Generally no, atheists do not believe that the universe was designed. A design implies a designer, and a designer of the universe would, by definition, be a supernatural being. There are different definitions of a god, but generally a supernatural being with the power to design or create a universe would be considered one, and atheists don’t believe that any gods exist.
There are many who claim that the universe shows evidence of being designed. In many cases, however, this evidence is just a misunderstanding of physics or a mis-characterisation of observation.
The physical constants of the universe are fine-tuned
Many claim that the physical constants of the universe are fine-tuned for life.
There are many numeric constants that must be used in the equations that we know govern the universe. These constants have very exact values, and there are quite a few of them. If any one of these constants had even a slightly different value, certain physical processes that occur in the universe would not be able to occur. For example, if the constants determining the relative strength of the strong nuclear force and electromagnetic force were changed slightly, nuclear fusion could not occur in stars. Nuclear fusion in stars is required for the production of heavier elements, upon which all life as we know it depends – thus, if fusion were not possible, life would not be possible.
This is how the argument goes. However, there are many problems with it. Firstly, physicists do not yet have a set of equations that completely describe the universe – a so-called ‘theory of everything’. The various theories we have so far work in some situations, but not others – they are, in a sense, a ‘best guess’ (a very well-informed guess), but they are not the ultimate truth of how the universe works. This means that the numeric constants that are put into them are there to make them produce useful predictions – they are there, in a sense, for convenience. The general opinion among physicists is that in a better theory of the universe, these constants would simply be inevitable results of more basic principles and laws. Thus, they would not be ‘fine-tuned’ – they would just appear that way because a more fundamental aspect of nature pushes them to those values.
Secondly, while nuclear fusion allows life as we know it to exist, it would be incorrect to assume that life as we know it is the only kind of life that could exist. Different values for physical constants might well allow a different kind of life to exist – indeed, they might even allow for more life to exist. It’s impossible for us to know – predicting whether complex life would exist for a different set of physical constants would require an extraordinarily powerful simulation – far more powerful than we are able to run (and possibly more powerful than we will ever be able to run).
The universe is perfect for life
Many claim that the universe is perfect for complex life, and that this is evidence it has been designed. This is very similar to the claim above, but it’s a distinct error of observation.
The universe is actually not perfect for life. The vast majority of the universe is completely inhospitable for life. Most of the universe is just empty space, in which no life can exist. Most planets cannot sustain life – even in our own solar system, only one can. And even on Earth, life exists in a very thin layer on the surface of the Earth. In most places on Earth, you need only go a few hundred metres up in the air or a few tens of metres down in the ground, and no life exists. Even then, there are parts of the surface of Earth, like the Sahara Desert, where very little life can exist.
Generally, liquids are required for life to exist, but liquids only exist under certain temperature and pressure conditions – usually on the surfaces of planets that have atmospheres.
So most of the universe is actually not suitable for life at all. A universe that was ‘perfect’ for life would surely be able to sustain life in a significant proportion of it.