Edwin Hubble Ushers In Modern Cosmology (the study of the universe as a whole)
Until the 1920's, scientists thought that the universe was in a "steady state". The Steady State Universe Theory was based on three assumptions, i.e. the universe was:
- Uniform - has the same general make-up throughout
- Static - not expanding or contracting
- Infinite - it extends forever, but there was only one galaxy - The Milky Way
If you look at the night sky, it seems easy to think that this is the way the universe "has" always looked and "will" always look. However, there was a problem pointed out by Heinrich Olbers in the 1800's - if the universe were infinitely large and infinitely old, then the whole sky should have a "glow". That is, at all points of sight there would eventually be a star. If the universe was "infinitely old" then the light from any star at any distance would have already reached us no matter how far the distance. So all points in the heavens should have some light.
However, the the sky between stars was "very dark", with no signs of any glow. This became known as Olbers' Paradox, but no one took the "obvious" solution seriously. In retrospect, the solution was that "the universe was expanding" and was not "infinitely old".
In 1922 to 1924, Edwin Hubble painstakingly developed a series of nebulae distance indicators using the 100 inch Hooker telescope at Mount Wilson Observatory in California. At the time Mount Wilson was the world's largest telescope (shown at the left below). Edwin Hubble, then a thirty-five year old scientist, presented a paper in January, 1925 that proved conclusively that the some of the faint stellar objects were much too distant to be part of our galaxy and were in fact "galaxies outside the Milky Way".
In 1922, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann Equations from Albert Einstein's equations of General Relativity. In 1924 Friedman published a paper indicating that the universe must be either expanding or contracting, but not static.
In 1927, George Lemaître, a Belgian physicist and Roman Catholic priest, interpreted Friedman's equations and proposed that the recession of nebulae was due to the "expansion of the universe". (Back then the word nebulae meant galaxies outside the Milky Way; today nebulae means large clouds of gas and dust.) However, both Friedman and Lemaître were mostly dismissed, even by Albert Einstein. But, in 1929 Hubble discovered a correlation between "nebulae distance and recession velocity" which meant that indeed "the universe was expanding", a shock to most scientists of that time.
Hubble's discoveries came to be known as Hubble's Law, which is: (a) all objects in deep space have a red shift (see next section) relative to earth and to each other; and (b) the velocity of galaxies receding from earth is proportional to their distance from the earth and other interstellar bodies.
Lemaître had already "theoretically predicted" the universe was expanding, but no one took his work seriously until Hubble's evidence was deemed un-refutable. Shortly thereafter, the Cosmological Principle evolved. The Cosmological Principle is that observers on earth do not occupy an unusual or privileged position within the universe. It means that the universe looks the same wherever you are in it and the same laws of physics apply everywhere. Hubble's lifetime of work fundamentally changed the scientific view of the universe.
Hubble worked toward the end of his life to get the Nobel Prize Committee to accept astronomy as an area of physics. He died in 1953 of a blood clot in his brain without succeeding. Shortly after his death, the Nobel Prize Committee began to recognize astronomy as a valid segment of physics, but the Committee does not award prizes to people who have passed away. Top
What exactly is a "red shift"?
In a phenomenon called "doppler shifting:", the wavelength emitted by something moving away from us is shifted to a lower frequency, and the wavelength of something moving towards us is shifted to a higher frequency. See the illustration to the left. A good example of this is the sound of an ambulance siren as it drives by. The pitch of the siren increases as the ambulance moves towards us, and decreases as it moves away. Although this example is for sound waves, the same effect occurs for all electromagnetic wavelengths including light.
The result is that wavelengths emitted by objects moving away from us are shifted towards the red part of the visible spectrum or red shifted. And the faster they move away from us, the more they are red shifted. See the picture to the right of a distant star moving away from us. The right portion is the distant star, the left portion is our sun, Likewise objects that are moving towards us are shifted towards the blue part of the spectrum and are blue shifted.
