Research interest

Image credit: © CTC, DAMTP, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 0WA.

Historically, astronomy started in the optical band – first with the naked eye and later with increasingly more powerful telescopes and detectors. This has provided us with many new insights, even though the optical wavelength range makes up only a tiny fraction of the entire electromagnetic wavelength range. Radio astronomy, just after world war-II, initiated the first expansion of the accessible wavelength spectrum for astronomers. This uncovered a completely new universe, as many astrophysical processes reveal themselves at very different frequencies. This revolution led to a number of fundamental discoveries like:

• microwave background and Big Bang,

• HI emission

• dark matter

• synchrotron emission and magnetic fields

• quasars, black holes and high-energy particle accelerators, pulsars and dense states of matter, organic molecules

Nowadays astronomy has successfully expanded into other domains of the electromagnetic spectrum, such as infrared, X-rays, and γ-rays. Opening up a new frequency window has always led to unexpected discoveries.

Matter-Radiation equality

The epoch at which radiation density is equal to the matter density is known as the matter-radiation equality epoch, i.e Ω_m(z) = Ω_r (z).

Since for radiation: H²Ω_r = H₀² Ω_r0/a⁴ and

For matter: H²Ω_m = H₀²Ω_m0/a².

We have Ω_r/Ω_m = Ω_r0/(Ω_m0 a).

Therefore the value of scale factor for which Ω_r / Ω_m = 1 ==> a = 8.4 ×10^-5/0.23 = 3.7 ×10^-4

Hence equivalent redshift is z ~ 2740. The differential equation for structure growth shows that density perturbations should grow ∝ a, in a matter-dominated universe. Explain why, despite this, one might expect that structures would not start to grow until the epoch of recombination at z ∼ 1100. High-density regions can be prevented from collapsing by their internal pressure. At radiation-matter equality, the radiation pressure (P_r = ε_r/3) is great enough to prevent the collapse of structures below the horizon size, although the gas pressure is not. While the baryonic matter is still fully ionized, the photons and baryons frequently interact, forming a single photon-baryon fluid; therefore, if radiation pressure prevents the photons from collapsing, the baryons won't either. At recombination, protons and electrons combine to form neutral hydrogen. Neutral atoms interact far less readily with photons, so the photons decouple from the baryonic matter. This means that the radiation pressure no longer acts to prevent baryonic structures from collapsing, so structures can start to grow. Non-baryonic dark matter is electrically neutral and therefore does not interact with photons. Therefore the radiation pressure has no effect on it, and collapse can start at matter-radiation equality. However, if the dark matter is relativistic at this time, it itself will have a "radiation" equation of state, and therefore its own pressure will prevent it from collapsing until its velocity has dropped below relativistic levels. We define dark matter to be "hot" if it is relativistic (v ∼ c) at z ∼ 3000 (i.e., T ∼ 10⁴ K) and "cold" if it is non-relativistic (v << c) at this temperature [dark matter which would be mildly relativistic, with v a significant fraction of 'c', is commonly called "warm"]. Therefore, if the dark matter is cold, it does not have a relativistic equation of state at matter-radiation equality, and so neither photon pressure nor its own pressure will prevent collapse.

Cosmic Magnetism

Observations indicate the existence of magnetic fields of varying strengths on different scales in the universe. While several theoretical models have been proposed in the literature to explain the origin of the magnetic fields at cosmological length scales, none of these mechanisms can be considered fully satisfactory in all respects. There are primarily two-generation mechanisms discussed in the literature to explain the origin of large-scale magnetic fields. The first one operates during the period of large-scale structure formation, and another one operates in the early universe, typically during inflation or during phase transitions. The seed fields produced in the latter one are subsequently amplified by astrophysical processes like dynamo mechanism and flux freezing in collapsed objects. Thus, the cosmologically generated magnetic fields from processes in the early universe become almost imperative to explain the presence of these fields at a sufficiently large length scale. The magnetic fields generated during the early universe are referred to as primordial magnetic fields (PMFs). These PMFs are believed to be one of the possible precursors of large-scale magnetic fields in the intergalactic medium. The existence of magnetic fields could have significantly affected the CMB, large-scale structure formation as well as the 21 cm line signal. These fields can induce velocity fluctuations between the ions and the residual electrons by the Lorentz force and heat up the gas in the IGM due to the frictional force between the charged and neutral particles. Further, if PMFs affected structure formation, it may be expected that their imprints may be left on the temperature and polarization anisotropies and the thermal spectrum of the CMB.

