Einstein
Friedmann
Lemaitre
Hubble
Early in the 20th Century,
physicists faced some perplexing problems and questions about
the universe itself, as knowledge of it began to unfold rapidly
and nearly continuously. Here are some:
It was the largest of these questions that began to be answered first! In 1915, Albert Einstein published a classical field theory of gravity, which after nearly 110 years is still the best and most accurate description of gravity that we possess. Einstein quickly noticed that his equations could be solved for the size and shape of the entire universe, if the values of various parameters were known. But he also quickly noticed that the equation did not have a stable solution! The universe either expanded or collapsed, but could not remain static. To solve this problem, Einstein inserted a fudge factor into the equation, a constant he called the Cosmological Constant, Λ. But other researchers, such as Alexander Friedmann (1922) and Georges Lemaitre (1927), explored the equation for an expanding universe... and efforts climaxed when astronomer Edwin Hubble discovered (1929) by direct observation that the universe in fact and indeed was expanding! [Hubble was also the first to show that our own Milky Way galaxy was not alone in the universe, but that the cosmos consists of limitless numbers of other galaxies.] On a smaller scale, in 1916, Karl Schwarzschild found a solution to Einstein's equations that we now recognize as the solution for a nonrotating black hole, and we also now know that a black hole is one of the inevitable end stages of the normal evolution of stars... the other two possible endpoints are neutron stars, and white dwarf stars.
| Einstein |
Friedmann |
Lemaitre |
Hubble |
How do stars work? That was the next major step taken in astrophysics progress. Sir Arthur Eddington proposed in 1920 that stars must generate energy by some kind of nuclear fusion. But! But! Two protons cannot be fused (2He does not exist), a proton and 4He cannot be fused (5B does not exist) and two helium nuclei cannot be fused (8Be does not exist). Just as bad is the fact that processes in which the nuclear force is involved take place in seconds at most. If a star operated by nuclear fusion it would use up its fuel in seconds as well... a billion year lifetime would be out of the question.
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 Bethe |
In 1938, Hans Bethe
built upon and completed earlier ideas suggested by George
Gamow and Carl
Friedrich von Weizsäcker, to show how stars like the sun
could successfully create nuclei up to 7B and 7Li.
The key to the reactions was the contribution of the weak
interaction, instead of just the strong nuclear interaction.
The first step in the energy generation of main sequence stars
like the sun is the incredibly rare decay of a proton into a
neutron, a positron and a neutrino. At the temperature and
density of the center of the sun (15 million K and 160,000
kg/m3), a proton has a 50% chance of undergoing
this process in 5 billion years! Note that we can instantly
guess from this that the life span of a star of solar mass is
only 10 billion years on the main sequence. (Bethe won
the Nobel prize for this work in 1967.)
The next big step was taken by
Margaret and Geoffrey Burbidge, Willy
Fowler and Fred Hoyle in a landmark review
paper published in 1957. This paper showed for the
first time in detail how nuclear reactions in stars in the
last stages of their lives could generate all the known
natural nuclei, with convincing predictions of the relative
abundance of each element. Fowler won the Nobel Prize in
1983, mainly for this work. By ordinary fusion, stars cannot
get past the nuclei of iron and nickel, which have the
highest binding energy of all nuclei. Thus, eventually
an inert core of iron and nickel nuclei forms in all very
old stars, and once that happens, the star is doomed... with
no central source of kinetic energy, pressure drops, and it
must undergo gravitational collapse.
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We now know that there are only three possible death stages for a star... low mass stars throw off mass in a “planetary” nebula, turn into white dwarves, sometimes explode further, and eventually go out. Stars of higher mass undergo a supernova explosion, leaving behind a massive core that either becomes a neutron star or a black hole. Collisions between neutron stars form nuclei that can't easily be formed any other way. Examples of all stages of this process are routinely observed by astronomers. To become a neutron star, a star has to have an initial mass of at least 10 solar masses, and to become a black hole a star has to have an initial mass of at least 20 solar masses. Do not confuse these initial masses with the final mass of the neutron star or black hole itself, after formation.
Astrophysicists, beginning in the late 1950s, did many increasingly realistic calculations of all stages of stellar evolution, and these results can be directly checked by observations of globular clusters, systems of stars which all formed at the same time from the same roughly spherical cloud of gas and dust. The appearance of the various stars in a given cluster depends entirely on the individual star's mass, as directly predicted by calculations, and there are 150 easily observable clusters in our galaxy alone.
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One of the most important developments in all the history of astrophysics occurred in 1964 when radio astronomers Penzias and Wilson (Nobel prize 1978) were given a microwave receiver by the Bell Telephone Company. They discovered a constant microwave signal coming from every point in empty space... the Cosmic Microwave Background. What they were seeing was the radiation emitted by the 3000 K universe at the moment it became transparent to visible light, at the age of about 380,000 years. The launching of three successive satellites, COBE in 1989 , W-MAP in 2001, and PLANCK in 2009, provided a successively more detailed idea of the temperature fluctuations in this Last Scattering Surface, fluctuations which give us a remarkably accurate idea of the composition of the whole universe at that time! |
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Here is a summary of facts about the universe obtained so far from satellite and ground-based observations. The universe is 13.787±0.020 billion years old, apparently infinite in size, and asymptotically flat, meaning that objects separated by large distances have precisely escape speed with respect to one another. The universe appears to be homogeneous... the same in all directions. The radius of the part of the universe we can directly observe today is 46.5 billion light-years. Only about 4.9% of the universe consists of ordinary matter and known particles, while 26.8% consists of unknown dark matter which apparently interacts only gravitationally, and 68.3% consists of so-called dark energy, which is very likely just Einstein's Cosmological Constant Λ, a fundamental constant of nature with units of pressure (energy per unit volume).
