Our universe is full of dead neutrinos.  Each cubic centimeter of the space around us contains at least 340 of them.  They are Big Bang leftovers, ephemeral relic neutral particles and anti-particles that possess a half-unit of spin and a small mass, but almost no kinetic energy.  We are fairly sure of their existence, based on the Standard LCDM Model of Cosmology.  (Note: "LCDM" here means the standard cosmological model based on Cold Dark Matter with a cosmological constant L, the Greek capital letter lambda). Checking this LCDM prediction is tricky, because low energy neutrinos are very elusive particles that are almost impossible to detect.  However, there are now other ways of confirming the presence of these Big Bang neutrino products.

It is now fairly well understood that about 379,000 years after the Big Bang, when the temperature of the hot plasma pervading the universe had fallen to about 3,000 K, the medium had cooled enough to allow the electrically negative free electrons and the electrically positive free protons to join and form neutral hydrogen atoms.  With that transformation, the light transmission of the plasma changed from opaque to transparent, and the thermal light that had been trapped in an electrical ping-pong game within the plasma was released in a flash of radiation that today, after cooling to 2.7 K, it is still visible as radio waves coming from all directions in space.

We call this radiation the cosmic microwave background (CMB).  In its early hot-plasma-filled stage the universe reverberated with acoustic compression waves, the so-called "Sound of the Big Bang" (see AV 169 in the October-2013 Analog).  These primordial vibrations left an imprint on the CMB that we can read today to deduce the parameters of the universe, extracting a "baby picture" of the cosmos taken at age 367,000 years.

The microwaves of the CMB were discovered by Arno Penzias and Robert W. Wilson in 1965.  The WMAP space mission of Princeton and NASA (2003) first produced a detailed map of the angular structure of the small thermal variations of the CMB.  A decade later, the Planck space mission of the European Space Agency (2013) did an even more detailed measurement that is now the "gold standard" for CMB research in cosmology.  This has led to our present understanding of the age and expansion rate of the universe and the dominance of dark matter and dark energy over normal matter.

What is perhaps not so well known is that only about one second after the Big Bang, when the plasma pervading the universe still had the huge temperature of 35,000,000,000 K, a similar process occurred involving the weak interaction, and it produced a flash of neutrinos and anti-neutrinos.  We call that radiation the cosmic neutrino background, abbreviated here as "CnB".  These relic neutrinos have cooled from a temperature of 35,000,000,000 K to less than 2 K, and they are all around us, with a density of 340 neutrinos per cubic centimeter of space.  The neutrinos forming the CnB come in six types: both neutrinos and anti-neutrinos of the electron, mu and tau flavors (n_e, n_m, and n_t), each type having a density of 56.7 per cubic centimeter.  Despite this very high particle density, the cosmic neutrino background has never been directly observed, because low energy neutrinos interact so very weakly with normal matter.

It is in principle possible that the electron neutrinos of the CnB could be detected in the KATRIN experiment presently being brought into operation in Karlsruhe , Germany .  The experiment, which is a laboratory attempt to directly measure the rest mass of the electron anti-neutrino, focuses on the energy-endpoint region (18.577 keV) of the beta decay of tritium, which is the mass-3 isotope of hydrogen.  The analysis looks for small probability shifts near the endpoint influenced by the neutrino rest mass.  If CnB electron-neutrinos were present in the vicinity of the tritium source in high enough densities, they could participate in the weak-interaction process of neutrino capture by tritium to produce ³He plus an electron.  In that case, the produced electrons would show up in the measurement as a sharp energy peak at 18.6 keV, slightly above the ³He endpoint energy.

Unfortunately, even though the density of CnB electron neutrinos is very high, it is not high enough to produce a visible peak in the KATRIN measurements.   Estimates by Amand Faessler and his colleagues indicate that at an average density of 56.7 CnB electron neutrinos per cm³, the rate of observation in KATRIN would be one count every 590,000 years.

Since neutrinos have a small gravitational mass, it is possible that the gravitational pull of our galaxy might cause the CMB neutrinos to cluster around the galaxy and reach a higher local density.  However, even if that effect boosted the CnB neutrino density by a factor of a million, as the most optimistic estimates suggest, the counting rate in the 18.6 keV peak for KATRIN would only be about one count per year.  Faessler, et al suggested that the CnB could be directly detected by boosting the tritium source strength in KATRIN by a factor of 100 or more. Unfortunately, that it not likely to happen because of its great technical difficulties.  Thus, direct observation of CnB neutrinos seems to be presently out of reach.

