Research Interests

I am a researcher at the field of theoretical cosmology. My work so far has focused on the investigation of various research topics in cosmic microwave background (CMB) physics. I am interested in aspects of both the primary and secondary CMB signals and, more generally, in the field of particle cosmology; signatures that can be probed by observable properties of the CMB, in particular. In recent years, I have been exploring various aspects of the possibility that gravitation is genuinely described by a Weyl-invariant (WI) version of general relativity (GR).

The CMB can be viewed as an experiment that has been in operation for the entire 14 billion years history of the expanding universe, and it is only here and now that we harvest and decode the data. The CMB, often supplemented by other cosmological probes, has already proved as a distinctively powerful probe of certain key cosmological parameters such as the energy composition and spatial curvature of the universe, and its reionization history. However, the elusive `holy grail' of cosmology is the energy scale associated with cosmic inflation. Unlike the amplitude of scalar (density) perturbations, `curl-type' tensor metric perturbations are determined solely by the energy scale of inflation in a model-independent fashion (assuming that perturbations are indeed generated during inflation).

Consequently, this minute degree-scale (`horizon scale') B-mode signal could potentially provide a clean measurement of the energy scale of inflation that could be as high as trillion times larger than achieved so far in the large hadron collider (LHC). Currently, this B-mode signal is being pursued by a few powerful telescopes, to which the Simons Observatory (SO) will join in the near future. Since the first B-mode detection on sub-degree scales by the POLARBEAR (PB) team, this secondary signal has been measured by a few other experiments with a high precision. The latter signal was induced via lensing of the dominant E-mode polarization by the intervening large scale structure and can potentionally inform us about, e.g. the sum of neutrino masses, dark energy, and curvature .

It also has the potential to address issues of more fundamental nature, such as the degree of lepton asymmetry in the early universe, validity of the CPT and parity symmetries (manifested as `cosmological birefringence' in the CMB), and potentially even put to test the cosmological principle.

It is an active field of research that draws together both theoreticians, experimentalists and phenomenologists, astrophysicists as well as particle physicists, in pursuing the valuable information encrypted in the feeble CMB temperature and polarization anisotropy at recombination, when the CMB photons decoupled from the plasma, approximately 400,000 years after the big bang (primary CMB), and later, a few billion years ago (secondary CMB) via lensing of the CMB by dark matter halos, and comptonization of the CMB by the hot intracluster (IC) gas in galaxy clusters, the Sunyaev-Zeldovich effect.

With the wealth of high-quality data we expect to have in the near future from CMB experiments (such as the PLANCK satellite as well as many powerful ground-based and baloon-borne CMB telescopes and data from other cosmological probes) we hope to perhaps unlock some of the deepest mysteries of the initial conditions of the universe and the laws of nature.

In a recent work I consider the possibility that at least on galactic and galaxy cluster scales the `Universal' gravitational constant, G, might actually vary (spatially) at a level of one part in 10,000. Reasonably accurate measurements of G have been carried out within our solar system but given that Newtonian gravity fails to satisfactorily describe, e.g. the kinematics of light lensed by galaxy clusters, as well as the motion of star clusters within galaxies, it is not observationally rulled out that G varies at this level on galactic and super-galactic scales. Within the realm of a WI version of GR dark matter (DM) phenomena could be explained on these scales as manifestations of spatial variation of G rather than by exotic, beyond-the-standard-model, particles. According to this proposal, the profile of the DM potential is precisely the spatial variation of G..

Another possibly interesting aspect of a WI version of GR relates to the `Hubble tension' between local measurement of the Hubble constant, H0, and inference from CMB anisotropy and polarization measurements. At present, this tension exceeds the 4-sigma level. Due to the modified spacetime geometry in a WI version of GR, photons experience an excess red/blueshift over that caused by Hubble expansion (similar to the `de-Sitter effect' in a static de-Sitter universe in GR). A small such tilt in the redshift history accumulates out to z~1100 to a level that reduces the tension to (statistically insignificant) slightly over 1-sigma level.

Meir Shimon

Contact

DEPARTMENT of ASTROPHYSICS
FACULTY OF EXACT SCIENCES
Tel Aviv University
Ramat Aviv, Tel Aviv
69978 Israel

Kaplun - Physics Building, room 517
Tel: (972)-3-6407414
Fax: (972)-3-6408179
Email: meirs[at]tauex.tau.ac.il