Expansion of the Universe
Until well into this century, it was not understood whether the great groupings of stars that were seen through telescopes were part of our own galaxy, or distant galaxies in their own right. This puzzle was finally resolved by using Cepheid variables to establish a distance to the objects like the â€œspiral nebulaâ€ in the constellation Andromeda, and to determine the size of our own galaxy. By around 1925, Hubble, Leavitt, Hertsprung, Shapley, and others had established conclusively that objects like the Andromeda â€œNebulaâ€ was in fact much further away than objects in our own galaxy and thus were themselves galaxies. Then, in the late 1920â€²s, Hubble, building on results obtained earlier by Slipher, combined Doppler shift measurements of radial velocities with distance measurements to conclude that almost all galaxies were flying away from the Milky Way, and that the velocity of recession was proportional to the distance from us: the further the galaxy from us, the faster it was receding. The Universe has 3 spatial dimensions, but it is easier to visualize an analogy to its expansion for the 2-dimensional surface of a balloon. There is no center. If you stand on any galaxy, all the others will appear to be moving away from you with a velocity proportional to the distance from you. An analogy in 2 dimensions is to put dots on the surface of a balloon and blow the balloon up. As it expands, there is no dot that is the â€œcenterâ€, but if you stand on any dot you will see all other dots moving away from you (and the rate at which they move away will be proportional to the distance. Dots close to you will be moving away slower than those further away). The expansion of the Universe appears to be like this, but in 3 rather than 2 space dimensions, which makes it much harder to visualize, but it is possible to describe it mathematically. It is often simpler to discuss things happening in the expansion of the Universe if we adopt a vantage point that is moving uniformly with the expansion. Astronomers call such a vantage point a comoving coordinate system. The balloon analogy in 2 dimensions illustrates simply the idea of comoving coordinates. If instead of viewing the expansion of the balloon from the outside I place myself on one of the dots on the balloonâ€™s surface, I appear to be stationary and I see the other dots moving away from me (and in my immediate area I see the a dimensional space) of a comoving volume. This means a volume of space that is moving uniformly with the expansion. A pparent curvature of the balloonâ€™s surface decreasing).
In the 1920s, Edwin Hubble, using the newly constructed 100â€³ telescope at Mount Wilson Observatory, detected variable stars in several nebulae. Nebulae are diffuse objects whose nature was a topic of heated debate in the astronomical community: were they interstellar clouds in our own Milky Way galaxy, or whole galaxies outside our galaxy? This was a difficult question to answer because it is notoriously difficult to measure the distance to most astronomical bodies since there is no point of reference for comparison. Hubbleâ€™s discovery was revolutionary because these variable stars had a characteristic pattern resembling a class of stars called Cepheid variables. Earlier, Henrietta Levitt, part of a group of female astronomers working at Harvard College Observatory, had shown there was a tight correlation between the period of a Cepheid variable star and its luminosity (intrinsic brightness). By knowing the luminosity of a source it is possible to measure the distance to that source by measuring how bright it appears to us: the dimmer it appears the farther away it is. Thus, by measuring the period of these stars (and hence their luminosity) and their apparent brightness, Hubble was able to show that these nebula were not clouds within our own Galaxy, but were external Hubbleâ€™s second revolutionary discovery was based on comparing his measurements of the Cepheid-based galaxy distance determinations with measurements of the relative velocities of these galaxies. He showed that more distant galaxies were moving away from us more rapidly: v = Hod.