摘要：If we could detect them, cosmic neutrinos would paint a picture of the universe in the instant after it began. Physicist Martin Bauer has come up with a plan to do just that

AN INSTANT after the big bang, a vast shower of particles was released into the cosmos. Ever since, they have been flooding through space, carrying with them secrets from the dawn of time. How we would love to capture one – but these messengers are as elusive as they come. They pass clean through matter: 100 trillion of them are streaming through your body every second and you never notice.

These ghostly things are neutrinos, elementary particles that we know exist but that are exceedingly difficult to detect. There are experiments around the world that can capture neutrinos released from the sun, in vast traps buried more than a kilometre underground, for instance. Yet neutrinos released just after the big bang have far less energy and have so far proved impossible to detect at all.

A fabulous prize awaits if we could spot them, though. They would paint us an unprecedented picture of the universe in its very first moments, hundreds of thousands of years earlier than we have ever been able to see before. It would transform cosmology.

For decades, any plans to detect these elusive particles have required technology beyond our wildest imagination. But now things have moved on and I have a suggestion for how we might just snare them. In the past, we have simply waited for neutrinos to come to us. Yet there may be a way of having our detectors race towards them.

Everything we observe today was once crammed into a much smaller space. When the universe was young, it consisted of a speck of extremely hot, dense plasma. Things were so cramped that subatomic particles constantly scattered off each other with such frequency and force that atoms couldn’t form without being immediately broken apart again. This primordial plasma was completely opaque. Photons, or particles of light, simply couldn’t travel very far without being absorbed or scattering off another particle.

As the universe kept expanding and the plasma cooled, it became less dense and so less likely for photons to get scattered or absorbed by other particles. This continued until a key moment was reached, 380,000 years after the big bang, when the photons could finally escape. Now, light could travel unobstructed through the cosmos. We can still see the photons released at that instant today: they form the universe’s oldest light, which we call the cosmic microwave background (CMB).

Cosmic microwave background

The CMB was discovered by accident. In 1964, when Robert Wilson and Arno Penzias set up their now-famous horn antenna to detect radio waves from early balloon satellites, the whole idea of the big bang was very much an open question. Even after accounting for all possible sources of noise, the pair heard a puzzling continuous background signal coming from all directions. It was there day and night. They eventually realised they were detecting the CMB and this was our first glimpse of the universe’s first light.

Since then, we have measured the cosmic background photons with high precision, and we continue to do so. This gives us a picture of the universe when it was 380,000 years old – a mere baby compared with the 13.8 billion years it is now. We see dense patches that later developed into galaxies, and patterns that tell us about the relative amounts of radiation, matter and dark matter in the early universe.

The CMB is the oldest light we have seen – and the oldest we ever will see. But there is another, untapped source of information about the early universe out there that could help us peer much further back in time.

Though neutrinos are decidedly strange, we have known about them for a long time. They were first hypothesised in 1930 and detected in experiments in 1956. They are created all the time as a consequence of radioactivity – even an ordinary banana produces streams of neutrinos as the potassium atoms inside it decay. However, neutrinos are extremely light and barely ever interact with anything else.

Read more: Neutrinos – the next big small thing

Because of their antisocial behaviour, neutrinos could move around unobstructed much earlier on in the early universe compared with light. While photons were constantly bumping into other particles, neutrinos could stream freely through that hot, dense plasma. The big bang theory predicts that neutrinos created in the first second of the universe would have immediately been able to escape the fog and they would still be flowing through the cosmos today. Rather than the CMB, we call this the CNB, or the cosmic neutrino background.

What cosmic neutrinos would tell us

Discovering the CNB would be huge. It would provide a completely new way of seeing how the universe developed. To understand why, we need to get to know a bit more about neutrinos and how they travel. The key difference between photons and neutrinos is that the former are massless, while the latter do have mass, albeit a very tiny one. In fact, neutrinos come in three different types, each of which has a slightly different mass. Because of this, background neutrinos would end up travelling at a range of speeds, all roughly 1000 times slower than light, as they stream towards us.

