Lectures

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	Lecture 41 - Eternal Inflation and Multiverse “It’s hard to build models of inflation that don't lead to a multiverse. It’s not impossible, so I think there’s still certainly research that needs to be done. But most models of inflation do lead to a multiverse, and evidence for inflation will be pushing us in the direction of taking [the idea of a] multiverse seriously.” ― Alan H. Guth In 1983, Andrei Linde forcefully argued that inflation (exponential expansion) does not require a cosmological phase transition, or any contrived potential. Rather, it is a ‘generic cosmological regime’. If one accepts this, then one of the possibilities is that “inflation is eternal” and the… Read More
	Lecture 40 - The Big and the Small The Inflationary Universe We do not yet have a convincing, detailed model of how and why inflation transpired because the models of particle physics that we have are not adequate at the huge energies of inflation., Understanding inflation requires a much better knowledge of particle physics than we have now, and possibly a full knowledge of string theory and quantum gravity." - Edward Witten (2002) In this lecture, ideas developed in the previous lecture will be invoked to understand why the universe might have initially expanded exponentially – the so-called inflationary phase. First, the idea of spontaneous symmetry breaking and phase transitions are reviewed. Then the concept of “… Read More
	Lecture 39 - The Big and the Small Unification of Forces Quests to show that the four basic forces are different manifestations of a single unified force follow a long tradition. In the nineteenth century, the distinct electric and magnetic forces were shown to be intimately connected and are now collectively called the electromagnetic force. , More recently, the weak nuclear force was united with the electromagnetic force. It is natural to suggest that a theory may be constructed in which the strong nuclear, weak nuclear, and electromagnetic forces are all unified. Formulation of such a theory is intricately connected to our understanding the nature of the forces/interactions in the early Universe. It is now clear that the large-scale behaviour of the universe was determined by the conditions that prevailed when the universe was 10-35 seconds old, when a UNIFIED FORCE described all interactions between the elementary particles. So, this lecture is devoted to a review of the microcosm.… Read More
	Lecture 38 - The BIG BANG The three main evidences that the Big Bang happened are a) the expansion of the universe, b) the Cosmic Microwave Background, and c) the Big Bang Nucleosynthesis. , Yet, "...the standard big bang theory says nothing about what banged, why it banged, or what happened before it banged. The inflationary universe is a theory of the "bang" of the big bang" (Alan Guth 1997). Around 1970, the following embarrassing questions were staring at us. (1) Why is our universe expanding? (2) How can our universe be so homogeneous and isotropic? (3) Why is the present density of the universe so close to the ‘critical density’ predicted by General Relativity – the density which… Read More
	Lecture 37 - The Accelerating Universe In the late 1990s, observation of distant supernovae established the fact that the Universe is, contrary to expectations, actually accelerating. Measurements of cosmic microwave background also strongly indicate that the Universe does not have large-scale curvature. , The obvious conclusion drawn from these is that only about 30% of the energy density of the Universe comes from baryonic matter that we are familiar with. The rest, that is 70% of the energy density of the Universe, must come from vacuum energy, metaphorically called as the `dark energy'. A surprising and startling discovery was made around the year 2,000 – the universe is not only expanding, but the expansion is accelerating! In the first half of this lecture, I describe how this great discovery was made. If the expansion is ‘accelerating’ then it implies two things, (i) There… Read More
	Lecture 36 - The Evolution of the Universe At a particular instant roughly 15 billion years ago, all the matter and energy we can observe, concentrated in a region smaller than a dime, began to expand and cool at an incredibly rapid ratei... Neutral atoms appeared in abundance only after the expansion had continued for 300,000 years and the universe was 1,000 times smaller than it is now., The neutral atoms then began to coalesce into gas clouds, which later evolved into stars. By the time the universe had expanded to one fifth its present size, the stars had formed groups recognizable as young galaxies."" P. J. E. Peebles et al., 1994 In the early universe, various elementary particles and radiation were in thermal equilibrium, with everything at the same temperature. The universe was opaque. At an age of roughly 350,000 years, electron scattering off photons became inefficient and the universe became transparent. Matter and… Read More
	Lecture 35 - The Expanding Universe "One could still imagine that God created the universe at the instant of the big bang, or even afterwards in just such a way as to make it look as though there had been a big bang, but it would be meaningless to suppose that it was created before the big bang. An expanding universe does not preclude a creator, but it does place limits on when he might have carried out his job!" ―- A Brief History of Time, Stephen Hawking" In 1922, Alexander Friedmann proved that an isotropic, homogeneous universe cannot be static. By 1929, Hubble demonstrated that the clusters of galaxies are receding from us. Even prior to that, George Lemaitre had argued that if the universe is expanding, then it must have been very small and… Read More
	Lecture 34 - Exoplanets The area of research that has attracted great attention in recent years is that of planet formation, recognised by the awarding of (2/3rd of) the 2019 Nobel prize to Michel Mayor and Didier Queloz ``for the discovery of an exoplanet orbiting a solar-type star''., In the past decade, NASA's Kepler Space Telescope has detected more than 3000 exoplanets ranging from sub-Earth-sized planets to huge gas giants that dwarf even our Jupiter, with densities ranging from that of styrofoam to iron. Astronomers find them close to their parent or host stars with scorching temperatures, to a great number in the so-called 'habitable zone' in which life (as we know it) can flourish. Even before Newton’s time, there were assertions that the stars were just like the sun, but far away. Some even went on to conjecture that there must be planets orbiting many of the stars. The first extrasolar planet was found in 1995. Today, nearly 6,000 exoplanets have been discovered. In this… Read More
	Lecture 33 - Celestial Masers A maser, like a laser, is a source of bright electromagnetic radiation but in the microwave regime. When microwave radiation is incident upon molecules present in space, sometimes they molecules respond by amplifying it, by producing a 'maser'. , Naturally-occurring masers are found near stars and supermassive black holes, as well as in the atmospheres of Jupiter and other Solar System objects. In the most dramatic cases, a water vapor maser can radiate more energy at a single wavelength than does the Sun over its entire visible spectrum. In a historic paper published in 1917, Einstein introduced the concept of “stimulated emission of radiation” by atoms and molecules. The first MASER (Microwave Amplification by Stimulated Emission of Radiation) based on this idea was made in 1953, and the first LASER was made in 1960. After the… Read More
	Lecture 32 - The Gamma Ray Universe Gamma rays, representing the most energetic part of the electromagnetic spectrum, are produced by the hottest and the most energetic objects in the universe. Because of their extremely small wavelengths, gamma-rays can not be captured and reflected by mirrors unlike visible light or X-rays., Moreover, gamma-rays coming from space are mostly absorbed by the Earth's atmosphere. So gamma-ray astronomy could not develop until it was possible to get our detectors above all or most of the atmosphere, using balloons or spacecraft. And we finally developed the ability to detect gamma-rays only in the 1960s, opening up a new window to the Universe. One of the new windows to the universe is Gamma Ray Astronomy. This branch of astronomy came of age during the past twenty years. Today, one can detect gamma rays with energies from MeV to TeV. Low energy gamma rays come from the interstellar medium, supernova remnants, etc. High energy gamma… Read More