Scientific Series

On April 4th, 2024, the Dark Energy Spectroscopic Instrument (DESI) released its first set of cosmological results based on measurements of the baryon acoustic oscillations (BAO) scale in the spatial distribution of galaxies and quasars, and in the Lyman-alpha forest. Those measurements constrain the expansion history of the Universe in the redshift range 0.1 < z < 4.16, with implications for studies of dark energy, neutrino cosmology and the Hubble constant. To make the most of the cosmological information content of the galaxy & quasar distributions, we now analyze the full shape of their power spectra, constraining the evolution of the large scale structure of the Universe over the range 0.1 < z < 2.1. In this talk, following a brief overview of the DESI instrument and observations, we will present the latest public results, discuss some of their main cosmological implications, highlight efforts towards the full shape results and expectations for the next data release.

It is commonly believed that logical states of quantum error-correcting codes have to be highly entangled such that codes capable of correcting more errors require more entanglement to encode a qubit. Here we show that this belief may or may not be true depending on a particular code. To this end, we characterize a tradeoff between the code distance d quantifying the number of correctable errors, and geometric entanglement of logical states quantifying their maximal overlap with product states or more general "topologically trivial" states. The maximum overlap is shown to be exponentially small in d for three families of codes: (1) low-density parity check (LDPC) codes with commuting check operators, (2) stabilizer codes, and (3) codes with a constant encoding rate. Equivalently, the geometric entanglement of any logical state of these codes grows at least linearly with d. On the opposite side, we also show that this distance-entanglement tradeoff does not hold in general. For any constant d and k (number of logical qubits), we show there exists a family of codes such that the geometric entanglement of some logical states approaches zero in the limit of large code length. This work was done by the collaboration with Sergey Bravyi, Zhi Li, and Beni Yoshida. (https://arxiv.org/abs/2405.01332)

Static patch holography is a conjectured duality between the static patch of an observer in de Sitter spacetime and a quantum theory defined on its (stretched) cosmological horizon. We illustrate from entanglement wedge reconstruction how a closed and connected de Sitter spacetime can emerge in this framework from the entanglement between the two holographic screens of two antipodal observers. In holographic spacetimes, a direct scattering in the bulk may not have a local boundary analog, imposing the existence of O(1/G) mutual information on the boundary. This statement is formalized by the connected wedge theorem, which is expected to hold beyond the AdS/CFT correspondence from which it originates. We consider scatterings in the static patch of an observer. We argue that for static patch holography to be consistent with the connected wedge theorem, causality on the stretched horizon should be induced from null infinity. In particular, signals propagating in the static patch are associated with fictitious local operators at null infinity. We present a sketch of proof of the connected wedge theorem in asymptotically de Sitter spacetime using induced causality.

The study of string scattering in general curved spacetimes presents a stark contrast to the relatively well-understood framework in flat space, where perturbative techniques and worldsheet methods can be used. However, the AdS/CFT correspondence offers a powerful indirect method to compute string amplitudes in AdS. In this talk, I will present recent developments in the AdS Virasoro-Shapiro program. The tree-level amplitude of four gravitons in AdS^5 x S^5 is mapped to a four-point correlator in N=4 SYM at large central charge, and studied by CFT methods combined with single-valuedness, which echoes its importance in flat space. The amplitude itself is defined and constructed as an expansion around flat space. While the goal is to compute it to all orders, attention can be placed on more tractable limits. I will present our results for the High Energy limit, with dual insights from the spacetime and worldsheet perspectives, and the richer case of the Regge limit, which encodes full information on the intermediate operators in the leading Regge trajectory.

Dark matter particle interactions could imprint characteristic signals in cosmic-ray and multi-wavelength observations of the sky. The central challenge is to distinguish these signatures from similar spectra produced by standard astrophysical processes, such as the life and death of stars and the interactions of cosmic rays with interstellar material. The GAPS Antarctic balloon payload, en route for its initial flight in December 2024, is the first experiment optimized specifically for low-energy cosmic antideuterons, an essentially background-free signature of dark matter, as well as antiprotons in an unprecedented low-energy region and leading sensitivity to cosmic anithelium. In this talk, I will detail the novel GAPS detection technique, its flight instrument, and the potential impact of these measurements in the coming years. 

Claims of a low clustering amplitude (S_8) at low redshifts from weak galaxy lensing measurements trace back nearly a decade, however, recent work suggests these results may be driven by large baryonic feedback or mischaracterization of linear alignments. I will present a complimentary approach to measure the evolution of S_8(z) using spectroscopically calibrated DESI galaxies and the latest CMB lensing measurements from Planck and ACT. These data are insensitive to many of the systematic complications present in galaxy lensing measurements, while our fiducial Hybrid Effective Field Theory model robustly regulates the information obtainable from smaller scales, such that our cosmological constraints are reliably derived from the (predominantly) linear regime. Our tomographic analysis of DESI Luminous Red Galaxies (LRG) prefers a slightly lower (5 − 7%) value of S_8 than primary CMB measurements with a statistical significance ranging from 1.8 − 2.3σ. Intriguingly, our lowest redshift LRG bin is most discrepant with a Planck cosmology, leaving open the possibility that structure growth is slowing down for redshifts z < 0.5. To address this possibility, I will conclude my talk with preliminary results from the DESI Bright Galaxy Survey, which enable tomographic S_8(z) measurements over the redshift range 0.1 \lesssim z \lesssim 0.4.