Recent headlines have proclaimed “Black Holes Don’t Exist!†They’re wrong. Black holes absolutely exist. We know this observationally. We know by the orbits of stars in the center of our galaxy that there is a supermassive black hole in its center. We know of binary black hole systems. We’ve found the infrared signatures of more than a million black holes. We know of stellar mass black holes, and intermediate mass black holes. We can even see a gas cloud ripped apart by the intense gravity of a black hole. And we can take images of black holes, such as the one above. Yes, Virginia, there are black holes.
So what’s with the headlines? It seems to start with an article about a new work concerning the formation of stellar mass black holes. The paper hasn’t been peer reviewed, but it is an extension of an earlier work by the same authors that has been peer reviewed. The focus of both of these papers is on the firewall paradox, specifically how Hawking radiation might affect the gravitational collapse of a star to form a black hole.
The firewall paradox is something that arises when you try to combine black holes with quantum theory. In quantum theory there are limits to what can be known about an object. For example, you cannot know an object’s exact energy. Because of this uncertainty, the energy of a system can fluctuate spontaneously, so long as its average remains constant. In 1974 Stephen Hawking demonstrated is that near the event horizon of a black hole pairs of particles can appear, where one particle becomes trapped within the event horizon (reducing the black holes mass slightly) while the other can escape as radiation (carrying away a bit of the black hole’s energy). These escaping particles have come to be known as Hawking radiation.
According to general relativity, if you were to fall into a black hole, you shouldn’t notice anything strange when you cross the event horizon. Yes, you might feelstrong tidal forces, but you’d feel those outside the black hole as well. But according to quantum theory if all this Hawking radiation is being created near the event horizon, then you should experience a firewall of quantum particles. The solution to this theoretical problem is still a matter of some debate. Some, such as Hawking and the authors of this new paper, feel that the Hawking firewall prevents black hole horizons from forming. Others, such as Sabine Hossenfelder argue that quantum theory doesn’t lead to a Hawking firewall. Just to be clear, I’m personally in the Hossenfelder camp.
In this new paper, the authors show that if the Hawking firewall idea is correct, then as a star starts collapsing at the end of its life, before it collapses into a black hole Hawking radiation starts kicking in, which pushes back against the collapsing star. So instead of collapsing into a solar-mass black hole, the star almost collapses into a black hole, Hawking radiation stops its collapse, and the stellar core then explodes. So the star dies in a supernova explosion, but no black hole is formed from its core.
This is interesting theoretical work, and it raises questions about the formation of stellar-mass black holes. But it doesn’t prove that stellar-mass black holes don’t exist, nor does it say anything about intermediate mass or supermassive black holes, which would form by processes other than stellar collapse. And of course the work depends upon Hawking’s take on firewalls to be correct, which hasn’t been proven. To say that this work proves black holes don’t exist is disingenuous at best.
So don’t buy into the hype. Black holes are real, this work is interesting, and the link-baiters should be ashamed of themselves.
Black Holes Exist, and We Have Some Massive Evidence
The center of our galaxy is about 27,000 light years away in the direction of the constellation Sagittarius. That isn’t a large distance on astronomical scales, but it is hidden by gas and dust so we can’t observe it in visible light. We can, however, observe it at infrared and radio wavelengths, and what we see is very interesting.
Over the past 20 years we’ve been able to individual infrared sources orbiting the galactic center. We now know these infrared sources are individual stars. Very near the center there is a cluster of about 100 stars known as the S-cluster. We’ve been able to determine their orbits, and they tell us something about the mass in the center of our galaxy.
In the figure below I’ve plotted the orbits of the five closest S-cluster stars. They have orbital periods ranging from 15 to 47 years. You can see that their orbits appear to be ellipses. This is just what you would expect if the central mass is very small (in size, not mass), and it means we can use Kepler’s laws of motion to determine the central mass.
Kepler’s third law says that if you take the cube of the semimajor axis (a measure of the size of the ellipse) and divide it by the square of its orbital period (how long it takes to complete one orbit), then you always get the same constant. If the semimajor axis is measured in AU and the period in years, then that constant is the mass of the central object.
Since we have determined the orbits of about 100 stars, we can calculate the central mass pretty accurately. What we find is that the central mass is about 4.3 million solar masses, give or take about 15,000 solar masses. That much mass in such a small volume means that the central mass must be a supermassive black hole.
That means we live less than 30,000 light years from a huge black hole. Hope you can sleep well tonight.
