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?
Artist's impression of a growing supermassive black hole. Image credit: NASA/CXC/A.Hobart
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.
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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.
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Quantum physicists regularly ask you with a straight face to accept what seems to be complete nonsense. Particles are also waves; cats are alive and dead at the same time. But some of the most incredible creatures of the quantum realm get far less attention than Schrödinger’s famous cat. They’re called virtual particles, and they might be the reason the universe exists in the first place. In the pencast below, I’ll explain the basics of virtual particles. Then read on to learn more.
While the Big Bang theory explains how the universe has expanded and cooled since it began, it is quite silent on what “pulled the trigger,” so to speak. We simply don’t know what started the process. How there could be nothing at one moment and an entire baby universe the next?
It turns out that getting something from nothing is just business as usual for virtual particles. The most straightforward way to explain virtual particles is by an example. Consider a particle collision in which one electron hits another and the two scatter. In the classical view, the electric field from one electron interacts with the other and the two feel a repulsive force. However, this approach neglects Einstein’s Nobel Prize realization that light—and, by extension, every electromagnetic field—is quantized. So a quantum treatment of electron scattering needs to include not only the quantum nature of the electrons, but also the quantum nature of the photon. We now treat electron scattering as the two quantized electrons exchanging a quantized photon and, in the process, changing their directions.
So how do virtual particles enter in? Well, you can calculate the properties of the photon that must be emitted to scatter the electrons. Simple energy and momentum conservation considerations tell us what the energy and momentum of the photon must be. However, when you do the calculation, you find that the photon has a mass! Since photons are massless particles, this seems to invalidate the whole idea. It sure sounds like physicists are pulling your leg, just to see how long it will be before somebody is willing to say that the subatomic Emperor has no clothes.
As crazy as this seems, it is true. To see how, we need to invoke another hard-to-swallow axiom of quantum mechanics: the Heisenberg Uncertainty Principle, named after its inventor, Werner Heisenberg. In classical physics, energy and momentum are always conserved. But Heisenberg spotted a loophole in this rule: in the quantum realm, energy and momentum don’t have to be conserved, as long as the non-conservation doesn’t persist for very long. It’s kind of like having a shady accountant. If you audit the books, the amount of money you send him has to agree exactly with the amount of money he uses to pay your bills. But, while he has your funds, he is free to temporarily lend or borrow money so that momentarily he will have the “wrong” amount of money. Further, the larger the amount of money loaned or borrowed, the shorter the period of time it will occur. Similarly, in the quantum realm, energy and momentum can briefly be “wrong,” but the larger discrepancy, the shorter the period of time for which it is allowed.
So in our example of electrons scattered by exchanging photons, the photon can briefly have the “wrong” amount of energy and momentum. Now, it is understandable if you find this a bit hard to take; perhaps an instance of physicists making stuff up to save their theories. And, truth be known, that would be my reaction if there were not an extensive list of experimental measurements that demonstrate that virtual particles exist. In fact, virtual particles play a critical role in most of the experiments performed at large particle physics laboratories like CERN, Fermilab and many other similar facilities.
While I’ve described the idea of a single virtual particle, the idea is actually much richer than that. Virtual particles also exist in association with real particles. For instance, suppose you have an ordinary, garden-variety, electron. A reasonable mental image of the electron would be a little subatomic marble, carrying electrical charge, mass and spin. Anyone with even a cursory understanding of quantum mechanics know that image is a bit dodgy, as electrons exhibit lots of crazy quantum behavior.
The life of an electron is much more complex than that, though. In addition to the usual quantum craziness, where an electron is both a particle and a wave and the position of the electron is generally indeterminate, electrons are surrounded by virtual particles. For instance, an electron can briefly emit a photon. That photon will be reabsorbed quickly in such a way that the energy and momentum conservation laws aren’t violated. But it gets crazier than that. The virtual photon can also turn into a virtual electron/positron pair. Thus, for a brief moment, what was once just an electron becomes an electron plus an additional electron and positron. As long as the virtual particles coalesce before the universe notices, it’s all within the rules. Indeed an electron never exists as a single “bare” electron. Rather, it is always enshrouded in an ephemeral cloud of virtual particles, flickering in and out of existence, and vastly complicating what an electron “really” is.
It might seem far-fetched, but experiments can actually detect the presence of this cloud. That is because every electron acts like a mini-magnet. We can calculate exactly how strong the magnet should be. But when we make very precise measurements of its strength, we find that the measured magnetic moment is about 0.1% off from the simple prediction. It turns out that when you take into account the virtual cloud around the electron, it exactly matches this small 0.1% discrepancy, showing that the cloud is definitely present. Further, the data and prediction exactly match to nine digits!
If your mind isn’t blown, wait…it gets crazier still. Empty space—that is, space that contains nothing—no energy, no charge, no matter, nothing—is filled with a writhing, active population of virtual particles that physicists call “the quantum foam,” with bubbles appearing and popping in wild abandon. At the subatomic level, space is never truly empty.
