There is ample corroborating evidence that our universe begun, some 13.7 billion years ago, as a point source of incredible density and it has been expanding ever since at an incredibly high speed. The universe's size is larger than 13.7 billion light years in radius. Its visible or reachable size is 13.7 light years. A light year is the space traveled by light in one year. Light travels 300,000 kilometers per second (187,500 miles per second! [1.87* 10**5]). A year contains 31,536,000 seconds [3.15 * 10**7] so a light year is 1.87 * 10**5 times 3.15 * 10**7, that is 5.9 * 10**12 miles. I.e. 5.9 billion miles. 6 billion miles more or less. The universe has a radius well in excess of 13.7 billion light years, that is, about 81 billion billion miles! A better grasp on such immensities is given in: scale of the universe
One may have reasonable doubts regarding mankind ability to actually know such things and in particular to measure such enormous distances. It turns out that, with our present state of knowledge, doing such measurements is a relatively simple matter. Explaining this is a good illustration of how Science works. Furthermore, it illustrates why we know such things in a very reliable factual manner.
The measurement of these enormous distances relies on the fact that the further an object the faster it recedes from us. In fact, the universe has been expanding since its beginning. The universe behaves like a balloon being inflated. If a bunch of dots are painted on it, the dots will fly away from each other as the balloon is inflated. From each dot the furthest dots appear to be flying away faster than the closer ones. The dots on the balloon correspond to the galaxies in the universe. The skin of the balloon (a spherically curved surface, i.e. two dimensional space) corresponds to the curved three dimensional space which is our universe.
The bottom line is that if we can accurately measure the speed of recession we can determine the distance of an object from us. It turns out that measuring the speed of recession is a relatively simple task. All we need to do is look at the light coming from the object and determine if its colors have shifted towards the red and by how much.
The reason for this is that the color of light depends on its wavelength, the shorter wavelengths correspond to the blue, violet (& eventually ultra-violet) portions of the spectrum. The longer wavelength correspond to the red (& eventually infra-red) portions of the spectrum. When light is emitted by an object flying away from us its waves are stretched. I.e. they move towards the red, in other words they are red shifted. A similar phenomenon occurs when we hear a train whistle which is higher pitched (shorter wavelength) when approaching, but suddenly drops in pitch once the train passes us and continues away from us. The change in pitch is proportional to the speed of the train.
So all we need do to measure the distance of a (very) remote object is to measure the red shift of its light, then a simple calculation will yield the object's distance. The degree of red shift is readily determined by passing the object light into a spectrometer (a fancy prism) which produces the spectrum of the light. The spectrum is a series of lines of different color. These lines are the sum of all the lines produced by the object atoms (quantum mechanics shows that atoms can only produce light at certain specific wavelength). Each element has its own characteristic spectrum or signature. The two key elements for stars & galaxies are hydrogen & helium. Those spectra are very well known so all one has to do, to determine the degree of red shift, is locate in the object's spectrum the hydrogen signature (or helium's) & measure how much the wavelengths have been stretched.
The bottom line is that we positively & factually (i.e. this is NOT a debatable opinion) know the universe is inconceivably large & our Earth is a totally negligible speck in this immense vastity. The Earth has a circumference of ONLY 25,000 miles (its radius is about 4,000 miles).
The fact that our Earth, & in fact the whole of our solar system, are less than a grain of sand is further apparent if one determines how many stars are in the universe. Present estimates are that the universe contains a few hundred billion galaxies (our galaxy is the Milky Way one can clearly see in the Summer sky). Galaxies typically contain 100 to 200 billion stars. So again the number of stars in the universe is of the order of 10,000 billion billion stars!
This immensity immediately should raise some considerable doubts on the claims made by virtually all the pre-scientific revelations about the central & important role Mankind plays in this world. we will come back to this issue which is not so simplistically dismissed.
The explosion of the universe from this point of nearly infinite density is referred to as the Big Bang. We have direct evidence of the Big Bang since some of our most potent radio telescopes can see the remaining radiation of the explosion at the edge of the universe. This is called the cosmic background. Its temperature has now cooled to a very cold 3 degrees Kelvin. We also have detailed theoretical calculations based on modern nuclear physics that predict how such a Big Bang would produce protons, neutrons & light atoms & their relative abundance. The predicted abundances, of approximately 80% & 20% for respectively hydrogen and helium & traces of all other atoms, jive very well with data from observational astronomy.
Additional details can be found in the following excellent course: Cambridge Cosmology
Before leaving this topic we need to make one more point. All this knowledge is extremely new. In fact we didn't even know about galaxies as separate systems from our own until the 1920's when Harvard's Harold Shapley first was able to provide crucial evidence of their existence. Less that 100 years is a blink of an eyelash in the history of mankind. Most of the revelations, in most cultures, are a few thousand years old. It is not surprising at all that this knowledge has had no impact on popular culture, but it eventually will.
Copyright sire.com 2010