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Cosmic microwave background trispectrum and primordial magnetic field limits
Pranjal Trivedi1, T R Seshadri, Kandaswamy Subramanian
1Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India. ptrivedi@physics.du.ac.in
Physical Review Letters
|September 26, 2012
Summary
Primordial magnetic fields leave non-Gaussian imprints on the cosmic microwave background (CMB). Analyzing the CMB trispectrum provides the tightest constraints to date on these early universe magnetic fields.
Area of Science:
- Cosmology
- Astrophysics
- Fundamental Physics
Background:
- Primordial magnetic fields are hypothesized to exist from the early universe.
- These fields can induce non-Gaussian signals in the cosmic microwave background (CMB) due to magnetic stresses.
- The temperature anisotropy in the CMB depends quadratically on the magnetic field strength.
Purpose of the Study:
- To compute a new measure of magnetic non-Gaussianity, the CMB trispectrum, on large angular scales.
- To use observational limits on CMB non-Gaussianity to constrain the strength of primordial magnetic fields.
- To establish the CMB trispectrum as a sensitive probe for primordial magnetic fields.
Main Methods:
- Utilizing the Sachs-Wolfe effect to source the CMB trispectrum.
- Calculating trispectra induced by magnetic energy density and magnetic scalar anisotropic stress.
- Applying observational constraints from WMAP data on CMB non-Gaussianity.
Main Results:
- The trispectra induced by magnetic energy density and anisotropic stress have typical magnitudes of ~10^-29 and ~10^-19, respectively.
- WMAP data allows for conservative upper limits of ~1 nG on the present value of primordial cosmic magnetic fields.
- Plausible sub-nG upper limits are achievable.
Conclusions:
- The CMB trispectrum provides the tightest upper limits to date on primordial magnetic fields on Mpc scales.
- This method surpasses the sensitivity of CMB bispectrum and power spectrum analyses for constraining primordial magnetic fields.
- The CMB trispectrum emerges as a novel and more sensitive probe for large-scale primordial magnetic fields.
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