## How Big is the Universe?

The Universe is big, but how big is it? That all depends on whether the Universe is finite or infinite. Even the word “big” is tough to get clear. Are we talking about the size of the Universe we can see, or the Universe’s actual size right now?

The Universe is big, but how big is it? And what the heck kind of question is that? Are elephants big? Trucks? Dinosaurs? Cheese? Is cheese big? How big is cheese? How big is big?

The word “big” is tough to get clear. Are we talking about the size of the Universe we can see, or the Universe’s actual size right now? This becomes even more complicated when we are trying to work under assumptions of either the Universe is finite or the Universe is infinite.

One difficulty with talking about the size, is that the Universe is expanding. Light takes time to travel from distant galaxies, and while that light travels, the Universe continues to expand. So our problem with talking about how big it is, is that there is no single meaning to distance when it comes to the universe. For this reason, astronomers usually don’t worry about the distance to galaxies at all, and instead focus on redshift, which is measured by z. The bigger the z, the more redshift, and the more distant the galaxy.

As an example, consider one of the most distant galaxies we’ve observed, which has a redshift of 7.5. Using this, we can determine distance by calculating how long the light has traveled to reach us. With a redshift of 7.5, that comes out to be about 13 billion years. You might think that means it’s 13 billion light years away, but 13 billion years ago the universe was smaller, so it was actually closer at the time the light left that galaxy. Using this, if you calculate that distance, it was only a short 3.4 billion light years away.

Now the galaxy is much farther than that. After the light left the galaxy, the galaxy continued to move away from us. It is now about 29 billion light years away. Which is definitely more than 13, and quite a bit more than its original 3.4.

Usually it is this big distance that people mean when they ask for the size of the universe. This is known as the co-moving distance. Of course, we can only see so far. So, how far can we see? The most distant light we are able to observe is from the cosmic microwave background, which has a redshift of about z = 1,000.

This means the co-moving distance of the cosmic background is about 46 billion light years. Sticking us at the center of a massive sphere, the currently observable universe has a diameter of about 92 billion light years. Even with this observed distance, we know that it extends much further than that. If what we could see was all there is, we would see galaxies tend to gravitate towards us, which we don’t observe.

In fact we don’t see any kind of galaxy clumping to a particular point at all. So as far as we know the universe could extend forever. It could be even stranger than that. Despite some media controversy, if the BICEP2 detection of early inflation is correct, it is likely the Universe undergoes a type of inflation with the intimidating moniker of “eternal inflation”. If it is the case, our observable universe is merely one bubble within an endless sea of other bubble universes. This is otherwise referred to as… the multiverse.

So, in the immortal words of Douglas Adams, “Space,” it says, “is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space”

What do you think? Does the Universe go on for ever? Tell us in the comments below. And if you like what you see, come check out our Patreon page and find out how you can get these videos early while helping us bring you more great content!

## What Are Comet Tails?

Comets are renowned for their big beautiful tails that stretch across the sky. But what’s in those things, anyway? And how can comets get multiple tails?

In the past, humans generally used one of two greetings for comets:
1. Dear God, what is that thing? Terrible omens! Surely we will all die in fire.
2. Dear God, what is that thing? Great omens! Surely we will all have a big party… where we all die in fire?

For example, the appearance of what came to be known as Halley’s comet in 1066 was seen as a bad omen for King Harold II. Conversely, it was a good omen for William the Conqueror.

Because of their tails and transitory nature, comets were long thought to be products of the Earth’s atmosphere. It wasn’t until the 1500s, when Tycho Brahe used parallax to determine a comet’s distance. He realized that they were Solar System objects, like planets.

So, good news, we no longer regard them as omens and everyone stopped panicking. Right? Wrong. When Comet Halley approached Earth in 1910, astronomers detected cyanide gas in its tail. French astronomer Camille Flammarion was quoted as saying the gas could “impregnate the atmosphere and possibly snuff out all life on the planet.” This caused a great deal of hysteria. Many bought gas masks and “comet pills” to protect themselves.

With the rise of photographic astronomy, it was found that comets often have two types of tails. A bright tail composed of ionized gas, and a dimmer one composed of dust particles. The ion tail always points away from the Sun. It’s actually being pushed away from the comet by the solar wind.

