Testing and constraining theories of particle physics through cosmology

My work on the early universe has concentrated on investigating the properties of a network of cosmic superstrings, the famous strings of string theory which are usually thought to be too small ever to detect. Well strangely enough, it is possible for such strings to not only be stretched and stabilised by the expansion of the universe. Once we accept this possibility, an obvious question is could we ever detect these strings? One possible way is seeing how they affect the cosmic microwave background (CMB) photons which were released in the early universe. This radiation would be affected by the presence of the strings in a particular way, depending on the strings properties, and leave a signature in the polarisation patterns seen in the CMB photons.

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The all-sky picture of our universe created from seven years of WMAP data. It shows 13.7 billion year old temperature fluctuations (the colour differences) that correspond to the seeds that grew to become the galaxies. Cosmic superstrings could be hiding in there! Image credit: NASA / WMAP Science Team.

What I have come up with are distinctive patterns that cosmic superstrings could leave in the CMB. Now over the next year or two as a number of CMB experiments, such as Planck begins to release its expected pristine data of the CMB you never know, they may see tell tale signs in the data telling us that strings really could be out there. The second area I have worked on is known as the cosmological constant problem. The universe is not only expanding today with distant galaxies getting ever further apart, they are accelerating away from each other. One question is what if the fuel driving the acceleration and overcoming the natural deceleration is associated with gravity. One solution is known as a cosmological constant, its presence fits the data perfectly, but here is the rub. When you estimate what value the cosmological constant should have today based on known physical arguments, it should be many orders of magnitude larger than what is observed. It makes very little sense it having the value we see. Many attempts have been made to address this problem, most of them involve making sure all the contributions almost cancel each other to a high degree of accuracy– but why should they?

My approach is different, allow the cosmological constant to be there, let it be as big as it wants, but crucially I need to hide it from gravity. In effect the cosmological constant has an invisibility cloak, known as a scalar field. The field shields us from the effect of the cosmological constant by responding to changes in it. Gravity doesn’t couple directly to the cosmological constant, but it does couple to the scalar field. What I have been doing with my collaborators is to demonstrate the mathematical consistency of the model. Now all I have to do is establish that there is a sensible cosmology associated with it.

Professor Ed Copeland
University of Nottingham

Ed was awarded a Research Fellowship grant in 2011.

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Found in Galaxy NGC 4526. Through many such observations, came the key evidence that the universe is accelerating today – we are still trying to work out why and how. Image credit: NASA, ESA, The Hubble Key Project Team, and The High-Z Supernova Search Team.