If the universe is "expanding", then any light waves from another galaxy will be stretched out to longer wavelengths. This is exactly what Hubble observed when he looked at the distant galaxies. He saw that their light was "red shifted", and therefore the galaxy clusters were moving away from each other. Top
An Expanding Universe Implies A Tiny Beginning - The Big Bang
In 1931 Georges Lemaître further postulated that the expansion of the universe, if projected back in time, would make the universe smaller and smaller until at some finite time (now estimated at 13.75 billion years ago) all the mass of the universe would be concentrated in a single point about the size of a ping pong ball. In 1949, Sir Fred Hoyle, an English astronomer and mathematician, coined the term The Big Bang in a BBC radio broadcast regarding the early growth of the universe. It immediately gained popularity and has since become the standard term for the universe's early growth period.
While believing that the universe was expanding, many scientists at the time could not accept that the universe began from a single point, known as a singularity in mathematics. Even Albert Einstein was among the doubters and had originally introduced a "Cosmological Constant" into his equations so that the universe would continue to be in a "steady state". He put the cosmic repulsion into his equations to prevent the universe from collapsing on itself from the gravitational pull of the matter inside it. (This was later removed by him.) Theories were put forth to explain how the universe could be unchanging while the galaxies were moving away from each other. These theories suggested the creation of matter between galaxies over time, so that even though galaxies grew further apart, new ones developed between them to fill the spaces they left behind. The resulting universe was in a "steady state" in the same manner that a flowing river is - the individual water molecules are moving away but the overall river remains the same. The discovery of the Cosmic Microwave Background (CMB) in the 1960's convinced most astronomers that the Big Bang Theory had merit. The "Hubble Expansion" is now one of three pieces of evidence that support the Big Bang Theory. (The other two are the Cosmic Microwave Background and Big Bang Nucleosynthesis.) Top
Big Bang Issues And Inflation
There is no "evidence" with respect to the absolute earliest instant of the Big Bang explosion. The Big Bang Theory can not and does not provide any explanation for the exact initial conditions; rather it makes a series of assumptions and describes the evolution of the universe going forward from that point.
While the early Big Bang Theory was based on Einstein's Theory of General Relativity, almost universally accepted by scientists of the day, there were some technical issues with the initial version of the Big Bang Theory. The original Big Bang Theory did not contain "inflation" and some scientists "rightly" poked holes in
the original theory. (See the Big Bang Theory page for a discussion of these issues.) Most scientists thought that the Big Bang Theory was correct in principle, but not complete. To confront the technical issues, the solution was just to "assume" the proper conditions at the start of the Big Bang process.
The solution to almost all of the Big Bang issues was to add an extremely fast "exponential" expansion, called "inflation", into the first micro-micro-seconds of the universe. Inflation caused the universe to grow by a factor of 100 billion trillion trillion in just a few microseconds. (Note that the universe was not expanding into empty space. Space-time itself was growing faster than the speed of light. Space-time is not limited by the speed of light, only objects "within" space-time are.)
While there is no concrete proof of inflation, recent satellites have provided evidence of its existence and have ruled out some competing theories. Inflation equations, which last for just a few seconds, were added to the very beginning of the Big Bang Theory in 1981 by Alan Guth, currently a Professor of Physics at MIT (and his alma mater). See the simplified diagram at the left by Alan Guth himself illustrating inflation. Inflation is now almost universally accepted as part of the Standard Model of the Universe. Top
Expansion Is Accelerating
In 1998 pictures from Hubble indicated that not only was the universe expanding, but the expansion was accelerating. Astronomers from the Supernova Cosmology Project (U. of California Berkeley Labs) and the High-z Supernova Search Team (Australian National University and John Hopkins University) were using "land-based" telescopes for preliminary data and then the Hubble Space Telescope for fine detail. After locating supernovas using the Keck Telescope in Hawaii (designed by Berkeley) and other telescopes where time could be scrounged, the teams used the the Hubble Telescope to study the most distant supernovas as they required much more accurate measurements than could be obtained from ground telescopes.
Instead of slowing down as everyone had expected, the dimness of the supernovas showed that they were 10% to 15% farther out than anticipated, indicating that expansion had accelerated over the ensuing billions of years. Far from decelerating under the influence of gravity as was commonly thought at the time, the expansion of the universe was in fact accelerating. In 2011, Saul Perlmutter, team leader of the Supernova Cosmology Project, along with Adam Riess and Brian Schmidt, team leaders of High-z, were awarded the Nobel Prize in Physics for their discovery.
For a great description of the the universe, see the Video by NASA's Dr. John Mather, winner of the Nobel Prize in Physics in 2006 for the confirmation of the CMB.