Locating the critical endpoint of QCD and the region of a first-order phase transition at finite baryon chemical potential is an active research area for QCD matter. We provide a gravitational dual description of QCD matter at finite baryon chemical potential and finite temperature using the non-perturbative approach from gauge/gravity duality. After fixing all model parameters using state-of-the-art lattice QCD data at zero chemical potential, the predicted equations of state and QCD trace anomaly relation are in quantitative agreement with the latest lattice results. We then give the exact location of the critical endpoint as well as the first-order transition line, which is within the coverage of many upcoming experimental measurements. Moreover, using the data from our model at finite baryon chemical potential, we calculate the spectrum of the stochastic gravitational wave background associated with the first-order QCD transition in the early universe, which could be observable via pulsar timing in the future.
We use AdS/QCD to analyze the quark and gluon scalar and pseudo-scalar condensates around static color sources described by a circular Wilson loop. We also derive the static dipole-dipole interactions between rectangular Wilson loops in AdS/QCD and discuss their relevance for static string interactions in QCD at strong coupling.
In the axiverse scenario, a massive scalar field (string axion) forms a cloud around a rotating black hole (BH) by superradiant instability and emits continuous gravitational waves (GWs). We examine constraints on the string axion parameters that can be obtained from GW observations. If no signal is detected in a targeted search for GWs from Cygnus X-1 in the LIGO data, the decay constant $f_a$ must be smaller than the GUT scale in the mass range $1.1\times 10^{-12}\mathrm{eV}<μ<2.5\times 10^{-12}\mathrm{eV}$. Constraints from invisible isolated BHs are also discussed.
We investigate the reach of the LIGO/Virgo/KAGRA detectors in the search for signatures of first-order phase transitions in the early Universe. Utilising data from the first three observing runs, we derive constraints on the parameters of the underlying gravitational-wave background, focusing on transitions characterised by strong supercooling. As an application of our analysis, we determine bounds on the parameter space of two representative particle physics models. We also comment on the expected reach of third-generation detectors in probing supercooled phase transitions.
We study the possibility of probing dark energy behaviour using gravitational wave experiments like LISA and Advanced LIGO. Using two popular parameterizations for dark energy equation of state, we show that with current sensitivities of LISA and Advanced LIGO to detect the stochastic gravitational waves, it is possible to probe a large section of parameter space for the dark energy equation of state which is allowed by present cosmological observations.
The ultimate aim of high energy heavy ion collisions is to study quark deconfinement and the quark-gluon plasma predicted by quantum chromodynamics. This requires the identification of observables calculable in QCD and measurable in heavy ion collisions. I concentrate on three such phenomena, related to specific features of strongly interacting matter. The observed pattern of hadrosynthesis corresponds to that of an ideal resonance gas in equilibrium at the pseudo-critical temperature determined in QCD. The critical behavior of QCD is encoded in the fluctuation patterns of conserved quantum nu…
String and M-theory compactifications generically have compact moduli which can potentially act as the QCD axion. However, as demonstrated here, such a compact modulus can not play the role of a QCD axion and solve the strong CP problem if gravitational waves interpreted as arising from inflation with Hubble constant $H_inf \gsim 10^{13}$ GeV are observed by the PLANCK polarimetry experiment. In this case axion fluctuations generated during inflation would leave a measurable isocurvature and/or non-Gaussian imprint in the spectrum of primordial temperature fluctuations. This conclusion is inde…
We study the spectrum of $H(m)=γ_5 W(-m)$ with $W(m)$ being the Wilson-Dirac operator on the lattice with bare mass equal to $m$. The background gauge fields are generated using the SU(3) Wilson action at $β=5.7$ on an $8^3\times 16$ lattice. We find evidence that the spectrum of $H(m)$ is gapless for $1.02 < m < 2.0$, implying that the physical quark is massless in this whole region.
We present a new signature by which to one could potentially discriminate between a spectrum of gravitational radiation generated by a self-ordering scalar field vs that of inflation, specifically a comparison of the magnitude of a flat spectrum at frequencies probed by future direct detection experiments to the magnitude of a possible polarization signal in the Cosmic Microwave Background (CMB) radiation. In the process we clarify several issues related to the proper calculation of such modes, focusing on the effect of post-horizon-crossing evolution.
