What is cosmology?
Observing sources of light from the night sky with the aim of understanding their origins has been something humans have done for millennia. Historical evidence exists which suggests ancient civilizations methodically recorded celestial objects. As our technology developed and our wider understanding of natural sciences blossomed, we have been able to devise models to explain the observations and make predictions which test these theories.
However, until recently in our history, this scientific discipline has been based on observations limited to within our own cosmic neighbourhood. Observing beyond our own Galaxy, the Milky Way, presents a huge technological challenge and requires detecting photons from other galaxies millions of light-years away which will therefore appear extremely faint. These complications mean that regular extra-galactic observations have only been a possible scientific discipline within the last 100 years when our telescopes have become sufficiently capable of detecting distant galaxies.
These distant observations present the possibility for us to study the Universe as one entity. We can now aim to build a representative map of large-scale cosmic structure and determine the matter from which this structure is built. Through detecting faraway photons which originated billions of years ago, we also peer deeper into our Universe’s past. Cosmologists then look to pursue questions about our Universe’s origins, evolution and predict its fate. These are the principal aims for the topic of cosmology.
Less than 5% of our present-day Universe is thought to be ‘ordinary matter’ which is compatible with the rest of our laws of Physics. [Credit: planck.cf.ac.uk]
While the topic of cosmology is a relatively young one by scientific standards, we have succeeded in developing a standard model to explain our Universe. On one hand, this model still has huge unanswered questions regarding the exact nature of 95% of the Universe’s energy and matter content, the so-called dark sector which includes dark energy and dark matter. However, on the other hand, with the assumption that we broadly understand how this dark sector behaves, our standard model proves successful at explaining our observations and replicating them with theoretical simulations.
The standard model of cosmology is not something that can predict the exact locations of every galaxy, cluster and void. Instead, it predicts statistical properties of fields and how the matter within them is distributed. Cosmological constraints and parameters are therefore inherently statistical and their precision is driven in large part by access to extensive data sets. The aim of modern, precision cosmology is therefore to record as large volumes of the Universe as possible to maximise statistical precision, placing tighter bounds on our statistical constraints. This in turn can potentially reinforce or rule out certain hypotheses on the nature of the dark sector. It also can reveal any tensions in independent measurements of the same parameters, thus highlighting areas of our standard model which might not be as robust an explanation of reality as we had hoped.