Suppose you were an unusual sort of person: one who wanted, above all else, to know the laws of physics. Not content with the rules governing just one sort of thing, a star or an atom or a galaxy, you want to know the fundamental rules behind everything in the universe.

A good reductionist, you know that smaller things are more fundamental: the rules of the parts of things determine the rules of the whole. Knowing about quantum mechanics, you know that the more precisely you want to pin down something’s position, the more uncertain its momentum will be. And aware of special relativity, you know that terms like “small thing” or “high momentum” are relative: things can look bigger or smaller, faster or slower, depending on how they move relative to you. If you want to find the most fundamental things then, you end up needing not just small things or high momenta, but a lot of energy packed into a very small space.

You can get this in a particle collider, and that’s why they’re built. By colliding protons or electrons, you can cram a lot of energy into a very small space, and the rules governing that collision will be some of the most fundamental rules you have access to. By comparing your measurements of those collisions with your predictions, you can test your theories and learn more about the laws of physics.

If you really just wanted to know the laws of physics, then you might thing cosmology would be less useful. Cosmology is the science of the universe as a whole, how all of the stars and galaxies and the space-time around them move and change over the whole history of the universe. Dealing with very large distances, cosmology seems like it should take you quite far away from universal reductionist physical law.

If you thought that, you’d be missing one essential ingredient: the Big Bang. In the past, the universe was (as the song goes) in a hot dense state. The further back in time you look, the hotter and denser it gets. Go far enough back, and you find much higher energies, crammed into much smaller spaces, than we can make in any collider here on Earth. That means the Big Bang was governed by laws much more fundamental than the laws we can test here on Earth. And since the Big Bang resulted in the behavior of the universe as a whole, by observing that behavior we can learn more about those laws.

So a cosmologist can, in principle, learn quite a lot about fundamental physics. But cosmology is in many ways a lot harder than working with colliders. In a collider, we can clash protons together many times a second, with measurement devices right next to the collision. In cosmology, we have in a sense only one experiment, the universe we live in. We have to detect the evidence much later than the Big Bang itself, when the cosmic microwave background has cooled down and the structure of the universe has been warped by all the complexities of star and galaxy formation. Because we have only one experiment, all we can do is compare different sections of the sky, but there is only so much sky we can see, and as a consequence there are real limits on how much we can know.

Still, it’s worth finding out what we can know.m Cosmology is the only way at the moment we can learn about physics at very high energies, and thus learn the most fundamental laws. So if you’re someone who cares a lot about that sort of thing, it’s worth paying attention to!