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...

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...