Rachel Bean : Research Interests

With recent unprecedented improvements in cosmological observations, the properties of the universe are being measured with increasingly exquisite precision. Two dramatic dark augmentations of a minimal theoretical picture of Standard Model matter interacting gravitationally through general relativity are required. There must exist a new component of the cosmic energy budget - dark matter - that supports the formation of large scale astrophysical structures and, secondly, a gravitational modification or energy component - dark energy - that drives late-time accelerated cosmic expansion.

The underlying physical origins of these two dark components is still unknown. Fundamental dark matter particle candidates might well be expected to have interactions beyond the purely gravitational. Such interactions could have astrophysical consequences, such as self-annihilation of the particle with the release of energetic photons, disruption of tidal streams of satellite galaxies around the Milky Way, and modifications to the distribution of dark and visible matter in galaxies and clusters of galaxies.

In this paper with Eanna Flanagan (Cornell), Istvan Laszlo (Cornell) and Mark Trodden (Syracuse), we place constraints on the size of extra non-gravitational interactions between dark matter and dark energy components using large scale cosmological observations. We find that these interactions can be at most 7% the strength of gravity. We also show that cosmological measurements constrain a specific type of interaction (a Yukawa interaction) between fermionic dark matter particles to be less than 5% the strength of gravity on scales of 10Mpc (~30 million light years), about the size of a large cluster of galaxies.

The degree of ionization (the ratio of ionized atoms to neutral ones) in the universe has varied over the course of its history. During the first 400,000 years the universe was a fully-ionized plasma. Around 400,000 years after the Big Bang, electrons combined with protons to form neutral hydrogen and the universe became effectively neutral. It remained neutral until about 400 million years after the Big Bang when photons from the earliest stars reionized the inter stellar medium.

The ionization history of the universe is of great importance to cosmologists as: 1) a sound understanding of the ionization history allows us to infer more about other cosmological properties we want to measure 2) it tells us about the presence of sources of ionizing radiation e.g. from the decay or annihilation of massive particles into a shower of photons (and other elementary particles) or from the evaporation of primordial black holes.

Silvia Galli and Alessandro Melchiorri (Rome), Joe Silk (Oxford) and I investigated the constraints we can place on the presence of additional ionizing sources from the Cosmic Microwave Background and galaxy correlation measurements.

We find that current data improves the upper bound we can place on how much extra sources can contribute to the ionization history. We also discovered, however, that allowing for such sources reduces the precision with which we can measure other important cosmological parameters, for example the properties of initial density fluctuations from inflation and the masses of neutrinos.

We believe that future small scale (arc-minute resolution) temperature and polarization measurements of the CMB will help mitigate this sensitivity to ionization history.

We also have two previous papers on this subject using earlier data from 2003 and 2007.

String theory is a way in which the fundamental forces, electromagnetism, the strong and weak forces and gravity might be unified within a single theory. Such a unification would occur at energies a billion times greater than the energy of cosmic rays and a billion billion times greater than those achievable in particle accelerators. There might be a way to test such high energies, however, by looking at the cosmological consequences of processes that happened just after the Big Bang, during the process of inflation, when particle energies were large enough.

In recent work, we considered the observational implications of string theory inflation features that are smooth function of length scale. In this work , Xingang Chen, Girma Hailu, Henry Tye and Jiajun Xu and I considered whether sharp 'kinks' in the primordial initial conditions, occuring at specific length scales, might be observed in the Cosmic Microwave Background . Girma and Henry showed in earlier work that such sharp features arise naturally in string theory models of inflation.

These sharp features show up in the statistical correlations of the CMB temperature fluctuations. The best measured statistic is the two-point correlation function (the average correlation in CMB temperature between two points on the sky separated by a given angle), however the string features also predict concurrent implications for higher order statistics, such as the three-point correlation function. The precision of these statistical measurements has improved dramatically in the last few years and promises to improve by a factor of ten in the next decade. This opens up the exciting prospect that we might find distinctive signatures of string theory in the CMB in the near future.

