Cosmology Timeline

• (350 B.C.) Aristotle proposed a cosmological model that held throughout much of antiquity and, with Christian modifications, during the medieval period as well. Aristotle’s universe is eternal, unchanging and perfect. Change is associated with the Earth, not the cosmos.
• (1576) Thomas Diggs proposes an infinite universe with Earth at the center.
• (1724 - 1804) Immanuel Kant also proposed that the universe is infinite, filled with “Milky Ways”, disk shaped swarms of stars. Except for the infinitude, this is roughly how we view the cosmos today.
• (1823) Olbers points out that an infinite universe should be ablaze with light. The fact that this is not what we observe leads his proposition to be called Olbers’ Paradox. Olbers himself thought that the explanation that we don’t see a blaze of light is that dust particles are obscuring our view of the more distant universe. However, we now know that dust would not obscure the view; it would simply absorb the tremendous radiation arriving from infinity, heat up and begin glowing.
• (1916) Einstein’s general relativity opened a new way to look at the universe. His theoretical model was guided by the Cosmological Principle - the proposition that universe is everywhere homogenous and isotropic. That is, matter in the universe is smoothly distributed throughout the extent of the cosmos: Wherever you are in the universe as an observer, you will see the same uniform distribution of galaxies (homogeneous) and it will look the same no matter which direction you look (isotropic). The Perfect Cosmological Principle adds the additional property that it will look the same at any point in time as well as space.
• (1927) Georges Lemaitre, a Belgian priest, used Hubble’s recent findings that the universe is expanding to propose that it began from an initial point of extreme density and expanded into what we see today. Lemaitre pointed out that the expansion of the universe explains Olbers paradox for an infinite universe. In an expanding and infinite universe, light from very remote galaxies would never reach us, because these galaxies would disappear over the expanding horizon.
• (1948) George Gamow cast Lemaitre’s theory in a more rigorous form. He was able to demonstrate that the known cosmic abundance of the elements (roughly 75% hydrogen and 25% helium) could be explained by assuming an initial universe of extreme density and temperature. He also showed that a remnant of this initial temperature should survive to this day and, in principle, could be detected.
• (1950) Fred Hoyle proposed an alternate cosmological theory known as the Steady State Universe. In this model the universe expands but the density remains the same, because new matter is continuously being created that “fills in the blanks”. Hoyle disliked Gamow’s universe because it violated the Perfect Cosmological Principle (a universe with a definite beginning clearly changes with time). Gamow and Hoyle used to debate on the BBC in the early 1950s, and Hoyle derisively called Gamow’s model the “Big Bang” approach to cosmology. Both Gamow’s theory and Hoyle’s had testable consequences. Gamow had predicted that radiation from the early universe should be still around at a much longer wavelength (corresponding to a very low temperature). Could anybody detect this? Hoyle, on the other hand, predicted that the universe looked the same at all times past and future. How do we look into the past? Simply find the most distant galaxies available as see if they look like galaxies in the local neighborhood. In the middle of the twentieth century there were technical restraints that left these questions ambiguous. This situation was soon to change, however.
• (1964) Arno Pezias and Robert Wilson, two Bell Labs engineers working on an extremely sensitive radio antenna, noted a faint background signal in the microwave region that seemed to be coming from space, equally in all directions. The signal corresponded to a temperature of 3 K, about what Gamow’s model predicted. This was considered very strong evidence for Gamow’s theory, now and forevermore known as the Big Bang, Hoyle’s flippant designation. Subsequent investigations, such as deep space observations of distant galaxies with the Hubble space telescope, have strengthened the evidence for the Big Bang. Today, virtually all cosmologists adhere to some form of this cosmological model.

The Cosmic Background Radiation

George Gamow and R. Alpper first proposed the idea that the universe is filled with detectable radiation from the Big Bang in 1948. They reasoned thus: The universe is expanding. Therefore, it is bigger today than it was yesterday and will be bigger tomorrow than it is today. Running the tape backwards, the universe was smaller yesterday than it is today and was smaller the day before yesterday than it was yesterday. If you run the tape back far enough, then all of the galaxies are together in the same place. You can almost think of it as a super version of how a star forms from a giant molecular cloud. As the galaxies run together, the universe gets hotter and hotter (matter falling through a gravitational field). The galaxies disintegrate into nothing but atoms and then into elementary particles. In effect, you have a giant “star” made up of all the matter in the universe. If you keep going you eventually end up with a “singularity”, a point of infinite density and temperature.

Ignoring the conceptual and computational problems with the singularity, Gamow and Alpher concentrated on what happened after the expansion began. At the initial extreme temperatures there were only photons of extremely high energy. These photons could produce matter in the form of matter and antimatter pairs: electrons and positrons (antielectrons), quarks and antiquarks. These matter and antimatter particles immediately collided to disappear in a shower of photons. However, as this “quark soup” cooled, more and more of the particles survived. Assuming a slight excess of matter over antimatter, after about one second electrons were surviving and quarks were combining to form protons and neutrons. For some time after the appearance of ordinary matter, photons still collided with great frequency with electrons, insuring that matter and photons stayed coupled together – the universe was opaque.

During this period, lasting about 300,000 years, the universe resembled the core of a star, fusing hydrogen into helium and producing energy as a byproduct. However, unlike a star, the universe was definitely not in hydrostatic equilibrium, and therefore, due to the expansion, the temperature soon dropped so these interactions ceased to occur. At that point atomic nuclei could combine with electrons to form ordinary atoms, and photons became decoupled from matter. The universe became transparent.

Gamow and Alpher estimated that the “star core” stage lasted just long enough to fuse about 25% of the mass of the universe (protons and neutrons) into helium nuclei (two protons and two neutrons) plus a small percent deuterium (one proton and one neutron) and lithium (three protons and four neutrons). The observed cosmic abundance of the elements is in agreement with this prediction.

Further, Gamow and Alpher concluded that the temperature of the universe at the time of the decoupling was approximately 3,000 K. Thus, the photons would have had wavelengths corresponding to this temperature (Wein’s Law). Since the universe is now about 1,000 times larger than it was then, the wavelengths of these photons have been stretched by a factor of 1,000, from about 1,000 nm to 1mm. The current wavelength of 1 mm corresponds to a temperature of around 3 K, thus it is often referred to as the “3 degree cosmic background radiation”. (Actually Gamow and Alpher estimated 25 K, but this was later corrected). This is the radiation detected by Penzias and Wilson in 1964. Their discovery was the single most important reason that the great majority of cosmologists abandoned the Steady State Universe and pledged allegiance to the Big Bang model.

(1) The universe from about 1 second to 300,000 years

Radiation Separates from matter

(2) The universe 300,000 years after the Big Bang

(3) The universe we observe today (estimated to be 15 billion years after the Big Bang)