You’d think that if empty space were filled with a constant roiling boil of quantum activity, you’d see it. The fact that you don’t could give you yet more reason to disbelieve, yet the effects of the quantum foam have been directly observed.
The first observation of the quantum foam came from tiny disturbances in the energy levels of the electron in a hydrogen atom. A second effect was predicted in 1947 by Hendrik Casimir and Dirk Polder. If the quantum foam was real, they reasoned, then the particles should exist everywhere in space. Further, since particles also have a wave nature, there should be waves everywhere. So what they imagined was to have two parallel metal plates, placed near one another. The quantum foam would exist both between the plates and outside of them. But because the plates were placed near one another, only short waves could exist between the plates, while short and long wavelength waves could exist outside them. Because of this imbalance, the excess of waves outside the plates should overpower the smaller number of waves between them, pushing the two plates together. Thirty years after it was first predicted, this effect was observed qualitatively. It was measured accurately in 1997.
Quantum foam also has astrophysical implications. In 1974, Stephen Hawking was thinking about quantum mechanics and black holes. He realized that the quantum foam would exist near the event horizon of the black hole. If an electron/positron virtual pair popped into existence just outside the event horizon, one of the two particles might spiral down and get trapped in the black hole, while the other would escape. As it happens, more energy would escape than be captured, so the energy of the black hole would get slightly smaller. Over the eons, this “Hawking Radiation” would cause the black hole to evaporate until it totally disappeared.
Virtual particles and the quantum foam are one of the craziest of the quantum phenomena. They have no classical analog and they certainly seem like something that physicists dreamed up to save the counterintuitive world of quantum mechanics. Borrowing from the movie “The Maltese Falcon,” quantum mechanics is said to be the dreams that stuff is made of, but virtual particles are no dreams. They have been experimentally observed, and indeed it could be that a quantum fluctuation similar to virtual particles was the thing that pulled the trigger on the creation of the universe itself: a crazy start for a universe where, we’re learning, the bizarre is the norm and dreams are reality.
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Alice in Quantumland: An Allegory of Quantum Physics
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Look out solid, liquid and gas: There’s a new form of matter in town. Actually, this “new” matter isn’t new at all—it is one of the most ancient forms of matter in the universe. Last seen more than 13 billion years ago, just millionths of a second after the Big Bang, this exotic stuff is making a comeback thanks to particle accelerators like the Relativistic Heavy Ion Collider (RHIC) on Long Island and the Large Hadron Collider (LHC) in Europe, where physicists can generate temperatures of more than a trillion degrees centigrade. These enormous temperatures allow scientists to push back the clock of the cosmos and witness matter in the extreme energy environment that existed within microseconds of the Big Bang.
At such high temperatures, the protons and neutrons inside atomic nuclei literally melt, releasing the quarks and gluons inside them and creating a form of matter called a quark-gluon plasma. You can think of the quarks as the “matter” particles and gluons as the particles of force that hold the protons and neutrons together. A reasonable mental image of a proton or neutron would be like a few flecks of Styrofoam (quarks) inside a lottery ball machine. The wind in the lottery machine is analogous to the force field, while the air molecules represent the gluons.
Under ordinary conditions, quarks and gluons are forever locked inside protons and neutrons. They’re like the water held frozen into ice cubes in a glass. But just as ice can be melted into water when energy is added to the system (by pouring hot tea over them, for instance) allowing the molecules from one ice cube to mix with molecules from other cubes, so too it is possible to melt protons and neutrons and have the quarks and gluons scamper around willy-nilly.
To melt protons and neutrons, you need to heat them up to about a trillion degrees. The only way to generate this kind of temperature is to smash together atomic nuclei at high velocities in huge particle accelerators. That’s what physicists are now doing at the LHC and the RHIC, accelerators that take atoms (lead and gold, as well as some others), strip off all of the electrons, and then slam the bare atomic nuclei together. The most violent of these collisions can heat up the nuclear matter enough to free the quarks and gluons to wander as they will. Though experimental calibration issues add some uncertainty to the mix, the current temperature record seems to belong to the ALICE experiment at the LHC, which measured an astounding 5.5 trillion degrees centigrade.
Studying the phase transitions of quark-gluon plasma allows us to understand the behavior of matter in the early universe, just fractions of a second after the Big Bang, as well as conditions that might exist inside neutron stars. The fact that these two disparate phenomena are related demonstrates just how deeply the cosmic and quantum worlds are intertwined. Credit: Brookhaven National Laboratory
While the first lab-made quark-gluon plasma was created in 2000, physicists are only now beginning to understand how this form of matter behaves. At the LHC and RHIC, they are mapping out in more detail the temperatures and pressures at which ordinary matter transforms into quark-gluon plasma. They are also tracing the boundaries between quark-gluon plasmas and even more exotic forms of matter like the stuff of neutrons stars, which is thought to be so dense that, at the center of the stars, quarks get “smooshed” together into an exotic kind of solid.
