Pulsar Timing Arrays: A Star-Way to New Physics

14 Aug 2023, 09:30 → 18 Aug 2023, 17:00 Europe/Berlin

2413/2-430 - MITP Seminar Room (MITP - Mainz Institute for Theoretical Physics, Johannes Gutenberg University Mainz)

The discovery of gravitational waves  from black hole and neutron star mergers has made a tremendous impact to contemporary astrophysics and in particular to the field of gravitational wave astronomy. This recent achievement has also drawn significant interest in related fields and here intertwined fields of cosmology and high-energy physics stand out. In this topical workshop we will focus on gravitational wave experiments that are sensitive in nHz region – Pulsar Timing Array Experiments (PTA). The main goal is to connect experts from several different fields working on gravitational waves in connection to PTAs and to stimulate  discussions and collaborations.

Monday, 14 August

09:30 Snacks

1 Welcome

2 Overview EPTA

The main objective of pulsar timing arrays (PTAs) is the regular monitoring of several tens of millisecond pulsars in the radio band in order to detect dynamic perturbations of the space-time. To date, pulsar networks are the only way to detect gravitational-wave (GW) radiation in the nanohertz frequency range. The most commonly considered source of metric perturbations at these frequencies is the GW background generated by the ensemble of supermassive black hole binaries, which are expected to be formed in the centres of galaxies. I will summarise the 25 years of work of the European pulsar timing array (EPTA), which collects pulsar data from the five largest radio telescopes in Europe. Details of the pulsar timing data processing, as well as the most up-to-date methods of astrophysical signal detection against correlated interference will be provided. At the end of the talk, I will discuss the signal and its characteristic features found in the data of all three regional PTAs (including EPTA) and its possible interpretation.

Speaker: Nataliya Porayko

pta_mainz_porayko.pdf

3 PTAs: where we are and where we are going

Speaker: Andrea Mitridate

mitp_2023.pdf

12:00 Lunch

4 Primordial gravitational waves in the nano-Hertz regime and PTA data -- towards solving the GW inverse problem

In recent years, several pulsar timing array collaborations have reported first hints for a stochastic gravitational wave background at nano-Hertz frequencies. Here we elaborate on the possibility that this signal comes from new physics that leads to the generation of a primordial stochastic gravitational wave background. We propose a set of simple but concrete models that can serve as benchmarks for gravitational waves sourced by cosmological phase transitions, domain wall networks, cosmic strings, axion dynamics, or large scalar fluctuations. These models are then confronted with pulsar timing data and with cosmological constraints. With only a limited number of free parameters per model, we are able to identify viable regions of parameter space and also make predictions for future astrophysical and laboratory tests that can help with model identification and discrimination.

Speaker: Eric Madge

14:30 Snacks

Tuesday, 15 August

09:30 Snacks

5 A Massive Black Hole Binary Interpretation to the GW Background

For decades a stochastic gravitational wave (GW) background in the nHz band has been theorized from binaries of (super)massive black holes (MBHs). The expected characteristics are broadly consistent with recent measurements from Pulsar Timing Arrays. In this talk, I will present the current state of predictions for MBH binary populations and their GW signatures. I will focus on realistic GW background spectra which can differ notably from the traditional, idealized power-law, and how these simulated spectra compare to current observations. Spectral characterization offers a method of discerning whether the background is indeed produced by binaries, or instead cosmological sources. The characterization of signal anisotropy on the sky, however, is likely the strongest determinant. I will also present models for the expected degree of anisotropy from MBH binaries, which are surprisingly close to current upper limits.

Speaker: Luke Kelley

2023-08-15 - Mainz - binary interpretation to the gwb.pdf

6 What can we learn about axions with PTAs?

Speaker: Geraldine Servant

PTA-MITP-2023-Servant.pdf

12:00 Lunch

7 Stop Horsing Around - Faster GWB Spectral Characterisation for your Cosmological Convenience

Strong evidence has been published for a nanohertz-frequency gravitational wave background by pulsar timing arrays (PTA), and there is considerable excitement as to its possible source(s), including cosmological sources. Spectral characterisation of GWB sources is challenging and slow in standard Bayesian analyses, in particular as PTA data sets continue to expand. In this talk, I will introduce a suite of faster and simpler refit techniques for spectral characterisation of the GWB to accelerate spectral characterisation. I will also present some preliminary results on the possibility of distinguishing between multiple background signals, and the possibility of determining a cosmological source for the GWB.

