Since I started my research activities when I was a graduate student, I have always enjoyed working on a range of problems in astrophysics and cosmology. My main motivation has been to learn on a broad range of phenomena in the Universe and to pursue some fundamental key questions that have arisen in our quest to comprehend the Universe: What is the dark matter, which constitutes 84 per cent of all matter in the Universe (the other 16 per cent being the ordinary matter we know, or baryonic matter, made of protons, neutrons and electrons)? What is the origin of the large-scale structure in the distribution of matter (galaxies and intergalactic gas) in the Universe? What are the key processes explaining the formation of galaxies and the black holes in their nuclei?
Some of the topics I have investigated in relation to these key questions are gravitational lensing by galaxy clusters and large-scale structure, the intergalactic medium and its ionization history, extragalactic absorption lines as a probe to the large-scale structure of the Universe, galaxy formation models, microlensing by stars and by dark matter structures in our Galaxy and in external galaxies, high-energy cosmic rays, and the stellar dynamics in the Galactic center.
A lot of my work has focused on cosmology in relation to the formation of galaxies, the distribution of gas in space and the epoch of reionization of matter in the universe. The Universe evolved as it expanded from an initially nearly homogenous state to the present highly inhomogeneous state: some matter has collapsed into galaxies and clusters of galaxies, and other matter is left in the intergalactic medium, which is the gas between galaxies. In the early Universe, when matter was still nearly homogeneous, the initial hot plasma made of atomic nuclei and electrons formed atoms for the first time when matter cooled down enough: this was the first ``combination'' of atoms, usually designated as the recombination epoch, which happened when the Universe was about 300 thousand years old. But matter was ionized again later as the first stars and quasars were formed and emitted ultraviolet ionizing radiation. I am interested in this process of ``reionization'', which we have learned took place when the Universe was between few hundred million to a billion years old, and the interpretation of observations of absorption spectra of distant sources caused by the intervening intergalactic medium, as well as high-density gas in the process of forming galaxies.
The intergalactic gas can be observed through absorption lines it produces on background sources. The most important of these absorption lines is the Lyman alpha line of hydrogen, an ultraviolet line at a wavelength of 1216 Angstroms, which is observed in luminous sources (usually quasars) at high redshift. My work in this area has concentrated on studying the interplay between the evolution of the intergalactic medium and the formation of galaxies.
My other main interest has been the nature of the dark matter and the primordial fluctuations that gave rise to the formation of galaxies and larger structures in the universe. Several observational tools can be used in astronomy to study this, among them: gravitational lensing by galaxies, clusters of galaxies and large-scale structure, microlensing of stars or quasars by any kind of compact object, the spatial distribution of galaxies, and the structure of dark matter halos studied through the dynamics of galaxies and hot gas in clusters.
Recently, I have become interested in a particular hypothesis for the nature of dark matter: axionic waves. These waves would be a new form of matter made of very light, electrically neutral particles called axions, with extremely feeble interactions with ordinary matter, and they would behave today as a classical wave that permeates space and accounts for the density of dark matter. The reason the axion is a very promising dark matter candidate is that it can solve a problem of the standard model of particle physics called the strong QCD problem, and at the same time it accounts for the formation of dark matter in the early Universe with this new field that is also naturally expected not to decay into anything else and to have extremely weak interactions with other matter. Nevertheless, these weak interactions, which would imply a modification of Maxwell's equations, are being searched by many experiments, one of which is the International AXion Observatory , which we are participating in at the ICCUB.
One interesting consequence in many models of axion dark matter is the formation of minihalos of dark matter on scales much smaller than that of galaxies: dark matter may form gravitationally bound objects as light as the typical mass of an asteroid, at a very early epoch much before any galaxies formed. Therefore, part of the dark matter might be today in the form of these small minihalos, rather than being in a diffuse form throughout the halos of galaxies. These minihalos might have some observational consequences in gravitational lensing, although with very small effects that are hard to detect.
Gravitational lensing is an area I have been involved with since my student days. Light from distant sources in the Universe is deflected when propagating toward us owing to the gravitational field of matter near the line-of-sight. The deflection of light can be studied from the distortions created on the images of these distant sources, and sometimes multiple images of a source are observed. This allows us to investigate the distribution of the masses responsible for the light deflection. Most of the mass in the Universe is dark matter, so it is not observed directly but can be detected through the gravitational influence it exerts on other matter. Therefore, gravitational lensing is a unique observational tool to investigate the distribution of dark matter in galaxies and clusters of galaxies.
Small scale structures in the distribution of dark matter, such as cold dark matter subhalos that collapsed on a smaller scale than the smallest galaxies and do not contain any stars, or the yet much smaller axion minihalos, are not detectable with standard dynamical methods that measure only the mass distribution on length scales of a whole galaxy or larger. They are also difficult to detect by gravitational lensing, because these small objects have a mass surface density that is far too small by itself to split a source into multiple images or cause high magnification and distortion of images. However, under some special circumstances their effects can be large, namely when the source is located close to a caustic of a larger scale lens. This happens in the so-called arcs, or highly magnified images of galaxies lensed by clusters of galaxies. In particular, occasionally the lensing magnification may become as high as a factor of thousands, making a single star at cosmological distance becomes visible to our telescopes. The small effects of axion minihalos may become detectable in this case.
I have also written a review article in Science on The Dark Age of the Universe, describing the period of time between the emission of the Cosmic Microwave Background and the formation of the first stars, the physics of how the first stars formed, and our present knowledge of the epoch of reionization. You can access the paper here for the Abstract and for the Full Text of the article.
J. Miralda-Escudé 2003, ``The Dark Age of the Universe'', Science, 300, 1904.