The expected fates of the Universe (top three illustrations) all correspond to a Universe where the matter and energy combined fight against the initial expansion rate.
In our observed Universe, a cosmic acceleration is caused by some type of dark energy, which is hitherto unexplained.
All of these Universes are governed by the Friedmann equations, which relate the expansion of the Universe to the various types of matter and energy present within it.
There’s an apparent fine-tuning issue here, but there may be an underlying physical cause.
(E. SIEGEL / BEYOND THE GALAXY)
The fact that our Universe has such a perfect balance between the expansion rate and the energy density — today, yesterday, and billions of years ago — is a clue that our Universe really is finely tuned. With robust predictions about the spectrum, entropy, temperature, and other properties concerning the density fluctuations that arise in inflationary scenarios, and the verification found in the Cosmic Microwave Background and the Universe’s large-scale structure, we even have a viable solution. Further tests will determine whether our best conclusion at present truly provides the ultimate answer, but we cannot just wave the problem away. The Universe really is finely tuned, and our existence is all the proof we need.