As the cosmic neutrinos speed through the universe, their course is bent by the gravitational pull of huge objects like galaxies they pass, an effect called gravitational lensing. This happens with light too, but because all light travels at the same speed, its path is bent in a way that depends on how massive the object was at one particular time, when the light ray zipped past. In contrast, cosmic neutrinos are travelling through space at different speeds and therefore they pass those huge objects at different times. This means that, if we could scan the sky for cosmic neutrinos, we could use them to glimpse the large-scale structure of the universe at different times. Think of it like this: if the CMB showed us a black-and-white photo of the early universe, cosmic neutrinos would produce a 3D movie in full colour.

Though we have never seen them, we have good reason to think cosmic neutrinos are there: their existence is as inevitable a consequence of the big bang as the CMB. And if they aren’t around, finding out would be an incredible discovery. It would be a direct contradiction of the established model of the big bang – a contradiction that would require exciting new physics to explain it. We simply must try to detect them.

But it won’t be easy. Although experiments have detected some types of higher energy neutrino, such as those produced by the sun, the energy of cosmic neutrinos would be a billion times smaller than the lowest energy neutrino from other sources that we have observed so far, making them inestimably hard to catch.

The earliest idea to find them was proposed in 1962 by the late Nobel-winning physicist Steven Weinberg. He was inspired by a technology that was used to first discover non-background neutrinos in the 1950s and later used to detect solar neutrinos.

It turns out that, if a neutrino hits an atom, it can be captured, imparting enough energy to transform one of the atom’s constituent particles, a proton, into another, a neutron. The chemical elements are defined by how many protons they have, so if this process happens, it produces a different chemical element – that is detectable and is a surefire sign that a neutrino was involved.

Read more: Your essential guide to the many breathtaking wonders of the universe

In 1970, physicists Raymond Davis Jr. and John Bahcall set out to find these signs by filling a tank with 380,000 litres of a liquid rich in chlorine. If a neutrino hit one of the chlorine atoms, it would turn it into an atom of argon. The only problem was that incoming cosmic rays – high-energy particles from space – could do the same thing. So the pair placed their tank in the decommissioned Homestake gold mine in South Dakota, almost 1500 metres underground. While cosmic rays were screened out, neutrinos sped through the ground and turned some of the buried chlorine into argon. By counting how many, Davis and Bahcall were able to work out the rate of flow of solar neutrinos – and net themselves the Nobel prize in physics in 2002.

Weinberg had a similar strategy in mind to discover cosmic neutrinos. But because they have such low energies, they simply aren’t powerful enough to convert protons into neutrons in any kind of stable atom. Because of this, Weinberg turned to tritium, a radioactive version of hydrogen with two neutrons and one proton. Tritium is unstable, which means it decays naturally by converting one of its neutrons into a proton, spitting out an electron at the same time. The electron is released with a known energy. But if a tritium nucleus absorbs a cosmic neutrino before it decays, the energy of the electron it spits out can exceed what would be expected because the neutrino puts in some extra oomph. Weinberg reasoned that, if we could measure the electron energy with extreme precision and we detected unusually energetic electrons being produced by tritium decay, we would have discovered the cosmic neutrino background.

How to detect cosmic neutrinos

All this would be extraordinarily challenging to pull off. Using 100 grams of tritium would yield just four neutrino absorptions – and so four unusually energetic electrons – per year. That is minuscule compared with the release of 100 trillion lower energy electrons per second via natural decay. Spotting those four rogue electrons would be nigh impossible and, currently, the technology is limited on all fronts. Yet there is an active international research collaboration called PTOLEMY that aims to build a prototype of such a detector.

In the meantime, other physicists have proposed different ways of detecting cosmic neutrinos – some more likely to prove successful than others. In 1974, the late physicist Reuven Opher first suggested measuring the pressure that cosmic neutrinos exert on a special probe called a torsion pendulum. But since neutrinos barely interact with matter, this pressure would be tiny and we would need to improve the sensitivity of the best measurements of such a set-up by a factor of a billion to have a hope of success.

