The age of the universe is known to be 12 to 15 billion years because, ever since the pioneering work of the American astronomers Vesto Slipher, Edwin Hubble, and Milton Humason in the first three decades of the twentieth century, the universe’s rate of expansion has been known. Measurements of galactic redshifts show that the farther away a galaxy is from us, the faster it recedes. In other words, redshift is proportional to distance. The data was collected by Hubble and Humason in 1931. Clearly, the relationship between distance and velocity of recession was a straight line. Therefore the equation representing this straight line is v = Hd, where v is the velocity of galactic recession, H is the slope of the line (now called the Hubble constant), and d is the distance between the observed galaxies and us. Velocity is expressed in kilometers per second, and d can be expressed in kilometers as well (although lightyears or megaparsecs— 1 megaparsec equals 3.26 million light-years— are more commonly used). Thus H, the Hubble constant, is expressed in kilometers times seconds 1 times kilometers 1 (or megaparsecs 1 ), which can be reduced to seconds 1 , which is of course the inverse of a time. Therefore the inverse of the Hubble constant, 1/H, gives the age of the universe in seconds— that is, the time elapsed between the Big Bang and now. Current estimates of the Hubble constant range between 50 and 100 km s 1 megaparsec 1 , with an apparent consensus at 80 km s 1 megaparsec 1 . The precise value of H is not known because of the difficulties and errors encountered when measuring galactic distances. The success our search for alien civilizations is intricately linked to an accurate perception of the age of the universe. Take the case of the early environment on a young earth. Earth’s atmosphere 4 billion years ago was very different from the one we know today. There was no oxygen, but other gases were present. In one scenario, these were methane (CH 4 ), water vapor (H 2 O), nitrogen (N 2 ), ammonia (NH3), hydrogen sulfide (H 2 S), and carbon dioxide (CO 2 ). Primeval hydrogen (H 2 ) and helium (He) were disappearing fast because Earth’s gravity was not strong enough to keep them in the atmosphere. Traces of helium would always be present, however, thanks to the radioactive decay of elements, such as uranium, thorium, and radium, in Earth’s interior. There were also oceans, whose geography we would not recognize today, since plate tectonics has moved the continents around. Volcanic activity contributed water, nitrogen, carbon dioxide, sulfur dioxide, and other gases to the atmosphere.
All of the above planetary activities affect the formation of the primary information bearing organic compounds that serve as the blueprint of known life. A very interesting deduction from the study of earth's chemical evolution is that the primitive atmosphere might have been a reducing one. In a reducing environment, oxygen is absent. This was one of the motivations that led to what is known as the Miller's Experiment. Miller had set up his experiment in a flask with various water, methane, ammonia and hydrogen gases. He also induced electrical discharges in experiment to simulate lightening. After several days of cycling the gases and sparking, Miller noticed that the condensed water in the tube had turned pink and subsequently orange-red. Clearly, some chemistry was taking place, as the original gases were completely colorless. Analysis of the solution revealed the presence of amino acids, the building blocks of proteins! Of the twenty amino acids found in proteins, ten were formed in Miller’s experiments. The chemistry that took place in these experiments is now understood. For example, the simple amino acid glycine results from the condensation of formaldehyde (formed from the sparked gases) with ammonia and hydrogen cyanide (also formed in the gas mixture) to produce the compound aminonitrile. Aminonitrile then reacts with water to form glycine. In addition to amino acids that make up proteins, gas-discharge experiments have also yielded the four nitrogenous bases, adenine (A), cytosine (C), guanine (G), and uracil (U), the building blocks of RNA. Adenine for example, results from the condensation of five molecules of hydrogen cyanide (figure 4.3). (It is unsettling to think that the poison used to execute prisoners in a gas chamber can lead to the synthesis of some of the building blocks of life!) Finally, many types of sugars were also synthesized in these experiments, including ribose, the sugar found in RNA. The startling results of Miller’s experiments have led to the notion that Earth’s primitive oceans accumulated more and more of the building blocks of life, amino acids, nitrogenous bases, and sugars, and became some sort of primordial or prebiotic soup. (Instead of the term soup, which suggests a chunky mixture— think about split pea with ham or chicken noodle soup!—I prefer the word broth.) Primordial broths of the Miller type have been replicated by many investigators using similar gas mixtures exposed to short-wave UV light or silent electric discharge, all undoubtedly present on primitive Earth.
Search for alien civilizations would be best served if focused on extrasolar planets that have been observed going through similar transformations. If the pattern of evolution of life on earth is truly universal, the process might have been replicated countless times across the cosmos that must have yielded intelligent life over time. An accurate realization of the age of the universe would give a quantitative indication of the multiplicity of the organic evolution on M-type planets and realistic odds of finding intelligent life one day.