We now know that a comet’s ion tail contains “volatiles” such as water, methane, ammonia and carbon dioxide. These volatiles are frozen near the comet’s surface, and as they approach the Sun, they warm and become gaseous. This also causes dust on the comet’s surface to stream away. The heating of a comet by the Sun is not uniform.

Because of a comet’s irregular shape and rotation, some parts of the surface can be heated by sunlight, while other parts remain cold. In some cases this can mean that comets can have multiple tails, which creates amazing effects where different regions of a comet stream off volatiles.

These ion tails can be quite large, and some have been observed to be nearly 4 times the distance of the Earth from the Sun. And even though they fill a great volume, they are also pretty diffuse. If you condensed a comet’s tail down to the density of water, it wouldn’t even fill a swimming pool.

We also now know that there isn’t a clear dividing line between comets and asteroids. It’s not the case the comets are dirty snowballs and asteroids are dry rocks. There is a range of variation, and asteroids can gain dusty or gaseous tails and take on a comet-like appearance. In addition, we’ve also found comets orbiting other stars, known as exocomets.

And finally one last fact, the term comet comes from the Latin cometa, which indicated a hairy star.

So, what’s your favorite comet? Tell us in the comets below. And if you like what you see, come check out our Patreon page and find out how you can get these videos early while helping us bring you more great content!

## Is Our Solar System Weird?

Is our Solar System normal? Or is it weird? How does the Solar System fit within the strange star systems we’ve discovered in the Milky Way so far?

With all the beautiful images that come down the pipe from Hubble, our Solar System has been left with celestial body image questions rivaling that of your average teenager. They’re questions we’re all familiar with. Is my posture crooked? Do I look pasty? Are my arms too long? Is it supposed to bulge out like this in the middle? Some of my larger asteroids are slightly asymmetrical. Can everyone tell? And of course the toughest question of all… Am I normal?

The idea that stars are suns with planets orbiting them dates back to early human history. This was generally accompanied by the idea that other planetary systems would be much like our own. It’s only in the last few decades that we’ve had real evidence of planets around other stars, known as exoplanets. The first extrasolar planet was discovered around a pulsar in 1992 and the first “hot jupiter” was discovered in 1995.

Most of the known exoplanets have been discovered by the amazing Kepler spacecraft. Kepler uses the transit method, observing stars over long periods of time to see if they dim as a planet passes in front of the star. Since then, astronomers have found more than 1700 exoplanets, and 460 stars are known to have multiple planets. Most of these stellar systems are around main sequence stars, just like the Sun. Leaving us with plenty of systems for comparison.

So, is our Solar System normal? Planets in a stellar system tend to have roughly circular orbits, just like our Solar system. They have a range of larger and smaller planets, just like ours. Most of the known systems are even around G-type stars. Just like ours.….and we are even starting to find Earth-size planets in the habitable zones of their stars. JUST LIKE OURS!

Not so fast…Other stellar systems don’t seem to have the division of small rocky planets closer to the star and larger gas planets farther away. In fact, large Jupiter-type planets are generally found close to the star. This makes our solar system rather unusual.

Computer simulations of early planetary formation shows that large planets tend to move inward toward their star as they form, due to its interaction with the material of the protoplanetary disk. This would imply that large planets are often close to the star, which is what we observe. Large planets in our own system are unusually distant from the Sun because of a gravitational dance between Jupiter and Saturn that happened when our Solar System was young.

Although our Solar System is slightly unusual, there are some planetary systems that are downright quirky. There are planetary systems where the orbits are tilted at radically different angles, like Kepler 56, and a sci-fi favorite, the planets that orbit two stars like Kepler 16 and 34. There is even a planet so close to its star that its year lasts only 18 hours, known 55 Cancri e.

And so, the Kepler telescope has presented us with a wealth of exoplanets, that we can compare our beautiful Solar System to. Future telescopes such as Gaia, which was launched in 2013, TESS and PLATO slated for launch in 2017 and 2024 will likely discover even more. Perhaps even discovering the holy grail of exoplanets, a habitable planet with life…

And the who knows, maybe we’ll find another planet… just like ours.

