The first 400,000 years after the Big Bang are inaccessible to us by using light; the material that filled the entire cosmos made it opaque. However, neutrinos interact very little with ordinary matter, so they could travel right through the opaque mess. Lots of these low-mass, fast-moving particles were formed in the first second after the Big Bang, so they could provide a sensitive probe of some of the very earliest moments in the Universe.

Unfortunately, these primordial neutrinos have never been detected directly, and they may have too little energy for us to ever detect them. But a new paper published in Physical Review Letters showed an unambiguous indirect detection using measurements of the cosmic microwave background light. This article marks the first clear measurement of the cosmic neutrino background, which is a significant confirmation of one of the major predictions of the Big Bang model.

The early Universe was a very different place. In the immediate aftermath of the Big Bang, matter and energy were compressed, and particles bashed into each other at higher energies than our colliders can achieve. But as the Universe expanded, it cooled off. Much like cooling a vapor turns it into liquid and then a solid, the entire cosmos underwent several profound changes as it expanded. Particles formed the first atomic nuclei, which later went from a hot plasma to stable atoms and so on.

One of those changes is called neutrino decoupling. It took place when the density of matter dropped enough that neutrinos stopped colliding with other particles regularly. This transition took place before the Universe was even one second old—before there were even protons around, much less atomic nuclei—as the average energy of each collision was high enough to destroy any particle like that.

After decoupling, all the neutrinos from the Big Bang were effectively freed to travel forever. Some still hit other particles, of course, but that became the exception. The freed neutrinos form a bath filling the cosmos, which is the cosmic neutrino background (CNB), by analogy with the cosmic microwave background. With the continued expansion and cooling of the Universe, the neutrino background is about 1.9°C above absolute zero, so it's slightly colder than the microwave background.

The biggest difficulty in characterizing the CNB is that neutrinos are far harder to detect than microwaves. Even with powerful neutrino observatories such as IceCube, researchers have only identified a relatively small number of neutrinos from deep space. For that reason, it's unlikely that we will observe the CNB directly anytime in the near future. It's a sad thought, one that makes cosmologists cry themselves to sleep: neutrinos from the first second after the Big Bang would be the earliest particles we can get and could provide evidence for the conditions that prevailed less than a second after the Big Bang.

Even though we may not see the CNB directly, few doubt it exists—it's a direct consequence of the Big Bang model and particle physics. Additionally, several important indirect observations suggest that the neutrino background exists, though interpreting those observations is ambiguous.

The new paper describes another indirect observation of the CNB, but one that is far less ambiguous. The measurement is based on the fact that even after neutrinos decoupled from matter, they were still a gravitational force to be reckoned with—at the time, one that was nearly as significant as ordinary matter. By contrast, in the modern Universe, neutrinos contribute much less than one percent to the total energy content, while ordinary matter contributes about five percent.

Before the cosmic microwave background formed about 380,000 years after the Big Bang, most ordinary matter (as opposed to dark matter) was in the form of hydrogen and helium plasma—a stew of protons, helium nuclei, and electrons. Since neutrinos move at nearly light-speed, they traveled faster than the speed of sound in this plasma. The sonic booms they produced caused very slight changes in the fluctuations in density in the primordial plasma, shifting the size and position of the fluctuations in a measurable way. (This is an oversimplification, kids—don't try this at home.)

The density fluctuations—sound waves—in the early Universe eventually led to the distribution of galaxies we see today. They also made their mark on the cosmic microwave background, producing the regions of slightly hotter and colder temperatures in the maps made by Planck and other observatories.

The authors used data from the Planck cosmic microwave background mission and simulated what the fluctuations would look like with and without the influence of the cosmic neutrino background. They also ran their simulations with different numbers of neutrino types; they found that a model with three "flavors" of neutrinos fit the data best, which agrees with nearly all current experiments. That's good: physicists like surprises, but not too many surprises.

Neutrino effects on the cosmic microwave background are subtle, but they are hard to mimic; the only other obvious candidate is a previously unsuspected form of dark matter that moves at nearly the speed of light. Given how unlikely that is, the newly reported effect is a small-but-unambiguous indirect detection of the cosmic neutrino background. With a number of ground-based telescopes performing similar observations, no doubt we'll see further refinement—and the ability to see more than just the presence of primordial neutrinos.

Physical Review Letters, 2014. DOI: 10.1103/PhysRevLett.115.091301 (About DOIs).