CMB Circular Polarization and Cosmic Birefringence: arXiv:1803.04505

One of my current research projects consists on exploring different conventional sources of Circular Polarization (CP) in the Cosmic Microwave Background (CMB). The CMB is known to be linearly polarized (at a level of ∼10%) due to anisotropic Thomson scattering. Therefore, any CP will come from propagation effects.

The most convenient mechanism (after the last scattering surface) to produce CP out of Linear Polarization (LP) is to use birefringence. Basically, a birefringent material has different indices of refraction, which then produce a phase shift in the LP that generates CP. Hence we are looking at propagation effects that can produce a small degree of circular polarization in a CMB photon. Note that since the Universe is isotropic and homogeneous any birefringence must come from perturbations (either matter or radiation).

The sources of cosmic birefringence that we have studied in our work are: spin-polarized hydrogen atoms (both at recombination and at the Cosmic Dawn), photon-photon scattering (non-linear response of the vacuum by creation and annihilation of pairs), static non-linear hyperpolarizability of hydrogen (non-linear response of bounded electrons, e.g. hydrogen atom potential is not a harmonic oscillator), and plasma delay (non-linear response of free electrons). These sources are conventional in the sense that no physics outside the Standard model is introduced.

The strongest source of cosmic birefringence we studied is the photon-photon scattering, which imprints a CP in the CMB at a level of ∼10^{-14} K (the LP of the CMB is ∼10^{-6} K). Even though different results float around in the literature for this particular effect, we made sure of explaining all apparent contradictions.

The level of CP in the CMB generated by conventional sources of cosmic birefringence is small both in comparison to relevant foregrounds (e.g. galactic synchrotron emission…) and current and near-term experimental sensitivities.

Reionization effects on Lyman-α forest: arXiv:1902.02892

After the CMB the Universe continues to cool down and form complex structures via gravitational instabilities. Eventually the first stars and galaxies form, and they start to emit UV photons. These UV photons heat up the surrounding Intergalactic Medium (IGM) to a few 10^4 K. This inhomogeneous phase transition leave an imprint on the IGM, even in cosmological scales. Small-scale structure that was previously gravitationally bounded can now diffuse away (Jeans mass increases with temperature, hence no longer bounded).

After reionization it is possible to probe the IGM using the absorption of neutral hydrogen in quasar spectra from rare overdense regions, i.e. the Lyman-α forest. However, in order to study the effect of patchy reionization in the IGM with Lyman-α forest one must focus on the high redshifts.

Recently, we estimated the impact of inhomogeneous reionization using small-box simulations to resolve the small-scale structure of the Lyman-α forest. In addition, we used semi-numerical big-box simulations capable of representing the effects inhomogeneous reionization. We find that the deviation in the 3D power spectrum of the Lyman-α forest at z = 4.0 and k = 0.14 1/Mpc ranges from 19 – 36%, with a larger effect for later reionization.

Revisiting Asteroid-Mass Primordial Black Hole Constraints:

The discovery of gravitational waves has led to the re-invigoration of Primordial Black Holes (PBHs) as dark matter candidates. Currently there are two “open” windows for PBHs to be the dark matter. The solar mass window is nowadays under constant scrutiny recently, and it seems that the overall result is the partial closing of this window. Besides, at lower masses we have another possible window. We revisit constraints in the latter window.

We focus on the capability of microlensing to constrain m_{pbh}<10^{-10} M_{sun}. We study the effects of diffraction at these lower PBHs masses. Also, we study the capture of PBHs by stars, and conclude that this mechanism is currently incapable of putting a constraint in this window. Furthermore, we analyze the survival of white dwarf stars to the passage of PBHs through them.

We conclude that none of the mechanism in the asteroid-mass window can constrain PBHs of these masses. Therefore, in principle, PBHs can *potentially *be the totality of dark matter.

Binary Neutron Stars Mergers:

Since the discovery of gravitational waves sourced by binary black holes mergers from the LIGO collaboration, the interest in other sources of gravitational waves has increased exponentially. Furthermore, LIGO has recently discovered it’s first neutron star binary merger, opening the window to a lot of new interesting physics and new opportunities.

However, there are several unknowns (for example the equation of state) when dealing with neutron stars mergers that might play an important role in the inspiral, merging and ringdown phases. One of such unknowns corresponds to a possible instability pointed out by Nevin Weinberg *et al*. Basically, this instability arises from the tidal interaction with the companion neutron star. The main consequence of this instability would be the additional flow (or loss) of energy put into the orbit, which then will provoke a tiny phase shift per cycle in the inspiral. However, in contrast to black hole mergers the number of cycles for neutron star mergers can be large (around 3000), thus a tiny phase shift can propagate into an order unity effect.

This phase shift will modify the parameters obtained by LIGO because their templates would be slightly wrong for this kind of system. Also, in the most extreme scenario it could made LIGO unable to detect a whole class of binary neutron star mergers, the ones with a strong instability (kind of like a Reynolds number, systems above that critical number will present important instabilities).

One of our current projects consists on exploring and understanding of the effects and causes of this instability from a more theoretical perspective, instead of simulations.

More coming soon.

Exact and Approximate Solutions of the Einstein Equations: arXiv:1405.2899, 1405.1776

During my undergraduate program I worked on this topic with Prof. Frutos and Prof. Bonatti in University of Costa Rica. The motivation for this topic consists on the problem regarding the interior matching of the Kerr metric.

The Kerr metric represents a massive spinning body (such as any physical star), the static limit is the expected Schwarzschild metric. However, both of these metrics are exterior metrics. There has not been an appropriate interior matching for the Kerr metric. One possible solution is to consider a metric that is more general than Kerr by modifying the geometry, i.e. more degrees of freedom to play with. However, does not matter what you do the new metric must be able to reproduce the Kerr metric at a certain limit.

For more information see the list of publications.

**A list of publications can be seen below.**

Articles:

- Montero-Camacho, P., Hirata, C. M., Martini, P., and Honscheid, K. (2019) Impact of inhomogeneous reionization in the Lyman-α forest.
*ArXiv:*1902.02892 - Montero-Camacho, P. and Hirata, C. M. (2018) Exploring circular polarization in the CMB due to conventional sources of cosmic birefringence.
*Journal of Cosmology and Astroparticle Physics,***08**, 040. doi:10.1088/1475-7516/2018/08/040. - Frutos-Alfaro, F., Montero-Camacho, P., Araya, M. and Bonatti-Gonzalez, J. (2015) Approximate Metric for a Rotating Deformed Mass.
*International Journal of Astronomy and Astrophysics,***5**, 1-10. doi:10.4236/ijaa.2015.51001. - Montero-Camacho, P., Frutos-Alfaro, F., Gutierrez-Chaves, C. and Cordero-Garcia, I. (2015) Slowly Rotating Curzon-Chazy Metric.
*Revista de Matematica: Teoria y Aplicaciones*,**22**(2): 265-274. doi:10.15517/rmta.v22i2.20833.