What say you? Where should we go looking for habitable worlds in this big bad universe of ours? Tell us in the comments.

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## What Is The Great Attractor?

There’s a strange place in the sky where everything is attracted. And unfortunately, it’s on the other side of the Milky Way, so we can’t see it. What could be doing all this attracting?

Just where the heck are we going? We’re snuggled in our little Solar System, hurtling through the cosmos at a blindingly fast of 2.2 million kilometers per hour. We’re always orbiting this, and drifting through that, and it’s somewhere out in the region that’s not as horrifically terrifying as what some of our celestial neighbors go through. But where are we going? Just around in a great big circle? Or an ellipse? Which is going around in another circle… and it’s great big circles all the way up?

Not exactly… Our galaxy and other nearby galaxies are being pulled toward a specific region of space. It’s about 150 million light years away, and here is the best part. We’re not exactly sure what it is. We call it the Great Attractor.

Part of the reason the Great Attractor is so mysterious is that it happens to lie in a direction of the sky known as the “Zone of Avoidance”. This is in the general direction of the center of our galaxy, where there is so much gas and dust that we can’t see very far in the visible spectrum. We can see how our galaxy and other nearby galaxies are moving toward the great attractor, so something must be causing things to go in that direction. That means either there must be something massive over there, or it’s due to something even more strange and fantastic.

When evidence of the Great Attractor was first discovered in the 1970s, we had no way to see through the Zone of Avoidance. But while that region blocks much of the visible light from beyond, the gas and dust doesn’t block as much infrared and x-ray light. As x-ray astronomy became more powerful, we could start to see objects within that region. What we found was a large supercluster of galaxies in the area of the Great Attractor, known as the Norma Cluster. It has a mass of about 1,000 trillion Suns. That’s thousands of galaxies.

While the Norma Cluster is massive, and local galaxies are moving toward it, it doesn’t explain the full motion of local galaxies. The mass of the Great Attractor isn’t large enough to account for the pull. When we look at an even larger region of galaxies, we find that the local galaxies and the Great Attractor are moving toward something even larger. It’s known as the Shapley Supercluster. It contains more than 8000 galaxies and has a mass of more than ten million billion Suns. The Shapley Supercluster is, in fact, the most massive galaxy cluster within a billion light years, and we and every galaxy in our corner of the Universe are moving toward it.

So as we hurtle through the cosmos, gravity shapes the path we travel. We’re pulled towards the Great Attractor, and despite its glorious title, it appears, in fact to be a perfectly normal collection of galaxies, which just happens to be hidden.

What do you think? What are you hoping we’ll discover over in the region of space we’re drifting towards?

And if you like what you see, come check out our Patreon page and find out how you can get these videos early while helping us bring you more great content!

## Are the BICEP2 Results Invalid? Probably Not.

Recently rumors have been flying that the BICEP2 results regarding the cosmic inflationary period may be invalid. It all started with a post by Dan Falkowski on his blog Resonaances, where he claimed that the BICEP2 had misinterpreted some data, which rendered their results invalid, or at least questionable. The story was then picked up by Nature’s Blog and elsewhere, which has sparked some heated debate.

So what’s really going on?

For those who might not remember, BICEP2 is a project working to detect polarized light within the cosmic microwave background (CMB). Specifically they were looking for a type of polarization known as B-mode polarization. Detection of B-mode polarization is important because one mechanism for it is cosmic inflation in the early universe, which is exactly what BICEP2 claimed to have evidence of.

Part of the reason BICEP2 got so much press is because B-mode polarization is particularly difficult to detect. It is a small signal, and you have to filter through a great deal of observational data to be sure that your result is valid.  But you also have to worry about other sources that look like B-mode polarization, and if you don’t account for them properly, then you could get a “false positive.” That’s where this latest drama arises.

In general this challenge is sometimes called the foreground problem.  Basically, the cosmic microwave background is the most distant light we can observe. All the galaxies, dust, interstellar plasma and our own galaxy is between us and the CMB.  So to make sure that the data you gather is really from the CMB, you have to account for all the stuff in the way (the foreground).  We have ways of doing this, but it is difficult. The big challenge is to account for everything.

