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Supersonic Neutrinos Detected via their Gravitational Impact on Sound Waves in the Primordial Plasma

A large part of the work of science is figuring out how to see what can’t be seen. How do we know what’s going on in the deep interior of the Sun? How can we know what was happening as the primordial fireball of the big bang expanded and cooled? How do we know the structure of the Hydrogen atom? We can’t visit the interior of the Sun or the very young universe, and we can't actually see Hydrogen atoms. The challenges the scientist has in these situations is somewhat analogous to those faced by a prosecutor (or judge and jury), trying to figure out what happened at the scene of a murder, when there are no witnesses and the murderer is not talking. Clues at the scene of the crime, maybe a fingerprint on a murder weapon, plus wounds on the body matching those one might receive from such a weapon, can sometimes be used to piece together a fairly reliable story of what transpired.

We cannot visit the early universe. But our understanding of the known laws of physics allows us to make predictions for things left over from the big bang, things we can observe today that are analogous to the fingerprints of our hypothetical crime scene.

Over the past fifty years more and more predictions related to events in the big bang have been confirmed. One prediction of our modern understanding of the big bang is that trillions of tiny particles called neutrinos, all produced in the first seconds of the big bang, are passing through our bodies every second. You might think that this would be an easy prediction to directly confirm or refute. But actually it's extremely difficult. The difficulty lies in the nature of neutrinos: they interact with other particles extremely weakly.

Recently my graduate students and I found a fingerprint from the Big Bang. (Of course I'm talking metaphorically here. In the hot, dense, very smooth, rapidly expanding conditions of the early uinverse there were no fingers!) We found an observational consequence of supersonic neutrinos. Our work appears in Physical Review Letters (as an Editor's Suggestion, no less) and has been blogged about by Andy Fell who gets paid to publicize work at UC Davis, and also blogged about by Ethan Siegel who has no connection to UC Davis but just thought it was really cool.

The fingerprint was found in exquisitely precise maps of the cosmic microwave background, the afterglow of the Big Bang, produced by the Planck collaboration using data from the Planck satellite.

Communicating precisely what we found in the data, and how that feature emerged from supersonic neutrinos, is beyond the scope of what I hope to achieve with this post. This is quite a technical discovery, in more ways than one. Instead, I provide context here for our discovery, and its implications. Important details will remain mysterious, for which I apologize.

What are neutrinos?

To understand neutrinos, it helps to know about the four forces in nature. Two of them everyone has directly experienced, whether they realize it or not. These are gravity and electromagnetism. Gravity is a very weak force, but it is a long-range one. Your body interacts, via gravity, with all the other masses in the whole solar system, and even beyond. For example, President Obama, whereever he is right now, is exerting a small gravitational force on you, pulling you toward him. Same goes for Donald Trump! Gravity does not care about your politics. You have not noticed the gravitational pull of The Donald because, as I said, gravity is very weak. But its effects can add up. Every atom of the Earth is exerting a teeny tiny pull on you. The net result, because there are a lot of atoms in the Earth, is a force you can feel.

If gravity were as strong as even the next strongest force, that pull of the entire Earth on you would crush you against the floor. But since gravity is a very weak force, you can resist that pull by balancing it with the electromagnetic forces operating betwween a much smaller number of atoms. If you are standing at the moment (good for you for having a standing desk!) there is a force that the floor exerts on your feet. This force is electromagnetic. Electromagnetic interactions between atoms on the bottom of your shoes, and atoms in the floor where your shoes contact the floor, keep your shoes (and you) from going through the floor, pulled downward by gravity. Just a small amount of material generates enough electromagnetic force to allow you to resist the gravitational pull of the whole earth. The strength of the electromagnetic interaction is much much greater than the strength of gravitational interactions. In principle, the electromagnetic force is a long-range force also, just like gravity, but with gravity everything pulls. With the electromagnetic force, some pairs of particles push eachother apart, other pairs pull together, and so we don't get everything adding up to work together, like with gravity. Effectively, electromagnetic forces in our daily experience are thus short range. Magnets are an exception to this rule, where interactions add up and you can feel, holding two magnets in your hand, the magnetic force being exerted over a distance.

The other two forces are ones we don't directly experience in daily life. One is the strong nuclear force. It is a bit stonger than the electromagnetic force. Without it, the nuclei at the centers of atoms would not be stable. So, it is indeed very important to daily life. If somehow the strong force all of a sudden stopped working, our bodies would completely disintegrate. The strong force is very short range however, with a range much smaller than the size of a typical atom. That is why you have no direct experience of it.

