A common objection is that if the anthropic principle dictates things are the way they are to allow intelligent life, then surely all the other galaxies, et al, are not needed? Of course, such a view disre- gards that there might be other forms of intelligent life needed. But further...
Quoting Gribbin and Rees, COSMIC COINCIDENCES. "A UNIVERSE BIG ENOUGH FOR LIFE
At first sight, it might seem that one planet like the Earth, circling one star like our Sun, would be sufficient to provide a home for life and the opportunity for intelligence to evolve. There is no way to set a precise figure on the extent of our Universe, and the number of stars and planets it contains, but at the very least it contains a billion billion (10^18) stars, and at least 1 percent of that number--some 10 million billion stars--are likely to be reasonably similar to our Sun. If we guessed that just 1 percent of these Sun-like stars actually possessed a retinue of planets that included a planet like the Earth, that would still provide a hundred thousand billion homes for life as we know it. This is a number so extravagantly large that it makes our place in the Universe seem utterly insignificant. And yet it may be necessary that all those billions of potential homes for life exist, simply because ONE home for life, our Earth, exists.
"Consider the implications in terms of the linear size of the Universe, rather than the number of stars it contains. Cosmologists estimate, for good, sound reasons, that the observable Universe is about 15 billion light years across. A light-year is simply the amount of distance light can travel in one year so it is no coincidence that this number is linked to the estimated age of the Universe--15 billion years. We can, in principle, "see" as far as light has had time to travel since the Universe began.
"The fireball of the Big Bang was a simple place, in the sense that matter was broken down into its component parts then. As the Universe expanded and cooled, those basic building blocks of matter formed into the simplest elements, hydrogen and helium. But studies of the light from very old stars show that scarely any heavier elements than these emerged from the Big Bang. The essential molecules of life, which include carbon, oxygen, nitrogen, and phosphorus, were manufactured by themonuclear processes inside stars AFTER the Big Bang. Our own Sun is not one of the first stars that formed when the Universe was young. These stars went through their life cycles, converting hydrogen and helium into more complex nuclei, and some of those stars then exploded as supernovae, scattering the fruits of stellar nucleosynthesis through the dust and gas clouds of the young Galaxy. Only later generations of stars, born out of collapsing fragments of those interstellar clouds, contained enough of the heavier elements to form planets like the Earth, and allowed life forms like ourselves to emerge.
"All that took time. In round terms, it takes a few billion years for a galaxy to form, for the first stars in it to process hydrogen and helium into heavier elements, live out their lives, and die in a blaze of glory, scattering those elements in the process. It then takes more time for new stars to form out of the debris, and for life to evolve on the planets circling those stars. In order for us to be here wondering about it all, the Universe MUST be about 15 billion years old, and therefore about 15 billion light years across!
"This insight demonstrate the power of anthropic reasoning. Simply from the fact that we are a carbon-based life form we can deduce that the Universe must be a certain size and a certain age. Sometimes, those who argue that the Universe cannot possibly have been designed, or created, expressly in order to produce a carbon-based, intelligent life form inhabitaing a single planet orbiting an ordinary star, point out the Universe seems ludicrously OVERdesigned for such a task bigger and older, and containing far more stars than seem necessary. Provided that the laws of physics had to be as they are, the argument falls down. Given the laws of physics that operate in our universe, all those billions of stars and billions of light-years are necessary for our existence."
To those who might think a single galaxy alone would be enough, I quote from Barrow and Tipler, THE COSMIC ANTHROPIC PRINCIPLE. "If our Universe were to contain just a single galaxy like the Milky Way, containing 10^11 stars, instead of 10^12 (10 to the 12th power) such galaxies, we might regard this a sensible cosmic economy with little consequence for life. But, a universe of mass 10^11 solar masses would, according to (formula) have expanded for only about a month. No observers could have evolved to witness such an economy-sized universe."
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