Cosmos

If the universe is vast and old and life is not astronomically rare, where is everyone? Every proposed answer — the Great Filter, the Zoo hypothesis, the Dark Forest — implies something extraordinary about our situation. The silence is the most interesting data point in the cosmos.

Stephen Hawking showed that black holes emit radiation and eventually evaporate. But if information cannot be destroyed, where does the information of everything that fell in go? Hawking and Penrose disagreed about this for decades. The resolution involves holographic principles that suggest the universe is, at some level, a projection.

The fundamental constants of physics — the gravitational constant, the speed of light, the mass of the electron — are set to values that allow matter, chemistry, and life to exist. Change any of them by a small fraction and the universe collapses, or expands into nothing, or never forms stars. Is this luck, necessity, or selection effect?

The laws of physics are symmetric in time — they work equally well forwards and backwards. And yet time has a clear direction: entropy increases, causes precede effects, we remember the past but not the future. Why? The best current answer is that the universe started in an extraordinarily low-entropy state. The Big Bang was the origin of time's arrow.

Jupiter's moon Europa and Saturn's moon Enceladus both have liquid water oceans beneath their icy surfaces, kept warm by tidal heating. Enceladus actively vents water vapor into space — organic molecules have been detected in the plumes. Life requires liquid water, energy, and chemistry. Both moons check all three boxes. We may not be alone in this solar system.

Everything we can see — every star, galaxy, and nebula — is approximately 5% of the universe's mass-energy content. The rest is dark matter (27%) and dark energy (68%), neither of which has been directly detected. The universe we know is the fringe. The main story is happening in matter and energy we cannot observe. Science rarely has a better plot twist.