The fourth and final force we call the weak nuclear force. It is called weak because it is considerably weaker than the strong nuclear force, and also the electromagnetic force. It is not as weak as gravity. We need it to understand some aspects of the decay of atomic nuclei. It is, like the strong nuclear force, a short range force. We don't have any experience of the weak force in our daily lives.

What does all this have to do with neutrinos? In the standard model of particle physics there is an up quark, a down quark, an electron, and an electron neutrino (see left column of the image below). This pattern is then copied twice, so that each one of these particles has a "cousin" that is heavier. For example, the cousins of the electron are the muon and the tauon. The cousins of the electron neutrino are the muon neutrino and the tauon neutrino. With the exception of the Higgs boson (that I won't be discussing here) these are all the matter particles we know about in nature and can directly detect. There is nothing on Earth, in our daily experience, that is not made out of these particles. There are also particles associated with each of the forces, called "gauge bosons."

The matter particles differ from eachother by mass, and by how strongly they participate in the four forces. Quarks participate in all four forces. Electrons (and their cousins) participate in everything but the strong force. They don't feel, or exert, the strong force. Neutrinos only participate in the weak force and the gravitational force.

Neutrinos can be produced in nuclear reactions. They are produced in the core of the Sun. They are produced in nuclear reactors on Earth. They are produced inside the Earth as radioactive elements inside the Earth decay. But, as I said, they interact very weakly. A typical Solar neutrino (one produced in nuclear reactions in the Sun), that happens to be headed straight for Earth will end up passing through the entire Earth. This seems strange to us because we are not used to, in our daily experience, dealing with particles that only interact via the weak force and gravity. We are only familiar with electromagnetic forces and gravity. A particle, such as an electron, or a proton (which is made out of quarks) would not pass right through the whole Earth. It would have strong and/or electromagnetic interactions with the Earth that would stop it in its tracks.

It actually is possible to detect neutrinos directly -- at least the ones produced in nuclear reactors or in the Sun. For these, one just has to build a big enough detector. The bigger you make it, the more likely that one of these rare interactions between a neutrino and ordinary matter actually occurs. The image to the right is from the inside of a big neutrino detector in Japan called Super Kamiokande. Usually it is filled with water. This is what the neutrinos interact with. The water tank is lined with photomultiplier tubes that record the light that results from these rare interactions. When this picture was taken, the detector was shut down so that the photomultiplier tubes could be cleaned. That's what the workers in the boat are doing.

Neutrinos in the Big Bang

Neutrinos were copiously produced in the big bang, when the Universe was as hot as, and hotter than, the core of the Sun. This was happening about 13.8 billion years ago. Just a few seconds into the expansion of the universe, the universe became sufficiently cool and rarified that neutrino production stopped. But the already-produced neutrinos remained. At least this is what we expect based on what we know about the laws of physics.

Although there are huge numbers of neutrinos flowing through us every second, their kinetic energy is very low and this, as it turns out, makes them even less likely to interact wtih ordinary matter. Direct detection of these neutrinos would be exceedingly difficult. (Although people are trying!)

Because they don't have much kinetic energy, or much mass either, neutrinos (despite their numbers) are a tiny component of the total mass/energy budget of the universe today. See the pie chart below. Neutrinos today only constitute about 0.3% of all the mass/energy in the universe. [That is not totally negligible, and actually does have observational consequences I might blog about in the future.]

Image credit: Lin Tao Wang (U. of Chicago)

The situation was different in the distant past. Neutrinos used to be a significant component of the total mass/energy of the universe. This was also true of photons (carriers of the electromagnetic force), which because they only contribute about 0.01% today don't even show up in the above pie chart. Back when the universe was about 1100 times less expanded than it is now, about 13.8 billion years ago, the composition was approximately that shown in the lower pie chart in the figure below and to the right.

If we take it back even earlier, the neutrinos and photons completely dominated the composition, with dark matter and atoms contributing a tiny fraction to the total. It is in this era, that cosmologists call the "radiation dominated era" that neutrinos left their fingerprints on the photons that we recently managed to see in the Planck data.