Dramatic improvements in the precision of Cosmic Microwave Background (CMB) observations (for example, from the WMAP satellite ) are providing unprecedented insights into the process, known as 'inflation', that, in the early universe, set up the initial fluctuations in the density of matter, that have since grown to form cosmic structure, such as galaxies.

The simplest theories of inflation contain a single particle, the 'inflaton', slowly evolving according to equations of motion governed by the particle's kinetic and potential energy. Because the particle is slowly evolving, the potential energy is the key factor and therefore observations are often directed to either constrain specific theories, in which the potential energy evolves according to a prescribed form, or reconstruct the potential energy making less assumptions about its exact form.

Recently alternative theories of inflation, such as brane inflation, have been proposed in which the particle does not have to evolve slowly. Efforts have likewise been directed to try and constrain parameters in these theories.

In collaboration with Daniel Chung (Wisconsin) and Ghazal Geshnizjani (Perimeter Institute), I've been considering how observations might be able to tell us about the nature of inflation without making theoretical assumptions, apriori, about the form of the theory.

In this work we develop a formalism to translate cosmological observations, using the CMB power spectra and higher order correlations, into general descriptive properties of the inflationary behavior, and in this way reconstruct the inflationary theory without bias.

We believe that the fluctuations in cosmic matter density, from which galaxies and clusters of galaxies evolve, were created through a process known as 'inflation'. This is an epoch, a trillionth of a second after the Big Bang, when the universe rapidly accelerated, and grew from quantum scales to scales where classical physics holds.

Although, conceptually, inflation fits observational data well, its theoretical origins remain a mystery. One possibility is that inflation generated as a result of the universe containing extra dimensions - so-called 'brane inflation'. In these theories, the observed 3-dimensional universe, is only part of the picture: it is a 'brane' (think of it as a membrane) on which matter exists, embedded in a higher dimensional 'bulk' through which the brane can move. We consider a scenario in which the brane is travelling down a deformation in the bulk called a 'throat' (like the finger in a rubber glove). This motion can generate inflation on the brane.

In recent work with Xingang Chen (Cornell), Hiranya Peiris (Cambridge) and Jiajun Xu (Cornell) and in earlier work in collaboration with Sarah Shandera (Columbia), Henry Tye (Cornell) and Jiajun Xu (Cornell), we've considered observational implications of brane inflation and compared them to current cosmological observational data from the Cosmic Microwave Background, galaxies and supernovae.

It is exciting that the data is getting precise enough that we can start to test the feasibility of these string-theory motivated models. Upcoming surveys which will measure the CMB to greater precision, will allow to tighten constraints on these models even further.

Most cosmological observations investigating dark energy focus on measuring the average density of the universe or observations in which the local over- or under-density of matter is small in comparison to the average (linear fluctuations). This includes the measurement of distant supernovae and the Cosmic Microwave Background.

In collaboration with Mark Trodden (Syracuse) and Eanna Flanagan (Cornell), I've been investigating theories which can give consistent theoretical predictions for the background expansion and have been studied as possible dark energy explanations. We study the behavior of these models as acceleration switches on however, and show that they suffer catastrophic gravitational instabilities, leading to large scale structure inconsistent with observations.

The instability arises because matter is coupled non-minimally to a scalar field, and the coupling and self-interaction potential of the scalar field combine to form an effective potential with a minimum. If the gradient of the coupling is steep enough, i.e. the coupling strength is large enough, then the scalar sits in the minimum, and this drives accelerated expansion. However, twinned with the acceleration, is a heightened growth rate for pretty much all cosmological scales.

The work is summarized in a short letter and a more detailed paper .

