1826-- Olbers' Paradox and the Necessity of a Beginning

A look  at the night sky suggests a Changeless Universe,  apart from local (small-scale) phenomena, such as the Clouds drifting across the Moon. Hence, suppose that on large-scale the Universe is static, infinite, eternal and uniformly  filled with stars. Then, one must reach the conclusion that every point on the night sky should be as bright as the surface of a star. The reason for this (seemingly crazy) conclusion is that the number of stars at any given distance increases with the square of the distance, while the intensity of light from a star decreases also with the square of the distance. In this way we should live in the center of a hollow blackbody whose temperature is (like that of the Sun) about 6000 degrees. This is Olbers' paradox, which can be traced as far back as Kepler in 1610, rediscussed by Halley and Cheseaux in the eighteen century, but was not popularized as a paradox until Olbers took up the issue in 1826. There are many possible explanations which have been considered. Here are a few:

      There's too much interstellar dust to see the distant stars.

      The Universe has only a finite number of stars.

      The Universe is expanding; distant stars are red shifted into obscurity.

      The Universe is young; distant light has not reached us yet.

The first explanation - Olbers' Solution - is just plain wrong. In a black body, the interstellar dust will gradually heat up as the medium absorbs the radiation. Thus, the clouds would glow as bright as the stars. The premise of the second explanation may be technically correct. But the number of stars, finite as it might be, is still large enough to light up the entire sky. The final two possibilities are surely correct and partly responsible. There are numerical arguments that suggest that the effect of the finite age of the Universe - proposed originally by Edgar Allan Poe (1948) - is the larger effect. We live inside a spherical shell of "Observable Universe". Historically, after Hubble discovered that the Universe was expanding, but before the Big Bang was firmly established, Olbers' paradox was presented as proof of special relativity; you needed the red-shift to get rid of the starlight. This effect certainly contributes, but the finite age of the Universe is the most important effect. Thus, if the age of the Universe is finite, then: The Universe had to have a Beginning, or a Genesis.

1916--Einstein published a new theory of gravity, called general relativity. His theory predicted a universe that would either expand or contract depending on the density of matter and energy within it. But, in those days, however, a dynamic universe was thought to be such a crazy idea that even Einstein could not believe the prediction of his own theory.

Therefore, Einstein, modified his theory to make it predict a static universe, that is, a universe that neither expanded nor contracted. Subsequently, Einstein would call this fudging of his equations the biggest mistake of his life.

1919--The British astronomer Sir Arthur Eddington led an expedition to West Africa to observe a solar eclipse and test Einstein's prediction of the bending of light by the warping of space and time near the Sun. The day after Eddington made public his confirmation of Einstein's prediction, Einstein became the most famous scientist in history.

1922--The Russian mathematician Aleksandr Friedmann abandoned Einstein's static universe model and worked out the mathematics and geometry of dynamic (that is, changing) universes. To make progress with the mathematics he made the following simplifying assumption:

the universe is isotropic (looks the same in all directions) from every vantage point, at all times. This implies a universe in which the matter and energy are uniformly distributed.

With this assumption (called by Einstein the Cosmological Principle, which earlier Einstein had arrived at on philosophical grounds) and using Einstein's equations of gravity Friedmann constructed a class of mathematical models that described expanding universes. The calculations were later repeated by the American Howard Robertson in 1935.

1924--Edwin Hubble, using work by Henrietta Levitt on Cepheid variables, measured the distance to 9 galaxies and proved that they are very distant.

1927--Abbe Georges Lemaitre (a Belgian priest) took seriously the idea of an expanding universe. He reasoned that if one went sufficiently far back in time all the matter we see in the universe must have been squeezed into a very small volume, a "Primeval Atom" which subsequently fragmented to form the galaxies and stars we see today.

Lemaitre derived a relationship between (what later turned out to be) Hubble's constant, H, and the age of the universe.

1929--Drawing on observations made by others as well as his own, Edwin Hubble concluded that the further away a distant galaxy is from us the greater its red shift, Z. The red shift is defined by

Z = (lo - le)/le

where lo is the wavelength of the light that reaches Earth and le is the wavelength of the light emitted at the source. The simplest way to obtain a red shift from the light emitted by an object is to have the object move away from the observer. A light source that moves away from us looks redder, while one that moves towards us looks bluer. This is an example of the Doppler effect.

Hubble proposed that the observed red shifts was evidence that the galaxies are receding from us, that is, evidence that the universe is expanding, just as Einstein's original equations predicted.

Einstein's (self-confessed) blunder was his earlier failure to accept this startling rediction. Had he been sufficiently bold he could have made one of the most extraordinary predictions of 20th century science: that the universe is expanding and came into being a finite time ago.

The velocity at which galaxies recede from us is called the recession velocity. Since the galaxies are receding from us, one might be tempted to draw the conclusion that the Earth is at the center of the expanding universe.  However, according to the cosmological principle, no place in the universe is privileged; in particular,  we do not occupy a privileged position; we are not at the center of the universe. If we moved to another part of the universe we should expect to see more or less the same thing: the galaxies would appear to recede from us.

1940s--George Gamow (a Russian and ex-student of Friedmann) and later Ralph Alpher and Robert Herman of Johns Hopkins University refined Lemaitre's idea of a primeval atom. Alpher and Herman reasoned that far back in the past particles of matter would be constantly colliding with each other. These collisions would generate a tremendous amount of heat that would manifest itself as photons of very short wavelengths. The temperature of these primordial photons would be billions of degrees.

But as the universe expands all length scales are stretched by the expansion including the wavelengths of the primordial photons. Recall, that the longer the wavelength the lower a photon's energy. Therefore, as the universe aged, and expanded, the photons would have progressively lower energies and would therefore grow ever colder. Alpher and Herman predicted that the universe should now be bathed in a feeble radiation whose temperature would be just a few degrees above absolute zero. This radiation would be literally the afterglow of the earlier extremely hot dense phase of the universe. Alas, for Alpher and Herman, their ideas were more or less forgotten.

1965--At Bell Labs in New Jersey, Arno Penzias and Robert Wilson were preparing a radio telescope to observe the Milky Way. They noted a persistent background noise wherever they pointed their telescope. They tried very hard to get rid of it, but couldn't. It finally dawned on them that this was not mere noise. In fact, they had discovered, by accident, photon radiation coming from outer space, that was not associated with any known astronomical objects. This radiation, which is in the microwave part of the electromagnetic spectrum, is now called the cosmic microwave background (CMB).

At the same time Bob Dicke and Jim Peebles (at Princeton), working on a suggestion by George Gamow that the universe might have been hot and dense in the past, were just getting ready to look for the afterglow radiation from this dense hot phase of the early universe when they were scooped by Penzias and Wilson. Sadly for Dicke and Peebles it was Penzias and Wilson who got the 1978 Nobel Prize for Physics for their accidental discovery of the microwave background!

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