The 4% Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality
Richard Panek
297 pages including notes and index
published in 2011
As a child reading Carl Sagan’s Cosmos it all seemed so simple. Some 13-14 billion years ago the Big Bang started the universe, which has been expanding ever since. In due time as it cooled off the first stars and galaxies appeared, formed from primordial hydrogen & helium gases through the power of gravitational attractions. The novas and supernovas from those first generations of stars would create the heavier elements needed for life to arise and ultimately the Solar System would form with the Earth being just right for us to evolve on. As Sagan explained, what would happen next depended very much on the total mass of the universe. If big enough, gravity would slow down its expansion and cause it to collapse, maybe triggering a new Big Bang. If small enough, the expansion could not be stopped and first galaxies, then stars would drift further and further apart until we were alone. If just right, expansion would stop but the collapse would be prevented. Knowing the mass of the universe therefore is important.
And this is where the problems arise: if you count up the mass of all the galaxies, stars, quasars, pulsars, etc, everything you can see with the electromagnetic spectrum, the amount of observable mass of the universe is just four percent of what it actually is according to all our other measurements. There has to be something else therefore that makes up the difference and it’s this something else is what is driving the fate of the universe. It’s the quest to discover what this might be that is the topic of The 4% Universe.
Panek starts this quest in 1964, just before the discover of the cosmic microwave background, following the story until he reaches the present consensus on what the universe looks like and what its likely fate is. Since this was published in 2011 the state of the art has moved on in the almost decade and a half since this was written, but that isn’t a major problem. Much of the The 4% Universe is about the journey of discovery and the people involved, how the then current consensus was reached, which is still interesting even if it has moved on since. Following a roughly chronological order, Panek divides the story in roughly four parts, looking at the different aspects of the problem of the universe’s missing mass. This turns out to be not just a problem for astronomers looking at stars and galaxies and finding too few, but also for cosmologists whose mathematical theories about the universe are directly impacted by this missing mass and particle scientists, whose models might hold the key to solving the problem.
As Panek puts the building blocks together, the picture that emerges is roughly that which you can read at the Wikipedia page on the Universe. That simple history Sagan showed me turned to be much more complicated. What you see in the night sky is not the entire universe and the largest parts of it cannot be seen at all: dark matter, some 23 percent of the universe and dark energy, the remaining 63 percent. And some of the things that we could see if we were close enough are forever out of our reach as they lay beyond the observable parts of the universe, expanding away from us too quickly to be reached by our light.
If you are more interested in cosmology than the history of our understanding of it The 4% Universe is the wrong book for you, as its understanding of it is outdated and its view of it rather high level; reading Wikipedia might be more useful. For me however it did a good job of showing that history and it was the first book to made me realise that doubts about that simple Big Bang model and questions about the mass of visible Galaxies were much older than I knew. It was also the first book for me that clearly spelled out that the cosmic inflation of the early universe happened at faster than light speeds. Obvious, you would think but I never quite got that the universe itself can expand faster than the speed of light and that its limit as the fastest speed possible only applies to objects within it.