St. Andrew’s is a university known for its impressive research, ranking third in the United Kingdom for the quality of its research projects. This will be the beginning of an ongoing series of articles about various research projects in the science schools of St. Andrew’s.
Dr. Rita Tojeiro, of the school of Physics and Astronomy, is working on a project to produce a three-dimensional map of the universe. This project has been running for nearly twenty years, mapping 5 galaxies a night to slowly build up an increasingly complex picture of the macroscopic distribution of galaxies. Dr. Tojeiro worked in the past on a similar project - Sloan Digital Sky Survey III - which was the largest ever map of this type, including 1.2 million galaxies. This number seems large - and it is - but it’s only a tiny sliver of the total number of galaxies we can possibly observe. The observable universe - the local spherical region of space encompassing everything we can actually see - contains hundreds of billions of galaxies at least, with the exact number unknown. Research thus continues to make even more detailed maps.
The current project is now drawing to a close, with its results expected to be published later this year. Dr Tojeiro is currently working on the next, even larger project - the Dark Energy Spectroscopic Instrument (DESI) survey. DESI uses far more advanced technology, to produce a vastly more enormous map of space.
As DESI’s name implies, these projects have more significance beyond simply mapping galaxies, although even being able to measure and plot the universe at such enormous scales is an impressive feat - they are important for the reason that they aid in the study of the elusive, poorly understood Dark Energy.
When the universe was young, it was composed of an incredibly hot, dense cloud of particles - the primordial plasma. At this point in time, atoms did not exist. The plasma consisted of electrons and baryons - the smaller constituent parts of atoms. Atoms would not come into existence, coalescing and combining out of the elements of this plasma, until much later.
Gravity, the inexorable force of attraction between all matter objects, tended to draw the plasma inwards - compressing and constricting it. However, the energy released by the hot plasma tended to push outwards; the result was a conflict in forces. The disturbances in the boiling, churning primordial plasma rippled outwards, forming waves. These are called Baryonic Acoustic Oscillations - BAOs, for short.
These waves propagated through the universe, and continued to do so until the plasma cooled to the point that atoms formed - as hydrogen began to come into existence, the waves froze in place, leaving their imprint in the distribution of the coalescing matter. This period is known as recombination.
The importance of BAOs is that they provide a ruler for the expansion of the universe. The universe is expanding, and has been since the moment of its birth - this growth is driven by a mysterious form of energy known as Dark Energy - ubiquitous, but poorly understood, the exact nature of Dark Energy is one of the most significant unsolved problems in physics.
These waves, frozen into the basic structure of the universe, are stretched by the expanding universe. By calculating the length of the waves at the point of recombination (many billions of years ago) and then measuring their length now by observing the distance between galaxies, the rate of expansion can be measured.
The research being carried out at St Andrew’s, in conjunction with other institutions, therefore aids us in understanding how and why the universe is expanding, going right to the core of one of the most important, and most poorly understood, areas in all of cosmology.