# Lorentz invariance violation in the neutrino sector: a joint analysis from big bang nucleosynthesis and the cosmic microwave background

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## Abstract

We investigate constraints on Lorentz invariance violation in the neutrino sector from a joint analysis of big bang nucleosynthesis and the cosmic microwave background. The effect of Lorentz invariance violation during the epoch of big bang nucleosynthesis changes the predicted helium-4 abundance, which influences the power spectrum of the cosmic microwave background at the recombination epoch. In combination with the latest measurement of the primordial helium-4 abundance, the Planck 2015 data of the cosmic microwave background anisotropies give a strong constraint on the deformation parameter since adding the primordial helium measurement breaks the degeneracy between the deformation parameter and the physical dark matter density.

## 1 Introduction

Neutrino oscillations have shown that there are small but nonzero mass squared differences between three neutrino mass eigenstates (see Ref. [1] and the references therein), which imply the existence of physics beyond the standard model of particle physics. However, neutrino oscillation experiments cannot tell us the overall mass scale of neutrinos. Fortunately, cosmology provides a promising way to determine or constrain the total mass of neutrinos by the gravitational effects of massive neutrinos which can significantly change the CMB power spectrum due to lensing [2, 3, 4, 5] and the formation of large-scale structures [6, 7], therefore, alter the cosmic microwave background (CMB) anisotropies and the large-scale structure distribution of matter (see [8] for a review). Recent Planck 2015 data combined with some low-redshift data give an upper limit on the total mass of neutrinos at \(95\%\) confidence level, \(\Sigma m_\nu <0.19\) eV [5].

Another possible signal of new physics is Lorentz invariance violation in the neutrino sector. The observed neutrino oscillations may originate from a combination of effects involving neutrino masses and Lorentz invariance violation [9, 10, 11, 12, 13, 14]. Lorentz invariance is a fundamental symmetry in the standard model of particle physics. Although present experiments have confirmed Lorentz invariance to a good precision [15], it may be broken in the early Universe when energies approach the Planck scale. The standard model itself is believed to be a low-energy effective theory of an underlying unified theory. Lorentz invariance violation has been explored in quantum gravity [16], loop quantum gravity [17], non-commutative field theory [18], and doubly special relativity theory [19]. Various searches for Lorentz invariance violation have been performed with a wide range of systems [20, 21].

The cosmological consequences of neutrinos with the Coleman–Glashow type [10] dispersion relation have been studied in Ref. [22]. As can be expected, the Lorentz-violating term affects not only the evolution of the cosmological background but also the behavior of the neutrino perturbations. The former changes the expansion rate prior to and during the epoch of photon–baryon decoupling, which alters heights of the first and second peaks of the CMB temperature power spectrum, while the latter alters the shape of the CMB power spectrum by changing neutrino propagation. Since these two effects can be distinguished from a change in the total mass of neutrinos or in the effective number of extra relativistic species, CMB data have been proposed as a probe of Lorentz invariance violation in the neutrino sector.

Moreover, since the Lorentz-violating term influences the abundances of the light elements by altering the energy density of the Universe and weak reaction rates prior to and during the Big bang nucleosynthesis (BBN) epoch, BBN provides a promising probe of Lorentz invariance violation in the neutrino sector in the early Universe [23]. In particular the BBN-predicted abundance of helium-4 is very sensitive to the Lorentz-violating term. It is well known that the primordial helium-4 abundance plays a non-negligible role at the recombination epoch because it has an influence on the number density of free electron. Therefore, Lorentz invariance violation affects the CMB power spectrum by changing the BBN-predicted abundance of helium-4. A reasonable way to test Lorentz invariance violation in the neutrino sector with CMB data is to take account of the BBN-predicted helium-4 abundance as a prior rather than fixing the helium-4 abundance.

In this paper we investigate constraints on Lorentz invariance violation in the neutrino sector from a joint analysis of BBN and CMB. In combination with the latest measurement of the primordial helium-4 abundance, the Planck 2015 data of the CMB anisotropies give a strong constraint on the deformation parameter. Adding the primordial helium measurement can break effectively the degeneracy between the deformation parameter and the physical dark matter density.

