Cosmology is the study of the cosmos. If we take the word "cosmos" to mean everything there is, then this makes cosmology a pretty ambitious subject. It's not even that long ago that cosmology was regarded as unscientific, something that belongs to the realms of speculation or even religion. Today, with sophisticated space and ground based experiments providing hard evidence about the Universe, this has changed somewhat, but there are still many deep questions that need to be asked.
George Ellis, a renowned mathematician and cosmologist, makes an interesting distinction between cosmology with a little c and cosmology with a big C. "Cosmology with a little c is looking at the large-scale structure of the physical Universe," he says. "You look at galaxies, you look at microwave background radiation, you look at stars and so on, and you see how all that fits together in the story of the evolution of the expanding Universe. Cosmology with a capital C, is when you add in ideas about the meaning of life, why we're here in the Universe, what, if anything, our existence in the cosmos has to say about meaning in life, or destiny, etc."
It's clear that, in order to deal with Cosmology with a big C, you need more than physics; you need philosophy. Cosmology with a little c is more amenable to the methods of science. But there is a major problem: there is only one Universe and we can't experiment with it. "In normal physics, you get a theory of how [something] works. Then you take many copies of that object and you do experiments on them, and you get a statistical result about how it behaves. When you've got only one Universe, all you can do is observe the one thing that exists, so you can't get any statistics. You can get statistics of aspects of the Universe, but you can't compare different universes with each other, so you can't get statistics of different Universe models. That makes cosmology different from every other science."
Another problem with studying the Universe is that it is very large and very old. There are reaches of it, both in time and in space, that we cannot see or experience directly, so how can we determine the laws of physics that govern them? "The laws of physics apply at the level of systems like the solar system or galaxies or so on," says Ellis. and "You can average them out to see what that says for the dynamics of the Universe at large." In essence, you try to see what you can deduce from the laws that describe smaller systems about larger systems.
But since the Universe isn't something we can reproduce in a lab and experiment with, the problem then is how to test the theories we develop. How can we know they are really true? One theory that is hotly debated but also untestable with current methods is that of the multiverse. The idea is that the there are many different universes, like there are many different bubbles in a bubble bath — we live in just one of them. In the others the constants of nature, such as the gravitational constant, or even the laws of physics, might be very different from ours.
Cosmologists have developed the multiverse theory for good reasons (see this Plus article). But as Ellis says: "There's absolutely no direct evidence and it never will be possible to get direct evidence. If you can't test the theory you have to say, 'Is this now a scientific proposal or a philosophical proposal?' In my view this is scientifically inspired philosophy, not science, because I think you should draw a hard line."
In view of this untestability some people have suggested that we should weaken the requirements of science. "If we have a really strong theoretical argument, [people have suggested] we should say, 'It's so good, we no longer need to test it in the way we've taken for granted up to now,'" explains Ellis. "I think that's very dangerous and I think it would allow all sorts of pseudo-sciences to be reclassified as science. I don't think we want to see that happen."
Cosmology needs philosophy as well as observations.
From a philosophical view point, there is another question hanging over our quest to find the laws of nature. What exactly is a law of nature? Is it something that is independent of us humans, something "god-given" that sits outside the cosmos and prescribes what goes on within it? Or is it something we humans have invented to describe the world we live in? "In some sense [laws of nature] are not part of the Universe," says Ellis. "They underlie the Universe because they control how matter behaves, but they are not themselves made of matter. Laws of physics aren't made of lead or uranium or something." In Ellis' view the laws of nature live in an abstract Platonic space that is independent of human existence. "The laws of physics do not depend on human beings for their existence. Our understanding of the laws does, so you must distinguish the laws per se and what we know about them. We don't know whether the laws are descriptive or prescriptive and that's one of the big difficulties; we can't tell."
It's a question that is very similar to one often posed about mathematics, which is, after all, the language in which we write the laws of nature we know of. Is mathematics something we have invented, or something that lives its own abstract existence and which we discover? "I believe that mathematics exists again in a Platonic space," says Ellis. "In other words, there's a mathematical reality which we explore with our minds, we understand it to some extent; and we don't to some extent. The fact the square root of two is irrational is a fact which exists independent of the human mind, and so we discover those things rather than inventing them. I believe in a Platonic world of mathematics and a Platonic world of physics. I'm not sure how they relate to each other."
Not everyone agrees with Ellis' Platonic view. Some say that mathematics as we know it is specific to us humans, shaped by how we perceive the world. The British mathematician Michael Atiyah once suggested that if we the world's intelligence resided in a single jellyfish living in the depths of an ocean, where there are no individual objects to count, the concept of the counting numbers, that appears so universal to us, would never arise. Similarly, there are people who believe that the laws of nature are human made attempts at describing the patterns we see in the world we live in, and nothing more.
Such thoughts may appear like idle musings, but philosophers argue that they have a real significance. The way you see a law of nature influences how you think about the world and the questions you ask about it — you will find out more about this in an upcoming article. And the question of how we test theories, or deal with the fact that they may not be testable, is clearly important if we are going to save the world from pseudo-scientific nonsense like intelligent design or astrology. We talked to Ellis at a meeting designed to address these and other philosophical questions thrown up by cosmology. For five glorious days the International Philosophy of Cosmology conference brought together cosmologists and philosophers on the beautiful island of Tenerife. We had the privilege of going along and we will bring you more articles and podcasts from the conference shortly. It's a fascinating subject, so stay tuned!
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You can also listen to our interview with Ellis in a podcast.