There is a lot of interest in extending cosmological observations to include those looking at correlations between large scale structure measurements at small scales (<10Mpc). In particular it is hoped that such measurements, which include galaxy surveys and weak lensing, (measuring the General Relativistic effect that light is distorted by a local gravitational field) will allow us to look for differences in these large scale structure observations that could help establish if dark energy is a result of modified gravity or standard GR with a cosmological constant.

On small scales, the over- and under-densities can be comparable or much larger than the average cosmic density and therefore linear perturbation theory doesn't work and we have to look at non-linear growth. In order to do this one has to conduct numerical simulations to model the growth over time and as a function of scale. A major impediment to applying current and prospective surveys to contrast different dark energy theories is a lack of non-linear simulations of growth in modified gravity theories.

Istvan Laszlo, an Astronomy graduate student working with me here at Cornell, and I investigated the non-linear growth in variety of modified gravity theories performing N-body simulations of growth in the modified theories. We considered models in which the Poisson equation, and how the gravitational potential is determined by the local density of matter, is altered, and models in which the way matter responds to the gravitational potential gradient is modified, through the presence of anisotropic shear stress.

We compared the results of the N-body simulations to analytical fits of Smith and Peacock and Peacock and Dodds , that are based on simulations of standard gravity. We find that the fits are remarkably good at modelling the growth in the modified theories, and therefore offer the possbility that the analytical fits can be applied to interpret current and future observationsin terms of modified gravity theories. This is important if we are to be able to make precise inferences about the nature of dark energy.

Scalar-tensor theories of gravity are modifications of General Relativity that have been investigated as possible solutions to the dark energy problem. One can view these theories from two different perspectives. In the Jordan frame, the theory is viewed as a modification to gravity, while matter is minimally coupled and follows geodesic paths. A conformal transformation, a redefinition of the gravitational metric, can alter the perspective so that gravity is normal General Relativity, but the matter fields are now non-miminally coupled to an additional scalar degree of freedom so that they no longer follow geodesic paths.

Nishant Agarwal, an Astronomy graduate student working with me here at Cornell, and I investigated the dynamical evolution of general scalar tensor theories to understand the necessary properties they must have to give cosmological evolution consistent with observations.

We established the critical points of the dynamical attactors, in the Einstein and Jordan frame (the physical results are frame independent), and found viable classes of theory that would give a transitory matter dominated era that would transition into an accelerated phase, either stable or unstable. This analysis technique is a power tool for discriminating between viable and non-viable models and quantifying criteria for cosmological consistency.

At the end of the 19th century, Lord Kelvin gave a talk about the "dark clouds" that hung over the physical laws of the time, including Newton's law of gravity. The orbit of Mercury had been measured to great precision and was found to be 'anomalous' to that predicted by Newtonian gravity. Many explanations for this discrepancy were made, including it being a result of an extra, hidden planet named `Vulcan'. By far and away the best explanation, however, (since it has no free parameters to `tweak') was that Newton's gravity had to be replaced with Einstein's theory of General Relativity.

Cosmologists are currently facing a simimlar predicament following the observation that the universe's expansion is accelerating; General Relativity with normal matter would predict the universe's expansion should be decelerating. The name `Dark Energy' has been coined to label what might be the cause of the acceleration. But does Dark Energy involve a modification of General Relativity by including new matter (a modern day equivalent to `Vulcan') or by modifying the theory of gravity itself (how matter distorts space). Can we hope to distinguish between these two possibilities?

With Pengjie Zhang (Shanghai Astronomical Observatory), Michele Liguori (Cambridge) and Scott Dodelson (Fermilab/ Chicago) I've been looking into whether we can use cosmological observations to discriminate between these two options.

By looking at the (in)consistency between the observed matter distribution and the measured gravitational lensing signal we propose that one might be able to distinguish between matter and gravity-based theories with upcoming experiments.

In 1998 measurements of distance supernovae suggested that the universe's expansion is accelerating, rather than decelerating as General Relativity predicts in a universe filled with normal matter. `Dark energy' is the name given to the mysterious origin of this acceleration. Theoretical cosmologists are trying to come up with possible explanations for these puzzling observations.

