New physics at the PTA frontier

External collaborators

  • NANOGrav collaboration
  • Andrea Mitridate, DESY Hamburg, Germany
  • Ken Olum, Tufts University, Massachusetts, USA
  • Stephen Taylor, Vanderbilt University, Tennessee, USA
  • ...


  • 2306.16377: PTArcade
  • 2306.16219: The NANOGrav 15-year Data Set: Search for Signals from New Physics


We are involved in the work of the NANOGrav collaboration, the North American Nanohertz Observatory for Gravitational Waves. NANOGrav is one of several existing pulsar timing arrays (PTAs) and as such part of IPTA, the International Pulsar Timing Array. NANOGrav and PTAs in general search for gravitational waves at nanohertz frequencies by monitoring an array of pulsars in the Milky Way: a background of gravitational waves permeating our Galaxy will stretch and squeeze the spatial distances between NANOGrav's radio telescopes on Earth and the pulsars in the array and will thus cause tiny distortions in the arrival times of the individual pulses. Einstein's theory of general relativity allows us to precisely describe this effect on the pulsar timing data and points to a unique signature in the cross-correlated data for pairs of pulsars that would be the telltale sign of gravitational waves. Compelling evidence for a signal exhibiting exactly this type of correlation pattern has recently been reported by NANOGrav and other PTA collaborations.

Within NANOGrav, we are responsible for the search for signals from new physics beyond the Standard Model of particle physics. These efforts are motivated by the fact that numerous extensions of the Standard Model predict the generation of a stochastic gravitational-wave background (GWB) in the early Universe. Such a primordial GWB, if it exists, would correspond to the gravitational analog of the CMB, the cosmic microwave background, which is composed of relic photons from the early Universe that traveled freely through the cosmos since roughly 380,000 years after the Big Bang. In our work, we analyze the most recent NANOGrav data addressing two key questions:

  1. Model fits: Given a model of new physics predicting the existence of a GWB signal from the early Universe, how well does it allow to fit the most recent NANOGrav data? What are the preferred regions in the model parameter space? And how does the interpretation of the NANOGrav data in terms of this model compare to the interpretation in terms of other models? Notably, does the cosmological model under consideration provide a better fit of the data than the astrophysical interpretation in terms of inspiraling supermassive black-hole binaries?
  2. Model constraints: Given a model of new physics predicting the existence of a GWB signal from the early Universe, which regions of parameter space can be safely ruled and declared unviable in view of the most recent NANOGrav data? That is, independently of the origin of any signal in the data, which parameter values predict a GWB signal that would be clearly in conflict with the observed data, e.g., because it is too strong and should have been seen in the data long ago?

In addressing these two questions, we explore a new frontier, the PTA frontier, in the hunt for new physics that is complementary to other frontiers of particle physics, such as the energy and intensity frontiers. We use the NANOGrav data in particular to search for clues about the physics of the early Universe at very early times and hence extremely high energies that are unattainable in terrestrial laboratory experiments. This includes ideas related to grand unification and even string theory that may be responsible for a GWB signal from the early Universe. Specifically, we have a strong interest in primordial gravitational waves from cosmic inflation, enhanced scalar perturbations, cosmological phase transitions, and cosmic defects such as cosmic strings and domain walls. While the focus of research area (1) "New physics at the PTA frontier" is on data analysis and phenomenology, we also work on theoretical questions regarding these gravitational-wave sources, which is the focus of research area (2) "Primordial gravitational waves".

Primordial gravitational waves

External collaborators

  • Wilfried Buchmüller, DESY Hamburg, Germany
  • Valerie Domcke, CERN, Switzerland
  • ...


  • 2207.03510: Gravitational Waves from Current-Carrying Cosmic Strings


Observations of our Universe in the electromagnetic spectrum can only extend up to the cosmic microwave background, the ultimate horizon at a distance of roughly 15 Gpc from beyond which no photon signal can reach us. To peak beyond this cosmic veil and probe the earliest moments of the cosmic evolution, we must rely on other messengers from the early Universe. A prime candidate in this respect are primordial gravitational waves, i.e., gravitational waves generated during the stage of cosmic inflation or during the Hot Big Bang, which propagate nearly unperturbed through the early Universe after their production and which hence, if registered in our detectors, can provide us with a pristine image of the birth of our Universe.

In our work, we are interested in various physical processes that can give rise to the production of primordial gravitational waves. A common feature of these processes is that all of them require new physics beyond the Standard Model of particle physics. In this sense, any search for gravitational waves from the early Universe is at the same time a search for new physics at very high energies. Our research activities focus on three possible sources of primordial gravitational waves:

  1. Cosmic strings: Cosmic strings are effectively one-dimensional topological defects that can be produced in the early Universe in consequence of symmetry-breaking phase transitions. They are closely related to vortex solutions in condensed-matter systems, such as Abrikosov vortices in superconductors. Once produced in the early Universe, cosmic strings organize themselves in a network consisting of long (potentially infinitely long) strings and smaller closed loops. Each individual loop radiates off gravitational waves, which in superposition results in a stochastic gravitational-wave background (GWB) signal. In our work, we are interested in (A) a better theoretical description of this signal as well as in (B) more realistic microscopic cosmic-string models that go beyond the simplest description in terms of one-dimensional Nambu-Goto strings. In the latter case, we especially focus on current-carrying cosmic strings and metastable cosmic strings, which are well-motivated by certain grand unified scenarios.
  2. Phase transitions: Cosmological phase transitions correspond to changes in the vacuum configuration of our Universe triggered by the decreasing temperature during the Hot Big Bang. In everyday life, similar transitions can e.g. be observed when supercooled water (maybe in a bottle you forgot in the freezer) suddenly turns from liquid water to solid ice. In the context of cosmology, strong first-order phase transitions are an attractive source of primordial gravitational waves. During the phase transition, bubbles filled by the new vacuum configuration expand in the ambient old vacuum configuration. When these bubbles collide and percolate, energy stored in the bubble walls is released in the form of gravitational waves. Similarly, the true-vacuum bubbles sweeping through the hot plasma can excite sound waves and ultimately turbulent plasma motion, both of which can also source gravitational waves. In our work, we study (A) the experimental sensitivity of upcoming gravitational-wave experiments to signals induced by cosmological phase transitions; we contribute (B) to refinements of the theoretical description of the expected signal; and we study (C) well-defined benchmark models that can give to a cosmological phase transition, such as the B-L extensions of the Standard Model, which promotes baryon-minus-lepton number B-L to the charge of a new Abelian gauge symmetry.
  3. Inflation: Cosmic inflation denotes a stage of exponential expansion in the early Universe prior to the Hot Big Bang. Today, the paradigm of cosmic inflation is a central element of inflationary cosmology, which incorporates a dynamical explanation of the initial conditions of old-school Big Bang cosmology. Indeed, cosmic inflation can explain the size, homogeneity, and isotropy of our Universe on cosmological scales, while at the same time, it sources small primordial density fluctuations that later act as the seeds of the nonlinear large-scale structure that we observe in the present Universe. In addition, inflation also stretches primordial tensor perturbations to super-horizon size, which then re-enter the Hubble horizon during the Hot Big Bang after inflation in the form of gravitational waves. The simplest models of inflation, standard single-field slow-roll inflation, only predict a weak gravitational-wave signal that is hard, if not impossible, to detect in pulsar timing arrays or interferometer experiments. The situation, however, drastically changes in many nonminimal models of inflation that predict a blue-tilted primordial gravitational-wave spectrum well within the reach of upcoming experiments. In our work, we are particularly interested in this class of models. In fact, research area (3) "Axion cosmology" is centered around models of axion inflation, an intriguing class of inflationary models with exciting predictions for the spectrum of primordial gravitational waves.

Axion cosmology

External collaborators

  • Eduard Gorbar, University of Kyiv, Ukraine
  • Marco Peloso, University of Padua, Italy
  • Lorenzo Sorbo, University of Massachusetts Amherst, Massachusetts, USA
  • Stanislav Vilchinskii, University of Kyiv, Ukraine
  • ...


  • 2206.01129: Axion Dark Matter from Frictional Misalignment


Axions—pseudo-Nambu–Goldstone bosons associated with the spontaneous breaking of anomalously violated global Abelian symmetries—are key players in the hunt for new physics. Originally proposed in the context of quantum chromodynamics (QCD), they may solve the strong CP problem in QCD, contribute to the relic density of dark matter, provide a form of quintessence responsible for the late-time acceleration of the Universe, or result in cosmic birefringence in the polarization of the cosmic microwave background. They are, moreover, well motivated from a theoretical perspective for several reasons:

  1. Axions are ubiquitous in field-theoretic and string-theoretic models of physics beyond the Standard Model.
  2. Axions are pseudoscalars; after the discovery of the Higgs boson (a scalar), the discovery of an elementary pseudoscalar is still pending.
  3. Many scenarios of new physics are motivated by new symmetry principles (e.g., supersymmetry). Axions come with their own unique symmetry principle: an approximate shift symmetry.
  4. The symmetry properties of axion ensure that they couple in a characteristic fashion to fermions and gauge bosons, which has interesting phenomenological implications.

The last point implies that the coupling between axions and other particles are typically suppressed by a large energy scale, the axion decay constant. This places axions at a frontier of particle physics where new physics is not necessarily related to large masses of new particles but rather to the feeble interaction with the Standard Model. In addition, the penultimate point renders axions interesting candidates for the inflaton, the (pseudo)scalar field responsible for driving the stage of cosmic inflation in the early Universe. Identifying the inflaton with an axion results in "natural inflation", where the flatness of the inflaton potential (required in order to realize slow-roll inflation) is protected by the approximate shift symmetry. On top, it is reasonable to assume that an axion inflaton field is coupled to the vector fields of an Abelian or non-Abelian gauge sector. Axion inflation thus represents a well-motivated example of a nonminimal model of inflation that goes beyond the usual toy models where the inflaton sector merely consists of one real scalar field and nothing else. In this sense, axion inflation coupled to visible or dark gauge fields may be regarded as a step towards a more realistic microscopic description of the inflaton sector, featuring more degrees of freedom and a richer phenomenology. Axion inflation can especially leave behind several primordial relics, such as primordial magnetic fields, primordial gravitational waves, and primordial black holes, all of which define important targets for future observations. Because of this, axion inflation provides an interesting playground to explore new theoretical tools and defines a valuable benchmark scenario for upcoming observations.

