The standard cosmological model determined from the accurate cosmic microwave background measurements made by the Planck satellite implies a value of the Hubble constant H0 that is 4.2 standard deviations lower than the one determined from Type Ia supernovae. The Planck best fit model also predicts lower values of the matter density fraction Om and clustering amplitude S8 compared to those...

Precision measurements of the Cosmic Microwave Background (CMB) have the potential to provide information about the birth and evolution of our universe. I will review how we extract cosmological parameters from the CMB from both temperature and polarization maps. The Atacama Cosmology Telescope (ACT) has been making measurements of the CMB since 2006. I will discuss the recent results from...

We present spherically symmetric solutions to Einstein’s equations, which are equivalent to canonical Schwarzschild and Reissner-Nordstrom black holes on the exterior, but with singular (Planck-density) shells at their respective event and inner horizons. The locally measured mass of the shell and the singularity are much larger than the asymptotic Arnowitt-Deser-Misner mass. The area of the...

It is well-known since the works of Utiyama and Kibble that the gravitational force can be obtained by gauging the Lorentz group, which puts gravity on the same footing as the Standard Model fields. The resulting theory - Einstein-Cartan gravity - happens to be very interesting. First, it may generate the electroweak symmetry breaking by a non-perturbative gravitational effect. In this way,...

I will discuss the current status of the secondary gravitational waves induced by the curvature perturbation and why they might be an important source of the cosmological stochastic gravitational wave background. As a practical example, I will use the latest NANOGrav results on the stochastic background of nanohertz gravitational waves to constrain the equation of state parameter of the early...

We will take a look at axion inflation in string theory, taking a somewhat eclectic approach guided by some mechanism classes and (semi-)explicit examples. Looking at models with either 1 or 2 axions, we will argue that (up to manifestly tuning for small-field models) inflation can arise from 2 different mechanisms - either monodromy, or hybrid inflation. Cautiously incorporating both known...

With black holes, what you see is not what you get. They are extreme structures of spacetime that represent the ultimate prison, from where even light cannot escape. After decades of being a subject of mathematical interest, recently black holes became a topic of direct observational studies, for which two Nobel prizes were awarded over the past three years. I will describe some of the most...

I will discuss how quantum fluctuations modify the Coleman theory of the decay of false vacuum

Polarised light of the cosmic microwave background, the remnant light of the Big Bang, is sensitive to parity-violating physics. In this presentation we report on a new measurement of parity violation from polarisation data of the European Space Agency (ESA)’s Planck satellite. The statistical significance of the measured signal is 2.4 sigma. If confirmed with higher statistical significance...

Cosmological Genesis is a scenario without initial singularity,

in which the Universe starts off from nearly Minkowski state with nearly vanishing energy density, then the energy density increases, the expansion rate grows; at some later epoch the energy density is converted into heat, and the conventional hot epoch begins (variant: at some later epoch energy density stops increasing and...

I plan to informally discuss several issues that have traditionally been raised in various approaches to quantizing gravity. They are invariably related to the concepts that are thought to be fundamental in one of the two theories (quantum and GR) but are (allegedly) at odds with the other one. I will discuss some of the key issues in my talk, such as Bell non-locality and the equivalence...

We are faced with an explosion of data in many areas of physics, but very so often, it is not the size but the complexity of the data that makes extracting physics from big datasets challenging. As I will discuss in this talk, data has shape and the shape of data encodes the underlying physics. Persistent homology is a tool in computational topology developed for quantifying the shape of data....

I will present new ideas about how the QCD axion or axion-like particles can make up the dark matter of our universe, and/or explain the origin of the primordial density perturbation. For axion dark matter, I will introduce a novel production mechanism that invokes a kinetic mixing between the axion and the inflaton. I will show that this mechanism opens up new windows in the axion parameter...

I will talk about the properties of the gravitational wave (GW) signals produced by first-order phase transitions during the inflation era. I will show that the power spectrum of the GW oscillates with its wave number. This oscillatory feature corresponds to the instantaneous nature of the first-order phase transition. I will also show that we can get information about how the universe evolves...

Gravitational-wave astronomy and new electromagnetic facilities allows us for unprecedented tests of the nature of dark compact objects and provide a novel way to search for new physics. I will give an overview of the many recent result in this area including, shadows, constraints on the multipolar structure, ringdown tests, gravitational-wave echoes, and tidal effects in binaries.

The frequency spectrum of the CMB was last measured in the nineties by the FIRAS instrument onboard COBE. It was found to be consistent with a perfect blackbody spectrum, up to <1e-4 relative deviations. Today, there is growing interest in re-exploring in more depth this aspect of the CMB, which is complementary to the well-studied CMB anisotropies. In this talk I will briefly review the...

Gravitational wave astronomy will transform astrophysics in many ways; can it do the same for particle physics? In this talk, I will describe how the gravitational waves emitted by binary black holes offer a new window onto physics beyond the Standard Model. I will focus on probes of ultralight bosons such as axion-like particles and dark photons, which can spontaneously form bound states...

I will review our current understanding of the initial conditions of the universe, and describe what information is available from current and future measurements of cosmological correlation functions. Then I will describe a new method to compute and constrain the possible shapes of those correlation functions, assuming they were generated during inflation. This ``cosmological bootstrap” draws...

When a black hole first forms, the properties of the emitted radiation as measured by observers near future null infinity are very close to the 1974 prediction of Hawking. However, deviations grow with time, and become of order unity after a time $t \sim M_i^{7/3}$, where $M_i$ is the initial mass in Planck units. After an evaporation time the corrections are large: the angular distribution...

I will discuss a novel mechanism of Dark Matter production through an inverse phase transition. I will focus on a simple Z_2-symmetric model of Dark Matter composed of a scalar singlet. Due to couplings to other matter fields, Z_2-symmetry is spontaneously broken at very early times, and the Dark Matter field is offset from zero. As the Universe expands, Z_2-symmetry is restored, and the Dark...