Speaker: William Lamb

presentation_mitp23.pdf

8 Pulsar Polarization Arrays

Pulsar timing arrays (PTAs) consisting of widely distributed and well-timed millisecond pulsars can serve as a galactic interferometer to measure gravitational waves. With the same data acquired for PTAs, we propose to develop pulsar polarization arrays (PPAs), to explore astrophysics and fundamental physics. As in the case of PTAs, PPAs are best suited to reveal temporal and spatial correlations at large scales that are hard to mimic by local noise. To demonstrate the physical potential of PPAs, we consider detection of ultralight axion-like dark matter (ALDM), through cosmic birefringence induced by its Chern-Simon coupling. Because of its tiny mass, the ultralight ALDM can be generated as a Bose-Einstein condensate, characterized by a strong wavy nature. Incorporating both temporal and spatial correlations of the signal, we show that PPAs have a potential to probe the Chern-Simon coupling up to ∼10−14−10−17GeV−1, with a mass range ∼10−27−10−21eV.

Speaker: Tao Liu

PPA_MITP_2023Aug 1.pdf

15:00 Snacks

Wednesday, 16 August

09:30 Snacks

9 Accurate calculation of the gravitational wave background from cosmic strings.

I will give an update on the work of my group to calculate
the gravitational wave background to be expected from a cosmic string
network. We start with a population of loops taken from string
network simulations, determine their evolution under gravitational
backreaction, compute their gravitational wave emission, and attempt
to extrapolate from simulation results to realistic cosmological
loops.

Speaker: Ken Olum

strings-talk.pdf

10 Constraining primordial non-Gaussianity with PTA's

We will discuss on primordial non-Gaussianity and primordial black holes at the PTA window.

Speaker: Daniel Figueroa

Dani_PTA_Mainz2023.pdf

12:00 Lunch

11 Testing gravity in the nanohertz GW regime using PTA correlations

The pulsar timing arrays' recent astronomical milestone of the detection of the stochastic gravitational wave background presents a new window of opportunity to study gravity in the nanohertz gravitational wave regime. In this talk, we present such tests focusing on both pulsar timing array measurements of the gravitational wave spatial correlation and the power spectrum. Our results suggest that PTAs can be its best version for testing gravity by focusing on the power spectrum.

Speaker: Reginald Bernando

TestingNanohertzGravity.pdf

14:30 Snacks

Thursday, 17 August

09:30 Snacks

12 Features of Early Universe Gravitational Waves in PTA datasets

The recently reported evidence for GWs in PTA datasets leaves the question of whether the signal is astrophysical or coming from the Early Universe. In this talk, I discuss several aspects of cosmological GW backgrounds that can help in addressing this question, and present results of dedicated searches.
In particular, I describe the imprint of the QCD crossover on GW spectra from transient early Universe processes. I then focus on two such sources: large curvature perturbations and cosmic domain walls. I discuss the relevance of complementary constraints on such scenarios for model comparison using PTA data.

Speaker: Fabrizio Rompineve

talkmainz.pdf

13 PBH bounds on scalar-induced GWs in the PTA band

Scalar perturbations source GWs at second order in cosmological perturbation theory. GW observations thus set bounds on (or give hints for) an enhanced scalar power spectrum at small scales. I will discuss the status of constraints on the scalar power spectrum set by PTAs as well as from primordial black hole (PBH) production, demonstrating that the latter seriously challenge an explanation of the recent PTA results as scalar-induced GWs.

Speaker: Valerie Domcke

mitp_sigw.pdf

12:00 Lunch

14 Implications for heavy primordial black holes from pulsar timing arrays

In this talk I evaluate whether the gravitational wave background for which a number of different pulsar timing arrays recently found evidence could be due to merging primordial supermassive black hole binaries. I find that for homogeneously distributed primordial black holes this possibility is inconsistent with strong cosmological and astrophysical constraints on their total abundance. If the distribution exhibits some clustering, however, the merger rate will in general be enhanced, opening the window for a consistent interpretation of the PTA data in terms of merging primordial black holes.

Speaker: Frederik Depta

pbh_pta_mitp.pdf

14:30 Snacks

Friday, 18 August

09:30 Snacks

15 Probing Dark Matter with Gravitational Waves in the LIGO, LISA and PTA Range

Speaker: Laura Sagunski

laura_sagunski.pdf

16 After nHz, microHz? Binary systems as a way to explore GWs in the microHz band

In this talk I will describe how binary pulsars, the Earth-Moon system and Earth-satellites system may by used as resonant absorbers of GWs in from microHz to miliHz. Current data may already provide the best constraints in this range. Quite remarkably, future data (that requires replacing the mirrors used for laser ranging at the Moon’s surface) may reach levels very relevant for the nHz detection or other signals. I’ll finish by mentioning other more futuristic ideas to tests GWs in this band

Speaker: Diego Blas

08mainz_blas.pdf

12:00 Lunch

14:00 Snacks