Soon after the BICEP2 results, another team noted a foreground effect that could effect the BICEP2 results. It involves an effect known as radio loops, where dust particles trapped in interstellar magnetic fields can emit polarized light similar to B-mode polarization. How much of an effect this might have is unclear. Another project being done with the Planck satellite is also looking at this foreground effect, and has released some initial results (seen in the figure), but hasn’t yet released the actual data yet.

Now it has come to light that BICEP2 did, in fact, take some of this foreground polarization into account, in part using results from Planck. But since the raw data hadn’t been released, the team used data taken from a PDF slide of Planck results and basically reverse-engineered the Planck data.  It is sometimes referred to as “data scraping”, and it isn’t ideal, but it works moderately well. Now there is some debate as to whether that slide presented the real foreground polarization or some averaged polarization. If it is the latter, then the BICEP2 results may have underestimated the foreground effect. Does this mean the BICEP2 results are completely invalid? Given what I’ve seen so far, I don’t think it does. Keep in mind that the Planck foreground is one of several foreground effects that BICEP2 did account for. It could be a large error, but it could also be a rather minor one.

The important thing to keep in mind is that the BICEP2 paper is still undergoing peer review.  Critical analysis of the paper is exactly what should happen, and is happening.  This type review used to be confined to the ivory towers, but with social media it now happens in the open.  This is how science is done. BICEP2 has made a bold claim, and now everyone gets to whack at them like a piñata.

The BICEP2 team stands by their work, and so we’ll have to see whether it holds up to peer review.  We’ll also have to wait for the Planck team to release their results on B-mode polarization. Eventually the dust will settle and we’ll have a much better handle on the results.

## How CERN’s Discovery of Exotic Particles May Affect Astrophysics

You may have heard that CERN announced the discovery (confirmation, actually. See addendum below.) of a strange particle known as Z(4430).  A paper summarizing the results has been published on the physics arxiv, which is a repository for preprint (not yet peer reviewed) physics papers.  The new particle is about 4 times more massive than a proton, has a negative charge, and appears to be a theoretical particle known as a tetraquark.  The results are still young, but if this discovery holds up it could have implications for our understanding of neutron stars.

The building blocks of matter are made of leptons (such as the electron and neutrinos) and quarks (which make up protons, neutrons, and other particles).  Quarks are very different from other particles in that they have an electric charge that is 1/3 or 2/3 that of the electron and proton.  They also possess a different kind of “charge” known as color.  Just as electric charges interact through an electromagnetic force, color charges interact through the strong nuclear force.  It is the color charge of quarks that works to hold the nuclei of atoms together. Color charge is much more complex than electric charge.  With electric charge there is simply positive (+) and its opposite, negative (-).  With color, there are three types (red, green, and blue) and their opposites (anti-red, anti-green, and anti-blue).

Because of the way the strong force works, we can never observe a free quark.  The strong force requires that quarks always group together to form a particle that is color neutral. For example, a proton consists of three quarks (two up and one down), where each quark is a different color.  With visible light, adding red, green and blue light gives you white light, which is colorless. In the same way, combining a red, green and blue quark gives you a particle which is color neutral.  This similarity to the color properties of light is why quark charge is named after colors.

Combining a quark of each color into groups of three is one way to create a color neutral particle, and these are known as baryons.  Protons and neutrons are the most common baryons.  Another way to combine quarks is to pair a quark of a particular color with a quark of its anti-color.  For example, a green quark and an anti-green quark could combine to form a color neutral particle.  These two-quark particles are known as mesons, and were first discovered in 1947.  For example, the positively charged pion consists of an up quark and an antiparticle down quark.

Under the rules of the strong force, there are other ways quarks could combine to form a neutral particle.  One of these, the tetraquark, combines four quarks, where two particles have a particular color and the other two have the corresponding anti-colors.  Others, such as the pentaquark (3 colors + a color anti-color pair) and the hexaquark (3 colors + 3 anti-colors) have been proposed.  But so far all of these have been hypothetical.  While such particles would be color neutral, it is also possible that they aren’t stable and would simply decay into baryons and mesons.

