Cheat sheet to the universe
CHEAT SHEET TO THE UNIVERSE
…FROM HOW IT BEGAN TO ITS ULTIMATE FATE
Our universe encompasses everything around us, with countless stars, planets and galaxies stretching as far as our telescopes can see and vast expanses beyond them that lie forever out of sight. It’s a big subject – the biggest of all – so join us as we break down the basics
Written by Giles Sparrow
WHAT IS THE UNIVERSE?
The universe is a vast expanse of space and time filled with matter and energy. The word comes from the Latin universus, meaning ‘all things’, although ours may in fact be just one of many universes in a so-called multiverse. Over the course of billions of years, the simple atoms that formed in the early history of the universe have come together to produce countless galaxies, stars and solar systems like our own, separated by enormous distances. Meanwhile, nuclear processes inside stars or those triggered by their violent deaths have steadily processed the first simple atoms to enrich later generations of stars – and the space between them – with heavier and more complex elements in an ongoing cosmic recycling scheme.
COSMIC DISTANCES
Astronomers put a scale to the universe by measuring the distance to remote galaxies. The most accurate way of doing this is to look for ‘standard candles’ – rare objects that are bright enough to see across vast distances and which obey physical rules that reveal their intrinsic luminosity, or light output. The most useful standard candles are variable stars called Cepheids, whose pulsation period is linked to their average luminosity. The distance to the host galaxies of these stars can be worked out by comparing their theoretical luminosity to their brightness measured on Earth.
While Cepheids are only detectable in the relatively local universe, the distance measurements they provide are enough to reveal a rule of thumb that can be applied more widely. This rule, known as Hubble’s law, links the distance of a remote galaxy to the speed at which it’s moving away from us. Hubble’s law applies because the universe as a whole is expanding, carrying distant galaxies away from each other like raisins in a rising cake. The wider the separation between galaxies, the faster they move apart, because there is more expanding ‘dough’ between them. What’s more, this expansion stretches light from fast-retreating galaxies into longer and redder wavelengths, creating a ‘redshift’ that can be measured on Earth and used as a proxy for distance itself. Broadly speaking, the farther away a galaxy lies, the greater the redshift in its light.
LIFE IN THE UNIVERSE
One of the most remarkable aspects of the universe is that it’s given rise – at least once – to intelligent life. Life of any kind is only possible because of a handful of basic parameters of nature, such as those which control the strength of forces within and between atoms and those that permit the existence of a stable universe at all. The precise values of these parameters allow the creation of stable elements, the existence of habitable planets and the formation of complex molecules.
Instinctively, it may feel like this level of ‘fine-tuning’ is too big a stroke of good fortune to happen by chance, but the so-called anthropic (human-centred) principle points out a possible flaw in this logic: we shouldn’t be too surprised that the universe seems fine-tuned for our existence because if its properties were slightly different, we simply wouldn’t be around to measure them.
A more immediate question, however, is whether life elsewhere in the cosmos is rare or widespread. Breakthroughs in recent decades, such as the discovery of abundant exoplanets orbiting other stars, have shown that raw materials and suitable habitats for life are certainly widespread, but we still can’t be sure that life will always arise wherever conditions are broadly hospitable or whether we’re overlooking some vital factor.