Hubble's Hubble Law: Our Expanding Universe (Interactive)

The Expansion of the Universe: From the Hubble Law to the Hubble Space Telescope, and Beyond(!)

Astronomical observations, going back to Edwin Hubble's days (the 1920s), clearly show that our Universe had a beginning in time, but also that it expands in all directions simultaneously, meaning that it has no single birth-place in space.  Unless we can understand time as fourth dimension, these ideas--a starting time, but not a starting place, are very hard for us to reconcile.  Human experience of daily life on Earth, where time and space are separate ideas, lead almost everyone beyond those of us versed in four-dimensional space-time to ask: 

If the Universe had a beginning, what was here *before*it? and If space is expanding, what’s it expanding in to? 

These two tricky questions can be avoided if a supernatural power pressed some kind of "start" button on time, and controls whatever is "beyond" our Universe.  In Ancient times, most thinkers believed in a finite Universe, controlled at least in part by supernatural forces.  As detailed in the "Path to Newton" iIn the 17th century, Newton and others showed how gravity controlled the motions of the planets as they orbit the Sun.  But, that did not mean that supernatural influences were off-the-table: Newton was primarily searching for ways to understand God.   In the 19th century, photography and spectroscopy entered astronomy, and ultimately changed everything.  Photography let us see objects and details we could only philosophize about before, and spectroscopy let us learn what they were made of, and how they moved.  Long-exposure photographs revealed beautiful nebulae, and millions and millions of faint stars.  Spectroscopy showed clear evidence that many of the elements we see on Earth are also everywhere in space (although, ultimately, definitely not in the same proportions—but that’s a story for another day), and it let us, via Doppler shifts, learn about motions.
Thus, by the early 20th century, it was possible to see and measure a very different Universe than was accessible to the Ancients or to Newton.   It had many more stars, and it had nebulae with crazy patterns—like spirals.  Debate about the true size and extent of the Universe began to spread.  Most famously, in 1920, Harlow Shapley and Heber Curtis debated whether observed “spiral nebulae” were just at the edge of our Milky Way, which essentially was the whole Universe (Shapley) or were in fact their own  “island universes” in sea of galaxies, of which the Milky Way was just one (Curtis). 

1924 cepheids Hubble, 1929 Universe expansion Hubble