There has been some experimental hints of tetraquarks, but this latest result is the strongest evidence of 4 quarks forming a color neutral particle.  This means that quarks can combine in much more complex ways than we originally expected, and this has implications for the internal structure of neutron stars.

Very simply, the traditional model of a neutron star is that it is made of neutrons.  Neutrons consist of three quarks (two down and one up), but it is generally thought that particle interactions within a neutron star are interactions between neutrons.  With the existence of tetraquarks, it is possible for neutrons within the core to interact strongly enough to create tetraquarks.  This could even lead to the production of pentaquarks and hexaquarks, or even that quarks could interact individually without being bound into color neutral particles.  This would produce a hypothetical object known as a quark star.

This is all hypothetical at this point, but verified evidence of tetraquarks will force astrophysicists to reexamine some the assumptions we have about the interiors of neutron stars.

Addendum: It has been pointed out that CERN’s results are not an original discovery, but rather a confirmation of earlier results by the Belle Collaboration.  The Belle results can be found in a 2008 paper in Physical Review Letters, as well as a 2013 paper in Physical Review D.  So credit where credit is due.

## We’ve Discovered Inflation! Now What?

Days like these make being an astrophysicist interesting.  On the one hand, there is the annoucement of BICEP2 that the long-suspected theory of an inflationary big bang is actually true.  It’s the type of discovery that makes you want to grab random people off the street and tell them what an amazing thing the Universe is.  On the other hand, this is exactly the type of moment when we should be calm, and push back on the claims made by one research team.  So let’s take a deep breath and look at what we know, and what we don’t.

First off, let’s dispel a few rumors.  This latest research is not the first evidence of gravitational waves.  The first indirect evidence for gravitational waves was found in the orbital decay of a binary pulsar by Russell Hulse and Joseph Taylor, for which they were awarded the Nobel prize in 1993. This new work is also not the first discovery of polarization within the cosmic microwave background, or even the first observation of B-mode polarization.  This new work is exciting because it finds evidence of a specific form of B-mode polarization due to primordial gravitational waves. The type of gravitational waves that would only be caused by inflation during the earliest moments of the Universe.

It should also be noted that this new work hasn’t yet been peer reviewed.  It will be, and it will most likely pass muster, but until it does we should be a bit cautious about the results.  Even then these results will need to be verified by other experiments.  For example, data from the Planck space telescope should be able to confirm these results assuming they’re valid.

That said, these new results are really, really interesting.

What the team did was to analyze what is known as B-mode polarization within the cosmic microwave background (CMB).  Light waves oscillate perpendicular to their direction of motion, similar to the way water waves oscillate up and down while they travel along the surface of water.  This means light can have an orientation.  For light from the CMB, this orientation has two modes, known as E and B.  The E-mode polarization is caused by temperature fluctuations in the CMB, and was first observed in 2002 by the DASI interferometer.

The B-mode polarization can occur in two ways.  The first way is due to gravitational lensing.  The first is due to gravitational lensing of the E-mode.  The cosmic microwave background we see today has travelled for more than 13 billion years before reaching us.  Along its journey some of it has passed close enough to galaxies and the like to be gravitationally lensed.  This gravitational lensing twists the polarization a bit, giving some of it a B-mode polarization. This type was first observed in July of 2013.  The second way is due to gravitational waves from the early inflationary period of the universe.  As inflationary period occurred, then it produced gravitational waves on a cosmic scale.  Just as the gravitational lensing produces B-mode polarization, these primordial gravitational waves produce a B-mode effect.  The discovery of primordial wave B-mode polarization is what was announced today.

Inflation has been proposed as a reason for why the cosmic microwave background is as uniform as it is. We see small fluctuations in the CMB, but not large hot or cold spots.  This means the early Universe must have been small enough for temperatures to even out.  But the CMB is so uniform that the observable universe must have been much smaller than predicted by the big bang.  However, if the Universe experienced a rapid increase in size during its early moments, then everything would work out.  The only problem was we didn’t have any direct evidence of inflation.

