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Physics LibreTexts

17.30 The Limits of Knowledge

The story of the universe in the first second of its existence is highly speculative. However, cosmologists pay attention to the scientific method in their discussions of the early universe, looking for theories that make unique and observable predictions. How far can we probe into the very early universe? Is there is a limit to our knowledge? These are some of the deepest questions we can ask in science.

Scientists believe the limit to our knowledge (and even our useful speculation) comes at the amazing temperature of 1032 K. This temperature occurred 10-43 seconds after the big bang, when the entire universe was 10-35 meters across. Imagine it: billions of galaxies in a space far smaller than the head of a pin! This time is called the Planck era, after one of the founders of quantum mechanics. All the forces of nature become equal at the temperature corresponding to the Planck era. The melting of gravity into the other three forces is the ultimate symmetry. However, we currently have no gravity theory that works under such extreme conditions. General relativity treats space and time as smooth and continuous. However, the very early universe was dominated by quantum fluctuations. Space was so curved that there was no distinction between a particle and the space it occupied. The universe was a seething cauldron of matter and energy appearing and disappearing out of a vacuum, and space and time twisted and fractured like foam.

In the inflationary big bang model, quantum fluctuations drive the rapid expansion that smooths and flattens the universe. The entire universe emerges from almost nothing. According to Alan Guth, the architect of the model, "the universe may be the ultimate free lunch." Andrei Linde, a Russian physicist working at Stanford, has proposed a variation of the model called chaotic inflation. In chaotic inflation, there are many "bubbles" in space-time during the Planck era. The bubbles are spawned by quantum fluctuations and each bubble may contain different forces and particles. In our bubble, the conditions were right for space to inflate and become the observable universe. The idea of many possible universes, most of which are stillborn, is one of the most bizarre in cosmology.

What is the ultimate theory of nature? Nobody knows, but it must involve unification between gravity and the world of the quantum. We are made of atoms and atoms are made mostly of electrons and atomic nuclei. The nuclei are made of protons and neutrons that are made of quarks. Current physics theories are incomplete because they treat electrons and quarks as particles that have no size. If a charged particle is a point, it must have infinite density of mass and electric charge. Physicists have been exploring theories that avoid this problem by treating particles as strings or membranes. These tiny entities can be open, or they can be closed in a loop; they can interact, merge, join, and split. Moreover they exist in more than three dimensions, most of which are unobserved by us. These speculative theories have great mathematical beauty, but no one knows if they apply to the real world. As modern scientists struggle toward an ultimate theory, we see an echo of the ancient Greek scientists like Democrats and Pythagoras. The quest continues to find the harmonies in nature.