Space Detective>
OJ287 is an interesting blazar. Typically a blazar is caused by a supermassive black hole in the center of a galaxy that is aligned so that the axis of rotation of the black hole points in our general direction. When the black hole is in a period of actively absorbing surrounding material (making it an active galactic nucleus, or AGN) it emits powerful jets from its poles. Since it is pointed in our general direction, we see an intense emission of energy known as a blazar.
OJ287 is different because it varies in brightness over a period of 11 – 12 years, with a sharp double spike in brightness when near maximum. Analysis of this variability during the 2005 outburst showed that the variation was due to a massive black hole of about 100 million solar masses orbiting a black hole of about 17 billion solar masses. Because the two are orbiting so closely, the smaller black hole intersects the accretion disk of the larger black hole, causing the characteristic double spike in the brightness. You can see how this works in the figure below.
Both of these black holes are unusually large. By comparison, the supermassive black hole in our own Milky Way galaxy is only about 4 million solar masses. The “small†companion in OJ287 is 25 times larger than that. The large companion is the largest known black hole in the universe.
There’s a certain amount of uncertainty to those measurements, particularly the larger one, because of the limited number of periods that have been observed. The blazar was first discovered as an intense radio source in a radio sky survey in the early 1970s, so there has only been about 3 periods to observe it.
But this is where things get interesting. Although OJ287 was discovered in the early 1970s, we actually have data on it dating back to 1891. That’s because it not only gives off intense radio waves when it flares up, it also brightens in the optical range. Although it wasn’t noticed as an object of interest, it would show up on photographic plates. So we actually have optical data on it for more than a century.
In a recent article in Astronomy and Astrophysics the authors did a bit of historical detective work. They studied photographic plates from the Harvard College Observatory, and found photographic images of OJ287 during its peak brightnesses of 1900 and 1913. From the plates they were able to gather about 500 historical data points. This was enough to determine the light curves of both events.
By comparing these century-old observations with modern observations, the authors were able to demonstrate that OJ287 is quasi-periodic. It has a roughly 12 year cycle, but not a fixed period. If it were a fixed period, then that would be evidence that the brightness cycle is driven by the orbit of the companion. But instead there is evidence that things are more complex.
This complexity seems to be due to the fact that the central black hole is rotating. This causes the orbit of the smaller black hole to precess through a process known as frame dragging. Given the size of the central black hole and the fact that black holes tend to rotate, this isn’t unexpected. There was in fact a model proposing this very effect in a 2010 paper. This new historical data supports that model.
The Harvard College Observatory has about 500,000 photographic plates spanning 120 years of observation. That’s a lot of data that can be mined for historical observations.
The Observable Evidence of a Million Black holes
n the center of most galaxies (including our own) is a supermassive black hole. These black holes can have masses of hundreds of millions of Suns. Some are more than a billion solar masses. Active supermassive black holes can be extraordinarily bright. When active, these black holes are surrounded by an accretion disk, which generates tremendous heat. Matter streams from their polar regions, creating huge jets of material that races away at nearly the speed of light.
How that energy is seen depends on how the galaxy (and hence the black hole) is oriented relative to us. If we view the galaxy edge on, then we mainly see the jets streaming outward, which produces intense radio energy, and we see them as radio galaxies. If the galaxy is tilted a bit toward us then we can see some of the accretion disk, which is so hot it gives off x-rays. These then appear to us as quasars. If our view is right above the pole of the black hole, then a jet is pointed in our direction and we see it as a blazar.
But this assumes we can actually have a clear view of things. Some galaxies are incredibly dusty, which means our view of the black hole and its accretion disk is obscured. Even if the black hole is active it would be hard to see it through all the dust of the galaxy. This is where infrared astronomy comes in handy.
Dust obscures shorter wavelengths of light, such as visible light and x-rays, but it doesn’t obscure longer wavelengths like infrared. When an active black hole is in a dusty galaxy, the energy it produces heats the surrounding dust, causing the dust to radiate in the infrared. As a result, the galaxies are somewhat hot, which is why they are known as hot Dust Obscured Galaxies, or hot DOGs (who said astronomers can’t have a sense of humor). These galaxies are not seen in the visible spectrum, but are very bright in the infrared.
You can see this in the image above, which shows a small region of sky surveyed by the WISE space telescope. The circles indicate where these hidden black holes have been detected. The images on the right show a close up of the center circle at different infrared wavelengths, going shorter to longer from top to bottom. You can see that even in shorter infrared the galaxy is not very visible, but moving to longer wavelengths the galaxy soon appears quite bright.