Black hole information is one of the greatest puzzles of theoretical physics from the 20th century that has persisted into the 21st century. After Stephen Hawking discovered black hole evaporation in 1974, in 1976 he predicted that black hole formation and evaporation would cause a pure quantum state to change into a mixed state, effectively losing information from the universe. In 1979 I...

Despite being arguably one of the hottest topics in the recent literature, there are several widely spread misconceptions concerning what the Hubble tension really is. Moreover, leaving these misconceptions aside, no compelling model to solve the Hubble tension has been found so far, despite a huge number of attempts (and false alarms). I will begin by explaining what the Hubble tension really...

Progress in cosmology over the past few decades has been quantified by the extent to which we can accurately measure “two-point functions” such as the power spectrum of galaxies; the shear-shear- correlation function; galaxy-galaxy lensing; and most famously the C_l’s of the anisotropies in the cosmic microwave background. New statistics are emerging though that offer potential to infer even...

Following the direct discovery of gravitational waves (GWs) by LIGO and Virgo, there are many opportunities to probe fundamental physics using GWs. These include using GWs from astrophysical sources to constrain the graviton mass and search for Lorentz violation, as well as searching for GWs from dark matter in merging neutron stars, from first-order phase transitions in the early Universe,...

Black Holes occupy a special place in the fascination of astronomers and physicists. From the most speculative mathematical physicist to the most sensible radio astronomer, everyone has their own narrative of what lies within a black hole, based on their own preconceptions. This is in contrast to the more empirical and agnostic approach that we take in studying (almost) everything else in...

We present novel symmetries of perturbation theory around rotating and non-rotating black holes in general relativity, and discuss their origins and implications for gravitational-wave astronomy. This is motivated by two special aspects of black hole perturbations in four dimensions: isospectrality of quasinormal modes and the vanishing of tidal Love numbers. There turn out to be off-shell...

In this talk , I shall report on how black hole is formed and how classical conserved quantities are defined in general relativity . These are joint works with Schoen , Mu Tao Wang , Poning Chen , and Ye Kai Wang .

It is generally believed that modification of general relativity inevitably introduces extra physical degree(s) of freedom. In this talk I argue that this is not the case by constructing modified gravity theories with two local physical degrees of freedom. After classifying such theories into two types, I show explicit examples and discuss their cosmology and phenomenology, such as possible...

Scalar fields with spatially varying background could modulate the reheating process, thereby leaving their imprints in the density perturbations. In this talk we discuss two scenarios using this mechanism to probe physics at very high scales. First, we introduce a “cosmological Higgs collider” where the SM-Higgs-modulated reheating allows us to discover heavy particles and to measure their...

We propose an experiment that the entanglement between two macroscopic mirrors suspended at the end of an equal-arm interferometer is destroyed by the noise of gravitons through bremsstrahlung. By calculating the correlation function of the noise, we obtain the decoherence time from the decoherence functional. We estimate that the decoherence time induced by the noise of gravitons in squeezed...

The cosmological coincidence problem is the question of why now? Why do we live at the dawn of dark energy domination, when the energy density of dark matter and dark energy are roughly comparable? In this talk, I will describe how the problem is significantly alleviated, if not entirely solved, in generic string theory models of dark energy.

In this seminar I will present the latest research and results on cosmic strings, which are line-like defects which may be formed in spontaneous symmetry breaking phase transitions in the early universe. Such phase transitions may have occurred at grand unification energy scales, and more generally at lower scales. Through their different observational consequences — which I will discuss here...

The discovery of the Higgs boson at the LHC marks the culmination of a decades-long quest for the final piece of the Standard Model. But the discovery of the Higgs also adds new urgency to the hierarchy problem, namely the question of why the Higgs boson is so light despite its unique quantum sensitivity to much higher energy scales. This puzzle is made all the more challenging by the lack of...

During the primordial universe such as the inflationary epoch, all particles with mass up to the Hubble parameter or higher are excited quantum-mechanically or classically. These particles left their imprints in the primordial density perturbations, as primordial features and non-Gaussianities, which may be probed by astrophysical observations of the large-scale structure of the universe...

Current measurements of the CMB anisotropies have given us unprecedented precision surrounding the standard ΛCDM model of cosmology and the parameters that make up this model. The data accrued by collaborations like Planck have even allowed us to test additional models of fundamental physics. These models have grown more recently in the context of diluting the tension between low-redshift and...

The information loss paradox is usually stated as an incompatibility between general relativity and quantum mechanics. However, the assumptions leading to the problem are often overlooked and, in fact, a careful inspection of the main hypothesises suggests a radical reformulation of the problem. Indeed, I will present a thought experiment that shows the existence of an incompatibility between...

Effective theories are being developed for quantum fields outside black holes, often with an unusual open system feel due to the influence of large number of degrees of freedom lying out of reach beyond the horizon. The absence of comparisons to simpler systems that share these features complicates the interpretation of what is found. This talk describes a simple model aimed to help remedy...

The primordial scalar power spectrum is well constrained over large scales, essentially by the observations of the anisotropies in the cosmic microwave background. However, the current bounds on the scalar power spectrum over small scales are considerably weaker. During the last few years, there has been an interest in examining scenarios which generate enhanced scalar power on small scales...

The cosmological constant provides a simple explanation for the observed late-time accelerated expansion of our Universe. Our lack of understanding of it, however, motivates the exploration of alternative explanations such as a modification of General Relativity at cosmological scales. I will first discuss how gravitational wave observations have severely challenged that concept. I will then...

In this talk, I will summarize several recent results about gravitational wave cosmology in the context of dark energy and inflation. In the first part of the talk, I will concentrate on astrophysical gravitational waves and will argue that the spatial clustering of gravitational wave sources provides a wealth of invaluable information concerning the propagation law of gravitational waves. I...

