The cosmic neutrino background (CNB or CB) is the universe's background particle radiation composed of
neutrino
A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
s. They are sometimes known as relic neutrinos.
The CB is a relic of the
Big Bang
The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
; while the
cosmic microwave background radiation
In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...
(CMB) dates from when the universe was 379,000 years old, the
CB decoupled (separated) from matter when the universe was just one second old. It is estimated that today, the CB has a temperature of roughly .
As neutrinos rarely interact with matter, these neutrinos still exist today. They have a very low energy, around 10 to 10
eV.
[ Even high energy neutrinos are notoriously difficult to detect, and the CB has energies around 1010 times smaller, so the CB may not be directly observed in detail for many years, if at all.][ However, ]Big Bang
The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
cosmology makes many predictions about the CB, and there is very strong indirect evidence that the CB exists.[
]
Derivation of the CB temperature
Given the temperature of the cosmic microwave background
In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...
(CMB) the temperature of the cosmic neutrino background (CB) can be estimated. It involves a change between two regimes:
;Regime 1: The original state of the universe is a thermal equilibrium, the final stage of which has photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
s and lepton
In particle physics, a lepton is an elementary particle of half-integer spin ( spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and neutr ...
s freely creating each other through annihilation
In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total energy a ...
(leptons create photons) and pair production
Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton. Pair production often refers specific ...
(photons create leptons). This was the very brief state, right after the Big Bang. Its last stage involves only the lowest-mass possible fermion
In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: spin , spin , etc. In addition, these particles obey the Pauli exclusion principle. Fermions include all quarks an ...
s that interact with photons: electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
s and positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...
s.
;Regime 2: Once universe has expanded enough that the photon+lepton plasma has cooled to the point that Big Bang photons no longer have enough energy for pair production
Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton. Pair production often refers specific ...
of the lowest mass / energy leptons, the remaining electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
–positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...
pairs annihilate. The photons they create cool, and are then unable to create new particle pairs. This is the current state of most of the universe.
At very high temperatures, before neutrinos decoupled from the rest of matter, the universe primarily consisted of neutrinos, electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
s, positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...
s, and photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
s, all in thermal equilibrium
Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be in ...
with each other. Once the temperature dropped to approximately ( K), the neutrinos decoupled from the rest of matter, and for practical purposes, all lepton and photon interactions with these neutrinos stopped.
Despite this decoupling, neutrinos and photons remained at the same temperature as the universe expanded as a "fossil" of the prior Regime 1, since both are cooled in the same way by the same process of cosmic expansion
The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion whereby the scale of space itself changes. The universe does not exp ...
, from the same starting temperature. However, when the temperature dropped below double the mass of the electron, most electrons and positrons annihilated, transferring their heat and entropy to photons, and thus increasing the temperature of the photons. So the ratio of the temperature of the photons before and after the electron–positron annihilation is the same as the ratio of the temperature of the neutrinos and the photons in the current Regime 2. To find this ratio, we assume that the entropy of the universe was approximately conserved by the electron–positron annihilation. Then using
:
where is the ''effective number of degrees of freedom'' and is the plasma or photon temperature. Once reactions cease, the entropy should remain approximately "stuck" for all temperatures below the cut-off temperature, and we find that
:
Here denotes the lowest temperature where pair production
Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton. Pair production often refers specific ...
and annihilation
In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total energy a ...
were in equilibrium; and denotes the temperature after the temperature fell below the regime-shift temperature , after the remaining, but no longer refreshed, electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
–positron
The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...
pairs had annihilated
The eighth season of the television series, '' Law & Order: Special Victims Unit'' premiered September 19, 2006 and ended May 22, 2007 on NBC. The series remained in its 10pm/9c Tuesday timeslot. With the introduction of a new partner for Detecti ...
and contributed to the total photon energy. The related temperatures and are the simultaneous temperatures of the photons () and neutrinos () respectively, whose ratio stays "stuck" at the same value indefinitely, after
The factor is determined by a sum, based on the particle species engaged in the original equilibrium reaction:
: + 2 for each photon (or other massless bosons
In particle physics, a boson ( ) is a subatomic particle whose spin quantum number has an integer value (0,1,2 ...). Bosons form one of the two fundamental classes of subatomic particle, the other being fermions, which have odd half-integer ...
, if any).[
: + for each electron, positron, or other ]fermion
In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: spin , spin , etc. In addition, these particles obey the Pauli exclusion principle. Fermions include all quarks an ...
.[
Whereas the factor is simply 2, since the present regime only concerns photons, in thermal equilibrium with at most themselves.][
]
So
:
Since the cosmic photon background temperature at present has cooled to [
] it follows that the neutrino background temperature is currently
:
The above discussion is technically valid for massless neutrinos, which are always relativistic. For neutrinos with a non-zero rest mass, at low temperature where the neutrinos become non-relativistic, a description in terms of a temperature is not appropriate. In other words, when the neutrinos' thermal energy ( is the Boltzmann constant
The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constant, ...
) falls below the rest mass energy in a low-temperature case one should instead speak of the neutrinos' collective energy ''density'', which remains both relevant and well-defined.
Indirect evidence for the CB
Relativistic neutrinos contribute to the radiation energy density of the universe , typically parameterized in terms of the effective number of neutrino species :
:
where denotes the redshift
In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in f ...
. The first term in the square brackets is due to the CMB, the second comes from the CB. The Standard Model
The Standard Model of particle physics is the theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions - excluding gravity) in the universe and classifying a ...
with its three neutrino species predicts a value of ,[ including a small correction caused by a non-thermal distortion of the spectra during e× e ]annihilation
In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total energy a ...
