Could Black Holes Give Birth to 'Planck Stars'?
Artist's impression of a spinning supermassive black hole with a surrounding accretion disk and relativistic jets.
How to measure the spin of a black hole: This chart illustrates the basic model for determining the spin rates of black holes. The three artist's concepts represent the different types of spin: retrograde rotation, where the disk of matter falling onto the hole, called an accretion disk, moves in the opposite direction of the black hole; no spin; and prograde rotation, where the disk spins in the same direction as the black hole.
Two models of black hole spin: Scientists measure the spin rates of supermassive black holes by spreading the X-ray light into different colors. The light comes from accretion disks that swirl around black holes, as shown in both of the artist's concepts. They use X-ray space telescopes to study these colors, and, in particular, look for a "fingerprint" of iron -- the peak shown in both graphs, or spectra -- to see how sharp it is. Prior to observations with NASA's Spectroscopic Telescope Array, or NuSTAR, and the European Space Agency's XMM- Newton telescope, there were two competing models to explain why this peak might not appear to be sharp. The "rotation" model shown at top held that the iron feature was being spread out by distorting effects caused by the immense gravity of the black hole. If this model were correct, then the amount of distortion seen in the iron feature should reveal the spin rate of the black hole. The alternate model held that obscuring clouds lying near the black hole were making the iron line appear artificially distorted. If this model were correct, the data could not be used to measure black hole spin.
This chart depicts the electromagnetic spectrum, highlighting the X-ray portion. NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton telescope complement each other by seeing different colors of X-ray light. XMM-Newton sees X-rays with energies between 0.1 and 10 kiloelectron volts (keV), the "red" part of the spectrum, while NuSTAR sees the highest-energy, or "bluest," X- ray light, with energies between 3 and 70 keV.
This image taken by the ultraviolet-light monitoring camera on the European Space Agency's (ESA's) XMM-Newton telescope shows the beautiful spiral arms of the galaxy NGC1365. Copious high-energy X-ray emission is emitted by the host galaxy, and by many background sources. The large regions observed by previous satellites contain so much of this background emission that the radiation from the central black hole is mixed and diluted into it. NuSTAR, NASA's newest X-ray observatory, is able to isolate the emission from the black hole, allowing a far more precise analysis of its properties.
What XMM-Newton saw: The solid lines show two theoretical models that explain the low-energy X-ray emission seen from the galaxy NGC 1365 by the European Space Agency's XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the measurements from XMM-Newton, which are explained equally well by both models.
Two X-ray observatories are better than one: NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, has helped to show, for the first time, that the spin rates of black holes can be measured conclusively. It did this, together with the European Space Agency's XMM-Newton, by ruling out the possibility that obscuring clouds were partially blocking X-ray right coming from black holes. The solid lines show two theoretical models that explain low-energy X-ray emission seen previously from the spiral galaxy NGC 1365 by XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the latest measurements from XMM-Newton, and the yellow circles show the data from NuSTAR. While both models fit the XMM-Newton data equally well, only the disk reflection model fits the NuSTAR data.
Black holes are vexing objects. Not only do they defy our everyday understanding about how the Universe should work, they confound even the most complex mathematical models. Science writers describe them as “gravitational behemoths” that warp spacetime so much that even light cannot escape the surrounding boundary, known as the “event horizon.” But what lies inside? Well, that doesn’t matter, we argue, everyday physics does not apply once you cross the event horizon.
The horribly unsatisfactory explanation has been battered in recent months when a serious physics confrontation entered the public arena. What goes on inside a black hole’s event horizon has actually caused a theoretical conflagration and now, two theoretical physicists have proposed a new idea that may marry quantum mechanics with gravity, extinguishing the tricky “firewall” and finding a solution to the “information paradox.”
A Brief History of Burning Black Holes
In January, British physics superstar Stephen Hawking published a short paper declaring “there are no black holes.” Of course, Hawking wasn’t saying that black holes didn’t exist, but that the physics of the black hole’s event horizon needs some tweaking.
The root of this issue can be found in a 2012 research publication by Joseph Polchinski and his team at the University of California in Santa Barbara. When tackling the thorny problem of whether or not black holes destroy information, they found that if black holes truly do not destroy information (a standpoint that Hawking himself reluctantly advocates) and that information can escape from the evaporating black hole via Hawking radiation, there must be a raging inferno just inside the event horizon called the “firewall.”
