CMD-HD and the First Moments of the Universe

Key CMB-HD Science Objectives

1. Measure the primordial non-Gaussian fluctuations in the CMB, characterized by the parameter fNL, with an uncertainty of σ(fNL) = 0.26, by combining the kSZ signal from CMB-HD with an overlapping galaxy survey such as LSST.

2. Remove 90% of the CMB B-mode fluctuations from gravitational lensing over half the sky, leaving only 10% remaining, i.e. achieve Alens = 0.1.

3. Probe the existence of primordial magnetic fields (PMFs) to find evidence for magnetogenesis in the early Universe and reveal the seeds of observed galactic magnetic fields.



A fundamental question is what happened right at the Big Bang and how did the structure we see (galaxies, planets, us) arise out of the homogeneous soup of the newborn Universe. Inflation is one compelling idea in which the Universe underwent a period of superluminal expansion a fraction of a second after the Big Bang; this expansion stretched out microscopic quantum fluctuations and froze them into the fabric of spacetime as macroscopic density fluctuations, seeding the structure we see today. However, we have no direct evidence that inflation actually occurred.

One prediction of inflation

Is that fluctuations of the quantum particle carrying gravity created gravitational waves that later got imprinted in the CMB. We can detect this imprint by looking for a well-defined signal in the polarized CMB light, in particular in the large-scale CMB B-mode fluctuations. The amplitude of this primordial B-mode fluctuation is given by the parameter r, and a number of well-motivated models predict r ∼ 3 × 10−3; thus σ(r) < 5 × 10−4 has become a desired target to definitively rule them out. However, to reach this target using a ground-based experiment requires removing the CMB B-mode fluctuations from gravitational lensing. In particular, for this σ(r) target, one must remove 90% of the lensing B-mode power, leaving only 10% remaining. CMB-HD is easily able to reach this Alens = 0.1 target.

In addition, inflation predicts the primordial CMB

Has small non-Gaussian fluctuations that can be characterized by a parameter called fNL. Reaching a target of σ(fNL) < 1 would rule out a wide class of multi-field inflation models, shedding light on how inflation happened. By combining the kinetic Sunyaev-Zel’dovich signal from CMB-HD with an overlapping galaxy survey such as LSST, one can cross this critical threshold, achieving σ(fNL) = 0.26. This cross-correlation could also resolve the physical nature of several statistical anomalies in the primary CMB that may suggest new physics during inflation and provide constraints on the state of the Universe before inflation.

References (see these works and references therein)

Sehgal, N et al, CMB-HD:
Astro2020 RFI Response, Feb 2020,

Sehgal, N et al, CMB-HD:
An Ultra-Deep, High-Resolution Millimeter-Wave Survey Over Half the Sky, September 2019,