However, the CnB shows up in other ways in our universe.  For example, in the hot dense environment of the early Big Bang, the high density of CnB electron anti-neutrinos means that protons have a significant chance of being transformed into neutrons by anti-neutrino capture, leading to an enhanced probability of forming neutron-containing deuterium, ³He, ⁴He, lithium, and other light-element nuclei in the early universe.  Model dependent nucleosynthesis calculations strongly suggest that the effects of the CnB are needed to explain the observed abundances of light elements in the universe.  However, these calculations are somewhat suspect because they over-predict the observed abundance of lithium by about a factor of 3.  Thus, it is difficult to make a convincing case for the presence of CnB based on them.

The CnB is also implicated in the damping of smallest-angle fluctuations in the cosmic microwave background seen in the Planck data.  However, this is rather indirect CnB evidence.  There are other possible explanations for this phenomenon, and so it cannot be taken as definitive evidence of the presence of the CnB.

This brings us to what is currently the best evidence that the cosmic neutrino background is really present in our universe.  Density fluctuations in the hot plasma of the early universe lead to mass-energy concentrations that resulted in galaxy formation.  Within the plasma, the information that a high density region was forming propagated at the speed of sound in the plasma, which is about 58% of the speed of light.  However, the neutrinos of the CnB propagated freely through the plasma just under the speed of light, providing a supersonic transmission of the same information.  It has been shown that this supersonic neutrino flow created a shift on the phases of plasma waves moving through the plasma.  These waves produced density fluctuations that subsequently became inscribed on the cosmic microwave background when the universe became transparent.  Therefore, a careful analysis of CMB data, for example from the Planck CMB measurements, allows the phase shift to be extracted.

Brent Follin and his colleagues at the University of California at Davis have analyzed the data from the 2013 Planck Mission measurements to extract these shifts in phase, and they have found very compelling and relatively model-independent evidence for the presence of the CnB in our universe.  Further, they found that the present temperature of these relic neutrinos have dropped to 1.96±0.02 K, almost precisely what is predicted by LCDM.  The authors point out that there are more precise results to come, because the polarization of the CMB microwaves, currently being measured and analyzed, should provide an even tighter case for the presence of the CnB in the early universe.

Further, the Follin CnB analysis also has something interesting to say about the number of distinct species of neutrinos.   In 2001 the Los-Alamos-based Liquid Scintillator Neutrino Detector (LSND), designed to detect the neutrino oscillations of mu neutrinos, reported an unexplained excess of electron neutrinos among those detected.  This was interpreted as a possible indication that the neutrino oscillations were being modified by a fourth "sterile" species of neutrinos in addition to the three neutrino species observed in the high energy physics laboratory (n_e, n_m, and n_t).  The phase shifts extracted from CnB analysis of Planck data depend strongly on the number neutrino species in which energy can be stored and propagated.  Consequently, the Follin analysis makes a convincing case that there are exactly three (not four) neutrino species, casting great doubt on the possible existence of a fourth sterile species of neutrinos.

Thus, we now have indirect evidence for the existence of a universe-filling neutrino signal originating during the first second of the Big Bang.  We can only fervently wish that there was some efficient method of directly detecting these relic neutrinos, because their angular structure would presumably contain the imprint of the acoustic oscillations of the primordial plasma of the universe at a much earlier time, only a single second after things first got started in the universe.

John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.

SF Novels by John Cramer: Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106. His new novel, Fermi's Question may be coming soon.

Alternate View Columns Online: Electronic reprints of 212 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: http://www.npl.washington.edu/av .

References:

The Cosmic Neutrino Background:

"First Detection of the Acoustic Oscillation Phase Shift Expected from the Cosmic Neutrino Background", Brent Follin, Lloyd Knox, Marius Millea, and Cen Pan, Physical Review Letters 115, 091301, (2015); https://arxiv.org/abs/1503.07863.

"Phases of new physics in the CMB", Daniel Baumann, Daniel Green, Joel Meyers, Benjamin Wallisch, Journal of Cosmology and Astroparticle Physics 01, 007, (2016); https://arxiv.org/abs/1508.06342

Detecting Cosmic Background Neutrinos:

"Search for the Cosmic Neutrino Background and KATRIN", Amand Faessler, Rastislav Hodák, Sergey Kovalenko, and Fedor Šimkovic, https://arxiv.org/abs/1304.5632.

Nucleosynthesis and Cosmic Background Neutrinos:

"Neutrinos and Big Bang Nucleosynthesis", Gary Steigman, Advances in High Energy Physics, Vol. 2012, Article ID 268321; http://dx.doi.org/10.1155/2012/268321 .

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