Title: Induced Gravitational Waves as Cosmic Tracers of Leptogenesis
Authors: Marco Chianese, Guillem Domènech, Theodoros Papanikolaou, Rome Samanta and Ninetta Saviano
First Author Institution: Scuola Superiore Meridionale, Naples, Italy and the Istituto Nazionale Di Fisica Nucleare Sezione di Napoli, Naples, Italy
Status: Available on arXiv
An unsolved problem in physics is the asymmetry in the abundance of matter and antimatter in the Universe; why is there more ordinary matter? Antimatter particles are the ‘twins’ of all fundamental particles with the opposite quantum numbers, and have been produced and studied in cosmological accelerators (they are even produced by bananas), but generally our Universe is composed of ordinary matter. A cosmological explanation is necessary so that the amount of matter and antimatter we observe today does not end up being the same. The generic name for theoretical processes that can lead to a cosmological asymmetry in the number of leptons (such as electrons, neutrinos, and their antimatter counterparts) and thus potentially also baryons (protons, neutrons) is called leptogenesis; it could explain the asymmetry.
Leptogenesis is related to the mysterious neutrino particle, one of the more poorly understood particles in the Standard Model. Neutrinos are weird in various ways, but one of the weird things about them is that they have very tiny masses compared to other particles. For example, neutrinos are no greater than two-tenths of a million times the mass of an electron. The Standard Model expects neutrinos to be massless; however, the observation of neutrino oscillations confirmed that they have non-zero masses. The masses of particles are related to how strongly they interact with a particle called the Higgs boson, discovered in 2012 at CERN. Stronger interactions with the Higgs lead to larger masses, so the neutrino might interact very weakly with the Higgs. But for neutrinos, this leads to some issues in describing other processes in which neutrinos interact with Standard Model particles and obtain mass. Neutrinos are left-handed particles, which means their spin (s=½) component of angular momentum always points in the direction they travel; right-handed neutrinos, which have their spins pointing the other way, have never been observed (but with the caveat that neutrinos are already notoriously difficult to observe).
These issues could be dealt with in a theory called the Seesaw Mechanism (see some nice articles for some more background here and here) in which right-handed neutrinos exist. These would be extremely heavy particles that would almost ‘balance out’ the masses with Standard Model neutrinos. This would explain their tiny masses and additionally allow for processes to occur in the early Universe that lead to matter and antimatter symmetry.
The authors of today’s paper look at how gravitational waves (GWs) could help us find the answer to matter and antimatter symmetry. In their work, they find that simple models for the Seesaw Mechanism could produce induced gravitational waves (IGWs) in the early Universe. GWs are propagating distortions in the fabric of spacetime, and are a prediction of the theory of General Relativity, directly detected by LIGO observations of a binary black hole merger. They are often created by mergers of extremely massive objects like black holes and neutron stars, and are a product of the energy released in these events. However, they can be created in various processes that generate motion in masses and thus can occur from various different phenomena (see more info here; in particular, the quadrupole moment of the mass distribution must be non-zero). Additionally, the authors find that the frequency of the gravitational wave signals may contain information about the epoch at which leptogenesis may have occurred, which depends on the energy scale for leptogenesis. Thus, detecting gravitational waves can allow us to search for evidence for leptogenesis via the Seesaw Mechanism, or rule out parts of the parameter space of the model if a signal isn’t detected.
In the concordance cosmological model \(\Lambda \)CDM, the Universe undergoes a period in which the expansion of the Universe is dominated first by the energy density of radiation, then matter, and then finally Dark Energy. However, in the models the authors consider in this paper for the Seesaw Mechanism, there could have been a very early period of matter domination, in which small perturbations grew to form large-scale structures. If this period lasted long enough, it could have produced IGWs, which could be detected. In this work, they determine the parameter space for which this epoch would have lasted long enough to produce a detectable IGW signal.
If this early matter domination period occurred, it is possible to determine a lower bound for the amplitude of the primordial matter fluctuations in the early Universe that would lead to the production of a IGW signal; this amplitude is a consequence of the initial conditions in the Universe and is usually denoted \(A_s \). The authors rely on numerical simulations from previous work in order to characterize the spectrum of resulting GWs, and are then able to relate this spectrum to the energy scale for leptogenesis and the primordial amplitude \(A_s\). The peak GW spectrum amplitude and frequency can be related to the leptogenesis energy scale. This result indicates that a full IGW spectrum would provide clear indications of the Seesaw Mechanism and thus leptogenesis.
Figure 1 below shows a plot of the plane of the GW spectrum amplitude vs frequency, with the shaded regions showing the sensitivity of various experiments for GW detection. The ‘BP’ points are benchmark points, showing various values for the expected GW spectrum at the peak frequency for the Seesaw Mechanism model with different values considered in the models parameter space; this demonstrates that the different parts of the parameter space of the model can be well distinguished because it leads to significantly different peak frequencies and amplitudes in the GW spectrum. The first four BPs are the more conservative scenarios for the model.
Summary
The authors are able to relate a detectable spectrum of IGWs in their leptogenesis model to the energy scale at which leptogenesis occurred. Observations of IGWs could thus allow one to determine or rule out part of the parameter space for such a model, allowing us to know more about how neutrinos get masses in a manner different manner the Standard Model expectation, and even explain how there is an asymmetry in matter and antimatter in the Universe. This motivates the development of experiments like LISA, which will try to measure gravitational wave signals with a space-based detector, and more research into such an exciting new field of multimessenger astrophysics.
Edited by Will Golay
Image credit: Artist’s impression of a gravitational wave event. NOIRLab, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
