M. Zemcov’s Research Group: Experimental Cosmology

The Extragalactic Background Light (EBL) is the combined brightness from all sources of light over the history of the Universe after the light from our solar system and Milky Way galaxy is excluded. The physical processes responsible for the production of these photons vary, and can tell us about the physical conditions at different times in the universe’s history. At optical and near infrared (IR) wavelengths, the EBL is due to the light from all the stars in the Universe, so teaches us about the history of baryons and nucleosynthesis across cosmic time, while in the far IR (FIR) the light arises from cold dust during star formation. The plot below shows the EBL’s relative brightness over the entire observable electromagnetic spectrum. Various peaks are visible, each corresponding to different physical processes in the universe.

Courtesy Hill, Matsui & Scott

The part of the spectrum with the most energy density - the largest peak on the plot - is around 1 mm; this is the cosmic microwave background radiation (CMB), which is the earliest light we can see in the universe. At shorter wavelengths than the CMB, labeled 'Infrared' in this plot, the light predominantly comes from dusty galaxies in the distant Universe that are vigorously forming stars at a fantastic rate. At shorter wavelengths still, labeled 'Optical' in the plot, the light is due to the light collectively emitted by all of 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 Optical and Infrared, particularly at wavelengths that are currently not so well understood.

Our research group is a world-leader in the field of spectrophotometric measurements of the EBL at a wide range of wavelengths. The spectrometric channels on CIBER-2 will measure the near IR EBL in the highly contentious range 0.6 to 2.2 microns, which is important as it straddles the expected peak in the EBL arising from stars in the universe. In addition, we are working on a small instrument to measure the IR background from a distant vantage point in the solar system to fly as part of the Interstellar Probe science payload.

The epoch of reionization was the era during which the first population of stars in the Universe, seeded by the same over-densities visible in the CMB, ionized the neutral hydrogen then ubiquitous in the Universe. The epoch of reionization is at the forefront of current cosmological research. The details of the process, such as when it began, how long it took, and what class of sources produced it, are not well understood. The image below shows our current understanding of the process of structure formation over cosmic history, and displays the epoch of reionization that began roughly 500 million years after the big bang and was complete about 500 million years later.

Courtesy SPHEREx Collaboration

A promising way to study the epoch of reionization is via the fluctuations it causes in the infra-red background. Several of the projects I am currently involved with are searching for these fluctuations, which will allow a much better understanding of the generation of the first galaxies in the Universe and the processes which led to them.

The Cosmic Infrared Background Experiment (CIBER) is a sounding rocket borne experiment which is optimized to search for the signatures of reionization in the near infrared background. With its first flight in early 2009 and second in the middle of 2010, the CIBER payload has been extremely successful. The continuation of this project, CIBER-2, has been qualified for launch and will fly in 2020. The photograph below shows team members C. Nguyen and M. Zemcov in front of the CIBER-2 instrument integrated into the Black Brant IX rocket payload during pre-flight tests at Wallops Flight Facility, Virginia.

CIBER-2 is specifically designed to help disentangle the reionization signal from emission from all of the galaxies closer to us. One of the primary CIBER results has been unexpectedly bright large-angle fluctuations at wavelengths of 1.1 and 1.6 microns, which may be identified with stars flung outside of galaxies or other new populations that result from large-scale structure formation. The CIBER-2 data set will give us our most complete view of the near-IR background to date.

Building on CIBER, SPHEREx, the Spectro-Photometer for the History of the universe, Epoch of Reionization, and ices Explorer, is an upcoming NASA Mid-scale Explorer mission (MIDEX) that will perform an all-sky spectral survey over the entire NIR wavelength range. SPHEREx is designed to map the large-scale structure of galaxies in the universe to help study the process of inflation, measure the light produced by stars and galaxies over time by using multiple wavelength bands to study galaxy clusters, and investigate how water and biogenic ices influence the formation of planetary systems by studying the abundance and composition of interstellar ices. Our group is at the front lines of this mission, currently helping to design the science case and data analysis pipeline ahead of a launch expected in 2024.

The epoch of reionization is also being studied in the sub-mm with the Tomographic Ionized-Carbon Mapping Experiment (TIME), a multi-pixel cryogenic grating spectrometer using superconducting bolometers. TIME will make line intensity maps of the redshifted 158 micron line of singly ionized carbon emitted by early galaxies as a result of star formation processes. A technique called line intensity mapping allows TIME to reach galaxy populations fainter than that of other instruments, and detect crucial ionizing systems such as dwarf and dust-obscured galaxies that may have been missed in other surveys. TIME is set to return to the KP12m telescope (photograph below with V. Bulter and M. Zemcov) in winter of 2023 for a dedicated science run with a full set of 2000 detectors.

FURTHER READING

• Wikipedia's Article on Reionization
• An Article on Reionization on LOFAR's Website
• Tom Abel's Home of The First Star
• White Paper on near IR Searches for Reionization
• Paper Describing CIBER's Science
• Paper on Line Intensity Mapping

The Sunyaev-Zeldovich Effect describes the scattering of cold cosmic microwave background photons off hot gas in the atmospheres of galaxy clusters. The SZ signal changes brightness with the mass of the cluster, but does not change as a function of redshift. This makes it an excellent tracer of large structures in the Universe, since it can be used to characterize the properties of a cluster at any distance. 

An important property of the SZ effect is that, because on average the CMB photons gain energy when scattered, the spectral shape of the SZ effect is a decrement in the temperature of the CMB at low frequencies, and an increment at high frequencies, crossing through a null at around 217 GHz. This behavior is exhibited in the figure below.

The Spectrum of the SZ Effect

Our group specializes in making measurements of the SZ effect in the sub-mm regime where it is an increase in the CMB temperature. Since there are very few ways to make small cold spots on the CMB background, but many to make small hot spots (including all of the galaxies in the Universe), these are challenging measurements. However, the potential payoff is important: subtle changes in the scattering physics due to the state of the electrons in the galaxy cluster atmosphere cause changes in the electromagnetic spectrum of the SZ effect. Though small, these changes can allow measurement of the temperature and velocity flows within the cluster medium, and even a measurement of the velocity of the cluster with respect to the CMB rest frame. These are shown in the figure above in the gold and blue lines; the region where this effect is largest is in the SZ increment. Though still an emerging field, these kind of measurements provide tight constraints on structure formation in the Universe which are independent of other measures.

We are currently leading efforts to measure the SZ effect using instruments like Herschel-SPIRE. The ultimate goal of this work is to map the SZ effect spectrum in many clusters, which will allow measurements of the spectral corrections to the SZ shape. Not only will this yield astrophysical and cosmological information, but also inform new instruments and measurement strategies as we progress.

FURTHER READING

• Wikipedia's Article on The Sunyaev-Zeldovich Effect.
• An SZ Primer from John Carlstrom's Group.
• NASA's explanation of the physics of the SZ effect.
• Mark Birkinshaw's Tour de Force Review Paper on the SZ effect.