In fact, physicists believe that there are many different phases of matter involving quarks. While I’ve focused on two states of matter, atomic nuclei and quark-gluon plasma, this just scratches the surface of the possible.
By studying quark-gluon plasma, physicists are able to explore a period in the history of the universe that has thus far eluded us—a period in which protons and neutrons, the basic ingredients of ordinary matter, were coalescing for the first time. Thanks to accelerators like the LHC and the RHIC, we are finally beginning to probe this pivotal chapter in the story of cosmos.
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Brookhaven Lab: Quark-Gluon Plasma: a New State of Matter
In this video, physicist Peter Steinberg explains the nature of a quark-gluon plasma.
Physics Central: Quark-Gluon Plasma
Learn the basics of quark-gluon plasmas in this explainer from the American Physical Society.
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Explore highlights from the August 2012 Quark Matter conference, held in Washington, DC.
Did the universe have a beginning? What, if anything, came before the Big Bang?
Today, we see galaxies rushing away from us in every direction, suggesting that, if you could press the rewind button on the entire universe, the whole thing would screech to a halt at a moment about 13.7 billion years in the past, when the entire cosmos was apparently compressed into a singularity—an infinitely small, dense point.
“How does the universe begin from such a state?” asks Alexander Vilenkin, a theoretical physicist at Tufts University. Indeed, the laws of physics as we know them break down around singularities, so physicists have devised a number of ways to sidestep the singularity problem.
One possibility is that the universe is cyclic: Every Big Bang expansion is followed by a contraction, ending in a “Big Crunch” from which a new Big Bang emerges, and so on and so on in an infinite series that extends eternally into the past and future. The idea was first proposed centuries ago, but received a fresh take from the physicists Paul Steinhardt and Neil Turok in 2002. There is a problem with this elegant idea, though: the second law of thermodynamics, which states that the total amount of disorder or entropy in a system increases over time—the party-pooper law that prevents the existence of perpetual motion machines. A universe that experienced repeated cycles of expansion and contraction would have get more and more disordered over time until it began completely disordered, something we do not see in our universe. One way to avoid such increasing entropy would be for the volume of the cosmos to increase with each cycle. However, if one ran this scenario backward in time, one would still be forced to conclude the universe began with a singularity.
If our Big Bang wasn't preceded by a Big Crunch, perhaps our universe instead existed as kind of dormant seed—“like a cosmic egg,” says Vilenkin—before suddenly breaking open in the Big Bang. But here, too, there is a problem: In the uncertain world of quantum physics, the “egg” couldn’t stay stable forever. It would have expanded and contracted and could have even collapsed into nothingness. "This means it couldn't have existed forever in the past," Vilenkin said, findings he and his student Audrey Mithani detailed in the January issue of the Journal of Cosmology and Astroparticle Physics.
But the same quantum fluctuations that could have cracked the cosmic egg could be birthing new universes as you read this, says Vilenkin. This idea, called eternal inflation, suggests that our universe is just one bubble within a larger multiverse which is perpetually popping out new bubble universes. Although inflation may have stopped in bubbles such as ours, new instances of inflation occur in the multiverse forever into the future, keeping the idea of eternal inflation true to its name. But what about the past? If one assumes that the multiverse is expanding and not contracting, then it had to have expanded from a certain point in time, Vilenkin explains. Even eternal inflation must have a beginning.
Even if the universe did have a beginning, it likely occurred so very far in the past that the cosmos might as well appear as if began an eternity ago, says theoretical physicist Leonard Susskind at Stanford University in California."We're talking about the beginning potentially occurring at time scales vastly, vastly larger than the age of our universe, longer than any time that you can name," Susskind explains. "Statistically, given this extremely long amount of time, we probably occurred very, very late in history, making us very far from the beginning, so most of the information about the beginning would be lost to us. I think we're really in the dark about what it would've been like."
Still, Vilenkin is hopeful that it might be possible to observe evidence of the beginning. In some versions of the eternal inflation model, bubbles occasionally collide, which we might detect as distortions in the cosmic microwave background radiation that pervades all of space. If there are a number of collisions between bubbles that are clumped together in one direction more than another, "that might be linked with the beginning of the universe," he said.
Vilenkin has no problem with the universe having a beginning. "I think it's possible for the universe to spontaneously appear from nothing in a natural way," he said. The key there lies again in quantum physics—even nothingness fluctuates, a fact seen with so-called virtual particles that scientists have seen pop in and out of existence, and the birth of the universe may have occurred in a similar manner.
"Of course, maybe someone will come up with another model of an eternal universe, and we'll have to start thinking about it all over again," Vilenkin said.
Editor's picks for further reading
arXiv: Eternal Inflation and Its Implications
Alan Guth, the physicist who originated the inflation hypothesis, summarizes the arguments for eternal inflation.
Edge: The Cyclic Universe
Neil Turok on the past and present of the cyclic universe model.
FQXi: Did the Universe Have a Beginning?
In this podcast, Alexander Vilenkin asks whether the universe could have existed forever into the past.