Assuming these new results hold up, now we do.  Not only that, we know that inflation was stronger than we anticipated.  The strength of the gravitational waves is measured in a value known as r, where larger is stronger.  It was found that r = 0.2, which is much higher than anticipated.  Based upon earlier results from the Planck telescope, it was expected that r < 0.11.  So there seems to be a bit of tension with earlier findings.  There are ways in which this tension can be resolved, but just how is yet to be determined.

So this work still needs to be peer reviewed, and it needs to be confirmed by other experiments, and then the tension between this result and earlier results needs to be resolved.  There is still much to do before we really understand inflation.  But overall this is really big news, possibly even Nobel prize worthy.  The results are so strong that it seems pretty clear we have direct evidence of cosmic inflation, which is a huge step forward.  Before today we only had physical evidence back to when the universe was about a second old, at a time when nucleosynthesis occurred.  With this new result we are now able to probe the Universe when it was less than 10 trillion trillion trillionths of a second old.

Which is pretty amazing when you think about it.

## Why the Asteroid Belt Doesn’t Threaten Spacecraft

When you think of the asteroid belt, you probably imagine a region of rock and dust, with asteroids as far as the eye can see.  Such a visual has been popularized in movies, where spaceships must swerve left and right to avoid collisions.  But a similar view is often portrayed in more scientific imagery, such as the artistic rendering above.  Even the first episode of the new Cosmos series portrayed the belt as a dense collection of asteroids. But the reality is very different.  In reality the asteroid belt is less cluttered than often portrayed.  Just how much less might surprise you.

The Sloan digital sky survey (SDSS) has identified more than 100,000 asteroids in the solar system.  Not all of these lie within the asteroid belt, but there are about 80,000 asteroids in the belt larger than a kilometer.  Of course there are asteroids smaller than that, but they are more difficult to detect, so we aren’t exactly sure how many there are.

We have a pretty good idea, however, because the observations we have indicate that the size distribution of asteroids follows what is known as a power law distribution. For example, with a power law of 1, for every 100-meter wide asteroid there would be 10 with a diameter of 10 meters and 100 with a diameter of 1 meter. Based upon SDSS observations, asteroids seem to follow a power law of about 2, which means there are likely about 800 trillion asteroids larger than a meter within the belt. That’s a lot of rock. So much that sunlight scattering off the asteroid belt and other dust in the solar system is the source of zodiacal light.

But there is also a lot of volume within the asteroid belt. The belt can be said to occupy a region around the Sun from about 2.2 to 3.2 times the distance from the Earth to the Sun from the Sun (AU), with a thickness of about 1 AU. A bit of math puts that at about 50 trillion trillion cubic kilometers. So even though there are trillions of asteroids, each asteroid has billions of cubic kilometers of space on average. The asteroid belt is hardly something you would consider crowded. It should be emphasized that asteroids in the belt are not evenly distributed. They are clustered into families and groups. But even such clustering is not significant compared to the vast space it occupies.

You can even do a very rough calculation to get an idea of just how empty the asteroid belt actually is. If we assumed that all the asteroids lay within a single plane, then on average there is 1 asteroid within an area roughly the size of Rhode Island. Within the entire United States there would be about 2000 asteroids, most of them only a meter across. The odds of seeing an asteroid along a cross-country road trip, much less hitting one, would be astoundingly small. So you can see why we don’t worry about space probes hitting an asteroid on their way to the outer solar system.  In fact, to get even close to an asteroid takes a great deal of effort.

## Planck “Star” to Arise From Black Holes?

A new paper has been posted on the arxiv (a repository of research preprints) introducing the idea of a Planck star arising from a black hole.  These hypothetical objects wouldn’t be a star in the traditional sense, but rather the light emitted when a black hole dies at the hands of Hawking radiation.  The paper hasn’t been peer reviewed, but it presents an interesting idea and a possible observational test.

When a large star reaches the end of its life, it explodes as a supernova, which can cause its core to collapse into a black hole.  In the traditional model of a black hole, the material collapses down into an infinitesimal volume known as a singularity.  Of course this doesn’t take into account quantum theory.

Although we don’t have a complete theory of quantum gravity, we do know a few things.  One is that black holes shouldn’t last forever.  Because of quantum fluctuations near the event horizon of a black hole, a black hole will emit Hawking radiation.  As a result, a black hole will gradually lose mass as it radiates.  The amount of Hawking radiation it emits is inversely proportional to its size, so as the black hole gets smaller it will emit more and more Hawking radiation until it finally radiates completely away.