We analyze gravitational particle production assisted by chemical potential. By utilizing the uniformly smoothed Stokes-line method and Borel summation, we gain insight into the fine-grained history of enhanced particle production. Analytic/semi-analytic formulae describing the production amount, time and width are obtained for both spin-1 and spin-1/2 particles in various FRW spacetimes. Our...

I will describe our recent work re-analyzing the GW data made public by the LIGO collaboration. More broadly I will discuss some of the outstanding questions related to binary black hole mergers, what the data might be saying and what we might expect in the near future. I will focus on what can be inferred from the spin measurements.

The BFSS matrix model is a proposed non-perturbative definition of M-theory in which space is emergent. In this talk, I shall present a new paradigm of early-universe cosmology in the context of the BFSS theory. Specifically, I will show that matrix theory leads to an emergent non-singular cosmology which, at late times, can be described by an expanding phase of Standard Big Bang cosmology....

The gauge dependence of the second order induced gravitational waves are widely recognized issue since GWs should be physical observable. While there have been several studies about the gauge dependence, I will talk about a new idea to solve the issue. I revisited the definition of nonlinear tensor modes and their gauge transformation from a covariant perspective and find a way to properly...

Cosmological observations and galaxy dynamics seem to imply that 84% of all matter in the universe is composed of dark matter, which is not accounted for by the Standard Model of particles. The particle nature of dark matter is one of the most intriguing puzzles of our time.

The wealth of knowledge which is and will soon be available from astrophysical surveys will reveal new information...

The present day distribution of dark matter on scales smaller than the mass scale of dwarf galaxies contain a wealth of information on the early history of the early universe as well as the nature of dark matter. This distribution is not reflected in the distribution of gas and stars because the amplitude of dark matter inhomogeneities on these scales are constrained to have little effect on...

In the approach to a singularity in general relativity, spacetime often becomes largely shear dominated and highly anisotropic. Any speculative cosmological scenario with a contracting phase prior to a Big Bounce must therefore address the issue of large anisotropies. In this talk, I will review this anisotropy problem and the status of isotropisation mechanisms in this context, which involve,...

An important and unresolved question in cosmology today is whether there is new physics that is missing from our current standard Lambda Cold Dark Matter (LCDM) model. Recent measurements of the Hubble constant, Ho -- based on Cepheids and Type Ia supernovae (SNe) -- are discrepant at the 4-5-sigma level with values of Ho inferred from measurements of fluctuations in the cosmic microwave...

I shall present a novel approach to disentangle two key contributions to the largest-scale anisotropy of the galaxy distribution: (i) the intrinsic dipole due to clustering and anisotropic geometry, and (ii) the kinematic dipole due to our peculiar velocity. Including the redshift or angular size of galaxies, in addition to their fluxes and positions allows us to measure both the direction and...

Among the many possible candidates for the nature of dark matter, one of the most well-motivated class of models and leading candidate is the ultra-light dark matter. This class represents the lightest possible dark matter candidates, and exhibits a wave-like behavior on galactic scales. This leads to a rich phenomenology on small scales that can potentially not only reconcile the CDM picture...

Recent years have seen great progress in probing gravitational physics on a vast range of scales, from the very largest cosmological scales to the microscopic ones associated with high energy particle physics. In this talk I will focus on how we can use these different systems synoptically to learn more about dark energy. Specifically, I will discuss the interplay of dark energy constraints...

A bright and statistically significant flux of GeV-scale gamma rays has been detected from the region surrounding the Galactic Center. While the spectrum, angular distribution, and intensity of this signal is consistent with the predictions of annihilating dark matter matter particles, it has also been suggested that these gamma rays could potentially be produced by a large population of...

In single field inflationary models that are capable of generating primordial black hole (PBH) populations, the power spectrum of curvature perturbation has interesting universal features such as the presence of a pronounced dip, occurring at scales much larger than the peak responsible for PBH formation. Focusing on the analytic framework of gradient expansion formalism, I will first discuss...

The discovery of astrophysical gravitational waves has opened a new avenue to explore the cosmos. I will discuss a few new frontiers in the field of physical cosmology and fundamental physics that can be explored using the LIGO-Virgo-KAGRA network of gravitational wave detectors. I will elucidate how the synergies between electromagnetic probes and gravitational wave probes will play a key...

I will review the prospects for future progress in cosmology. I will give examples of two futuristic experiments. One can obtain the dark ages signature via low frequency radio astronomy on the lunar far side. Attainable angular resolution potentially opens up huge numbers of modes to provide a new and robust probe of inflationary cosmology. A second direction involves a far infrared telescope...

Inflation predicts a stochastic background of gravitational waves. In this talk I will discuss how anisotropies in the gravitational wave energy density can be a powerful tool in characterizing the inflationary gravitational wave background and potentially distinguishing it from backgrounds due to other sources.

In this presentation, we discuss the information loss paradox of black holes in the light of the Euclidean path integral approach. This provides an interesting idea to understand the entanglement entropy and the Page curve. In order to make the discussion better, perhaps we further need to provide some quantum boundary conditions for the singularity. Finally, we compare our results to the...

Our local Universe is anisotropic. On the contrary the CMB indicates that the early Universe was isotropic. A convergence between the two is expected in the standard model of cosmology. In this talk, I explore various observational probes to search for the cross-over between these two domains.

In this talk, I will discuss how a newly proposed gravitational theory (arXiv: 2007:00082) could lead to a solution for the dark matter problem by reducing to Milgrom’s Modified Newtonian Dynamics at the scale of galaxies and to the LambdaCDM model on cosmological scales. I will show that it (i) leads to correct gravitational lensing on galactic scales, (ii) propagates tensor modes at the...

We will discuss the possibility that dark matter is composed of sufficiently light particles that it effectively behaves as a collection of waves. We will review the particle physics motivations and the rich wave phenomenology, and discuss the implications for astronomical observations and experimental detection.

I discuss various aspects of very light scalar dark matter, including axions. I begin by reviewing the properties of light scalar dark matter in superfluid condensates, and the relation to classical field theory. I review how such condensates are spatially localized clumps, which may be present in the galaxy. I then discuss the interesting possibility of parametric resonance of scalar axion...

