These upper bounds tell us that neutrinos have to be the lightest of all Standard Model particles, more than six orders of magnitude lighter than the electron!
Pinning down the exact masses, and understanding the mechanisms by which neutrino mass is generated are among the most interesting questions in the field of particle physics today. Many experiments are underway, or are being planned, expressly to answer these questions see, for example, this recent Symmetry magazine article.
Surprisingly, information about these extremely light particles is also imprinted on the largest cosmological scales of the Universe that we can observe. This is because neutrinos, by number, are the second most abundant particles in the Universe, with their number density being only slightly lower than that of photons.
These large number densities mean that neutrinos make up a non-negligible fraction of the total energy density of the Universe. In fact, the more massive the neutrinos, the larger the energy fraction as a function of the total energy in a given volume of the Universe contained in these particles. Until recombination the point at which the cosmic microwave background, or CMB, becomes free to travel unimpeded through space , neutrinos were relativistic and contributed to the expansion of the Universe as radiation instead of matter.
Thus precise measurements of the CMB can give us information about the number of neutrino species, but not their masses.
The best-studied effect of massive neutrinos is the way they affect the shape of the "matter-power spectrum" at low redshifts. The matter-power spectrum is a measure of how all the matter in the Universe clusters at different scales. This is usually plotted as a function of the wavenumber k , which is roughly the inverse of a given length scale R. So, in the figure above, large physical length scales correspond to small values of k , and small scales correspond to large values of k.
As a consequence of their low masses, neutrinos can, on average, move much faster at the same temperature than heavier particles like Cold Dark Matter CDM or baryons.
Cosmic neutrino background - Wikipedia
Therefore, unlike the CDM, which clusters down to very small scales due to the low velocity and mutual gravitational interactions of the constituent particles, neutrinos barely cluster together on small scales. However, on very large scales - larger than the scale up to which an average neutrino particle can travel given its thermal velocity which is referred to as the free-streaming scale —neutrinos behave just like CDM and baryons. This leads to the behavior shown in the figure, which shows the ratio of power spectrum from a cosmology with massive neutrinos to the power spectrum of a cosmology with massless neutrinos.
We hold fixed the total amount of matter in the Universe in both cosmologies—i.
- Neutrino Cosmology | UCL Astrophysics Group - UCL - London's Global University.
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-  Neutrino cosmology and Planck.
On very large scales small k , removing CDM and replacing it by neutrinos has no effect, as they both behave similarly in terms of clustering. This ensures that the ratio is exactly one. On small scales, removing a clustering component like CDM and replacing it by non-clustering neutrinos, damps the power spectrum, i. This is seen both in analytic linear perturbation theory calculations black curve , as well as in fully nonlinear calculations calibrated from cosmological simulations red curve.
The total amount of structure damping on small scales, as well as the scale at which the ratio starts to deviate from unity gives information about the mass of neutrinos. Heavier neutrinos lead to more damping on small scales, but the scale at which start damping the power spectrum becomes smaller. Various current and future cosmological experiments look to measure this characteristic damping effect of neutrinos on the matter-power spectrum using different observables.
For example, upper bounds have already been set on the neutrino mass through precise measurements of the lensing of the primary CMB signal, measurements of the Lyman alpha forest power spectrum, galaxy clustering and galaxy-galaxy lensing in photometric surveys, and Baryon Acoustic Oscillation studies. To interpret the measurements from these surveys, one needs very accurate theoretical predictions for the power spectrum on all measurable scales. On large scales, where the evolution of Large Scale Structure is mostly linear, analytic calculations can predict the power spectrum very accurately.
On small scales, gravitational collapse introduces non-linearities which require full cosmological simulations for modeling their evolution. Therefore, cosmological simulations which include the effects of massive neutrinos are extremely important to be able to extract useful information about neutrino masses down to small scales.
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For cosmologies with just CDM, N-body simulations are known to produce very precise and accurate predictions in the non-linear regime. However, massive neutrinos have proven to be somewhat difficult to incorporate into these simulations precisely because they have such large thermal velocities. Since neutrinos are fermions, these thermal velocities are drawn from the quantum distribution for fermions, known as the Fermi-Dirac distribution. One of the standard approaches in literature has been to sample this underlying distribution randomly at every point on a grid on which the initial conditions are generated—i.
This procedure, of course, needs to be repeated at every point on the grid, so, in principle, one would need multiple neutrino particles starting off at every point in space to get a good approximation of the distribution of neutrinos throughout the simulation volume. Unfortunately, the number of particles needed for the randomly drawn distribution to be a good approximation of the true distribution throughout the evolution of the simulation far exceeds the maximum number of particles that can be handled even in the largest simulations run to date.
In fact one would need about a million times as many particles as used in the largest simulations to reach the required accuracy. In the absence of sufficient numbers of particles, the random nature of thermal velocities means that the neutrino particles in the simulations just zoom about in random directions.
This leads to the situation that the density of neutrino particles in different parts of the simulation volume is just a random number, and not a true representation of what the physical density is expected to be. This problem is called the shot noise problem for fast moving particles in N-body simulations.
While this problem was recognized more than a decade ago, most neutrino simulations were run with this method—the hope being that neutrinos formed a small enough fraction of the matter density that large errors in the local neutrino density would not affect predictions for cosmological observables at a level that can actually be observed in experiments.
However, cosmological surveys like DESI and LSST will be able to measure the power spectrum on small scales at a very high degree of accuracy, and therefore it has become imperative to investigate whether the shot noise problem in the neutrino simulations can indeed be ignored, or if new methods are needed to describe the power spectrum down to small scales. We showed that the shot noise could be completely removed by changing the method for generating the initial thermal velocities of the neutrino particles.
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Instead of sampling the Fermi-Dirac distribution randomly to assign the initial thermal velocities of neutrinos, we sampled the distribution in a regular and repeatable manner at every point on the initial grid. A way to understand how this helps is the following: in the absence of physical perturbations, sampling the distribution in a regular manner ensures equal numbers of neutrino particles moving in and out of any given patch in the simulation volume at any later time.
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Neutrino cosmology Lesgourgues, Julien. Subjects A limited number of items are shown. Click to view More Neutrinos. Neutrino astrophysics. Summary "The role that neutrinos have played in the evolution of the universe is one of the most fascinating research areas that has stemmed from the interplay between cosmology, astrophysics and particle physics.
In this self-contained book, the authors bring together all aspects of the role of neutrinos in cosmology, spanning from leptogenesis to primordial nucleosynthesis, their role in the CMB and structure formation, to the problem of their direct detection. The book starts by guiding the reader through aspects of fundamental neutrino physics, such as the standard cosmological model and statistical mechanics in the expanding universe, before discussing the history of neutrinos in chronological order from the very early stages until today.
This timely book will interest graduate students and researchers in astrophysics, cosmology and particle physics who work with either a theoretical or an experimental focus" Notes Includes bibliographical references pages and index. Related Electronic Resources Cover image. Contents Preface -- 1. The basics of neutrino physics -- 2.