Read more: The race to see the start of time in the first light of the universe

Then, there is the idea proposed by physicist Thomas Weiler in 1984, which involves counting cosmic rays as a different proxy for cosmic neutrinos. It turns out that high energy neutrinos can hit the atmosphere and produce a cosmic ray that then shoots down to Earth. We can reliably detect these cosmic rays. But there is a rare process in which a cosmic neutrino hits one of the high energy neutrinos and the two annihilate, meaning that fewer cosmic rays are created. So, in theory, we could determine the presence of the cosmic neutrinos by a dip in the rate of incoming cosmic rays. If this sounds like a long shot, it certainly is. We would need to run a detector for centuries before we managed to see the dip.

In short, all the ideas for snaring the CNB have rather serious difficulties. Which is why, recently, my PhD student Jack Shergold and I have come up with an alternative possible route to discovery. It is based on overcoming the primary obstacle of each proposed experiment: that cosmic neutrinos carry so incredibly little energy.

Editorial use only View of the giant tank of dry-cleaning fluid which forms the Homestake detector for solar neutrinos, seen shortly after construction in the Homestake Gold Mine, South Dakota, in 1967. The detector is the work of physicist Raymond Davis Jr. Still in operation in 1993, it has found only about one third of the neutrinos predicted by the standard theory of nuclear interactions in the Sun. The tank contains perchlorethylene. Neutrinos occasionally interact with a chlorine-37 nucleus in this fluid and transform it into an argon-37 nucleus; the argon decays reveal how many such interactions have occurred.

In normal neutrino detectors, huge vats of atoms are sitting still waiting for relatively fast-moving solar neutrinos to strike them. When it comes to cosmic neutrinos, they are moving much slower and have less energy, so the collisions are harder to detect. We can’t do anything about the neutrinos. But what about the target atoms? Could we somehow accelerate them so that when they collide with cosmic neutrinos the overall crash is harder?

My radical plan to snare cosmic neutrinos

Accelerating atoms is difficult, since they are electrically neutral. This means they aren’t influenced by the strong electromagnetic fields that are used in accelerators such as those at CERN to get charged particles, like protons, to near the speed of light. Yet there is a trick we can pull: if we strip off some of an atom’s outer electrons, this produces a charged version of the atom called an ion. This can be accelerated using common technology and the fact that an electron is gone makes little difference to any interactions with neutrinos.

Last year, Shergold and I considered a situation in which we use an accelerator to speed up a large number of ions. Cosmic background neutrinos would flood through the accelerator all the time, in the same way that they pass through your body. We calculated that it would be possible to design a set-up like this so that we have collisions with enough energy to detect cosmic neutrinos.

If we employed the chlorine-to-argon transition used in the Homestake experiment, that would require an accelerator with an energy a million times greater than that of the Large Hadron Collider (LHC). This is, of course, a huge problem.

Read more: The CERN particle accelerator that will breathe new life into physics

But there are other kinds of ions that might work. For example, we could accelerate helium ions which, if struck by a neutrino, would transform into tritium. To get this working, we would need an accelerator only about 100 times more powerful than the LHC. Not a piece of cake, but not out of the question either. Another challenge would be to accelerate the large numbers of ions needed; it would be many more than typically used in particle-smashing experiments.

We are still at an early stage with the research in this area. The trick will be to find ions that will interact with neutrinos at as low an energy as possible. That way, we would need fewer of them – perhaps many orders of magnitude fewer.

Finding ions with the correct properties to make this a reality may not be any easier than realising Weinberg’s experiment. Yet the challenge is different and much less well explored, which makes it worth investigating.

Some of the biggest discoveries in physics – from the Higgs boson to gravitational waves – were decades in the making. It will likewise take time to corner the first cosmic neutrinos. But we could be one breakthrough away from finding the ghostly particles that carry the universe’s earliest secrets.