I will discuss how early dynamics of the axion can naturally induce a matter-kination era inside the standard radiation era. The matter-kination era imprints a smoking-gun gravitational-wave peak on the irreducible inflationary GW background as well as on the local/global cosmic-string gravitational-wave background, whose position depends on kination's energy scale and duration. Remarkably,...

I will describe how some of the fine-tuning problems of the early dark energy solution to the Hubble tension can be addressed using couplings to other fields already present in cosmology. I will discuss the formulation, the cosmology, and the constraints on such models, arising from both observational and theoretical considerations.

Observations of the cosmic microwave background and measurements of the large-scale structure of the universe have revealed the initial fluctuations that grew to form galaxies. I will review measurements showing that these fluctuations were Gaussian random phase and that the basic properties of the universe appear to be described by the Lambda Cold Dark Matter model. I will report recent...

I will discuss the possibility that the Hubble tension is the signature of a fast triggered phase transition in the dark sector. Such a phase transition is called New Early Dark Energy (NEDE) and must have taken place just before recombination at the eV scale to resolve the tension fully. After discussing the cosmological NEDE phase transition, I will discuss the details of possible particle...

I will discuss our recent work Phys.Rev.Lett. 128 (2022) 4, 041301 in which we present a simple class of mechanical models where a canonical degree freedom interacts with another one with a negative kinetic term, i.e., with a ghost. We prove analytically that the classical motion of the system is completely stable for all initial conditions, notwithstanding that the conserved Hamiltonian is...

The “singularity theorems” of the 1960s, demonstrated that large enough celestial bodies, or collections of such bodies, would, collapse gravitationally, to singularities, where the equations and usual assumptions of Einstein’s classical theory of general relativity cannot be mathematically continued. These singularities are normally expected to lie deep within what are now referred to as...

We study D dimensional pure Einstein gravity theory in a region of spacetime bounded by a generic null boundary. We show besides the graviton modes propagating in the bulk, the system is described by boundary degrees of freedom labeled by D surface charges associated with nontrivial diffeomorphisms at the boundary. We establish that the system admits a natural thermodynamical description....

The Cold Spot is a puzzling large-scale feature in the Cosmic Microwave Background temperature maps and its origin has been subject to active debate. As an important foreground structure at low redshift, the Eridanus supervoid was recently detected, but it was subsequently determined that, assuming the standard LCDM model, only about 10-20% of the observed temperature depression can be...

The standard approach to cosmological observables involves a homogeneous and isotropic background model on top of which small linear perturbations are considered. While the need to go beyond this first-order approach is universally recognized when dealing with cosmic structures, there are several other consequences of "going beyond linearity" which have become an important tool in cosmological...

Utilizing toy models from AdS/CFT to fluid gravity, we consider whether non-perturbative effects in near-horizon states of quantum gravity could give rise to effects in the infrared, that are possibly observable in terrestrial experiments.

After introducing the standard model of cosmology and its parameters, I will discuss two important tensions between early and late-time measurements, namely the H0 tension and the sigma8 tension. Considering a small late-time deviation of the standard model, I will drive fully analytical conditions that any late-time dark energy model has to satisfy in order to solve both tensions...

We will discuss the state of the art of primordial black holes in view of the gravitational wave current and future measurements.

I will argue that effective point particle field theory will inevitably break down in a rapidly expanding universe. Hence, a non-perturbative formalism is required to understand the very early universe. A new approach based on matrix theory (a proposed non-perturbative definition of string theory( will be discussed, which yields an ``emergent" cosmology.

Our motion through the Universe generates a dipole in the temperature anisotropies of the Cosmic Microwave Background (CMB) and also in the angular distribution of sources. If the cosmological principle is valid, these two dipoles are directly linked, such that the amplitude of one determines that of the other. However, it is a longstanding problem that number counts of radio sources and of...

Gravity and general relativity are considered as an Effective Field Theory (EFT) at low energies and macroscopic distance scales. The effective action of the conformal trace anomaly of light or massless quantum fields has significant effects on macroscopic scales, owing to its describing light cone singularities not captured by an expansion in local curvature invariants. A compact local form...

We revisit the quantum cosmological constant problem and highlight the important roles played by the dS horizon of zero-point energy. We argue that fields which are light enough to have dS horizons of zero-point energy comparable to the FLRW Hubble radius are the main contributors to dark energy. On the other hand, the zero-point energy of heavy fields develop strong nonlinearities on...

Since the first gravitational-wave detection of a binary black hole merger in 2015, the LIGO-Virgo-KAGRA collaboration have observed gravitational waves from almost 100 merging systems. That number is expected to increase significantly over the coming years as these experiments become even more sensitive. The increased number of detections, and the improved sensitivity of these instruments,...

After discussing the necessary condition for a naturally small cosmological constant, I present a string theory motivated supergravity model that satisfies this condition. This leads to the axi-Higgs model that offers a resolution to the Hubble tension, the Lithium puzzle in big bang nucleosynthesis and the isotropic cosmic birefringence, with predictions to be tested in the near future. For...

The interplay between dark matter and black holes remains largely unexplored. Dark matter can in principle be *made of* black holes, as long as these are primordial, i.e. they are formed in the very early universe. Dark matter can also *accumulate around* black holes, and modify the rich phenomenology exhibited by these objects. After an overview of the status of dark matter searches, I will...

Although black holes can be the remnants of dead stars, it is also possible that some predate stars. Such primordial black holes (PBHs) are a special (non-particle) dark matter candidate, and they could also explain some of the unexpected properties of the black hole mergers that LIGO-Virgo-KAGRA have detected. I will summarise the evidence and challenges behind this claim, linking PBH...

Astrophysical black holes are surrounded by accretion disks, jets, and coronae consisting of magnetized, (near)-collision-less relativistic plasma. They produce observable high-energy radiation, and it is currently unclear where and how this emission is exactly produced. The radiation typically has a non-thermal component, implying a power-law distribution of emitting relativistic electrons....

