Research
The history and structure of the universe are determined by constituents that cannot be directly seen: dark matter, dark energy, and perhaps an inflationary scalar field. I am broadly interested in what we can learn about these constitutents from directed observations of massive galaxy clusters, surveys of galaxies, the cosmic microwave background (CMB) , as well as the atomic and more general baryonic physics that make possible all of these windows onto unknown physics. I also maintain an interest in the epoch between inflation and big-bang nucleosynthesis (BBN), particularly in empirically allowed alternatives to the conventional thermal history of the universe during this epoch. My research requires theoretical and computational work, as well as data analysis.
More precisely, my Ph.D. thesis will have 5 main sections, ordered here by centrality to my current efforts: the effect of high-excitation states on cosmological hydrogen recombination, the echo of halo merger histories in dark matter halo concentrations, new telescope searches for decaying thermal relic axions, large-scale-structure (LSS) constraints to axions in modified thermal histories, and observational constraints to 'fat graviton' theories.
I maintain an interest in Lyman Limit systems.The effect of high-n states on cosmological hydrogen recombination
Completed work on recombination:
This paper is on the arXiv (http://arxiv.org/abs/0911.1359) and has been submitted for review to Physical Review D.The well-known pattern of bumps and wiggles in the CMB angular power spectrum comes from acoustic oscillations in the baryon-photon plasma before primordial hydrogen first becomes neutral ('recombination'). The May 2009 launch of the Planck satellite has pushed us deeper into the era of precision cosmology. Many hope to rewind this snapshot of the universe at ~300,000 years after the beginning to better understand the extremely high-energy physics responsible for inflation and cosmological density perturbations. Errant models of recombination could mimic the effect of variations in the value of
(which reflects the influence of the inflationary power spectrum on CMB fluctuations), 
(which reflects the influence of re-ionization by the first stars, galaxies and quasars on CMB fluctuations), and 
; accurately estimating these parameters from Planck data thus depends crucially on getting the recombination history right. This requires an understanding of the process of hydrogen recombination with an order of magnitude better accuracy than previous work, which has already shown the importance of highly excited states with n>100. The question of convergence with n remains unsolved. The recombining plasma dramatically falls out of statistical equilibrium at late times, making accurate calculations of the free electron fraction computationally demanding if highly excited states are to be included. I have developed a new cosmological recombination code, RecSparse, which exploits dipole selection rules and sparse matrix techniques to include states as high as n=300, separately follow all relevant states, all while keeping recombination calculations computationally tractable. This code also includes, for the first time, the effect of electric quadrupole transitions on cosmological recombination. I have modified the CMB anisotropy code CMBFast to include these effects and establish the importance of high-n states for predictions of the multipole moments that will be observed by Planck. At Fisher Matrix level, hydrogen recombination is converged well enough for Planck data analysis for a maximum n value of n>128.
My chief collaborator in this effort is Prof. Christopher M. Hirata.
Future work on recombination:
The effect of atomic collisions on this process is still poorly understand, largely because small impact parameters and non-adiabatic impulses invalidate simple estimates of the relevant collisional rates. I aim to improve theoretical calculations of these rates in future work to definitively address the question of high-n convergence. I also believe similar sparse matrix techniques could be fruitfully applied to accelerate computations of the distortion to the CMB blackbody from atomic recombination lines. These distortions may someday be detected and yield a view of the baryon-photon plasma behind the veil of the last scattering surface. We see tentative evidence for population inversion between some states in our results. If this lead to cosmological masing, this could make it easier to detect CMB spectral distortions from recombination. Thus, we plan to explore if this radiation stays coherent, and assess the affect of collisions on population inversion. Finally, Yacine Ali-Haimoud has done extremely careful analytic work on the effect of line-overlap at high n on the radiative transfer. In the near future, I will include this additional physics in RecSparse. I will also explore mock Planck data sets using the CosmoMC package to determine how badly biased CMB parameter estimates will be if this additional piece of recombination physics is not taken into account.
The echo of halo merger histories in dark matter halo concentrations
Dark matter halos appear to be well described by a universal density profile, parameterized by two numbers: the halo concentration c, which describes how centrally condensed a halo is, and its mass M. These two numbers are inversely correlated. This is thought to result from the fact that more massive halos collapse at later times, in a more diffuse universe. I am working to understand the considerable scatter in the mean c(M) relationship by combining a simple energy-conserving model for the properties of a merger remnant halo with halo merger trees derived from the extended Press-Schechter formalism and the Millenium simulation. This scatter has considerable implications for the properties of galactic disks and other galaxy observables predicted by semi-analytic models of galaxy formation such as GalForm. I will test this model using the results of dedicated two-halo merger simulations recently performed by Dr. Stelios Kazantzidis.