This paper is organized as follows. In Sect. 2 we parameterize Lorentz invariance violation in the neutrino sector. In Sect. 3, we derive the weak reaction rates in the Lorentz-violating extension of the standard model and calculate the BBN prediction of the helium-4 abundance. In Sect. 4, we derive the Boltzmann equation for neutrinos in the synchronous gauge and calculate the CMB power spectrum. In Sect. 5, we place constraints on the deformation parameter using the Planck 2015 data in combination with the latest measurement of the primordial helium-4 abundance. Section 6 is devoted to our conclusions.

## 2 Deformed dispersion relation

*E*is the neutrino energy,

*m*the neutrino mass, \(p=\left( p^ip_i\right) ^{1/2}\) the magnitude of the 3-momentum, and \(\xi \) the deformation parameter characterizing the Lorentz symmetry violation.

*a*, the number density \(n_\nu \), energy density \(\rho _\nu \) and pressure \(P_\nu \) for neutrinos with (1) are given by

*q*the magnitude of the comoving 3-momentum, and \(\epsilon =\sqrt{m^2a^2+\left( 1+\xi \right) q^2}\) the comoving energy.

## 3 BBN prediction

The abundances of the light elements produced during the BBN epoch depend on the competition between the nuclear and weak reaction rates and the expansion rate of the Universe. In the standard cosmological scenario and in the framework of the electroweak standard model, the dynamics of this phase is controlled by only one free parameter, the baryon to photon number density.

*N*species of nuclides whose abundances \(X_i\) are the number densities \(n_i\) normalized with respect to the baryon number density \(n_B\),

*i*involved in the reaction.

In order to calculate the abundance of the light elements produced during the BBN epoch, we modified the publicly available PArthENoPE code [26] to appropriately incorporate the Lorentz-violating term in the neutrino sector. As shown in [23], the BBN-predicted abundance of helium-4 is sensitive to the deformed parameter \(\xi \) due to the fact that it is mainly determined by the neutron-to-proton ratio, which is related to the weak reaction rate, neutrino number density and expansion rate of the Universe. In our analysis we shall focus on the helium-4 abundance that influences the CMB power spectrum.

## 4 CMB power spectrum

*h*and \(\eta \) are the two scalar modes of the metric perturbations in the Fourier space

*k*. To avoid the reflections from high-\(\ell \) equations by simply set \(\Psi _\ell =0\) for \(\ell >\ell _\mathrm{max}\), such a Boltzmann hierarchy is effectively truncated by adopting the following scheme [28]:

*q*dependence in the neutrino distribution function so that

*q*,

*C*is a dimensionless constant determined by the amplitude of the fluctuations from inflation, \(R_\nu \equiv \rho _\nu /(\rho _\gamma +\rho _\nu )\), and \(\omega \equiv \Omega _m H_0/\sqrt{\Omega _\gamma +\Omega _\nu }\), which corresponds to the matter contribution to the total energy density of the Universe. In the radiation-dominated era the massive neutrinos are relativistic. The initial condition of \(\Psi _0\) for massive neutrinos is related to \(\delta _\nu \). Then, using Eqs. (19) and ignoring the mass terms in the differential equations of high-order moment, we get the initial conditions of \(\Psi _\ell \) for massive neutrinos:

Mean values and marginalized 68% confidence level for the deformation parameter and other cosmological parameters, derived from the Planck data in combination with baryon acoustic oscillation data, the JLA sample of Type Ia supernovae and the measurement of the helium-4 mass fraction. For comparison with BBN consistency, fixing \(Y_\mathrm{p}=0.24\) is also considered

Model | Planck \(2015+\mathrm{JLA}+\mathrm{BAO}\) | Planck \(2015+\mathrm{JLA}+\mathrm{BAO}+Y_\mathrm{p}\) | |
---|---|---|---|

\(Y_\mathrm{p}=0.24\) (fixed) | BBN consistency | BBN consistency | |

\(\Omega _{\mathrm {b}}h^2\) | \(0.02127\pm 0.00046\) | \(0.02212\pm 0.00025\) | \(0.02227\pm 0.00021\) |

\(\Omega _{\mathrm {c}} h^2\) | \(0.1124\pm 0.0061\) | \(0.1163\pm 0.0040\) | \(0.1197\pm 0.0018\) |

\(100\theta _{\mathrm {MC}}\) | \(1.04181\pm 0.00098\) | \(1.04173\pm 0.00101\) | \(1.04090\pm 0.00050\) |

\(\tau _\mathrm{re}\) | \(0.071\pm 0.018\) | \(0.076\pm 0.018\) | \(0.077\pm 0.018\) |

