When the Universe began from its hot, dense, rapidly expanding state — from the Big Bang — there was no way of knowing it would have turned out this way. The Universe could have recollapsed almost immediately, or it could have rapidly expanded away into oblivion before a single atom formed. Instead, the initial expansion was almost perfectly balanced by the matter and energy present in the Universe, where the density of matter and radiation was almost exactly at that one, critical value it would take for them to eventually cancel each other exactly. As the Universe aged for seconds, then days, then years, then millions and billions of years, still the expansion rate and density dropped, approaching zero.
And then, right around the time our Sun and Earth were being formed, something funny happened: the distant galaxies that had been slowing down in their apparent recession from us all started to speed up once again. Somehow, the Universe began accelerating.
We can measure how the Universe’s expansion is speeding up, and what we find is 100% consistent with a phenomenon that has many names but that all describe the same thing:
• a cosmological constant,
• energy inherent to space itself,
• vacuum energy,
• or an unchanging scalar field.
There are some small wiggle-room parameters for how it might change over time — perhaps a slowly-varying scalar field, or some model of quintessence is allowed — but for the most part, it looks like dark energy is really, really constant. It’s only because the energy density of the other components of the Universe drop as it expands that dark energy ever becomes important.
Yet as the volume of the observable Universe increases, the matter density drops, and the radiation density drops even faster, since its wavelength stretches to lower energy states. But because dark energy is a property of empty space itself, as the Universe expands, the density stays the same. After the first few thousand years of the Universe, matter becomes more important than radiation. After a few billion, dark energy passes matter. And by time we get to today, we can see what the fate of our Universe looks like: a lonely, empty void where everything beyond our gravitationally bound local group eventually disappears from our reach.
One of the biggest unsolved question in theoretical physics, then, is why? What causes this dark energy in the first place? What force compels the Universe to accelerate? And why is the apparent energy inherent to space some value other than zero?