[Paper Review] Gravitational waves and fundamental physics

Michele Maggiore|ArXiv.org|Feb 15, 2006

Computational Physics and Python Applications2 references3 citations

TL;DR

This paper explores how gravitational waves (GWs) serve as a unique probe for fundamental physics, particularly in extreme astrophysical environments like neutron stars and the early universe. By analyzing GW signals from neutron star oscillations, core transitions, and stochastic backgrounds, the study demonstrates that future detectors—especially advanced LIGO and LISA—can test quantum chromodynamics at high densities and constrain cosmological models, with sensitivity reaching $ h_0^2\Omega_{\rm gw} \sim 3.7 \times 10^{-11} $, enabling detection of primordial GWs from phase transitions or string cosmology.

ABSTRACT

I give an overview of the motivations for gravitational-wave research, concentrating on the aspects related to ``fundamental'' physics.

Motivation & Objective

• To investigate how gravitational waves can reveal insights into non-perturbative quantum chromodynamics (QCD) in neutron star cores.
• To assess the potential of gravitational wave observations to test the strange quark matter hypothesis and hybrid star formation.
• To evaluate the sensitivity of upcoming GW detectors to stochastic gravitational wave backgrounds from the early universe.
• To establish observational bounds on the effective number of neutrino species ($N_\nu$) using pulsar timing and CMB anisotropies.
• To determine whether cosmological models involving phase transitions or string theory can produce detectable stochastic GW backgrounds.

Proposed method

• Analyzes normal modes of neutron stars as probes of their internal structure, especially during corequakes or crustquakes.
• Uses the strange quark matter hypothesis to model quark-core formation and its implications for neutron star stability and GW emission.
• Applies bounds on $ h_0^2\Omega_{\rm gw} $ from pulsar timing arrays and CMB B-mode polarization to constrain early-universe GW backgrounds.
• Evaluates detector sensitivities using correlation techniques between two detectors over long integration times.
• Derives theoretical limits on $ h_0^2\Omega_{\rm gw} $ from cosmological models, including electroweak phase transitions and string cosmology.
• Compares predicted GW energy densities with observational bounds, using $ N_\nu $ as a proxy for extra relativistic degrees of freedom.

Experimental results

Research questions

• RQ1 Can gravitational waves from neutron star oscillations reveal the presence of quark matter cores via the strange quark matter hypothesis?
• RQ2 To what extent can the energy density of a stochastic gravitational wave background constrain the number of effective neutrino species ($N_\nu$)?
• RQ3 Can future gravitational wave detectors such as advanced LIGO and LISA detect primordial gravitational wave backgrounds from early-universe phase transitions?
• RQ4 How do the sensitivities of ground-based and space-based interferometers compare in probing low-frequency stochastic GW backgrounds?
• RQ5 What are the theoretical predictions for stochastic GW backgrounds in models beyond the Standard Model, such as string cosmology or electroweak phase transitions?

Key findings

• The advanced LIGO detector can achieve a sensitivity of $ h_0^2\Omega_{\rm gw} \simeq 5.1 \times 10^{-9} $, enabling detection of stochastic GW backgrounds from cosmological sources.
• Third-generation ground-based detectors could reach $ h_0^2\Omega_{\rm gw} \simeq 3.7 \times 10^{-11} $, probing previously unexplored regions of parameter space.
• LISA, operating at lower frequencies, can achieve $ h_0^2\Omega_{\rm gw} \simeq 10^{-11} $ as a single detector, making it highly sensitive to primordial backgrounds.
• Pulsar timing arrays provide bounds on $ h_0^2\Omega_{\rm gw} $ at frequencies $ f \sim 10^{-9} - 10^{-8} $ Hz, with constraints on $ N_\nu $ up to 7.1 if electron neutrino chemical potentials are considered.
• CMB measurements constrain stochastic GW backgrounds via B-mode polarization, with current bounds labeled as 'CMB' in Fig. 2.
• Theoretical models such as string cosmology and electroweak phase transitions predict GW backgrounds detectable by LISA and advanced LIGO, depending on model parameters.