We exhibit a mechanism which dynamically adjusts cosmological constant toward 0+. The adjustment is quantum-mechanical, discharging cosmological constant in random discrete steps. It renders de Sitter space unstable, and triggers its decay toward Minkowski. Since the instability dynamically stops at vanishing cosmological constant, the evolution favors the terminal Minkowski space without a...

I will describe a proposal for a unified dark sector model in heterotic string theory with the following features: The model-independent axion descending from the Kalb-Ramond 2-form field is identified with the dark-matter field, and the real part of a Kahler modulus field associated with the radius of one of the extra spatial dimension accounts for dark energy. The expectation value of the...

The TianQin Project plans to deploy around 2035 three satellites to form an equilateral triangle constellation, TianQin, in an orbit centered on the Earth with an altitude of about 105 kilometers, to detect gravitational waves in space. TianQin is expected to open the gravitational wave detection window in the frequency band of 10-4 Hz ~ 1 Hz, opening our eyes towards the nature of gravity,...

That event horizons generate quantum correlations via the Hawking effect is well known. In this talk, I will argue that the creation of entanglement in Hawking's process very much depends on the environment surrounding the horizon. In fact, I'll show that such entanglement can be modulated as desired, by appropriately illuminating the horizon. I will further apply these ideas to analog event...

I will show that primordial quantum diffusion unavoidably generates non-Gaussian, exponential tails in the distribution of inflationary perturbations. This type of non-Gaussianity cannot be captured by the usual perturbative parametrisations, and it leaves specific imprints on the statistics of cosmic structures that I will discuss.

There has been a lot of interest in theories that modified Einstein gravity but which reduce to general relativity in certain limits. Such modified gravity theories predict the existence of a fifth force. There are severe constraints on fifth forces from solar system observations. However there has been a recent resurgence of interest in modified gravity theories with the advent of...

In recent years, causality has become a popular criterion to distinguish between EFTs arising from physical and unphysical high-energy theories. A direct way to ensure a given EFT is causal is to demand a lower bound on scattering time delays, which essentially bounds the propagation speed averaged over the entire trajectory. In flat space, this is unambiguously dictated by the Minkowski light...

Gravitational waves can probe cosmic structures via gravitational lensing in ways that are highly complementary to electromagnetic signals: 1) their low frequency and phase coherence makes them sensitive to wave-optics diffraction and frequency-dependent effects, 2) weak interactions with matter allow them to probe dense regions, such as the cores of galactic halos and 3) accurate waveform...

I will review some modern applications of effective field theories outside their traditional particle physics domain. In particular, I will focus on spontaneous symmetry breaking for spacetime symmetries. The effective theories for the associated Goldstone excitations capture the low-energy/long-distance dynamics of a number of physical systems, from ordinary macroscopic media (solids, fluids,...

Primordial scalar perturbations are always considered as a source in the study of large-scale structures. Being the dominant ones at first order in perturbation theory, they have also encouraged the study of generation of second order gravitational waves from them. We seek to investigate the opposite effect, i.e. if gravitational waves can have an observable contribution on the matter power...

In this talk I will first review the recent development and opportunities addressing the profound puzzle of matter-antimatter asymmetry in our Universe, as a general motivation. Then I will introduce a cosmological probe for a compelling solution to the puzzle, leptogenesis, which is generally challenging to directly test due to the very high energy scales involved. In particular, we propose a...

I review a recent approach to connecting quantum gravity and the real world by deconstantizing the constants of nature, and using their conjugate as a time variable. This is nothing but a generalization of unimodular gravity. The wave functions are then packets of plane waves moving in a space that generalizes the Chern-Simons functional. For appropriate states they link up with classical...

The remarkable progress in cosmology over the last decades has been driven by the close interplay between theory and observations. Observational discoveries have led to a standard model of cosmology with ingredients that are not present in the standard model of particle physics – dark matter, dark energy, and a primordial origin for cosmic structure. Their physical nature remains a mystery,...

Optimal Transport Theory is a field of Mathematics that describes the cost-effective transfer of probability distributions, and provides connections between probability theory, geometry, partial differential equations, and of course optimisation. Over more than two centuries active research this field has fuelled great advances, and more recently has led to understanding relations with other...

After decades, we still lack a proper understanding of the quantum nature of gravity. Nonetheless, we have already seen many theoretical hints that gravity does not easily fit in the quantum mechanical framework. In this talk, I will discuss the issues associated with gravitating vacuum energy and take that as empirical evidence of the breakdown of QFT in the presence of gravity. Then, I will...

Observations of the universe have revealed a surprising economy in its basic laws and structure. In this light, Latham Boyle and I have reconsidered cosmology’s central puzzles, aiming to find simpler, more principled and more predictive solutions. From an improved understanding of the big bang singularity, we were able to explain the dark matter as consisting of a stable, massive RH neutrino....

Inflationary models predicting a scale-dependent large amplification of the density perturbations have recently attracted a lot of attention because the amplified perturbations can seed a sizable amount of primordial black holes (PBHs) and stochastic background of gravitational waves (GWs). While the power spectra in these models are computed based on the linear equation of motion, it is not...

Observations of the Cosmic Microwave Background (CMB) have cemented the notion that the large-scale Universe is both statistically homogeneous and isotropic. But is it invariant also under mirror reflections? Recently, observations of CMB polarization (through birefringence) and the distribution of galaxies (through four-point functions, or trispectra) have challenged this notion, and give...

As of today, the Advanced LIGO and Virgo gravitational-wave (GW) detectors have cataloged nearly 100 GW detections from various compact object mergers. These discoveries began the endeavors to search for other kinds of GW sources. Among these, the Stochastic Gravitational-Wave Background (SGWB), because of the superposition of individually undetectable cosmological and/or astrophysical...

