Michael Mortonson






Cluster abundance fitting functions
ModeCode inflation module for CAMB/CosmoMC
Reionization module for CAMB/CosmoMC

Contact information

mmortonson {at} berkeley {dot} edu

Lawrence Berkeley National Lab
Berkeley, CA 94720

Useful links

I am a postdoc at UC-Berkeley and an affiliate of LBNL. Currently, I am working primarily with Uros Seljak on finding methods to improve constraints on cosmological models using observations of the large-scale structure of the Universe. My research interests span a variety of topics in theoretical cosmology, including tests of dark energy models and signatures of reionization and inflation in the cosmic microwave background. Previously, I was a CCAPP postdoctoral fellow at Ohio State. I completed my Ph.D. in physics at the University of Chicago, and before that was an undergraduate in Course 8 at MIT.

A few recent results from my research are summarized below. For more details, click on the links to my publications and talks in the sidebar on the left.

Observational Probes of Cosmic Acceleration

Collaborators: David Weinberg, Daniel Eisenstein, Chris Hirata, Adam Riess, Eduardo Rozo

Preprint: arXiv:1201.2434

Our review of observational methods for studying cosmic acceleration and improving constraints on theories of dark energy and modified gravity is now available on arXiv. We hope this will be a useful resource for graduate students and other researchers who are interested in learning about the basic observational techniques. In the review, we summarize the basic principles, the current status of observations, and the main challenges that future experiments are likely to face for Type Ia supernovae, baryon acoustic oscillations, weak gravitational lensing, galaxy cluster abundances, and other probes of acceleration. We also present a new, comprehensive set of forecasts for the constraints on cosmological parameters that these observational methods will provide within the next 5 to 10 years.

Consistency tests of dark energy theories

Collaborators: Wayne Hu, Dragan Huterer, Ali Vanderveld, Tim Eifler
Papers: [1], [2], [3], [4], [5], [6]

Two of the main methods we have to try to understand cosmic acceleration are measurements of cosmological distances as a function of redshift and measurements of the growth of large-scale structure in the universe. Both distance and growth are affected by theories of dark energy or modified gravity that attempt to explain cosmic acceleration, but some theories that make the same predictions for distances make different predictions for growth, and vice versa. So, by measuring distances and growth, we can distinguish among the different theories.

Typically, distances are easier to measure accurately than the growth of structure. Distance indicators such as Type Ia supernovae (SNeIa), baryon acoustic oscillations (BAO), and the cosmic microwave background (CMB) currently measure the distance-redshift relation with an accuracy of a few percent or better over much of the history of the universe. For the simplest theory of cosmic acceleration, a spatially flat universe with a cosmological constant and cold dark matter (flat ΛCDM), we found that these measurements of distances can be used to make predictions for the growth of structure with percent-level precision.

The figure on the right shows the flat ΛCDM predictions from current SNeIa, BAO, and CMB data, plus a measurement of the Hubble constant (H0). Shading shows the regions with 68% confidence level (CL), and solid curves mark the boundaries of the 95% CL regions. The top panel shows predictions for the growth function G and for the same function with an alternate normalization G0, the middle panel shows predictions for the differential growth rate fG and the growth index γ (often used as a diagnostic for modified gravity), and the bottom panel shows predictions for the extrapolation of distance D to high redshifts and for the Hubble expansion rate H.

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Signatures of reionization in the CMB

Collaborators: Wayne Hu, Cora Dvorkin, Hiranya Peiris
Papers: [1], [2], [3], [4], [5], [6], [7]

During the epoch of reionization, hydrogen atoms in the intergalactic medium are ionized by radiation from the first stars and quasars. The resulting free electrons rescatter photons from the CMB, reducing the amplitude of CMB temperature anisotropies and generating additional CMB polarization anisotropies on large scales. The effect on the angular power spectrum of the CMB temperature (CTT) is difficult to distinguish from changes to the amplitude of primordial fluctuations from inflation, but in the E-mode polarization power spectrum (CEE) reionization produces a detectable feature on large scales (low ℓ). From WMAP measurements of the amplitude of this feature, we know that the total optical depth to reionization is approximately τ=0.09, which implies that reionization occurred at a redshift of about 10.

While measurements of CEE can determine the total optical depth τ fairly well, it is more difficult to tell whether the transition from a neutral medium to an ionized medium happened quickly or slowly. The figure on the right shows two very different ionization histories with similar values of τ (see inset) and their corresponding polarization power spectra. Although there are clear differences in the spectra at ℓ<30, data from WMAP are not precise enough to distinguish between these models (see points and error bars, showing 3-year WMAP data). Until the large-scale polarization measurements improve with data from upcoming experiments like Planck, the detailed evolution of the ionized fraction during reionization will remain largely unknown. Given this uncertainty, most analyses of CMB data assume a simple form for the ionization history (e.g., an instant transition).

In a series of papers, we studied the impact of this uncertainty in the ionization history on the estimated values of cosmological parameters using a general parametrization of the ionization history based on principal components. With current WMAP data, the impact is fairly small: relative to an analysis that assumes instant reionization, accounting for uncertainty in the ionization history slightly increases both the central value of the estimated optical depth (to about 0.10) and the uncertainty in τ. Even this small change can significantly affect parameters correlated with τ. For example, evidence for deviations from a scale-invariant primordial power spectrum weakens when reionization uncertainty is considered. Future CMB polarization measurements from Planck and other experiments will improve constraints on reionization but will also be more susceptible to assumptions about the form of the ionization history; for Planck, accounting for uncertainty in the evolution of the ionized fraction is expected to increase the error on τ from 0.005 to 0.01.

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