ONE of the best parts of sharing my scientific interests with the public is how engaged people are with the ideas. In my time writing for New Scientist, I have received lots of lovely and thoughtful notes from diligent readers. Occasionally, there is a question that I quietly respond to in a future column. Since I only write once a month, I can’t address all of the questions, but I was particularly pleased last month to get an email asking me in what ways calculations in cosmology are similar to calculations involving fluids. The person who sent the question is a plumber, and I was gratified that they saw the clear connection between their trade and mine because the answer is yes, they are quite similar.

To explain, it is helpful to say that, of course, one of the wonderful things about physics is that it is consistent. General relativity is as true here on Earth as it is in distant regions of space where dark matter so heavily distorts space-time that it acts like a funhouse mirror, creating gravitational lenses.

My former colleagues at the Massachusetts Institute of Technology, David Kaiser, Alan Guth and the late Andrew Friedman, have helped conduct experiments which show that quantum mechanics can be tested using supernovae – exploding stars at the end of their lives. In other words, all of our experimental data indicates that the laws of physics we learn here on Earth seem to apply everywhere.

Obviously this is good news. If we thought the rules changed in different places, we wouldn’t know how to interpret the cosmos, since there are so many phenomena that we can’t get close to or reproduce in the lab, like supernovae and the neutron stars they sometimes leave behind. Importantly, these phenomena are really complex, so even when using the laws of physics that we know, we look for simplifications.

It turns out that in astrophysics, fluids are actually one of the most important tools that allow us to gain insight into systems without having to reinvent the wheel every single time.

You might object to this because your gut intuition is that outer space is nothing like water coming out of a tap. But there are some similarities. Like water, matter in the cosmos is a substance that deforms under the application of an external force, for example, gravity. This is essentially the formal definition of a fluid.

There is another way to make the case for why matter in the cosmos can be treated like a fluid: the first law of thermodynamics.

“All of our data indicates that the laws of physics we learn here on Earth seem to apply everywhere”

The idea behind this law is that in isolated thermodynamic systems – ones where heat and temperature are of particular importance – energy is conserved.

The first law of thermodynamics tells us that the total change in energy of a system is equal to the difference between the energy that is given to the system in the form of heat and the amount of energy that the system releases in the form of exerting force on its surrounding environment.

We will have to fudge a little on what exactly energy is because even to a professional physicist it is a bit of a tautological idea, but you can think of an object’s energy as its ability to exert force or produce heat.

And you may have heard before that one of Albert Einstein’s great contributions with special relativity was articulating that there is actually a clear way of converting mass to energy and vice versa, through his famous equation, E = mc².

So, if we accept that energy and matter are functionally equivalent, we find ourselves in a situation where it seems clear that we can apply the first law of thermodynamics to the cosmos.

Thus, like water, matter in the universe is conserved – for the most part at least, aside from some quantum flickers here and there – being neither created nor destroyed.

It is actually the case that using the mathematical form of the first law of thermodynamics and taking the expansion of space-time into account, we can derive what is often called “the fluid equation”. This explains how the density of the universe changes as it expands and is identical to what I might use to describe fluid flow here on Earth.

This might seem strange, but I find it reassuring. So many things about the world are uncertain, yet knowing just a few rules and some mathematics opens up the ability to describe vast swathes of the universe.

Today, so much of research in cosmology, for example studying the evolution of galaxies that I described in my last column (9 January, p 20), relies on using computers to solve complicated versions of the fluid equation. These computer codes follow the flow of particles as they create the beautiful structures we call galaxies.