My chief collaborator in this effort is Dr. Andrew Benson.Telescope searches for decaying thermal relic axions
Completed work:
This work is detailed in astro-ph/0611502 and was published in Physical Review D.One leading dark matter candidate is the axion, a hypothetical particle proposed to deal with the strong-CP problem; more specifically, the axion is enlisted to dynamically drive the electric-dipole moment of the neutron to zero, thus satisfying experimental constraints without unsightly fine-tuning. The axion mass is apriori unconstrained; although constraints abound. In the ~eV mass window, axions would be thermally produced before BBN with relic density roughly equal to that of neutrinos or baryons. Such axions would not be cosmologically dominant, but they would be cosmologically important. Due to their two-photon coupling, such axions would decay to produce mono-chromatic line emission. I led a team (Prof. Marc Kamionkowski, Giovanni Covone, Prof. Jean-Paul Kneib, Eric Jullo, and Prof. Andrew Blain) to search for axionic decay emission in the galaxy clusters A2667 and A2390 using the VIMOS IFU at the VLT in Chilé. Our sensitivity improved on past work for two reasons. First of all, we have a 10-m class telescope at our disposal with plenty of integration time. Second of all, strong lensing maps tell us where this decay line is strongest, allowing optimal extraction of any putative axion signal. Our results impose the strongest existing limits on the two-photon coupling of the axion in the 4.5-7.7 eV mass window, shown below along with an image of A2390.
Ongoing work:
This work is described in my successful VLT VIMOS proposal for ~20 hours of observation time to look for decaying relic axions in the high-z galaxy cluster RDCS 1252. My collaborators on this work are Prof. Marc Kamionkowski, Giovanni Covone, Prof. Jean-Paul Kneib, and Eric Jullo.The above plot highlights the fact the utility of a telescope search for decaying relic axions in the 8-14 eV, using optical IFU observations of a z~1 galaxy cluster. Although any faint signal would fall off with redshift, the higher-redshift observations would probe an axion mass
. Since the intensity of the axion decay signal at fixed rest-frame wavelength scales as 
here, the optical telescope axion searches in the 8-14 eV window would be roughly two orders of magnitude more sensitive (in terms of the model-dependent two-photon coupling parameter 
) than existing limits, obtained using measurements of the diffuse extra-galactic background radiation (DEBRA) and limits derived using the lifetimes of helium-burning stars.To this end, I put together a proposal to obtain 15-20 hours of VIMOS IFU spectroscopy of the galaxy cluster RDCS 1252, once the highest-redshift known galaxy cluster, in order to search for decaying relic axions. The proposal was finally approved and observations obtained by spring 2008. Using Rosati's weak lensing maps, I am working closely with Eric Jullo to perform a search for decaying relic axions in this galaxy cluster. This window is particularly interesting, because axions in this mass window could be a considerable fraction of the cosmological dark matter, as we know from the following expression for their relic density:
It is often said that axions in this mass window are already highly constrained by stellar lifetimes and DEBRA measurements. Reports of their demise, however, are greatly exaggerated. The reason is that the two-photon coupling of the axion is highly dependent on the axion model ; the coupling also depends on the poorly-known up-down quark mass ratio (lattice QCD constraints still disagree by as much as 50%!). With a moderate amount of fine tuning on the theoretical side, existing experimental constraints still allow an axion in this mass window, and indeed significantly relax many astrophysical axion constraints . Thus, an axion in the 8-14 eV mass window, if weakly coupled to photons, could be almost all of the dark matter, and would (as discussed below) be sufficiently cold to escape limits from the galaxy correlation power spectrum and large-scale structure.
LSS constraints to axions in modified thermal histories
Completed Work:
This work is detailed in arXiv:0711.1352, and was published in Physical Review D. This work was done in collaboration with Tristan Smith andProf. Marc Kamionkowski.Most experimental searches for axions and astrophysical constrains depend on their two-photon coupling (e.g. ADMX, CAST, PVLAS, gamma-eV, globular cluster stellar evolution, and our telescope searches). Since this number is model-dependent and may even vanish in some parts of theoretical parameter space, it is important to look for evidence of axions using couplings not subject to this problem. The axion-hadron couplings do not vanish in any allowed region of parameter space. For example, consider axion interactions with nucleons.
Nuclear resonances in Li, Fe, Kr in the sun would produce a beam of axions detectable via the same resonances on Earth. Some constraints using these techniques already exist and work is ongoing to explore more of the axion parameter space with these techniques.The hadronic couplings of axions are also responsible for the rates keeping them in thermal equilbrium in the early universe.