The stochastic gravitational waves background is a rich resource of cosmological information, encoded both in its source statistics and its anisotropies induced by propagation effects. During their journey, the gravitational waves constituting the stochastic background encounter cosmic structures, which are able to modify the observed signal. According to the ratio between the wavelength of...

Mass density profiles are key ingredients of many astrophysical and cosmological data evaluations, for instance mapping the mass distribution in galaxies or galaxy clusters using strong gravitational lensing or kinematical information from spectroscopically inferred velocity dispersions. Yet, how accurate and precise are interpretations of such observations if their mass models are based on...

Although Hawking radiation was derived from an effective-theory calculation, many believe that it is robust, and insensitive to UV physics. We analyze Hawking radiation in more detail, paying attention to its time dependence, and find that it is in fact sensitive to UV physical effects. We conclude that the effective-theory prediction is not reliable. We will also comment on the applications...

High-energy particle theory motivates that a very light (m<<10^{-12} eV) field may exist, in addition to the usual standard model spectrum. If so, such a field could contribute to the dark matter and dark energy of the universe at a broad range of epochs. In this talk, I will explore the impact of such a field as a dark matter component (though its impact on CMB anisotropies), as a contributor...

I will review recent progress to address the generation of primordial non-Gaussianity during cosmic inflation. I will focus my attention on the origin of non-Gaussian signals that are poorly parametrized by the bispectrum (the three-point function). I will show that there are various types of non-Gaussianities that can arise during inflation that, to be understood, require taking into account...

There are persistent tensions in flat LCDM cosmology, most notably H0 and S8 tension. Modifications of the flat LCDM model designed to alleviate one tension typically exacerbate the other. Returning to basics, I will argue why evolution of cosmological parameters with effective redshift is expected in LCDM cosmology. I will review observations supporting this claim.

We describe how the standard tools of holography might be used to define microscopic models of big-bang cosmology. We consider models where a bubble of the cosmological spacetime is embedded in an asymptotically AdS spacetime, and models where an asymptotically AdS Euclidean spacetime obtained by analytically continuing the cosmological spacetime is described via a Euclidean CFT construction....

This talk will present theories of modified gravity that contain a single scalar degree of freedom, in particular the very general framework of Degenerate Higher-Order Scalar-Tensor (DHOST) theories, which includes and extends Horndeski and Beyond Horndeski theories. I will then discuss cosmological aspects of these theories, notably the dynamics of cosmological perturbations. I will also...

A gravitational wave from a binary black hole merger is an important probe to test gravity. Especially, the observation of ringdown may allow us to perform a robust test of gravity as it is a superposition of excited quasi-normal (QN) modes of a Kerr black hole. The excitation factor is an important quantity that quantifies the excitability of QN modes and is independent of the initial data of...

For more than a century now, our inference of the mass distributions (including dark matter) in galaxies have been based on modeling the positions and velocities of stars, i.e., using kinematic analyses, which assume equilibrium. These kinematic estimates can be inaccurate for a time-dependent potential, and there are now many lines of observational evidence that show that our Galaxy has had a...

I will show that the quantum field theory that reproduces classical general relativity, has a slight but nontrivial generalization. The Hilbert space allows for states that violate some of Einstein‘s equations without creating additional degrees of freedom. This amounts to a “classical background“ which is non-dynamical, but effectively redshifts like dark matter. I will go over a toy...

The evolution of the baryonic (normal) matter in the Universe is an excellent probe of the formation of cosmic structures and the evolution of galaxies. Over the last decade, considerable effort has gone into investigating the nature of baryonic material, theoretically and observationally. The technique of intensity mapping (IM), which measures the integrated emission from sources over a...

There has been renewed interest recently in the possibility of producing primordial black holes from large density perturbations after a period of inflation in the very early universe. Such large fluctuations would be the result of very rare, extreme excursions in the fields driving inflation which may not be well described by standard perturbation theory techniques. I will discuss a...

Despite overwhelming observational evidence for the existence of dark matter, its identity and properties remain a mystery. Recently, bosonic ultralight fields with masses below 1 eV are gaining a lot of attention, as they are well motivated by cosmology. Laser interferometers are sensitive to oscillations from such fields that change the interference fringe. Recently, we have proposed to...

Pulsar timing arrays (PTAs) consisting of widely distributed and well-timed millisecond pulsars can serve as a galactic interferometer to detect gravitational waves. With the same data acquired for PTAs, we have proposed Ref. [2] 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...

The Stochastic Gravitational Wave Background is one of the main targets of present and future detectors. Characterising its properties is crucial to pin down its origin and distinguish among the various possible sources. In this talk I will mainly consider the Cosmological Gravitational Wave Background (CGWB) and discuss a variety of new observables that can help reaching such a...

Axions and axion-like particles (ALPs) are a prominent dark matter candidate, drawing motivation in part from the axiverse of string theory. However, the string axiverse is not the only game in town: In this talk I will discuss axion-like particles that emerge as pions of a QCD-like dark sector. In a dark Standard Model (SM) wherein all 6 quark flavors are light while the photon is massive —...

If gauge fields are coupled to an axion field during inflation, they can lead to unique observational signatures. However, this system often shows strong backreaction effects, invalidating the standard perturbation theory approach. I will present the first nonlinear lattice simulation of an axion-U(1) system during inflation. The simulation is used to fully characterize the statistics of the...

I will discuss recent and ongoing work focused on attempts to restore concordance amongst cosmological data sets, motivated by discrepancies between some measurements of the cosmic expansion rate (H_0) and the matter clustering amplitude (S_8). Particular attention will be paid to scenarios invoking new physics in the high-redshift universe, including models featuring interactions between the...

The field of primordial non-Gaussianities is twenty years old. During that time, cosmologists have built a dictionary between the physics active during inflation and higher-order correlation functions of primordial density fluctuations. I will argue that this dictionary is far from complete, with theoretical predictions available only in restricted classes of theories.

To fill in this gap,...

