The preponderance of the lightest elements, H and He, not only in our Sun, but in the entire visible universe, begs for an explanation. How did there come to be so much of these simple atoms, and what controls the relative distribution of all atoms that were available to build up the Earth? What process could produce so much H and He, and so little Fe, yet lead to a huge concentration of iron in the Earth's core? These questions drive us to perhaps the biggest scale we can consider: what is the Universe?

The answer has come in stages, with two key advances enabling progress: The first was the ability to estimate distances to very distant objects, and the second was to assess how those objects are moving relative to the Earth. The first issue, measuring distances, exploits the changing position of the observers on the spinning Earth, or the changing position of the Earth itself, relative to a planet or star that serves as a reference point. By looking at the angle (the parallax) subtended by two observing positions relative to a known reference point while imaging a very distant object, one can estimate distance. The details can be found in any introductory cosmology book. From such measurements, we know that the Milky Way Galaxy in which our solar system resides is about 100,000 light years across, and that it is 2,000,000 light years to the next nearest galaxy, the Andromeda Nebula. (A light year is the distance traveled at the speed of light in one year. Given that the speed of light is about 300,000 km/s, this is a long way).

In addition to measuring the distance to an object, we must determine whether the object is moving toward or away from the Earth. This is essential for looking either forward or backward in time to understand how the Universe is evolving, a clear key to understanding what the Universe is. The important notion here is the concept of the Doppler Shift.

Assume some distant object is X km away from an observer, and the object radiates a particular light wave with a wavelength, Ltrue. If the object is stationary, the next peak of the wave will come in one period, T, later, where T is the time between peaks, and the observer will measure the true period of the waves. Now assume that the object is moving relative to the observer at velocity V. In the time between radiating one wave peak and the next, the object will move a distance dX = VT. The observer will now see the second peak arrive at a time after it is radiated. This is simply because the wave travels a different distance than it did for the earlier peak. If the object is moving away from the observer, the observed wavelength is longer than the true one, which shifts the observation toward the red (long wavelength) end of the spectrum. If the object is moving toward the observer the wavelength will be shorter, toward the blue end of the spectrum.

Armed with the ability to measure distances, and the knowledge that certain fundamental radiation excitations of materials in the universe have predictable wavelengths, Edwin Hubbell in 1929 found a remarkable thing: Looking at the distant galaxies in the Universe, the light from them is shifted toward the red end of the spectrum. In every case! Also, the further the galaxy is, the more red shifted the spectrum is, with the amount of shift increasing proportional to distance. This provided strong evidence that the Universe is expanding. It also supports the Cosmological Principle, which is the notion that the Universe should look the same to observers in all galaxies if the scale is big enough and motions are less than the speed of light. The expansion of the Universe is often likened to how any point on a balloon moves relative to all other points on the balloon when it is inflated. All other positions move away from a reference point, with increasing velocity at larger distances, and the same behavior is found for any reference point on the balloon.

An expanding Universe has profound implications for what the Universe is, but the most dramatic is to consider what happened long ago, effectively to take the measure velocities and run all the galaxies back through space. The systematic convergence (letting the air out of the balloon), brings all points together again on a time scale of 12-20 billion years ago. This leads to the notion that all material in the Universe was in one place, say 15 billion years ago, and has been spreading outward ever since, creating space as it goes. It was projected outward by the BIG BANG, and you and me and everything around us, every atom was there! It was quite a moment, I'm sure.

So, our expedition from speculating about rocks and toenails has brought us up to the most dramatic catastrophe of all, the massive explosion that produced the Universe. Recent tests of Hubbell's observations by the ironically-name Hubbell telescope have raised some tough questions about this, as they suggest an age of the Universe of only 8 billion years, younger than some estimates of ages of pulsars in the Universe, so not all details are sewed up.

"In the beginning, there was an explosion. An explosion which occurred everywhere, with every particle rushing apart from every particle."

-- Steven Weinberg, The First Three Minutes

The Big Bang happened, symmetry was broken and matter began to form, creating space with great density.

0.01 s after the Big Bang, the temperature was 100 billion degrees C. No atoms existed, only constiuent pieces, electrons, positrons, neutrinos, photons. The density was 4 billion g/cm3.

1 s after the Big Bang the volume of space was growing, rapidly decreasing the density of the Universe and letting it cool to only 10 billion degrees C.

180 s after the Big Bang, at a temperature of 1 billion degrees, protons and neutrons became stable forms of matter, providing H and He nuclei. In this instant the Universe became comprised of 73% H, 27% He.

From there on, expansion, cooling, aggregation into galaxies and stars took place. From an instant of 10,000 years after the Big Bang to the present, events have radiated light propagating in the now transparent Universe (low enough density so that photons could survive), and we can see it all.

Looking at the stars is like looking back in time, as we look to more and more distant objects we are seeing events longer and longer ago. This is because it has taken longer for light from distant objects to reach us today, so that light had to have left the object longer ago. Can we see all the way to the Big Bang?

In part, the answer is yes. In 1965 Bell Lab and Princeton astrophysicists observed the Background Radiation. This is the "original flash" of light, radiation persisting from the Big Bang once the Universe became transparent. It is so far red-shifted that we see it to day as light in the radiowave (microwave) spectrum. It is the flash of the Big Bang!