Nobody likes Dark Energy. The name itself, like Dark Matter, tells us that we don’t know what it is. It’s a placeholder name for some anomalies between theory and observation. It started in 1998 with some distant supernovae that suggested that the rate at which the universe is expanding is increasing. There is a type of supernova (Type Ia) that is used as a so called ‘standard candle’. They always give off the same amount of light and so by measuring how bright they appear to us we can tell their distance. And the further away they are, the further back in time we see them, and from this (and some snazzy mathematics) it was becoming clear that the universe wasn’t just expanding, but getting quicker and quicker at expanding. Suddenly we were living in a special time.
It’s a general assumption in physics that we don’t live in a particularly special place or at a particularly special time. Sure we happen to live on a planet that supports life, and we live after a certain time – certainly life as we know it could not exist until the first generation of stars had burned the higher elements into existence. But we could safely have lived a few billion years ago and the universe would be much the same, and we could live a few billion or trillion years into the future and everything would be essentially the same as now.
Dark Energy changes this. Now the past is receding with an alacrity that will one day hide the evidence of the Big Bang. A lot of what we know about the early universe comes from observations of the Cosmic Microwave Background radiation. But with the acceleration of the spreading of space the radiation will one day slip away and no longer be a clue that we can look at to discover our past. Less than a hundred years ago it was thought that the Milky Way Galaxy was the entire universe – a massive collection of stars surrounded by an infinite empty void. Improved telescopes have shown us that this is not the case. The 400 billion stars of our galaxy have cousins in the hundreds of billions of other galaxies we have seen. Yet in the dim and distant future when space is stretched further and further our local group of galaxies will be all that is visible to the civilisations that exist then. Not for them the discovery of receding galaxies, no clues to lead them to the idea of a singularity from which all we know has come. Indeed by then the local group of galaxies may all have merged into one super galaxy and the ideas of the early twentieth century, the island universe surrounded by infinite void, will be in exact accord with the evidence.
Unless… What if we are not living in a special time, but a special place. Every way we look the universe is the same, but what if we were in the centre of a region of space which was emptier than normal. In astronomy we look far away and look back in time, but what if it’s not that things were different back then, but that things are different ‘over there’. If the rests of the universe is denser than where we are it will restrict the expansion of the universe. Our observations say that the universe did not expand as quickly in the past, but maybe it’s just that the universe didn’t expand so quickly in denser areas. If this is the case then we will have no need of dark energy to make sense of things. Within ten years we will have telescopes capable of measuring enough of a sphere around us to test if this theory is true. Then we will know whether we live in a special place, or just in a special time.