Our group focuses on making precision measurements of the Cosmic Microwave Background (CMB) using world-leading international CMB experiments in Chile and the South Pole. The CMB is our first image of the infant Universe — when the Universe was a mere 0.003% of its current age — and provides stringent tests of our understanding of how the Universe began and how it has evolved. We work on all stages of the analysis of these Petabyte-sized data sets, from experimental characterization to cosmological interpretation.
I am happy to discuss potential research projects with prospective Masters or PhD students. There are also opportunities for enthusiastic undergraduates looking to do a research project in cosmology, typically in either your second or third year of study.
Dark Energy
In 1998, observations of supernovae led to the discovery that the expansion of the Universe was accelerating with time, instead of decelerating as one would expect due to gravity. The source of this acceleration, which we call Dark Energy, is one of the biggest mysteries in cosmology today.
Galaxy Clusters
Galaxy clusters are the most massive collapsed objects in the Universe today, with masses from a 100 to a 1000 or more times the mass of the Milky Way. As they fall on the extreme tail of the mass distribution, the number counts of galaxy clusters are exquisitely sensitive to anything that affects structure growth in the Universe — such as Dark Energy.
In the Sunyaev-Zel’dovich (SZ) effect, the (cold) photons of the cosmic microwave background (CMB) inverse Compton scatter off the very hot electrons of the intracluster medium. This leaves a deficit of CMB photons below 217 GHz. The figures to the left show one of the first four galaxy clusters discovered via the SZ effect (Staniszewski et al., ApJ 2009). The top panel is an optical image of the cluster, SPT-CL 0517-5430; the bottom panel is the SZ signal seen by the South Pole Telescope.
The SZ effect is a superb tool for finding massive clusters, especially at high redshifts, since unlike optical or X-ray signals, the SZ signal doesn’t dim with redshift. The SZ flux is also closely tied to the overall cluster mass, making SZ surveys a great way to create mass-limited samples of galaxy clusters.
There are MsC and PhD projects related to all of these experiments.
CMB-S4:
Simons Observatory:
Simons Array:
SPT-3G:
2032 – (expected)
2024 – (expected)
2023 –
2017 –
The South Pole Telescope is a 10m telescope at the geographic South Pole. It is midway through the SPT3G survey, which is making precise measurements of the polarized CMB anisotropies. The next camera, SPT3G+, is being designed currently. A prototype CO line intensity mapping receiver, SPT-SLIM, will observe in the 2024-2025 Austral summer.
The main science goals are twofold. First, studying the gravity waves produced during Inflation using degree-scale polarized CMB anisotropies. Second, reconstructing a map of all structure in the Universe from z=0 to 1100 using arcminute-scale CMB anisotropies. This map will be invaluable for studying the growth of structure and tightly constraining the sum of the neutrino masses.
Additionally, the SPT surveys are producing a catalog of galaxy clusters selected via the Sunyaev-Zel’dovich (SZ) effect. The SZ signal is nearly independent of redshift, and the SZ surveys are uniquely powerful at finding high-redshift galaxy clusters. Notably, the SPT survey area overlaps the on-going optical Dark Energy Survey which will greatly assist in studying the galaxy clusters in both surveys. The dark energy constraints from clusters are competitive with supernovae, and a combination of clusters and supernovae can help distinguish between the dueling paradigms of modified gravity and dark energy.
All three telescopes of the Simons Array have now been installed in Chile. The first receiver is currently taking engineering data to prepare for science observations, while installation of the second receiver is underway. The Simons Array builds upon the success of POLARBEAR, by adding 2 more telescopes and expanding each camera from ~1400 detectors to ~7500 detectors. The full Simons Array will have more than 22 thousand bolometers, split between 95, 150, 220 and 280 GHz! The experiment will conduct two surveys – a wide survey covering half the sky aimed at mapping the mass in the Universe through gravitational lensing, and a deep survey aimed at discovering gravitational waves from inflation.
The University of Melbourne is a partner on the Simons Observatory, a precursor to the planned CMB-S4 experiment. The Simons Observatory will combine one Large Aperture Telescope with six Small Aperture Telescopes to make low-noise maps of half the sky while searching for the inflationary gravitational wave signal on a smaller, ultra-low-noise region.
Photo: The Receiver of the Large Aperture Telescope being installed in the telescope in Aug. 2023.
Past Experiments
The standard model of Cosmology invokes the idea of Inflation to explain why the Universe is flat, as well as other problems.
During Inflation, the Universe undergoes an extremely rapid, exponential expansion, as illustrated by the blue band in the plot to the left. This rapid expansion stretches out the radius of curvature of the Universe, pushing it towards flatness.
Inflation predicts a nearly-invariant spectrum for the primordial density fluctuations, a prediction that has now been confirmed by observations. These fluctuations are the seeds that eventually grow into galaxies and the other structures we see today.
The rapid stretching of space-time during Inflation should lead to a cosmic background of gravitational waves, with wavelengths comparable to the size of the Universe. These gravitational waves have been called the “smoking gun” of Inflation, and were one of the first predictions of Inflation models. However, they are yet to be observed. Detecting these “inflationary gravitational waves” would conclusively prove that Inflation occurred.
The most-promising way to detect these gravitational waves is through the unique imprint they would leave in the polarization pattern of the cosmic microwave background. Searches for the Inflationary gravitational waves is one of the key science goals of all the experiments we work on in the Observational Cosmology group at Melbourne.
Explore the South Pole, through a collaboration with Google’s street view project. This virtual tour was filmed in 2014 and shows you the South Pole Telescope and other cosmic microwave background telescopes located in Dark Sector of the South Pole Station. The main station (where people live) is on the other side of the runway.
You can find more images of the South Pole Telescope on Google maps.
Christian Reichardt
313 David Caro
+61 3 8344 4136
christian.reichardt@unimelb.edu.au
Current members
Behzad Ansarinejad Postdoctoral Scholar 2021 –
Mahsa Rahimi Postdoctoral Scholar 2021 –
Eduardo Schiappucci PhD 2020 –
Justin Clancy MSc, PhD 2020 –
Michael Doohan PhD 2021 –
Kevin Levy PhD 2022 –
Jin Lee MSc 2023 –
Junhao Zhan MSc 2024 –
Past members
Federico Bianchini Postdoctoral Scholar 2016 – 2020
Nikhel Gupta Postdoctoral Scholar 2018 – 2020
Srinivasan Raghunathan Postdoctoral Scholar 2015 – 2018
Lennart Balkenhol MSc, PhD 2023
Prakrut Chaubal PhD 2023
Thi Anh Pham PhD 2020
Sanjaykumar Patil PhD 2019
Arwa Abdulghafour MSc 2022
Mitchell de Zylva MSc 2019
Bryce Murphy MSc 2018
Dylan Sutton MSc 2016
Anthony Doucouliagos MSc 2016
Fanuel Rumokoy MSc 2015
Christian Reichardt
christian.reichardt@unimelb.edu.au
313 David Caro
+61 3 8344 1436