. The radiation density had a major impact on various physical processes in the early universe, leaving potentially detectable imprints on measurable quantities, thus allowing us to infer
Inferences are steps in reasoning, moving from premises to logical consequences; etymologically, the word '' infer'' means to "carry forward". Inference is theoretically traditionally divided into deduction and induction, a distinction that in ...
the value of from observations.
Big Bang nucleosynthesis
Due to its effect on the expansion rate of the universe during Big Bang nucleosynthesis
In physical cosmology, Big Bang nucleosynthesis (abbreviated BBN, also known as primordial nucleosynthesis) is the production of nuclei other than those of the lightest isotope of hydrogen (hydrogen-1, 1H, having a single proton as a nucleus) du ...
(BBN), the theoretical expectations for the primordial abundances of light elements depend on Astrophysical measurements of the primordial and abundances lead to a value of = at 68% c.l.,[ in very good agreement with the Standard Model expectation.
]
CMB anisotropies and structure formation
The presence of the CB affects the evolution of CMB anisotropies as well as the growth of matter perturbations in two ways: Due to its contribution to the radiation density of the universe (which determines for instance the time of matter–radiation equality), and due to the neutrinos' anisotropic stress which dampens the acoustic oscillations of the spectra. Additionally, free-streaming
In astronomy, a free streaming particle, often a photon, is one that propagates through a medium without scattering.
Use in defining surfaces
Defining an exact surface for an object such as the Sun is made difficult by the diffuse nature of matt ...
massive neutrinos suppress the growth of structure on small scales. The WMAP
The Wilkinson Microwave Anisotropy Probe (WMAP), originally known as the Microwave Anisotropy Probe (MAP and Explorer 80), was a NASA spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic mic ...
spacecraft's five-year data combined with type Ia
A Type Ia supernova (read: "type one-A") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white ...
supernova
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...
data and information about the baryon acoustic oscillation
In cosmology, baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter (normal matter) of the universe, caused by Acoustics, acoustic density waves in the primordial plasma of the early universe. In the ...
scale yielded = at 68% c.l.,[ providing an independent confirmation of the BBN constraints. The ]Planck spacecraft
''Planck'' was a space observatory operated by the European Space Agency (ESA) from 2009 to 2013, which mapped the anisotropies of the cosmic microwave background (CMB) at microwave and infrared frequencies, with high sensitivity and small angu ...
collaboration has published the tightest bound to date on the effective number of neutrino species, at = .[
]
Indirect evidence from phase changes to the Cosmic Microwave Background (CMB)
Big Bang cosmology makes many predictions about the CB, and there is very strong indirect evidence that the cosmic neutrino background exists, both from Big Bang nucleosynthesis
In physical cosmology, Big Bang nucleosynthesis (abbreviated BBN, also known as primordial nucleosynthesis) is the production of nuclei other than those of the lightest isotope of hydrogen (hydrogen-1, 1H, having a single proton as a nucleus) du ...
predictions of the helium abundance, and from anisotropies in the cosmic microwave background
In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...
. One of these predictions is that neutrinos will have left a subtle imprint on the cosmic microwave background (CMB). It is well known that the CMB has irregularities. Some of the CMB fluctuations were roughly regularly spaced, because of the effect of baryon acoustic oscillation
In cosmology, baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter (normal matter) of the universe, caused by Acoustics, acoustic density waves in the primordial plasma of the early universe. In the ...
. In theory, the decoupled neutrinos should have had a very slight effect on the phase
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
* Phase space, a mathematic ...
of the various CMB fluctuations.[
In 2015, it was reported that such shifts had been detected in the CMB. Moreover, the fluctuations corresponded to neutrinos of almost exactly the temperature predicted by ]Big Bang theory
The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
( compared to a prediction of 1.95 K), and exactly three types of neutrino, the same number of neutrino flavours currently predicted by the Standard Model
The Standard Model of particle physics is the theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions - excluding gravity) in the universe and classifying a ...
.[
]
Prospects for the direct detection of the CB
Confirmation of the existence of these relic neutrinos may only be possible by directly detecting them using experiments on Earth. This will be difficult as the neutrinos which make up the CB are non-relativistic, in addition to interacting only weakly with normal matter, and so any effect they have in a detector will be hard to identify. One proposed method of direct detection of the CB is to use capture of cosmic relic neutrinos on tritium
Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus o ...
i.e. 3H, leading to an induced form of beta decay
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
.[
The neutrinos of the CB would lead to the production of electrons via the reaction
:
while the main background comes from electrons produced via natural beta decay
:
These electrons would be detected by the experimental apparatus in order to measure the size of the CB. The latter source of electrons is far more numerous, however their maximum energy is smaller than the average energy of the CB-electrons by twice the average neutrino mass. Since this mass is tiny, of the order of a few eVs or less, such a detector must have an excellent energy resolution in order to separate the signal from the background. One such proposed experiment is called PTOLEMY, which will be made up of 100 g of tritium target.][ The detector should be ready by 2022.][
]
See also
*Cosmic background radiation
Cosmic background radiation is electromagnetic radiation from the Big Bang. The origin of this radiation depends on the region of the spectrum that is observed. One component is the cosmic microwave background. This component is redshifted pho ...
*Dark matter
Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...
*Diffuse supernova neutrino background The diffuse supernova neutrino background (DSNB) is a theoretical population of neutrinos (and anti-neutrinos) cumulatively originating from all of the supernovae events which have occurred throughout the Universe.
Sources
An individual supernova w ...
*Gravitational wave background
The gravitational wave background (also GWB and stochastic background) is a random gravitational-wave signal potentially detectable by gravitational wave detection experiments. Since the background is supposed to be statistically random, it has ...
Notes
References
{{Cosmology topics
Physical cosmology
Cosmic background radiation
Neutrino astronomy