Mapping the Invisible

Let’s Start at the Very Beginning…


13.7 billion years ago the Big Bang happened: the birth of the Universe. The building blocks of nuclei (protons and neutrons) formed, before combining to form the nuclei of the first elements of the Universe. However, the temperature and pressure of the Universe was so high atoms couldn’t form, since electrons weren’t able to combine with the nuclei. Instead, a hot ionised plasma fog was formed, scattering radiation with each forced separation of electrons from a nucleus.


This then created an impenetrable wall of radiation which lasted for 380,000 years, before it was then cool enough for electrons to finally meet with nuclei to form neutral atoms (the epoch of recombination - the biggest reunion in the history of the Universe, it could be said). This radiation left a mark on the Universe, still visible to this day in the form of the Cosmic Microwave Background Radiation (CMBR) [1][2].


The CMBR is the key to understanding everything that comes later. Hidden within it are seemingly random fluctuations in the amount of radiation emitted: the weaker the radiation, the more the photons had to fight their way through the impenetrable fog and loose energy because of it, meaning weaker radiation shows denser areas of matter.


And these minute fluctuations in density, quantum fluctuations, were formed in random incidences in the first event of rapid inflation in the 0.00000000000000000000000000000001 seconds (that’s 32 zeros: a one hundred-nonillionth), with an increase in size by a factor of 1,000,000,000,000,000,000,000,000,000,000 (1 with 30 zeros: a nonillion). This increase dramatically amplified the small quantum fluctuations to an enormous scale, leading to denser areas of matter and sparser areas of matter. The denser areas then became denser and denser due to gravitational attraction, leading to what we see today: the cosmic web [3].


The Cosmic Microwave Background Radiation (CMBR)

The Cosmic Microwave Background Radiation (CMBR)

Credit: NASA/WMAP Science Team


Universe Wide Web


Over the next 13.7 billion years or so, these quantum fluctuations kept on amplifying, ordering objects, such as galaxies, into thick long strings (filaments), creating voids of near-empty space (where the laws of physics may not be the same as here on Earth). These filaments then condensed into large galaxy clusters, which are where galaxy groups (such as the Local Group, made of galaxies such as the Milky Way, Andromeda, and Triangulum) congregate, making up thousands of galaxies with a collective mass of between 100 trillion and 1 quadrillion times the mass of our Sun, and around 30 million light years across [4].


As time goes on, the filaments will feed into the galaxy clusters, forming galaxy superclusters, at more than 100 million light years across. Eventually, these superclusters will become islands of galaxies, separated by voids of very low density, containing isolated stars and galaxies[4][5]


There’s one problem though: there’s not enough stuff to produce that much of a pull to create such structures.


Computer simulation of the 3 Universe, at 3 billion years old, where filaments and voids are forming, along with areas of slightly higher densities: galaxy clusters.

Credit: ESO, CC BY 4.0


The Invisible Web


The answer lies in dark matter. Making up 27% of the Universe and 80% of matter in the Universe, dark matter creates a skeleton for the cosmic web to form around. And around galaxies dark matter clusters further, creating dark halos (the Milky Way has a halo which is 300,000 light years across, and possibly even further -the Milky Way itself is only 100,000 light years across), linking galaxies together gravitationally [6][7][8][9][10].


But, if it’s invisible, how come we’ve got pictures of it? There are a number of methods.


Chandra X-Ray Image of Abell 2029: a cloud of hot gas where multiple galaxies lie

Chandra X-Ray Image of Abell 2029: a cloud of hot gas where multiple galaxies lie.

Credit: NASA/CXC/UCI/A.Lewis et al


What’s not there?


The first method is to observe how the stars and gas within galaxies move about. If the amount of matter which can be seen doesn’t account for how the stars and gas move, then there’s something at play, and that something could