M. Zemcov’s Research Group: Experimental Cosmology

Overview

Dr. Michael Zemcov is an experimental astrophysicist at the Center for Detectors of the Rochester Institute of Technology whose scientific background and interests are centered on cosmological observations including studies of diffuse radiation in the cosmos, particularly the cosmic microwave and infra-red backgrounds. Currently, his focus is on the measurements of cosmic large scale structure, the epoch of reionizationsecondary anisotropies in the cosmic microwave background, and the history of the star formation in the Universe. Dr. Zemcov has extensive experience with instrumentation, observation and data analysis throughout the electromagnetic spectrum from the optical to the radio, with particular emphasis on the infra-red and sub-mm/mm regimes. He is currently involved in a number of projects in a variety of roles, ranging from technology development to the scientific interpretation of data from mature instruments.

Research

A major focus of my current research is observation of the epoch of reionization. This was the era during which the first population of stars in the Universe, seeded by the same over-densities visible in the cosmic microwave background radiation, ionized the neutral hydrogen then ubiquitously present in the Universe. The epoch of reionization is at the forefront of current cosmological research, and what little is known about it suggests it was virtually complete at the earliest times we are able to probe with today's best optical instruments. The details of the process, such as when it began, how long it took, and what class of sources produced it, are not known. The image below shows our current understanding of the process of reionization, which began roughly 500 million years after the big bang and was complete about 500 million years later.

Reionization Era Chart

A promising way to study the epoch of reionization is using 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, is currently being designed and will fly in 2013. In addition, we are working on a small space mission to measure the near infrared fluctuations from a distant vantage point in the solar system such as Saturn named ZEBRA. The future of reionization studies in the infrared is promising.

FURTHER READING

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 behaviour is exhibited in the figure below.

The Spectrum of the SZ Effect

SZ Effect Spectrum Chart

I have concentrated much of my SZ research on making measurements of the spectrum of the SZ effect, particularly in the sub-mm regime where it is an increment in the CMB temperature. These are challenging measurements, 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 sub-mm and radio sources in the Universe). However, the potential payoff is rewarding: 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. 

I am currently leading efforts to measure the SZ effect using instruments likeHerschel-SPIREZ-Spec, and SCUBA-2. 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

SZ-RELATED PHOTOS
Group photo for the SZ2011 conference in Santander, Spain. 

Group Photo for SZ2011 Conference

A photograph of the Dark Sector at the Amundsen-Scott South Pole Station taken circa 2007 by Iceman. Many historic measurements of the SZ effect and CMB were taken with the various telescopes visible in this image, including QUaD in the upper middle, with Viper just below it, both part of the Martin A. Pomerantz Observatory. Further up from that is what remains of the old AST/RO telescope buried by snow. At the bottom is the new dark sector lab which houses BICEPand SPT.

Amundsen-Scott South Pole Station

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.

Galaxies in the Universe Plot

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.

FURTHER READING

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. 

Timeline of the Universe Graphic

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. 

Zodiacal Light Artist Rendition

About Dr. Zemcov

 

Michael Zemcov is a research professor at the Center for Detectors and the School of Physics and Astronomy whose primary focus is experimental astrophysics.  His research centers on instrumentation for cosmological observations, including the cosmic microwave and infra-red backgrounds. He develops instruments and data analysis methods for a variety of platforms, including ground-basedsub-orbital rockets, and orbital observatories.  Currently, his scientific focus is on the epoch of reionizationsecondary anisotropies in the cosmic microwave background, and studies of the history of the star formation in the Universe using novel techniques and experiments. He has extensive experience with instrumentation, observation and data analysis for astrophysics throughout the electromagnetic spectrum from the optical to the radio, with particular emphasis on the infra-red and sub-mm/mm regimes. His research group is currently involved in a number of projects in a variety of roles, ranging from technology development to the scientific interpretation of data from mature instruments.

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