Is Dark Energy Evolving?

Life is shaped by choices. The fundamental debate is whether to live life for learning or for pleasure. This weekend, my existential tension boiled down to a simple question: why am I writing my next book and my next scientific paper rather than having fun in the sun?

After some contemplation, I came up with the realization that learning is pleasure. But there is another benefit to writing. Most people will live in the future and I wish to communicate my thoughts to those who will be born long after I am gone. I weigh my priorities in life based on the number of people who might benefit from my actions.

There are currently 8.1 billion people on Earth, about 7% of the total number of humans who have ever lived since the Big Bang, 117 billion. Based on the star count from the Gaia sky survey, the number of stars in the Milky-Way galaxy is comparable to this total value within a factor of a few. This implies that for the foreseeable future, Milky-Way stars could be named after each person who ever lived on Earth.

But there are two important distinctions between stars and people. First, in difference from people, most of the Milky-Way stars were already born. The reservoir of cold interstellar gas that can make new stars is significantly depleted. The same holds true throughout the cosmos. The average rate of star formation per unit mass in the Universe peaked 9 billion years ago and declined by a factor of ten by the present time, 13.8 billion years after the Big Bang. The Sun formed in the last third of cosmic history, 4.6 billion years ago, halfway back to the time when most stars were born. This establishes the rationale for the Galileo Project’s search for interstellar technological objects: advanced technological civilizations could have existed billions of years before us. It takes chemical rockets less than a billion years to cross the Milky-Way disk from one side to the other.

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