Under the framework of gravitational effective field theory, we propose a theory agnostic strategy of searching for higher-spin particles with gravitational waves from compact binary inspirals. Using this strategy, we analyze gravitational wave signals from the binary black hole merger events GW151226 and GW170608, as well as the binary neutron star merger event GW170817. We find that the existence of higher-spin particles with mass ranged from $10^{-12} {\rm eV}$ to $10^{-11} {\rm eV}$ is strongly disfavored by these events unless the particles precisely combine within a supersymmetric superm…
The direct detection of gravitational waves offers an exciting new window onto our Universe. At the same time, multiple observational evidence and theoretical considerations motivate the presence of physics beyond the Standard Model. In this thesis, we explore new ways of probing particle physics in the era of gravitational-wave astronomy. We focus on the signatures of ultralight bosons on the gravitational waves emitted by binary systems, demonstrating how binary black holes are novel detectors of this class of dark matter. We also discuss probes of other types of new physics through their fi…
The sources of the observed ultra-high energy cosmic rays must also generate ultra-high energy neutrinos. Deep inelastic scattering of these neutrinos with nucleons on Earth probe center-of-mass energies $\sqrt{s} \sim 100$ TeV, well beyond those attainable at terrestrial colliders. By comparing the rates for two classes of observable events, any departure from the benchmark (unscreened perturbative QCD) neutrino-nucleon cross-section can be constrained. Using the projected sensitivity of the Pierre Auger Observatory to quasi-horizontal showers and Earth-skimming tau neutrinos, we show that a …
We present the relation between the sphaleron energy and the gravitational wave signals from a first order electroweak phase transition. The crucial ingredient is the scaling law between the sphaleron energy at the temperature of the phase transition and that at zero temperature. We estimate the baryon number preservation criterion, and observe that for a sufficiently strong phase transition, it is possible to probe the electroweak sphaleron using measurements of future space-based gravitational wave detectors.
Black holes are the most compact objects in the Universe. According to general relativity, black holes have a horizon that hides a singularity where Einstein's theory breaks down. Recently, gravitational waves opened the possibility to probe the existence of horizons and investigate the nature of compact objects. This is of particular interest given some quantum-gravity models which predict the presence of horizonless and singularity-free compact objects. Such exotic compact objects can emit a different gravitational-wave signal relative to the black hole case. In this thesis, we analyze the s…
We perform a detailed analysis on a primordial gravitational-wave background amplified during a Kasner-like pre-inflationary phase allowing for general triaxial anisotropies. It is found that the predicted angular distribution map of gravitational-wave intensity on large scales exhibits topologically distinctive patterns according to the degree of the pre-inflationary anisotropy, thereby serving as a potential probe for the pre-inflationary early universe with future all-sky observations of gravitational waves. We also derive an observational limit on the amplitude of such anisotropic gravitat…
Primordial black holes (PBHs) are supposed to form from the collapse of over-densed regions generated by large scalar curvature perturbations in the radiation dominated era. Despite decades of various independent observations, the nature of dark matter (DM) remains highly puzzling. Recently, PBH DM have aroused interest since they provide an attracting explanation to the merger events of binary black holes discovered by LIGO/VIRGO and may play an important role on DM. During the formation of PBH, gravitational waves will be sourced by linear scalar perturbations at second-order, known as the s…
We analyze a possibility to probe beyond-Riemann gravity (BRG) contributions, introduced by Kostelecky and Li (see Phys. Rev. D 103, 024059 (2021) and Phys. Rev. D 104, 044054 (2021)) on the basis of the Effective Field Theory (EFT) by Kostelecky Phys. Rev. D 69, 105009 (2004). We carry out such an analysis by calculating the BRG contributions to the transition frequencies of the quantum gravitational states of ultracold neutrons (UCNs). These states are being used for a test of interactions beyond the Standard Model (SM) and General Relativity (GR) in the qBOUNCE experiments. We improve by or…
Stochastic gravitational wave backgrounds, predicted in many models of the early universe and also generated by various astrophysical processes, are a powerful probe of the Universe. The spectral shape is key information to distinguish the origin of the background since different production mechanisms predict different shapes of the spectrum. In this paper, we investigate how precisely future gravitational wave detectors can determine the spectral shape using single and broken power-law templates. We consider the detector network of Advanced-LIGO, Advanced-Virgo and KAGRA and the space-based g…