~eV axions would thermalize through interactions with pions and freeze-out in the early universe, leaving behind a relic population that is relativistic when structure forms, suppressing the galaxy correlation power spectrum on small scales. This is similar to the effect of standard model neutrinos, and indeed the best constraints to the absolute scale of neutrino masses comes from cosmology. This was used by Hannestad and collaborators to place an upper limit on the axion mass of 1.4 eV using WMAP1 and the Sloan Digital Sky Survey (SDSS). They evaluated constraints leaving both the freeze-out temperature (parameterized by gs, the number of relativistic degrees of freedom at axion freeze-out) and the relic density in axions as free parameters, even though in standard QCD axion models, there is a one-to-one correspondence between the axion mass and its decoupling temperature. The constraints they derived are shown below:
The combined purple-pink region is excluded at 2 standard deviations. If axions froze out earlier (higher gs) than in the usual scenario, their free-streaming length would be small enough that it falls into a regime that cannot be probed by the galaxy auto-correlation power spectrum. In fact, the flattened contour at high gs corresponds to the minimum length scale at which SDSS accurately measures the galaxy auto-correlation power spectrum. At this point, constraints to thermal axions are completely relaxed. This got us thinking: Is there any way to change the cosmological scenario to relax constraints on axions without spoiling the successes of the standard big-bang cosmology? In other words, is there a physically plausible way to realize the high gs (at freeze-out) part of the allowed parameter space?
I realized that the answer is yes! Prior to BBN, we really know very little about the expansion history of the universe. If the universe is dominated by an unstable scalar of mass in the TeV range, its decay will generate entropy, delaying the start of the standard radiation dominated expansion phase of the universe until the radiation has a temperature of T> ~ 4 MeV, just in time for BBN not to be messed up. This will wash out the abundance of any species that freeze out at earlier times and leave the relevant particles much colder than species coupled to the plasma. Prior work on these low-temperature reheating (LTR) or entropy-generating cosmologies showed that they provide an excellent way to widen the parameter space available for a slew of dark matter candidates, such as WIMPs, sterile neutrinos, non-thermal axions, standard-model neutrinos, and even WIMPzillas! Considering low-temperature reheating models is not just an intellectual exercise. They are a real possibility. Light (~TeV) scalars abound in some versions of string theory, and they may dominate the energy of the universe at some time.
Unresolved, however, was the question of how constraints to thermally-produced 1-20 eV axions change in such scenarios. Using the standard pionic interaction rates, I wrote code to calculate the abundance and free-streaming length of axions in such unconventional scenarios. I re-mapped the constraints of Hannestad et al. into the space of abunance/free-streaming length, using them in concert with my code to obtain new constraints to thermal axions in an entropy-generating cosmology, shown below:
The red region is excluded by WMAP1+SDSS, while the yellow region is excluded by the simple requirement that thermal axions not have an abundance greater than the total dark matter abundance. As you can see, at sufficiently low reheating-temperatures, the cosmological constraints to thermal axions are almost completely relaxed. This follows from the smaller free-streaming length and lower abundance of axions in these scenarios, as shown below:
I also considered the promise of future surveys (LSST, better probes of the power-spectrum on small scales such as Ly-alpha forest measurements) for improving these constraints. I found that entropy generation is crucial to relax constraints; less dramatic changes in the expansion history, such as a kination phase dominated by the kinetic energy of a scalar field, offer far less dramatic changes in constraints.
Ongoing work:
With Scott Watson and Tristan Smith, I am trying to establish just big wide a parameter space (in reheating temperature, up/down quark mass ratio, axion mass, axion model parameters) is allowed for ~eV axion models, taking into account telescope searches, cosmological constraints, CAST results, globular cluster evolution, and the diffuse extra-galactic background radiation. We are particularly interested in the possibility that a 10-20 eV axion could be a viable `very warm' dark matter candidate. Work by Lesgourges and others has generalized the pink-purple parameter space plot shown above to include WMAP 5-year data, and we hope to extend previous results accordingly.
Observational constraints to 'fat graviton' theories
This work is detailed in astro-ph/0606133 and was published in Physical Review Letters.Raman Sundrum and others have postulated that the seemingly arbitrary milli-eV energy scale implied by the phenomenon of cosmic acceleration could be explained if gravity is a low energy effective field theory in which gravitons cease to gravitate past some critical momentum. Such a theory can be derived from a Lagrangian density non-linear in the d'Alembertian.
Robert Caldwell and I showed that if the cutoff scale scale corresponds to a milli-eV, existing x-ray lenses wouldn't be possible. To do this, we did a tree-level calculation (first pair of diagrams immediately below) of the cross-section for gravitational lensing (analagous to Rutherford scattering) in linearized quantum gravity with a cutoff, and then extended the calculation to all orders in perturbation theory (series of diagrams below the first pair). This calculation only converges if we restrict ourselves to the eikonal (geometric-optics) limit, which certainly applies in the astrophysical scenarios of interest.
We also show below the x-ray lensing contours of the lensing system Q0957+661 provided to us by George Chartas for our analysis:
Lyman-limit absorbers
Michael Strauss and I have compiled the largest existing sample of Lyman Limit systems, using SDSS data. We're analyzing our sample to understand the statistical properties of high column-density neutral hydrogen absorbers in the early universe.
Last updated on September 10, 2009