Because black holes don’t last forever, this has led Stephen Hawking and others to propose that black holes don’t have an event horizon, but rather an apparent horizon.  This would mean the material within a black hole would not collapse into a singularity, which is where this new paper comes in.

The authors propose that rather than collapsing into a singularity, the matter within a black hole will collapse until it is about a trillionth of a meter in size.  At that point its density would be on the order of the Planck density.  When the the black hole ends its life, this “Planck star” would be revealed.  Because this “star” would be at the Planck density, it would radiate at a specific wavelength of gamma rays.  So if they exist, a gamma ray telescope should be able to observe them.

Just to be clear, this is still pretty speculative.  So far there isn’t any observational evidence that such a Planck star exists.  It is, however, an interesting solution to the paradoxical side of black holes.

## Why Hawking is Wrong About Black Holes

A recent paper by Stephen Hawking has created quite a stir, even leading Nature News to declare there are no black holes. As I wrote in an earlier post, that isn’t quite what Hawking claimed.  But it is now clear that Hawking’s claim about black holes is wrong because the paradox he tries to address isn’t a paradox after all.

It all comes down to what is known as the firewall paradox for black holes.  The central feature of a black hole is its event horizon.  The event horizon of a black hole is basically the point of no return when approaching a black hole.  In Einstein’s theory of general relativity, the event horizon is where space and time are so warped by gravity that you can never escape.  Cross the event horizon and you are forever trapped.

This one-way nature of an event horizon has long been a challenge to understanding gravitational physics.  For example, a black hole event horizon would seem to violate the laws of thermodynamics.  One of the principles of thermodynamics is that nothing should have a temperature of absolute zero.  Even very cold things radiate a little heat, but if a black hole traps light then it doesn’t give off any heat.  So a black hole would have a temperature of zero, which shouldn’t be possible.

Then in 1974 Stephen Hawking demonstrated that black holes do radiate light due to quantum mechanics. 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.  What 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).

While Hawking radiation solved one problem with black holes, it created another problem known as the firewall paradox.  When quantum particles appear in pairs, they are entangled, meaning that they are connected in a quantum way.  If one particle is captured by the black hole, and the other escapes, then the entangled nature of the pair is broken.  In quantum mechanics, we would say that the particle pair appears in a pure state, and the event horizon would seem to break that state.

Last year it was shown that if Hawking radiation is in a pure state, then either it cannot radiate in the way required by thermodynamics, or it would create a firewall of high energy particles near the surface of the event horizon.  This is often called the firewall paradox because according to general relativity if you happen to be near the event horizon of a black hole you shouldn’t notice anything unusual.  The fundamental idea of general relativity (the principle of equivalence) requires that if you are freely falling toward near the event horizon there shouldn’t be a raging firewall of high energy particles. In his paper, Hawking proposed a solution to this paradox by proposing that black holes don’t have event horizons.  Instead they have apparent horizons that don’t require a firewall to obey thermodynamics.  Hence the declaration of “no more black holes” in the popular press.

But the firewall paradox only arises if Hawking radiation is in a pure state, and a paper last month by Sabine Hossenfelder shows that Hawking radiation is not in a pure state.  In her paper, Hossenfelder shows that instead of being due to a pair of entangled particles, Hawking radiation is due to two pairs of entangled particles.  One entangled pair gets trapped by the black hole, while the other entangled pair escapes.  The process is similar to Hawking’s original proposal, but the Hawking particles are not in a pure state.

So there’s no paradox.  Black holes can radiate in a way that agrees with thermodynamics, and the region near the event horizon doesn’t have a firewall, just as general relativity requires.  So Hawking’s proposal is a solution to a problem that doesn’t exist.

What I’ve presented here is a very rough overview of the situation.  I’ve glossed over some of the more subtle aspects.  For a more detailed (and remarkably clear) overview check out Ethan Seigel’s post on his blog Starts With a Bang!  Also check out the post on Sabine Hossenfelder’s blog, Back Reaction, where she talks about the issue herself.