Various field strength correlators are investigated in the maximal Abelian projection of pure SU(2) lattice gauge theory. High precision measurements of the colour fields, monopole currents, their curls and divergences allow for detailed checks of the dual superconductor scenario. On this basis, we perform a Ginzburg-Landau type analysis of the flux tube profile from which we derive the size of the penetration length, LAMBDA = 0.16(2) fm, and coherence length of the monopole condensate wave function, XI = 0.27(3) fm. The ratio of these numbers is KAPPA = LAMBDA/XI = 0.59(13) which is below the…
We report on a novel phenomenon of particle cosmology, which features specific cosmological phase transitions via quantum tunnelings through multiple vacua. The latter is inspired by the axiverse ideas and enables us to probe the associated new physics models through a potential observation of specific patterns in the stochastic gravitational waves background. Multiple vacua may induce the nucleation of co-existing bubbles over the phase transition epoch, hence enhancing the overall process of bubbles' nucleation. Our detailed analysis of semi-analytical and numerical solutions to the bounce e…
New AdS/QCD backgrounds have been proposed to describe the spectrum of heavy vector mesons via the implementation of additional energy scales on the bulk geometry of the soft wall model. The extra energy scales are needed to include the decay constants of hadronic states when describing the radial excitations of the heavy meson. Here we analyze one model that introduces an ultraviolet cutoff on Anti de-Sitter space and a model that consider a dilaton profile modified by the addition of an extra term and no cutoff. For each one of these two models we consider the presence of a semi-classical st…
We present an alternative way of tracing the existence of a scalar field based on the analysis of the gravitational wave spectrum of a vibrating neutron star. Scalar-tensor theories in strong-field gravity can potentially introduce much greater differences in the parameters of a neutron star than the uncertainties introduced by the various equations of state. The detection of gravitational waves from neutron stars can set constraints on the existence and the strength of scalar fields. We show that the oscillation spectrum is dramatically affected by the presence of a scalar field, and can prov…
The next generation of instruments designed to measure the polarization of the cosmic microwave background (CMB) will provide a historic opportunity to open the gravitational wave window to the primordial Universe. Through high sensitivity searches for primordial gravitational waves, and tighter limits on the energy released in processes like phase transitions, the CMB polarization data of the next decade has the potential to transform our understanding of the laws of physics underlying the formation of the Universe.
A salient feature of String/Gauge duality is an extra 5th dimension. Here we study the effect of confining deformations of AdS5 and compute the spectrum of a string stretched between infinitely massive quarks and compare it with the quantum states of the QCD flux as determined by Kuti, Juge and Morningstar in lattice simulations. In the long flux tube limit the AdS string probes the metric near the IR cutoff of the 5th dimension with a spectrum approximated by a Nambu-Goto string in 4-d flat space, whereas at short distance the string moves to the UV region with a discrete spectrum for pure Ad…
A gravitational observatory such as LISA will detect coalescing pairs of massive black holes, accurately measure their luminosity distance and help identify a host galaxy or an electromagnetic counterpart. If dark energy is a manifestation of modified gravity on large scales, gravitational waves from cosmologically-distant spacetime sirens are direct probes of this new physics. For example, a gravitational Hubble diagram based on black hole pair luminosity distances and host galaxy redshifts could reveal a large distance extra-dimensional leakage of gravity. Various additional signatures may b…
Many motivated extensions of the Standard Model predict the existence of cosmic strings. Gravitational waves originating from the dynamics of the resulting cosmic string network have the ability to probe many otherwise inaccessible properties of the early universe. In this study we show how the spectrum of gravitational waves from a cosmic string network can be used to test the equation of state of the early universe prior to Big Bang Nucleosynthesis (BBN). We also demonstrate that current and planned gravitational wave detectors such as LIGO, LISA, DECIGO/BBO, and ET/CE have the potential to …
The gravitational interactions of elementary particles are suppressed by the Planck scale M_P ~ 10^18 GeV and are typically expected to be far too weak to be probed by experiments. We show that, contrary to conventional wisdom, such interactions may be studied by particle physics experiments in the next few years. As an example, we consider conventional supergravity with a stable gravitino as the lightest supersymmetric particle. The next-lightest supersymmetric particle (NLSP) decays to the gravitino through gravitational interactions after about a year. This lifetime can be measured by stopp…