When WISE completed its full sky survey, about 1.6 million “hidden†black holes were discovered. Some of these are billions of light years away, which will help give us a better understanding of how these supermassive black holes evolve within galaxies.
Black holes moving the Path of Gas Clouds
Last September a planet-massed gas cloud known as G2 made a close approach to the supermassive black hole at the center of our galaxy. At minimum distance it will pass within about 260 AU of the black hole, which is about a third as close as any other object so observed. It will be close enough that it will enter the hot accretion region of the black hole, and may provide the first observation of matter as it is absorbed by the black hole. You can see observations of the gas cloud over the past several years in the figure below.
Because the cloud is diffuse, rather than a compact object like a star, it will also provide a way to probe the region around the supermassive black hole. It is thought that stellar mass black holes might orbit close to the supermassive black hole, but these would be difficult to observe directly. However if any interact with the G2 cloud during close approach, their effects will be observed.
The reason why it is so difficult to observe has nothing to do with the supermassive black hole itself. It is only 26,000 light years away, which is rather close on astronomical terms. The problem is that it’s obscured by gas and dust, which blocks most light in the visible spectrum. To observe objects near galactic center, astronomers look for infrared and radio emissions from the region, which penetrate the dust more easily. But to get high resolution images, arrays of telescopes must be used, such as the Very Large Telescope (VLT) array.
Currently the highest resolution observations are made in the infrared, and they resolve objects to about the size of Mercury’s orbit. In the future we may have even better ways to peer behind the veil. But this Fall, no less than six major observatories will be keeping an eye on the center of our galaxy to see what happens to this planetary cloud.
What Could No Black Holes Mean?
Due to the news of a radical new theory proposing that the universe began from a hyper-dimensional black hole. Most of the reports seem to stem from an article posted a while back on the Nature blog, which references the original paper. So let’s have a little reality check.
No one is abandoning the big bang model. The original paper hasn’t even been peer reviewed yet and the paper doesn’t present a radical new theory to overturn the big bang. What the paper is actually about is higher-dimensional gravitational theory.
The standard theory of gravity (general relativity) describes our universe as a geometry of three-dimensional space with one dimension of time. This is sometimes called 3 + 1 space, and it gives a very accurate description of the universe we observe. But theorists like to play around with alternative models to see how they differ from regular general relativity. They may look at 2 + 1 space, a kind of flatland with time, or 2 + 2, with two time dimensions. There isn’t necessarily anything “real†about these models, and there certainly isn’t any experimental evidence to support anything other than 3 + 1 gravity, but alternative models are useful because they help us gain a deeper understanding of general relativity. In this particular paper, the authors were exploring 4 + 1 gravity. That is, a five-dimensional universe with 4 spatial dimensions and 1 time.
Back in 2000, another team of authors proposed a model where our regular 3 + 1 gravity could be treated as a brane within a larger 4 + 1 universe. It is similar to the way a 2 + 1 universe could be imagined as a 2-dimensional surface (the brane) within our 3-dimensional space. In the 2000 paper, the authors showed that a particular 4 + 1 universe with a 3 + 1 brane could give rise to the type of gravity we actually see.
The new paper takes this model one step further. In it, the authors show that 4 + 1 gravity allows for the existence of black holes. So if a 4 + 1 universe had large stars, some of those stars could collapse into a 4-dimensional “hyper black holeâ€. Like black holes in regular general relativity, these hyper black holes would have a central “singularity†of extremely dense and hot matter/energy. The authors then went on to show that a hyper black hole with the right conditions could not only create a three-dimensional brane, but the new brane would look very similar to the early universe we actually observe.
In other words, if we imagine a five-dimensional 4 + 1 universe, and if such a universe could create stars that collapse into hyper black holes, and if a particular hyper black hole had the right energy, then it might be possible for for such a hyper black hole to produce a 3 + 1 brane-universe with a beginning that looks like a big bang. That’s a lot of ifs.
Just to be clear, this is good theoretical work. The model is interesting, and it shows a curious connection between the universe we observe and higher-dimensional gravity. It could also address some of the issues in cosmology, but it also predicts the universe is flat, which as I mentioned yesterday may not be the case. The authors note this problem, and are careful not to make broad claims. They also outline possible ways that such a model could be tested. This is what good theoreticians do.
But currently there is no experimental evidence to support higher-dimensions, much less hyper black holes. So don’t toss the big bang or black holes just yet.
There Are Such Things as Black Holes
