Michael Zemcov

The Extragalactic Background Light

The Extragalactic Background Light (EBL) is the integrated light of all sources of photons along a line of sight through the Universe, when the emission from our solar system and own galaxy is excluded. The physical processes responsible for the production of these photons vary. In the near infrared (IR), the EBL is due to the light from all the stars in the Universe, and so can tell us a great deal about the baryons and nucleosynthesis across cosmic time, while in the far IR (FIR) the photons arise from cold dust during star formation. The plot below shows the power falling on a square meter as a function of the wavelength (or via the Planck relation, energy) of the light.

One can immediately see that the part of the spectrum with the most energy density is around 1 mm; this is the cosmic microwave background radiation discussed elsewhere in these pages. At shorter wavelengths than the CMB, labelled 'FIR' in this plot, the light is predominantly from dusty galaxies in the early Universe which are vigorously forming stars at a fantastic rate. A shorter wavelengths still, labelled 'visible' in the plot, the light is due to direct emission from the stars in the Cosmos. FIRAS on COBE measured the FIR EBL spectrum to amazing accuracy; various projects I work on are aiming to do the same in the IR/visible, which as you can see from the plot are currently not well understood.

The Low Resolution Spectrometer on CIBER will measure the near IR EBL in the crucial range 0.7 to 2 microns for the first time. This wavelength regime is important as it straddles the expected peak in the EBL from reionization, and yet has not ever been measured with direct photometry. At longer wavelengths, I am involved with projects aiming to understand the source population which creates the FIR background, including SPIRE and SCUBA-2. Further in the future, it may be possible to repeat the measurement of FIRAS to further constrain the color of the EBL spectrum.

EBL Images

A brief history of the Cosmos showing where each component of the EBL arises. The long-wavelength CMB is produced at very early times, while the IR EBL is produced by more local sources since the epoch of reionization.


Measuring the EBL from within the solar system is difficult - this images shows an artist's rendition of trying to look out of the Zodiacal Light. ZEBRA will get out of the majority of the Zodiacal Light dust cloud to get an unobscured view of the EBL.