The Cosmology of the Early and Late Universe

We live in a Universe in which distant galaxies are accelerating apart from one another, but the cause for this can not be the ordinary matter present in it as the universe should be slowing down due to the attractive pull of gravity on the matter it contains. Whatever is causing this acceleration must be a new substance that we have not come across before in nature, we call it Dark energy (DE). The best candidate for this is the Cosmological Constant (CC), however, there can't be much of it around today because if there was, it would have caused galaxies to blow apart by now. The problem is reconciling the observed amount of DE with the predicted amount that should be present in the form of a CC. The latter is much higher and reconciling this has been a problem for many years. Recently, we proposed a resolution to the problem in which the CC is cancelled, effectively removing it. We intend to develop our understanding of the mechanism involved in resolving this problem and establish whether it can be found to exist in particle theory models of our Universe. This leaves open the question of what is the nature of DE and we will be working on models proposed to account for this in which the force driving the acceleration is screened from view in regions of high density such as here on Earth. Such models are hard to rule out, but we have established a number of features they have that would allow us to test for them in the laboratory and on galactic scales. There is an alternative point of view to DE driving the acceleration of our universe, and that is that on large scales we need to modify Einstein's General Theory of Relativity (GR). This is a big field of enterprise and we are heavily involved in working on models in which GR is modified. We will be testing many of these models, both on vast cosmological scales and by seeing how they effect the formation of massive objects like neutron stars. We will also constrain models of Dark Matter, the key component of the universe which keeps galaxies together. We have yet to find out what DM really is, and one of our aims is to help uncover the mysterious particle that is binding our galaxy together. We will test whether it can be in the form of primordial black holes formed in the early universe or maybe as an extremely light particle called the axion.

Many people think the early Universe underwent a period of almost exponential expansion for a brief period of time, known as Inflation. We will work on models of inflation to establish whether or not successful models can be found in string theory, something that has recently been questioned by many. We will also ask how did the universe establish that there was more matter than antimatter in it, something known as Baryogenesis.

A new area which has emerged here at Nottingham recently is the possibility of testing theories of gravity in the laboratory by creating analogue experiments. Although at a completely different energy scale to the early universe, they enjoy similar equations, hence the hope that by doing such experiments we can mimc early universe effects and even use the results to make predictions about those earliest moments. Another area we are developing at Nottingham for the first time is to use the power of Machine Learning to create neural networks to analyse data. We are hoping to apply this new technology to search for cosmic strings, dark energy and provide new ways to analyse astronomical data. It is very exciting.

In Quantum Gravity, we attempt to establish a connection between General Relativity and Quantum Mechanics. We are developing models of cosmology that allow us to talk about the earliest moments in the universe where the role of Quantum Mechanics is vital and intend to develop methods to constrain and test these models with astronomical data such as the fluctuations seen in the thermal radiation associated with the Big Bang.

Who might benefit from this research?

Key beneficiaries are astronomers, atomic/cold atom physicists, particle cosmologists, mathematicians and Particle physicists. Our work is interdisciplinary, using astrophysical data to constrain fundamental models of particle physics, and in doing so we introduce mathematics that will be of interest to that community. We are invited to give plenary talks at a number of major particle physics, astronomy and mathematics conferences, including: NAM2016, LOOPS17, Canada RQI-N 2016 Conference, Testing Gravity 2017.

All of the applicants have given plenary talks. Amongst these: Green gave a review talks on 'Astrophysical constraints on dark matter', (Lisbon Dec 18), and on PBH as DM candidates (KITP Apr 2018, Barcelona 2017). Burrage gave plenary talks on testing for screened DE at CERN, Vancouver and Grenoble to audiences of particle physicists, experimentalists on precision tests and neutron physicists. Copeland was a plenary speaker at NAM2016, the primary International Astronomy conference held in the UK. Gielen was a plenary speaker at Loops '17, Warsaw (July 2017), the largest international non-string QG conference. Padilla gave invited plenary talks on the CCP at Paris, Odense, Liverpool (2016), Belgrade. He gave a series of invited lectures on the cosmological constant at Prague (Sep 2018). Louko gave invited talks on the Unruh effect to conferences and workshops in Ontario (2016), York (2017,2018), Newfoundland, Kyoto (2017). Weinfurtner gave 25 conferences and workshops talks and 13 colloquia. Her work on analogue gravity is truly multidisciplinary and she has spoken at meetings on Black Holes (Harvard 2018) and at the Institute for Quantum Computing (Waterloo 2016). Sotiriou is invited regularly to speak at conferences from quantum gravity and theoretical physics to relativistic astrophysics and gravitational waves,

Another group of people who we believe we may well have an impact on is the General Public. Cosmology in particular holds an impressive fascination with the public. The Sixty Symbols videos that we run in Nottingham are subscribed to by over 750k people, and the particle physics and cosmology inspired videos are particularly popular.

How might they benefit from this research?

The academic community will benefit because our results will hopefully enable them to make progress in their own fields. For example our work on dark energy may well provide insights for the astronomical community in searching for ways of probing for dynamical dark energy, or for cold atom physicists in considering new experiments to search for it. The work on Quantum Gravity and the CCP may well have an impact on mathematicians - the proposed solutions for example may lead to new insights in mathematics and particle physics. Our work on Machine Learning opens up a new world of possibilities linking directly into all the disciplines including the new one of Machine Learning itself. It should be of interest to computer scientists interested in the impact of neural networks on machine learning. Weinfurtner's work has led to invitations to be an Independent Assessor for the European Space Agency (ESA) on their `new science ideas' programme.

Our Sixty Symbols videos are very popular and we know from feedback have an impact on people, to the extent that a number of people turn to studying physics having watched them. As an example of their popularity, three extended videos on topics covered in this proposal (Inflation, Cosmic Strings and Dark Energy) made by Copeland have been viewed over 1.7 Million times. [https://www.youtube.com/playlist?list=PLcUY9vudNKBMxm0HSZCDU7rMAWOt7zLgp]
Our video on the Case for String Theory has been viewed 535k times. (https://www.youtube.com/watch?v=Q8ccXzM3x8A)