Pedagoguery

One hundred years ago, our understanding of the universe was drastically different from what it is today. Back then, it was believed that the Milky Way galaxy was alone, the only collection of stars in a universe that was static and eternal, surrounded by an infinite void. It took a number of different discoveries to change this view. First came Einstein's Theory of General Relativity. When the equations for this theory were first solved, it was discovered that a static universe was impossible. Einstein added the cosmological constant to the theory to correct this “flaw”. Then came the discovery in the that “spiral nebulae” were actually galaxies in their own right, that they existed far outside the confines of our own, and that they were receding from us at speeds that depended on their distance. The discovery of the cosmic microwave background radiation further revolutionized the study of cosmology, decisively deciding in favor of the Big Bang model over the Steady State model. The most recent discovery major discovery in cosmology, however, may presage the end of the science altogether, and that is the discovery that the expansion of the universe is accelerating.

The first three discoveries I mentioned above serve as three of the pillars of cosmology. General Relativity provided the theoretical framework that allows us to describe the large scale evolution of the universe. The proper classification of galaxies allows astronomers to discover the expansion of the universe, thereby making better sense of General Relativity. Finally, the cosmic microwave background radiation allowed us to probe back to the early stages of the universe. There is a fourth pillar, and that is the observation that in the early stages of the universe, conditions were right for nuclear fusion, and thus after the universe was about three minutes old, it consisted of 76% hydrogen, 24% helium, and a trace of lithium.

All four of these pillars are being endangered by the accelerating expansion of the universe. In such a universe, any galaxy that is not gravitationally bound to ours will eventually be accelerated outside of our local horizon. Those galaxies that are bound gravitationally to ours will eventually merge with ours, resulting in a large elliptical supergalaxy. 100 billion years from now, there will be no other galaxies visible to us, and therein lies the end of cosmology as a science.

The second of the pillars of cosmology in such a universe will be gone. Without visible galaxies, we lose our signposts displaying the expansion of the universe. You can't measure a void. Without the ability to measure the expansion of space, the role of General Relativity in cosmology also ends.

The cosmic microwave background similarly suffers. As the universe expands, it gets further redshifted. It has already suffered a redshift of a factor of about 1000. By the time the universe is 100 billion years old, it will have been redshifted into invisibility.

The final pillar of cosmology is in danger not from the expansion of the universe, but from the actions of the stars themselves. As they age and die, they change the chemical composition of the universe. We already see some evidence of that. In the present day, the abundance of hydrogen is 70%, down from 76%. Helium is 28%, up from 24%, and heavy elements are at 2%, up from a trace. By the time the universe is 1 trillion years old, we expect that the relative abundances of hydrogen, helium, and heavy elements will be 20%, 60%, and 20% respectively. All traces of the big bang nucleosynthesis will have been obliterated.

Ironically, what we will be left with will resemble the universe that was thought to exist 100 years ago – a single galaxy in an endless void. All traces of the origin of the universe will have been wiped out by the passage of time and the expansion of the universe. We truly do live in a privileged time in our universe.

Next time, how Venus lost its water.