The Epoch of Reionization Overview

The “Epoch of Reionization” is a period of the universe, when the first stars and galaxies were starting to form. Prior to this epoch, the universe was dark, suffused with a dense, obscuring fog of primordial gas. As the first stars switched on, their ultraviolet energy began to reionize the cosmos, punching ever-larger holes in their murky surroundings. Eventually, the effect of these young, massive stars and their infant galaxies enabled light to shine freely through space.

Thermal and ionization history:

For its first 370,000 years, the universe was filled with a hot, dense fog of ionized gas. As the universe cooled and expanded, electrons and protons were able to combine to form the first neutral atoms. When this happened, thermal energy from the Big Bang was able to travel freely throughout the universe. This high-energy radiation has, over cosmic time, cooled and been red-shifted by a factor of more than 1,000 due to the expansion of the universe. We see the remnants of this today as the cosmic microwave background (CMB). Studies of the CMB, beginning in the early 1990s, revealed that the very early universe, though generally smooth, contained tiny density fluctuations (on the order of one part in 100,000) that rippled through space. More fine-grained studies by instruments, such as WMAP, helped better establish the composition of the universe and fixed its age at 13.7 billion years. After the CMB became imprinted on the universe, the cosmos became opaque at shorter wavelengths due to the absorbing effects of atomic hydrogen. This began a long period known as the Dark Ages, so-called because of the absence of stars and the extremely dense intervening neutral hydrogen gas. Over time, areas of high-gas density began to collapse under gravity, and the neutral matter in the universe began to clump together. Eventually, these regions cooled and collapsed, igniting nuclear fusion in their cores and leading to the first stars and galaxies. As the first stars emerged, their energy heated the surrounding medium, once again ionizing the hydrogen in the universe. At first, these areas were like small bubbles of ionized gas surrounding bright energy sources. As these bubbles grew and punched ever-larger holes into the neutral universe, they eventually began to overlap, enabling ionizing radiation to travel farther and farther through space. The first stars also significantly altered the chemical makeup of the cosmos. Through nucleosynthesis in the cores of these short-lived stars—which may have been as much as 1,000 times more massive than the sun—and as a result of powerful supernovae, a fraction of the universe’s initial constituents of hydrogen and helium were converted into carbon, oxygen, iron and other heavier elements. Once the majority of the universe was reionized, approximately one billion years after the Big Bang, light across much of the electromagnetic spectrum could travel unimpeded through the cosmos, eventually revealing the universe as we see it today. Peering into the Reionization era has been both a challenge and a quest for astronomers, with today’s best telescopes giving tantalizing clues as to the nature of these early stars and the assembly of the very first galaxies. Studies of the distant and ancient universe with the next generation of ground-based observatories will be able to probe further into the epoch of reionization.

After recombination at a redshift of ~1100, the baryonic matter in the Universe remained neutral until the first stars and galaxies formed. The nature of the sources responsible for the re-ionization of the intergalactic medium (IGM) is still the subject of a lively debate but most theoretical models adopt stellar sources. These have to be constrained on the basis of the available observations. Several alternative processes/contributions that may enhance the high redshift ionizing photon emission have been proposed. In addition to stellar-type sources, a contribution to the ionizing photon budget could also come from an early population of mini-quasars powered by intermediate-mass black holes. Finally, another possible contribution to reionization at high redshift could come from decaying particles and neutrinos.

Leading questions for these future optical, infrared, and radio observatories include:

• What is the role of galaxies in reionizing the universe? To understand the role of early galaxies in reionization, astronomers first need a more accurate census of these galaxies as far back in time as possible. This will require exquisitely sensitive and fine-grained, near-infrared studies, which will be possible with future large, ground-based observatories equipped with adaptive optics.

• What are the processes that drove galaxy formation? The majority of galaxies in the early universe are expected to be relatively small, making studying their shape and movement extremely difficult. With higher-resolution images and Doppler studies of their internal velocities, astronomers will be able to better understand how these galaxies emerged from the primordial gas, and how they evolved into the large galaxies we see in the universe today.

• What is the process of early star formation? The process of star formation is difficult to unravel, even, in nearby star-forming regions today. Current conditions that affect star formation include the cooling of superheated gas, feedback from supernovae, and even the effects of supermassive black holes that are believed to reside at the center of most galaxies today. In the very early universe, these complex conditions become even more muddled. These early galaxies would have contained massive stars formed from primordial gas (referred to as Population III stars). They would be unlike any stars in the universe today and would likely have formed and evolved under radically different conditions.