In our work, we are interested in various aspects of axion physics in the early Universe, including (A) relaxion models addressing the hierarchy problem of the Standard Model, (B) axion dark matter and its various production mechanisms in the early Universe, and (C) the rich phenomenology of axion inflation. In the latter case, we focus on the development of new theoretical tools that allow us to describe the highly nonlinear dynamics of axion inflation and the application of these tools to specific realizations of axion inflation, including the generation of primordial gravitational waves, primordial black holes, etc. A scenario of particular relevance in this respect is axion inflation coupled to the Standard Model hypercharge sector, which offers the possibility to produce primordial (hyper)magnetic fields during inflation. Axion inflation is in this case then responsible for primordial (hyper)magnetogenesis, which in turn can have interesting implications for the generation of the baryon asymmetry of the Universe, which is a central topic of research area (4) "Early Universe".

Early Universe

External collaborators

  • Valerie Domcke, CERN, Switzerland
  • Kohei Kamada, University of Tokyo, Japan
  • Kyohei Mukaida, KEK, Japan
  • Masaki Yamada, Tohoku University, Japan
  • ...


  • 2304.06612: Chiral Magnetohydrodynamics with Zero Total Chirality
  • 2210.06412: Wash-In Leptogenesis after Axion Inflation
  • 2208.03237: A New Constraint on Primordial Lepton Flavour Asymmetries


In the first moments of its existence, the Universe was filled by a hot dense plasma of relativistic particles scattering with each other at extremely high energies. The early Universe can hence be thought of as a particle physics laboratory that allows us to ponder over the interactions of elementary particles at energies far beyond the reach of terrestrial experiments. To describe the dynamics of the Hot Big Bang and the preceding stage of cosmic inflation, it is thus necessary to follow an interdisciplinary approach combining techniques from particle physics and cosmology. This is accomplished in the discipline of particle cosmology, which aims at a better understanding of the early Universe based on the methods of quantum field theory and string theory. A central ambition of particle cosmology is, in particular, to shine more light on new physics beyond the Standard Model that may have governed the evolution of the early Universe, including phenomena such as:

  1. Cosmic inflation: The stage of exponential expansion prior to the Hot Big Bang responsible for setting the initial conditions of the subsequent cosmological evolution (homogeneity, isotropy, primordial perturbations, etc.).
  2. Preheating and reheating: The transition between cosmic inflation and the Hot Big Bang, i.e., the stage in the expansion history dominated by a thermal bath composed of relativistic radiation.
  3. Baryogenesis: The dynamical generation of the primordial asymmetry between matter and antimatter in our Universe starting from matter-antimatter-symmetric initial conditions after cosmic inflation.
  4. Dark matter: The mysterious form of matter that reveals its existence through gravitational effects in countless cosmological observations but which otherwise seems to lack any appreciable interaction with Standard Model particles.
  5. Dark energy: The driving force behind the accelerated expansion of the present Universe, which is probably either related to the energy density of the vacuum (i.e., a cosmological constant) or a dynamical scalar field (i.e., quintessence).

While we are broadly interested in all of these topics among several others, questions related to the generation of the baryon asymmetry of the Universe play a particularly important role in our research. In this context, we study a broad range of baryogenesis scenarios, ranging from baryogenesis in grand unified theories over baryogenesis from helical hypermagnetic fields to different models of baryogenesis via leptogenesis. Here, the last scenario refers to the idea that the primordial asymmetry between matter and antimatter is in fact first created in the lepton sector and then transferred to the baryon sector by nonperturbative Standard Model processes in the early Universe. Many leptogenesis models involve heavy sterile neutrinos, which establishes an intriguing connection between early-Universe cosmology and neutrino physics.  

In our work, we explore the phenomenology of existing variants of leptogenesis, including models of thermal, nonthermal, resonant, and spontaneous leptogenesis, and we propose new scenarios for the generation of the baryon asymmtry that come with their own unique phenomenology and predictions for upcoming experiments and observations. An example for the latter would be wash-in leptogenesis, a new leptogenesis scenario that we recently proposed and that generalizes thermal leptogenesis to arbitrary chemical background configurations in the early Universe. The mechanism of wash-in leptogenesis is representative of particle interactions in the early Universe that may collectively be referred to as Big Bang chemistry, the interplay between the chemical potentials of all particle species during the Hot Big Bang.

Big Bang chemistry is concerned with questions such as:

  1. How does one achieve a chemical composition of the primordial plasma featuring a baryon asymmetry of exactly the right size?
  2. What are the phenomenological implications if individual particle species have large chemical potentials in the early Universe?
  3. Can we constrain the maximally allowed size of individual chemical potentials in the early Universe based on other considerations?

The exploration of these questions is partially still in its early stages; the further development of Big Bang chemistry is guaranteed to keep us busy for many years to come.