We come from the void and we’ll go back there.
This Eastern saying could well be the motto of modern cosmology. The events that led to the formation of galaxies, stars, planets and living beings might be just an interlude between two eternal “non-places, non-times” – one before the Big Bang and one after all the matter in the universe has been destroyed at the end of cosmic evolution.
The Universe’s End
We’ll kick things off at the end, since we can see signs of this happening pretty clearly (spoiler: no immediate danger, not today or in the next 10 billion years). For around a hundred years (the anniversary is in 2029), we’ve known that the universe is expanding, which means the distances between galaxies get bigger over time. For example, a galaxy 100 megaparsecs away from our Milky Way – that’s about 330 million light-years – is moving away from us at a speed of about 7,000 km/s. But in 1998, two research groups working separately figured out that the universe not only expands, but it’s doing so at an accelerating rate. They figured this out by studying a specific type of supernova called Type Ia, which has a brightness that doesn’t change. By measuring how much light we get from these supernovas, we can figure out how far away they are. And by combining that with the speed of light from when the light was emitted, we can work out a relationship that shows how fast the universe is expanding.
The idea of acceleration was a real game-changer for cosmologists, who had to totally rethink their science. Accelerated motion needs a repulsive force, but gravity, the only fundamental force on cosmological scales, is actually attractive.
So, what’s causing cosmic acceleration?
Lambda, The Answer
The discovery was a total surprise, but the answer had been there the whole time, just waiting to be pulled out of the drawer of great ideas. Albert Einstein came up with the idea as early as 1917, but he got it wrong. He thought there was a ‘cosmological constant‘, which is a kind of energy that fills space and is the same everywhere. He thought this would help him to get his equations of general relativity and his ideas about space and time to fit together. But then Georges Lemaître and Edwin Hubble found out that the universe isn’t static, which meant that Einstein’s constant was a bit of a silly idea. Still, the cosmological constant, also called Lambda, had a weird property: its energy is totally uniform, but it doesn’t move objects (forces come from changes in potential energy) yet it affects gravity like any other form of energy. According to the rules of general relativity, the Lambda constant and its associated pressure act like negative mass, which means they exert repulsion instead of attraction. This is exactly what was needed following the discoveries of 1998.
The Evolution of the Universe and Its Components
The Lambda constant, or “vacuum energy” as it’s also known, really lives up to its name. If we strip away everything – atoms, electrons, protons, neutrons, electromagnetic radiation, even dark matter and the Higgs boson – what’s left is the cosmological constant. And this can’t be got rid of because it’s not linked to particles, it’s just there, spreading everywhere. Vacuum energy is a property of space itself, not matter, and it increases as space expands. Its density stays the same (that’s where the name “cosmological constant” comes from), but the density of matter goes down in an expanding universe, just like a gas becomes rarer as it spreads out in space. This is why the cosmological constant is so important in cosmology: as space expands, its constant density gradually overtakes the decreasing density of matter in stars, gas, dust, and galaxies. At the moment, the Lambda constant is about 70% of the total energy in the universe, but this is set to rise to 100%. When that happens, the rest of the matter, which will probably all end up as black holes, will be spread out over infinite distances.
The Primordial Void: The Beginning and the End
The void is not just the end of the universe, but also how it started. Some theories say the universe went from one state of void to another, so you could say it leapt quantum-style from one to another. The first and last voids are often called “false vacuum” and “true vacuum”.
One of these transitions might have caused the universe to expand really quickly in its early days, a process called “inflation“. Other transitions might have happened later as the universe got bigger and cooler. These things are a lot like the changes you get when you go from liquid to solid, which are called “phase transitions”. Just as ice forms when crystals gradually join together to make a solid, cosmic transitions might generate regions of the “new” phase with different properties to the surrounding phase.
As these regions gradually join together, they might create new phenomena that we could observe. For example, they could perturb space and create diffuse gravitational waves strong enough to be detected by future experiments like ESA’s LISA satellite. Or they might leave behind remnants of the transition, known as “topological defects,” which, like inextricable knots in fabric, survive every transformation and exert gravitational influence on ordinary matter.
A hundred years after Einstein’s constant, we are still uncovering the rich phenomenology of the void.Just as the invention of zero revolutionised ancient mathematics, the discovery of the void as a dynamic entity has compelled physics and cosmology to revise their theoretical foundations. Today, the nature of the void, how it changes and its physical properties are among the main reasons for lots of big experiments in science.
