Alumna Update: Chandrima Ganguly
In this week’s post alumna Chandrima Ganguly (2013) poses the question of where do we all come from? As she pursues her Phd at Cambridge University in Cosmology she explores the origin story of the universe and challenges how we perceive it.
Of Bounces and Bangs: how much do we know about how we all got here?
How did the Universe begin? This is a question that has puzzled people from as long ago as anyone has records of things people liked to think about. The ancient Greeks, Romans, Aryan tribes on the Indian subcontinent all speculated about its origin. Even in the absence of cosmological precision data that is at our disposal today, many theories of the very early Universe that are being debated today at first approximation deeply resemble the theories that these ancient philosophers thought about. For example, a model that has seen several resurrections is the cyclic model of the Universe - it was first thought of in the Rigveda in ancient India between 1200 − 1500 BC, but also been explored by Einstein and Tolman in the 1930s and most recently by Steinhardt, Turok and collaborators in the early 2000s. This interest in our origin story is very natural and fundamental to the human experience as is exemplified by this very long standing curiousity in the history of our Universe. Fortunately, the way we set out to answer these questions has changed significantly over the centuries. We no longer rely solely on philosophical thought to answer our questions. We have the mathematical machinery through General Relativity, Particle Physics, Quantum Field Theory as well as differential geometry and statistics to create a consistent model. We also have recourse to several high precision observations of the radiation left over from the very early Universe, or of the patterns in which galaxies are distributed in our observable Universe. Further observations are anticipated regarding our neutrino background and higher precision observations of large scale structure of galaxies and all of these observations together give us a way to confirm or reject our mathematical hypotheses of the very early Universe.
Our current understanding of the Universe says that it is approximately 13.8 billion years old. It evolved from an early dense hot state, the relic radiation from which has given us a way of studying patterns in it and trying to test our zoo of early Universe theories. It has also given Penzias and Wilson who accidently stumbled upon its existence the 1978 Nobel Prize for Physics. I speak of course, of the Cosmic Microwave Background Radiation (CMB). One fact that is glaringly obvious from studying the temperature of the light(photons) in the CMB is that the Universe is very homogeneous and isotropic on large scales. This just means that the Universe looks basically the same whichever direction you look on large scales. This gives us the first clue in constructing our ‘beginning’ story for the Universe. If we assume that the initial state of the Universe was not homogeneous and isotropic, and really we have no reason to assume that this must be so, something must have happened since the beginning of the Universe to make it as uniform as it is today.
Light travels at a finite speed. So all the information about objects that we get through light bouncing off of it, is actually information slightly in the past. This doesn’t make a big difference because although light does have a finite speed, things we can see in everyday life are close enough to us to make the time in which the light bouncing off of them reaches us almost instantaneous. This is not true for objects on a cosmic scale. So if we observe a supernova explosion in a neighbouring galaxy, the explosion has actually occurred in the past. Thus cosmological observations are taking us through a time machine. This is why we study the CMB in such great detail - because it contains the imprint of the hot dense initial state that our Universe evolved from. There is another interesting device that helps us know when the event from which the light is reaching us occurred. Much like the way sirens from ambulances seem less shrill if the car is going away from us, light from distant cosmic objects is shifted to the red end (lower frequencies). This tells us that the Universe is expanding and also the amount of redshifting tells us how long ago the particular event we are observing the light from occurred.
Now we have an additional ingredient for our cosmological model - not only must it produce a homogeneous and isotropic cosmology but also a Universe that is expanding in an accelerated manner. Some other information can be deduced from studying the patterns in the CMB - for example the global geometry of the Universe is largely flat, it is mostly empty i.e. composed of Dark Energy which is believed to be responsible for the current expansion. The CMB does yield some patterns and fluctuations on smaller scales - and any consistent model of the very early Universe would seek to reproduce these patterns when evolved through.
So which models of the early Universe do we think about? Broadly they can be classified as (a) inflationary cosmology - the Universe has always expanded so inevitably must have begun from a singular point (a point where our current understanding of the laws of physics don’t make sense and the general consensus is that we can understand it better in a more complete theory of quantum gravity) (b) a Universe that went through alternating phases of expansion and contraction while going through a ‘bounce’ where it is of minimum size. Although the Universe is at its smallest size, it still has finite volume at this point and we can still use our current understanding of physics in this regime. There are various combinations of these scenarios and a large chunk of work that theoretical cosmologists do is to both refine these theories mathematically and also to calculate observational signatures to be confirmed by future detectors and experiments. In my work, I have taken a fairly agnostic approach to this problem. I study non-singular bouncing cosmologies. The question I seek to answer is - is it possible to construct a non-singular cosmology that reproduces the current Universe today? The advantage of this type of scenario is that we would not need a still-elusive theory of quantum gravity to describe the complete history of the Universe. The disadvantage of this scenario is that research in it is still quite preliminary and there are many conceptual and theoretical difficulties with it, which make the calculation of robust observational signals difficult. My research has shown that the inclusion of friction-like forces in these models make it more likely for them to resemble the Universe we live in today.
So overall, it’s a very exciting time to be alive as a cosmologist. There are lots of new experiments being planned to launch with even a new sector of observing opening up - gravitational wave astronomy. It will give us a way of determining with greater accuracy which models are more consistent with our current Universe and maybe give us a better idea of how to answer the question that I started with. Where did we all come from?
