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Nikhil Padmanabhan Lecture

Nikhil Padmanabhan’s talk was on the evidence for the Standard Model of Cosmology, and discussed a universe composed primarily of dark energy and dark matter. Nikhil presented an overview of matter and energy in the universe, and asked two of the major questions in cosmology pertaining to dark matter and dark energy.

The first question related to dark matter in the universe - How much does the universe weigh? The mass and curvature of the universe are closely related in determining its density. The density of the universe is what ultimately determines its fate. There is a critical density on which the fate of the universe resides: if the density of the universe is greater than this critical density, the universe will eventually collapse in on itself. If the universe is less dense than this critical density, it will expand forever. If the density of the universe and critical density are roughly equal, then it will also expand forever, but approach a zero velocity over an infinite amount of time. These relate to what are known as closed, open, and flat universes and knowledge of their composition is very important.

Nikhil explored various methods of weighing objects in the universe such as counting photons and measuring observable traits such as temperature, velocity, and gravitational lensing. He explained how to determine the mass of an orbiting object with Newton’s Law and compare it to an expected rotation curve. This proves that there must be dark matter that accounts for the extra gravitational force in our galaxy. Regions of space containing dark matter were shown in addition to computer simulations of how these regions were created.

The final step in weighing the universe involves looking at the large-scale structure that arose from small fluctuations in the early phase of development. A computer simulation aided the evolution of these fluctuations into large-scale dark matter filaments. Galaxies form on the denser portions of the filaments, and put many constraints on the composition of dark matter. The dark matter structures could not exist if it was very light, baryonic, or if it interacted strongly with itself. This tells us that the composition of the universe is mostly dark matter and fits well with observed galaxy distributions from the Sloan Digital Sky Survey.

The second major question in cosmology is - How fast is the universe expanding? To answer this question, Nikhil explained how the apparent size and brightness of a Type 1A supernova could determine how fast the universe is expanding. Type 1A supernovae occur when a white dwarf accretes an excessive amount of mass from a binary companion. It then undergoes a type of runaway fusion, unbinding itself in a single luminous explosion. All supernovae of this type have the same peak luminosity so they are referred to as “standard candles” for cosmology. An expansion rate of the universe is obtained by comparing the relative magnitudes and redshifts of the Type 1A supernovae. These results have shown that on a large scale, everything in the universe is accelerating outward, which necessitates the existence of an antigravity force. This force is called dark energy and is thought to make up 74% of the Universe.

The formation and detection of the Cosmic Microwave Background was one of the last topics of discussion. This description focused on a few similarities between the formation of the CMB and large-scale dark matter structure formation as well as their detection. The lecture concluded with an overview of the projects at Berkeley Lab pertaining to the studies of dark matter, the CMB, and dark energy. As his final thought, Nikhil gave an optimistic view on where the Standard Model of Cosmology and astrophysics research is headed and encouraged others to join the field of cosmology.