The story I'm giving you is an interesting and lively one, with a cast of characters with a dynamically evolving relative importancet. But the reason I tell it to you with such confidence is the wealth of data that we have to back it up. It's not just a wealth of data --it's also the simplicity of the underlying theory, a theory that also manages to explain the result of every particle physics experiment on Earth, as well as excruciatingly precise details of the motions of the planets.

All the same, we want to keep testing our story. Maybe it's wrong in some important way. (In fact, we know it can't be the final answer.) Maybe there's some exciting discovery lurking right around the corner, waiting to reveal itself to a sufficiently insightful analysis! That's the potential reward that keeps many of us going, working hard to build instruments, to make measurements, and to analyze data.

One of these predictions to test is an effect that neutrinos have on the photons at very early times. The particular effect we detected is way too difficult to explain in detail, so I will not even try. I'm just going to try to get across the general idea.

The photons that dominated the energy budget of the early universe were part of, and trapped in, a plasma. A plasma is a state of matter in which the electrons are separated from the atoms. The photons (particles of light, the carriers of the electromagnetic force) readily scatter off of free electrons. Because of this, the photons do not travel freely, like they do through the gas filling the room you are in. The early universe was opaque. One could not see anything (of course if one were there, one would get quickly fried to a crisp). But if you could somehow survive the temperatures that make an inferno seem like a day in Antractica, you could hear things. There were sound waves in the plasma. We can see effects of these sound waves in maps of the cosmic microwave background (CMB; see this post for explanation of CMB maps).

The speed of sound in this plasma was not your ordinary 1100 feet per second that you may know is the sound speed in air at atmospheric pressure and room temperature. That's pretty fast. It means sound waves in Earth's atmosphere travel about one mile in five seconds. You may have used this fact to estimate the distance to a lightning strike. No, the speed of sound in the primordial plasma was nearly the speed of light divided by the square root of 3, or about 107,000 miles per second. That's the maximum speed by which disturbances in the plasma can propagate. If you imagine throwing a stone into the plasma at one point in space, ripples would spread out from it at about 107,000 miles per second.

As fast as this is, it's not the ultimate speed, the speed of light. That's about 186,000 miles per second. In the plasma, even the light can't travel this rapidly, because it is trapped in the plasma, rapidly scattering off of free electrons. It can't travel in straight lines. But something can travel in straight lines, right through the plasma: the neutrinos. Because they only interact via the appropriately-named weak interaction! The fact that disturbances in the neutrinos can propagate at speeds faster than the sound speed (hence I call them supersonic neutrinos) has an unusual consequence on the nature of the sound waves in the plasma. This is the part that is really hard to explain -- even to an audience of experts. In fact, I know I could understand it better myself than I currently do.

You might wonder, if the neutrinos are streaming right through the plasma like it's not even there, how do they end up influencing the plasma? The answer is they do so gravitationally. It's the gravitational forces associated with supersonically-propagating variations in neutrino density that impact the plasma in ways we have now seen in the Planck data.


We have figured out an amazing amount about the laws of nature, and about the origin of our universe. We know so much, in fact, and our measurements are becoming so precise, that we are able to dig out subtler and subtler effects from the data. We've had evidence for a long time that neutrinos were copiously produced in the big bang. To extend our crime scene analogy, evidence before this was like the fingerprint of neutrinos. We had the fingerprint. Now we have even better data from the crime scene, and we've been able to show that the criminal's DNA was at the crime scene. Our finding is more like the latter -- better, more precise, data has revealed what we expected to find. We believe we are seeing the expected impact on the plasma, of the neutrinos streaming through it at the speed of light, a speed faster than the sound speed in the plasma.

Again, I have not explained what that signature is, or how exactly we see effects of sound waves in this plasma. That should still be mysterious to the reader.

In some way's nothing's new. In another way though, that we can tease these tiny effects out of the data underscores how well we understand what is going on. A juror who is not impressed with a partially-smudged fingerprint, may be swayed by DNA evidence.

Also, as I said in the beginning, the main thing driving our field forward is not a desire to confirm all these predictions about the big bang. We want to see the predictions fail. Because then we will have found clues to new physics. We will be discovering something truly new. We will have an opportunity to be the first to be able to explain such data and what they are telling us about the cosmos and about the laws of nature. Fame and glory beckons!

Until then though we must content ourselves with marveling at how well our predictions are working. We are stuck on this tiny planet, thinking about events billions of years in the past, observing them with light that has traveled to us from the edge of the observable universe. And we're getting it right. That says something remarkable about the universe, and also about human minds.

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