The cosmic chasm

The cosmic chasm

Pedro G Ferreira 

Physics as we know it is elegant and exquisitely accurate. It tells almost nothing about the deepest riddles of the Universe

These observations are useful in that they confirm the laws of physics that we know. The problem is that they don’t offer much guidance about how to approach data that simply don’t fit with what we already suspect to be the case. One such puzzle concerns the very early Universe – the speculative bit of the boring model I described above. With two more decades of exquisite measurements of the cosmic microwave background, we can now pinpoint the properties of the Universe in the first fractions of a second of its evolution. Aficionados of this period of cosmological history are overwhelmingly convinced that the Universe underwent a very rapid period of expansion, which imprinted what were once quantum ripples in space-time and matter onto the largest scales, and seeded the patterns that we can now see in the relic radiation. Dubbed the theory of inflation – because the Universe inflated uncontrollably during that period – it’s very elegant and mathematically simple, but also highly speculative. The reality is that we have no idea what actually happened at the beginning of the Universe; if it was inflation, what was driving it; how it came about; and how it ended.

If what happened at the beginning of the Universe is a mystery, what’s happening now is no less puzzling. We’ve come to realise that most of the matter in the Universe, the stuff that gravitates around galaxies, for example, is dark. In other words, it doesn’t emit or reflect light like the particles from the Standard Model, or the atoms and molecules that we’re familiar with from the lab. This dark matter outnumbers ordinary matter by a factor of six to one, and underpins any explanation of why the Universe looks the way it does on the largest scales. We know almost nothing about what it actually is, apart from the fact that it coalesces under the pull of gravity. Whether it’s one type of particle or an amalgam of different types of particles, whether it could be something more complex and substantial, like a sea of black holes, or some kind of quantum field, the explanation is completely up for grabs. And that’s been the situation since the idea of dark matter first took off more than 50 years ago.