And herein lies a paradox. If we view a black hole as an object governed by general relativity, should an unfortunate astronaut get dragged across the event horizon, they shouldn’t experience anything out of the ordinary (“no drama”); he or she will just drift on through, into the black hole, where, eventually, intense tidal forces gruesomely “spaghettify” them. But if we view black holes as objects governed by quantum mechanics, and they conserve information, that astronaut will immediately get incinerated by Polchinski’s firewall (the antithesis of “no drama”). The two theories are symptomatic of our growing unease with the compatibility of general relativity and quantum mechanics, and black holes have become the front line of this battle.
Hawking therefore came forward with a possible (unpublished) solution last month: perhaps the black hole’s event horizon isn’t the definite boundary that theoretical physicists think it is. Perhaps the event horizon should be replaced with an “apparent horizon,” a consequence of the chaotic mess of information inside the event horizon. This may stick a Band Aid over the problem, but in a new (unpublished) paper submitted to the arXiv preprint service, two theoretical physicists have come up with an alternative idea.
Not Such a Singularity
The conventional view of a black hole is that it is composed of two key components: the singularity and the event horizon. Everything else is just details. The event horizon is the distance from the singularity where gravitational forces are so strong that even light cannot escape. The singularity is an infinitely dense point where all the matter of the black hole is concentrated. However, the singularity assumes that there is no quantum structure that can compete with the inward forces created by gravity.
Spotted the problem here? Yes, once again the black hole has become a battleground for quantum mechanics and gravity.
Carlo Rovelli from the University of Toulon, France, and Francesca Vidotto from Radboud University in The Netherlands have found a potential solution to the “Firewall problem” by throwing out the concept of a singularity and replacing it with an extreme class of “star” — the Planck Star.
Rovelli and Vidotto looked at this problem from a different perspective. While working on models of a collapsing universe — i.e. the opposite to the Big Bang, known as the Big Crunch — they found that the fundamental quantum structure of the Universe prevents an infinitely dense singularity from forming. The collapse of the Universe therefore reaches a fundamental density, causing the universal collapse to rebound, or “bounce.” This bounce could spawn a cyclical universe where “Big Bounces” lead to cosmological inflation, then contraction and ultimately a Big Crunch… and the process starts all over again.
Say if a similar model can be used to describe a black hole?
A Planck Star Rises
If a massive star explodes as a supernova, creating a black hole in its wake, what if the superdense material that formed the black hole actually didn’t form a “singularity”? Sure, the material is unimaginably dense, but the object in the core of the black hole still has structure. Rovelli and Vidotto argue that the inward force of gravity is counteracted by the quantum structure of the Planck density.
If we were to zoom in, far beyond the size of quantum particles, it is theorized that we will reach a fundamental scale known as the Planck length. Should matter be compressed to these scales, rather than disappearing into an “infinitely dense” singularity — a solution that doesn’t make a whole lot of sense — perhaps the contraction stops at the Planck density, creating a “Planck Star” and the object rebounds, or “bounces.” From the perspective of the Planck Star, it will be a very short-lived affair; it’s collapse and bounce would occur rapidly. But to outside observers elsewhere in the Universe (i.e. us), as space-time surrounding the Planck Star is so extremely warped, time dilation makes the black hole (and the Planck Star it contains) seem static and unchanging.
Over time, as the black hole loses mass to Hawking Radiation and the Planck Star continues to expand after the rebound, the event horizon of the black hole will slowly contract, eventually reaching the surface of the Planck Star contained within. At this point, argue the researchers, all of the information the black hole ever consumed over its lifetime will be suddenly released to the Universe — solving the “information paradox.” What’s more, we should be able to detect this deluge of information.
“(Planck Stars) produce a detectable signal, of quantum gravitational origin, around the 10-14cm wavelength,” they write. This signal could embody itself in cosmic rays of energies in the GeV range, a signal that can be easily detected by gamma-ray observatories.
Like Hawking’s “grey hole” paper, Rovelli and Vidotto’s work is currently not peer reviewed, so we are hearing arguments fresh from the theoretical community. But it would be interesting if we can link the emergence of Planck Stars from beyond black hole event horizons through the detection of gamma-rays from deep space, a field of study that has more than its fair share of mysterious signals.