Humans and the History of the Universe

It's been about twelve years since we were able to estimate the age of the universe with reasonably high precision using astronomical data. The answer is 13.7 billion years. By measuring light from distant objects, astronomers are making measurements of the universe as it was in the past. This is possible due to fact that it takes time for light to travel from the distant objects. For instance, we view the sun as it was eight minutes in the past. (Equivalently, if the sun turned off, we would not know it for eight minutes.) We see other stars around our galaxy as they were thousands of years ago, and distant galaxies as they were billions of years ago. In fact we can look almost all the way back to the beginning, when the universe was only a few hundred thousand years old—less than 1/10,000th of its current age. It's important to stress that we are actually witnessing events as they happened in the past. This is a great advantage for astronomers: it would be as if a historian could step into a time machine and return to witness events first hand instead of having to rely on incomplete and biased records. This is how we are confident in our knowledge of cosmological history.

The second fact that allows us to understand the universe is that, whichever way we look, the universe is essentially the same. This is called the Copernican Principle, named after the philosopher who argued that the Sun does not circle the Earth. Just as the location of the Earth is not particularly special, neither is any other spot. The importance of this can be appreciated after considering how physical laws are established. To be accepted, a physical theory must agree with many experiments. A single experiment can be subject to error, so many are needed to produce convincing results. In a laboratory, one can repeat an experiment as many times as needed. Not so with the cosmos. The astronomer cannot repeat the evolution of the universe. Fortunately though, because every location in the universe at a given epoch is similar to every other location, the astronomer can obtain the average properties of the universe at a given epoch by averaging said properties over their values at the many independent, but similar, locations.

In this way, astronomers have measured the average properties of the universe over most of its 13.7 billion-year history. This is what we see: at the earliest times the universe was filled with a hot plasma consisting mainly of protons, electrons, and radiation. As the universe cooled the protons captured the electrons and formed atoms. Eventually these atoms condensed through gravitational collapse into stars. We have witnessed that most of the stars in the universe formed many billions of years ago resulting in giant galaxies with hundreds of billions of stars (much larger than our Galaxy). At present day these giant galaxies have further coalesced gravitationally into clusters of galaxies a thousand times the mass of a single galaxy. It is in this hierarchical fashion, from atoms, to stars, to galaxies, to clusters of galaxies, that the beautiful structure we see in the present universe has formed.