The Cosmology of the Early and Late Universe

Our Universe is accelerating, distant galaxies move apart with ever increasing velocities, yet we don't know the source of this acceleration, referring to it as Dark Energy (DE). A key goal of our work is to pin down the nature of this enigmatic substance, including the possibility that it is signalling a breakdown of Einstein's description of gravity on the largest scales. The cosmological constant (CC) is the simplest DE candidate, yet the value it would have to have to be driving the acceleration is at least 60 orders of magnitude lower than we would naturally expect it to have based on the quantum theory of matter. As Pauli once observed, the electron's contribution to the CC should result in a universe no larger in size than the earth-moon distance ! We have developed two approaches to deal with the CC and will be analysing their cosmology. In both cases, we consider controlled deviations from Einstein's original model of gravity, allowing us to eliminate these dangerously large contributions to the CC. Alongside these models, we continue to ask questions about the nature of DE, and other novel long distance phenomena known as chameleons, developing new approaches to consider their effects on particles in accelerators, as well as on cold atoms in laboratory vacuum chambers.

Dark matter (DM( is an illusive substance difficult to see because it does not interact electromagnetically, and yet it completely dominates the matter content of the Universe driving the dynamics of galaxies. There are currently two main ways we use to detect DM particles: one is deep underground in direct detection experiments where the particle collides with a nucleus of a heavy atom and the recoil of the nucleus is picked up in sensitive detectors, and the other is through the indirect measurements of decay products such as very high energy gamma rays that are produced as a result of DM particles annihilating in the sun. Up until now these two approaches have been somewhat exclusive of each other using different approaches to their analysis. In this proposal we intend to develop technology that will allow for the consistent comparison of direct and indirect detection data, thereby bringing these communities together and increasing the chance of detection.

It is quite possible that the Early Universe was populated by two or more types of strings, fundamental strings whose modes of oscillations provide us with the particles we see today, and cosmic strings formed during phase transitions. Both of these strings would evolve as the universe expands and we can model their evolution. Because they are very massive (a km of cosmic string would have the mass of the moon for example), they perturb the spacetime in which they move leading to distortions in the temperature of the light in regions close to the strings. We search for this distortion in maps made from the remnant radiation from the Big Bang known as the Cosmic Microwave Background (CMB). However, the most recent maps from the Planck satellite, in which we have played an important role in analysing, there is no direct evidence of the strings. It doesn't mean they are not there, although they may not be, but it does mean they are not as massive as we thought. However, there are other signals they can leave in the polarisation of the remnant radiation and we intend to obtain the unique signals cosmic strings and cosmic superstrings would leave in such a map and then search the Planck data to see if they are found lurking in there - the first direct evidence of string theory.

One of the key problems facing modern physics concerns reconciling two of the pillars of twentieth century physics, General Relativity which governs the motion of objects on macroscopic scales and Quantum Mechanics which governs objects on the atomic scale. The effort to reconcile these is known as Quantum Gravity and in this proposal we are aiming to develop a number of approaches with that as the ultimate goal.

Who might benefit from this research?

The most obvious group of people that will benefit from our research are particle physicists, atomic/cold atom physicists, cosmologists, astronomers and mathematicians. Our work is interdisciplinary, we use astrophysical data to constrain fundamental models of particle physics, and in doing so we introduce mathematics that will be of interest to that community. This interest is seen in the fact we are invited to give plenary talks at all the major particle physics, astronomy and mathematics conferences, for example: SUSY2014, LOOP2015, COSMOS2014. Padilla was invited to present his work on the CCP in a series of lectures to the particle physicists at CERN as well as to cosmologists and astronomers in Mexico. Copeland presented his work to particle phenomenologists at SUSY2014 and will be a plenary speaker at NAM2016, the primary Astronomy conference in the UK.

Another group of people who we may well have an impact on is the General Public. Science and cosmology in particular holds an impressive fascination with the public. The Sixty Symbols videos that we run in Nottingham are subscribed to by over half a million people, and the particle physics 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 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 following his IOP award have been viewed over 870,000 times.
[https://www.youtube.com/playlist?list=PLcUY9vudNKBMxm0HSZCDU7rMAWOt7zLgp]