\(n_\mathrm {s}\) | \(0.9583\pm 0.0069\) | \(0.9679\pm 0.0049\) | \(0.9659\pm 0.0044\) |

\(\ln (10^{10} A_\mathrm {s})\) | \(3.064\pm 0.038\) | \(3.084\pm 0.035\) | \(3.087\pm 0.035\) |

\(H_{\mathrm 0}\) (km \(\mathrm s^{-1}\) \(\mathrm Mpc^{-1}\)) | \(65.92\pm 1.76\) | \(67.25\pm 1.00\) | \(68.01\pm 0.63\) |

\(\sigma _\mathrm {8}\) | \(0.840\pm 0.014\) | \(0.846\pm 0.016\) | \(0.841\pm 0.015\) |

Age (Gyr) | \(14.191\pm 0.35\) | \(13.937\pm 0.19\) | \(13.767\pm 0.068\) |

\(\Omega _\mathrm {\Lambda }\) | \(0.691\pm 0.008\) | \(0.694\pm 0.007\) | \(0.693\pm 0.007\) |

\(\Omega _\mathrm {m}\) | \(0.309\pm 0.008\) | \(0.306\pm 0.007\) | \(0.307\pm 0.007\) |

\(\xi \) | \(0.101\pm 0.091\) | \(0.042\pm 0.049\) | \(-0.002\pm 0.014\) |

In order to compute the theoretical CMB power spectrum, we modified the Boltzmann CAMB code [29] to appropriately incorporate the Lorentz-violating term in the neutrino sector. As pointed out in [22], the effects of the Lorentz-violating term on the CMB power spectrum are distinguished from a change in the total mass of neutrinos or in the effective number of extra relativistic species [30, 31, 32, 33, 34]. Therefore, the measurements of the CMB anisotropies provide a cosmological probe of Lorentz invariance violation in the neutrino sector. Moreover, we calculate the matter power spectrum today for \(\xi =-0.1, 0, 0.1\). From Fig. 2 we find that increasing \(\xi \) can enhance the matter power spectrum at small scales. It is well known that variances in the helium-4 abundance modify the density of free electrons between helium and hydrogen recombination and therefore influence the CMB power spectrum. However, the effects of the Lorentz-violating term on the BBN-predicted helium-4 abundance were not considered in [22]. In the next section, we shall put constraints on the deformation parameter from a joint analysis of BBN and CMB.

## 5 Joint analysis from BBN and CMB

*h*is the dimensionless Hubble parameter defined by \(H_0=100h\) km \(\mathrm s^{-1}\) \(\mathrm Mpc^{-1}\), \(\Omega _bh^2\) and \(\Omega _ch^2\) are the physical baryon and dark matter densities relative to the critical density, \(\theta _\mathrm {MC}\) is an approximation to the ratio of the sound horizon to the angular diameter distance at the photon decoupling, \(\tau _\mathrm{re}\) is the reionization optical depth, \(n_s\) and \(A_s\) are the spectral index and amplitude of the primordial curvature perturbations at the pivot scale \(k_0=0.05\) Mpc\(^{-1}\).

We use the recently released Planck 2015 likelihood code and data, including the Planck low-\(\ell \) likelihood at multipoles \(2\le \ell \le 29\) and Planck high-\(\ell \) likelihood at multipoles \(\ell \ge 30\) based on pseudo-\(C_\ell \) estimators [5]. The former uses foreground-cleaned LFI 70 GHz polarization maps together with the temperature map obtained from the Planck 30 to 353 GHz channels by the Commander component separation algorithm over 94% of the sky. The latter uses 100, 143, and 217 GHz half-mission cross-power spectra, avoiding the galactic plane as well as the brightest point sources and the regions where the CO emission is the strongest. “Planck 2015” denotes the combination of the low-\(\ell \) temperature-polarization likelihood and the high-\(\ell \) temperature likelihood.