General inhomogeneous cosmologies give rise to differential cosmic expansion which differs from that of Friedmann-Lemaitre-Robertson-Walker (FLRW) models. Even models with an average isotropic homogeneous isotropic expansion law on > 100/h Mpc scales will generically have expansion laws which differ from FLRW plus local Lorentz boosts. That is, they differ from the conventional "kinematic...

One of consequences of General Relativity is that vacuum black holes are remarkably simple macroscopic objects. They are thus ideal laboratories to test the underlying theory and to search for new degrees of freedom. I will discuss recent progress in black hole spectroscopy and in our understanding of black hole environments using gravitational-wave observations.

Single field models of inflation capable to produce primordial black holes usually require a significant departure from the standard, perturbative slow-roll regime. In fact, in many of these scenarios, the size of the slow-roll parameter ∣η∣ becomes larger than one during a short phase of inflationary evolution. In order to develop an analytical control on these systems, I explore the limit of...

Detecting gravitons is a big challenge. In this talk, we discuss possible experiments for the graviton detection. We consider macroscopic quantum phenomena and its relation to gravity. First of all, we will discuss quantum noise of gravitons in a detector. Then, we will explain how the decoherence due to noise of gravitons can be used for the graviton search. We also argue that high frequency...

It's known that some FLRW inflationary models admit spacetime extensions through the big bang. For k = -1 FLRW spacetimes, they are known as "Milne-like spacetimes." For k = 0 FLRW spacetimes, they are known as "past-asymptotically de Sitter" spacetimes. In both cases, a new set of coordinates shows that the big bang is a coordinate singularity for these spacetimes, and, in both cases, the...

Inflationary spacetimes have been argued to be past geodesically incomplete in many situations. However, whether the geodesic incompleteness implies the existence of an initial spacetime curvature singularity or whether the spacetime may be extended (potentially into another phase of the universe) is generally unknown. Both questions have important physical implications. In this talk, we will...

Dipole cosmology is the maximally Copernican generalization of the FLRW paradigm that can incorporate bulk flows in the cosmic fluid. In this talk, I first discuss how multiple fluid components with independent flows can be realized in this set up. This is the necessary step to promote “tilted” Bianchi cosmologies to a viable framework for cosmological model building involving fluid mixtures...

Phantom fields have been widely invoked as a source of dark energy in cosmology, but rarely taken seriously as quantum theories. The vacuum is automatically unstable to production of negative-energy ghost particles plus normal particles, requiring such theories to be effective only, below some UV cutoff. I will present recent cosmological constraints arising from the vacuum instability, both...

The quantization of gravity is widely believed to result in gravitons -- particles of discrete energy that form gravitational waves. But their detection has so far been considered impossible. Here [1] we show that signatures of single gravitons can be observed in laboratory experiments. We show that stimulated and spontaneous single-graviton processes can become relevant for massive quantum...

Pulsar Timing Array (PTA) collaborations around the globe recently announced compelling evidence for a gravitational-wave background (GWB) at nanohertz frequencies. This breakthrough achievement has important implications for astrophysics, as the GWB signal, if genuine, is likely to originate from a cosmic population of supermassive black holes orbiting each other at the centers of galaxies....

The cross-correlation between the cosmic microwave background (CMB) E- and B-mode polarization can be used to probe parity-violating physics in the Universe. Parity-violating processes such as a Chern-Simons coupling to axion-like particles or the Faraday rotation induced by primordial magnetic fields are expected to rotate the plane of linear polarization and produce a non-null EB...

based on Phys. Rev. Lett. 130 (2023) 221502 (arXiv:2305.18521 [gr-qc])

This talk presents a new avenue to black hole evaporation using a heat-kernel approach in the context of effective field theory analogous to deriving the Schwinger effect. Applying this method to an uncharged massless scalar field in a Schwarzschild spacetime, we show that spacetime curvature takes a similar role as the...

The QCD axion, serving as a classical dark matter candidate, has a close intriguing interplay with cosmic inflation, a leading paradigm to understand the origin of our universe. In this talk, I will discuss two novel effects of interaction between the inflaton and the Peccei-Quinn (PQ) scalar field (the phase becomes the axion after symmetry breaking). First, the inclusion of the leading...

We start with a gentle introduction to the spectral approach to (noncommutative) geometry. We insist on the presence of fermions, so that a central role is played by a Dirac operator, acting on a one-particle Hilbert space. The combination with a coordinate algebra then allows for a full reconstruction of the spacetime geometry from the spectral data. Moreover, when allowing for noncommutative...

In this talk, I will discuss an upcoming paper that shows how knowledge of the ultra-violet asymptotics of the effective action can offer a shortcut for the computation of certain processes in the effective theory of single clock inflation that involve loops of particles of arbitrary spin, in particular, with respect to the logarithmic running of correlation functions. In doing so, this work...

The Hawking evaporation of primordial black holes (PBH) reheats the Universe locally, forming hot spots that survive throughout their lifetime. We propose to use the temperature profile of such hot spots for light PBHs to calculate the decay rate of metastable vacua in cosmology, avoiding inconsistencies inherent to the Hartle-Hawking or Unruh vacuum and thermal loop correction which suppress...

We study the impact of a binary companion on black hole superradiance at orbital frequencies away from the gravitational-collider-physics (GCP) resonance bands. A superradiant state can couple to a strongly absorptive state via the tidal perturbation of the companion, thereby acquiring a suppressed superradiance rate. Below a critical binary separation, this superradiance rate becomes...

I will talk about a model in which an inflaton, through an axionic coupling to a U(1) gauge field, causes an amplification of the gauge field modes that strongly backreact on its dynamics. In particular, I will discuss, using analytical formulae, an instability of the system that emerges when one considers the evolution of the gauge field modes coupled to the inflaton zero mode, treating...

The separate-universe approach gives an intuitive way to understand the evolution of cosmological perturbations in the long-wavelength limit. It uses solutions of the spatially-homogeneous equations of motion to model the evolution of the inhomogeneous universe on large scales. We show that the separate-universe approach fails on a finite range of super-Hubble scales at a sudden transition...

