It was nearly a full century ago that enough data came in to map out both the distance to a significant number of extragalactic objects and also measure the amount that their light was redshifted: interpreted as an apparent recession speed away from us. When the distance and recession velocity of those objects were plotted against one another, an intriguing relationship emerged: the farther away we looked, the faster an object seemed to be moving away from us. Twice as far meant twice as fast; ten times as far meant ten times as fast.
This linear relationship became known as Hubble’s Law (or more recently, the Hubble-Lemaître Law, since Georges Lemaître derived it first), with the slope of that line known as the Hubble constant. However, it turns out that this is a complete misnomer: not only does the expansion rate of the Universe change with time, but it isn’t even constant across all of space right at this very moment. In other words, there’s nothing constant about the Hubble “constant” at all!
The variation of the expansion rate with time was predicted from the star, but the variation in space came as a surprise: only with the discovery of dark energy did we realize this was the case. And yet, dark energy may turn out to be the Hubble constant’s ultimate savior, as if it truly is a cosmological constant, then one day, the Hubble parameter will, far into the future, actually become a constant after all. Here’s the science of what’s really going on.
Hubble’s discovery of a Cepheid variable in the Andromeda galaxy, M31, opened up the Universe to us, giving us the observational evidence we needed for galaxies beyond the Milky Way and leading us, in short order, to the discovery of the expanding Universe. Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA); Illustration via NASA, ESA, and Z. Levay (STScI) Observationally, the story is seemingly straightforward.
Beginning in the 1920s, our telescope technology became advanced enough — with large-enough aperture telescopes and the development of photographic plates — that we could start to resolve and identify individual stars in objects (i.e., galaxies) that were outside of our own Milky Way. The first such star was identified by Edwin Hubble in 1923: a Cepheid variable star, or a special class of star where, if you could measure the amount of time it took Then, you could compare the observed brightness of that star (which you measured) with the intrinsic brightness that you just figured out, and that allows you to figure out the distance to it: the same way that, if you had a light bulb or candle with a known intrinsic brightness, then seeing how bright it appears would allow you to figure out how far away it is. This method for figuring out a distance in astronomy — the standard candle method — has since been extended to far more classes of objects than just Cepheid variable stars, and is one of the most useful tools in an observational astronomer’s arsenal.
As was first noted by Vesto Slipher back in the 1910s, some of the objects we observe show the spectral signatures of absorption or emission of particular atoms, ions, or molecules, but with a systematic shift toward either the red or blue end of the light spectrum. When combined with distance measurements for those objects, this data gave rise to the initial idea of the expanding Universe: the farther away a galaxy is, the greater its light will appear redshifted to our eyes and instruments. Then, in addition to figuring out the distance to that object, you can measure the light coming from that object as well.
Extract — continue reading at the source.