We use the “Joint Light-curve Analysis” (JLA) sample of type Ia supernovae, which is constructed from the SNLS and SDSS supernova data, together with several samples of low-redshift supernovae [36]. Baryon acoustic oscillation (BAO) measurements are another important astrophysical data set, which are powerful to break parameter degeneracies from CMB measurements. We use BAO measurements of \(D_V/r_\mathrm{drag}\) from the 6dFGS at \(z_\mathrm{eff}=0.106\), SDSS Main Galaxy Sample at \(z_\mathrm{eff}=0.15\), BOSS LOWZ at \(z_\mathrm{eff}=0.32\), and BOSS CMASS at \(z_\mathrm{eff}=0.57\) [5]. Here \(D_V\) is the effective distance measure for angular diameter distance, \(r_\mathrm{drag}\) is the comoving sound horizon at the end of the baryon drag epoch and \(z_\mathrm{eff}\) is the effective redshift. In our analysis we also use the helium-4 mass fraction measurement of \(Y_\mathrm{p}=0.2449\pm 0.0040\), derived from helium and hydrogen emission lines from metal-poor extragalactic HII regions and from a regression to zero metallicity [37].

In Table 1 we list our results. With the combined data of Planck \(2015+\mathrm{JLA}+\mathrm{BAO}\), the deformation parameter is estimated to be \(\xi =0.042\pm 0.049\) if the helium-4 abundance is set from the BBN prediction. For comparison, if the helium-4 abundance is not varied independently of other parameters, with the same data set we find \(\xi =0.101\pm 0.091\), which indicates that the constraint of \(\xi \) is not improved compared to the result derived in [22] from the 7-year WMAP [38] in combination with lower-redshift measurements of the expansion rate. The reason is that the estimated values of cosmological parameters are biased by fixing \(Y_\mathrm{p}=0.24\). The gray and red regions in Fig. 3 show 68 and 95% contours in the \(\xi -\Omega _c h^2\) plane setting \(Y_\mathrm{p}=0.24\), derived from Planck 2015+JLA+BAO and from \(\mathrm{WMAP7}+\mathrm{H}_0+\mathrm{BAO}\) [22], respectively. We see that the Planck 2015 data marginally favor a positive \(\xi \) compared to the 7-year WMAP data. The addition of the latest measurement of the helium-4 abundance leads to a strong constraint on the deformation parameter \(\xi =-0.002\pm 0.014\). As shown in Fig. 3, adding the \(Y_\mathrm{p}\) data breaks the degeneracy between the deformation parameter and the physical dark matter density. This result shows that no signal of Lorentz invariant violation is detected by the joint analysis of CMB and BBN. Figure 4 shows the constraints in the \(\xi -\Omega _bh^2\) plane, \(\xi -H_0\) plane, \(\xi -n_s\) plane and \(\xi -A_s\) plane from Planck \(2015+\mathrm{JLA}+\mathrm{BAO}\).

## 6 Conclusions and discussions

We have studied the cosmological constraints on Lorentz invariance violation in the neutrino sector by a joint analysis of CMB and BBN. Instead of fixing the value of the helium-4 abundance, we have applied the BBN prediction of the helium-4 abundance determined by \(\xi \) and \(\Omega _b h^2\) to calculate the CMB power spectra. Using the Planck 2015 data in combination with the JLA sample of type Ia supernovae and the BAO feature, we have put a constraint on the deformation parameter. Adding the measurement of the helium-4 abundance breaks the degeneracy between the deformation parameter and the physical dark matter density and so improves the constraint by a factor 3.

Compared to cosmological bounds on the Lorentz-violating coefficient, observations of high-energy astrophysical neutrinos give stronger constraints. As listed in Table XIII of Ref. [13], the coefficient is constrained down to \(\mathcal{O}(10^{-9})\) from the time-of-flight measurements, under the assumption that neutrino oscillations are negligible. Cohen and Glashow have argued that the observation of neutrinos with energies in excess of 100 TeV and a baseline of at least 500 km allows us to deduce that the Lorentz-violating parameter is less than \(\mathcal{O}(10^{-11})\) [39]. The present work provides a new way to probe the signal of Lorentz invariance violation in the early Universe, which can in principle be used to constrain \(\xi \) in the sterile neutrino sector [40, 41]. Although present cosmological data are too weak to yield competitive constraints, future measurements of CMB and BBN offer prospects for placing stringent constraints.

## Notes

### Acknowledgements

Our numerical analysis was performed on the “Era” of Supercomputing Center, Computer Network Information Center of Chinese Academy of Sciences. This work is supported in part by the National Natural Science Foundation of China Grants Nos. 11575272, 11690021, 11690022, 11335012, 11375247, 11435006 and 91436107, in part by the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant Nos. XDB23030100 and XDB21010100, and by Key Research Program of Frontier Sciences, CAS.

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