In recent work, with David Bernat (Cornell), Alessandra Silvestri (Syracuse), Levon Pogosian (Simon Fraser) and Mark Trodden (Syracuse), I've been looking at whether modifications to Einstein's General Relativity could be the origin of dark energy.

We studied a class of models, known as 'f(R) gravity', which can give rise to perceived acceleration today by modifying how the expansion of space time is related to the density of matter.

We found that, although the theories can give rise to acceleration, their predictions for how the density fluctuations give rise to large scale structure (such as galaxies and clusters of galaxies) are inconsistent with observations.

These conclusions suggest that it will be hard to generate an f(R) theory that is in agreement with all cosmological observations.

One possible origin of dark energy is that it is a new type of matter, known as a `scalar field'. The density of the universe is very low today in comparison to the early universe, when matter was formed. For the scalar field to dominate the universe today it has to have an extremely light mass, which is difficult to motivate theoretically. One way to generate the light mass is to couple the scalar field to another particle of light mass, for example the neutrino.

With collaborators Axel De La Macorra (UNAM), Alessandro Melchiorri (Rome) and Paolo Serra (Rome) I investigated whether combining current ground-based, direct observations of neutrino properties from particle detectors, when combined with indirect cosmological neutrino observations could tell us something about the nature of this coupling.

When averaged over cosmological scales, scales bigger than millions of lightyears (much larger than the size of galaxies or clusters of galaxies) the universe appears to have a pretty even density. We know that this is not the case, however, on smaller scales (since we have over densities in the form or galaxies, stars and planets).

We believe that the fluctuations in density, from which galaxies and clusters of galaxies evolve, were created right after the Big Bang through a process known as `inflation'. This is an epoch, a trillionth of a second after the Big Bang, when the universe rapidly accelerated, and grew from quantum scales to scales where classical physics hold.

Simple models of inflation, based on theories with a single scalar field, predict that the fluctuations were generated in an entropy conserving or `adiabatic' process. Many other models, in which multiple scalar fields were involved, or in which matter was produced after inflation, predict additional non-adiabatic or `isocurvature' fluctuations. These isocurvature fluctuations would lead to observational differences in the cosmic microwave background fluctuations.

With Joanna Dunkley (Princeton) and Elena Pierpaoli (USC) I looked at constraints we can place on isocurvature fluctuations using the 3-year WMAP data. On single isocurvature modes the observational constraints are very tight. If one allows multiple isocurvature modes, however, cancellation between them means that a surprisingly large component of the initial conditions can come from isocurvature fluctuations.

The cosmic microwave background (CMB) is relic radiation emitted just 400,000 years after the Big Bang. It gives us the earliest observations of the universe; prior to its formation the universe was an opaque plasma of hot ions. The CMB also gives us an integrated history of the universe since it was formed, since the CMB photons have witnessed many epochs in the universe's history on their journey to us today, for example: the formation of galaxies, the first stars forming, and the recent period of accelerated expansion.

Recently, I was a member of the NASA Wilkinson Microwave Anisotropy Probe (WMAP) science team analyzing data from WMAP satellite and understanding its cosmological consequences.

The results after 3 years of observations made a particularly important development that much improved our understanding of the universe: WMAP measured not only the temperature of the CMB photons but also their polarization. The polarization gives us much more information about the epoch of the first star formation, when light emitted by the early stars ionized the inter stellar medium, known as `reionization'.

Usually many theoretical models can fit the data rather than just one, this is known as `degeneracy', and limits how much we can learn from observations. By better tying down when reionization occurs we reduce these degeneracies and can learn a lot more about other cosmological parameters e.g. the different types of matter in the universe, and how the initial flucutations in the density of matter were produced in the early universe.

Check out the cool CMB data (yes data can be cool!) and guides to the CMB at the Legacy Archive for Microwave Background Data Analysis (LAMBDA) .