Non-propagating fields have a wide presence in the literature, from non-propagating form fields in quantum hall systems to non-propagating 4-form fields in supersymmetric theories. Among these, the cuscuton field theory stands out as an extension of general relativity that avoids introducing additional propagating degrees of freedom, making it highly applicable in cosmology. Its simplicity...

In this talk, we revisit motivation from String Theory for new phases of cosmology – prior to inflation. Cosmic inflation offers a causal way to predict initial conditions for the growth of structure and density fluctuations in the cosmic microwave background and large-scale structure formation. However, asymptotic de Sitter space possesses a past cosmological (physical) singularity implying...

Gamma-ray observations of blazars suggest the existence of the intergalactic magnetic fields and their origin is interest for both astro physicsts and cosmologists. Among several proposals, magnetogenesis in the early Universe is an interesting option since it might also be a probe for the physics beyond the Standard Model of particle physics. Recently, it has also been proven that the baryon...

All the existing derivations of the black hole entropy formula S=A/4G have something unsatisfactory to them, and this dissatisfaction has stimulated much investigation into the structure of spacetime. In this talk I will describe a very broad statistical interpretation of this formula. Together with Ana Climent, Javier Magan, Martin Sasieta, and Alejandro Vilar Lopez, we have refined and...

The gravitons produced during the early stages of the evolution of the space-time curvature represent an ideal triple point where theoretical physics, high-energy physics and cosmology meet for different purposes. After a general introduction, I will focus on some recent results and argue that relic gravitons will represent, in the years to come, the sole direct probe of the the...

We recently showed that superradiance generates entanglement in generic situations. On the other hand, the entanglement generated during Hawking evaporation is a crucial aspect of the Hawking effect, and is influenced by rotational superradiance induced by the black hole ergoregion. In this talk, we will leverage Gaussian quantum information techniques to describe the Hawking process for a...

I will discuss the possibility of an eternal universe, a universe with no first moment and no end. The talk will focus on eternal inflation and the key role that inflation plays in resolving cosmological singularities. I will describe how proposed no-go theorems, such as the famous theorem of Borde, Guth and Vilenkin (BGV) are circumvented or obviated. Our exploration encompasses eternal...

Both single- and multi-field models of inflation might lead to enhanced scalar fluctuations on scales much smaller than those seeding the large-scale structure formation. In these scenarios, it is possible that the spike of power at high wavenumber might induce large corrections to the scalar power spectrum, e.g. in the form of loop corrections, potentially endangering the perturbativity of...

In this talk, we will study the massless limits of two cases of Proca theory -- in the presence of self-interactions, and with non-minimal coupling to gravity. In contrast to its massless counterpart, this theory propagates an additional longitudinal mode in flat spacetime. Due to this, conventional methods indicate that the perturbative series is singular in mass. In the presence of...

Next generation gravitational wave (GW) observatories will have sensitivities capable of obtaining polarization data of the GWs, which is one way to constrain parity violating models beyond General Relativity. Interestingly, this can also serve as a novel way to probe dark matter physics, in particular models which include nonminimal couplings between the metric and dark matter. In this talk,...

Gravitational particle production is the process by which particles are created due to the expansion of spacetime during inflation. In this talk we will discuss aspects of gravitational particle production of dark photons, a popular dark matter candidate, with a particular focus on dark photons with nonminimal couplings to gravity. I will first show that the inclusion of nonminimal couplings...

I will present recent work on the formation of primordial black hole dark matter and the resultant gravitational wave (GW) signal, in which my collaborators and I performed an MCMC analysis of a simple—yet generic—multifield inflation model characterized by two scalar fields coupled to each other and non-minimally coupled to gravity. The model was fit to the Planck 2018 CMB data. In...

Gravitational waves (GWs) are an ideal observable to probe the properties of gravity, from the strong to the weak-field regimes. In particular, by studying their cosmological propagation, we can test the fundamenal properties of gravity on large scales, such as parity symmetry. In this talk, I will discuss the signatures that parity violation would leave on gravitational waves from coalescing...

We present a new way to study cosmic inflation with gravitational waves (GWs). The gravitational signal is generated thanks to nonlinear structures in the inflaton field, called oscillons. This novel probe allows us to test models of inflation which are challenging to constrain with CMB experiments. We also present a novel induced-GW signature, called Universal GWs, associated with all...

Cosmic birefringence is a parity-violating phenomenon that rotates the plane of linear polarization of the CMB photons. Recently, a nonzero isotropic cosmic birefringence (ICB), its overall rotation angle from the last scattering surface to the present, with a statistical significance of 3.6 sigma has been reported for the latest joint analysis of Planck/WMAP data. In this talk, we will...

The minimal coupling of massless fermions to gravity does not allow for their gravitational production solely based on the expansion of the Universe. In this talk I will explain that changes in the presence of realistic and potentially detectable stochastic gravitational wave backgrounds. Next, I will discuss the resulting energy density of Weyl fermions at 1-loop. If the initially massless...

The cosmological evidence supporting the existence of dark matter is extremely compelling. And while we have known how dark matter fits into the standard model of cosmology, Lambda CDM, direct observation still eludes us despite a global experimental effort ruling out large regions of once promising parameter space. This has motivated study of alternative dark matter models. One such model is...

We study the fluctuations in the vacuum zero point energy associated to quantum fields and their statistical distributions during inflation. We show that the perturbations in the vacuum zero point energy have large amplitudes and are highly non-Gaussian. The effects of vacuum zero point fluctuations can be interpreted as the loop corrections in primordial power spectrum and bispectrum....

Among the many possible candidates for the nature of dark matter, one of the most well-motivated class of models and leading candidate is the ultra-light dark matter. This class represents the lightest possible dark matter candidates, and exhibits a wave-like behavior on galactic scales. This leads to a rich phenomenology on small scales that can potentially not only reconcile the CDM picture...