matches up nicely with the latest data from the Planck space observatory, which made the most precise map ever of the cosmic background radiation. The new black hole model doesn’t agree as closely with the Planck data. Plus, physicist Paul Halpern points out, “The authors have constructed something that needs to be closely manipulated and tweaked to get the parameters that inflation gets very naturally.”
Halpern also isn’t convinced that the problems the new model sets out to solve are truly so dire. The naked singularity that plagues the equations of general relativity might melt away once we have a theory that combines general relativity with quantum mechanics, says Halpern. The Big Bang singularity “doesn’t really trouble people who think there will eventually be a theory of quantum gravity.” Plus, Halpern points out, there are practical limits on our ability to observe the naked singularity at the beginning of the universe. “As we go back in time, it’s harder and harder to make observations.”
It doesn’t seem likely that the Big Bang is going to be dethroned by the Big Black Hole anytime soon. But, says Halpern, “It’s important to be generous in terms of allowing for a wide range of theoretical models. You never know which will help us, ultimately.”
1The paper has been prepared for submission to the Journal of Cosmology and Astroparticle Physics, though it has not yet been peer-reviewed and published.
Author’s picks for further reading
arXiv: 4D Gravity on a Brane in 5D Minkowski Space
In this academic paper, physicists Gia Dvali, Gregory Gabadadze, Massimo Porrati propose the brane world scenario on which the new paper is based.
The Nature of Reality: A Journey Into Extra Dimensions
In this video pencast, theoretical physicist Delia Schwartz-Perlov explains what physicists talk about when they talk about extra dimensions.
The Question of Cosmic Censorship
Physicist Roger Penrose on the problem of naked singularities and the possibility that physics may prevent them through “cosmic censorship.”