During the 1920s, astronomer Edwin Hubble made unprecedented discoveries at Mt. Wilson Observatory concerning the large-scale structure of the universe. Sometime later Walter Baade, working at Palomar and at Mt. Wilson, discovered the existence of two populations of Cepheid variables. His observations of these populations allowed him to recalculate the distance scale previously established by Edwin Hubble and, in so doing, also to double our understanding of the size of the known universe.
Allan Sandage was considered the preeminent observational cosmologist of the later part of the 20th century. He built upon the work of Edwin Hubble and Baade. In a paper published in 1958, he again increased the size of our perceived universe and brought the value of the Hubble Constant to 75km/s/Mpc, close to its modern value. As part of this work, while using the Hale Telescope, Sandage described and measured the linearity and isotropy of expansion of the universe. The universe was essentially similar in all directions, an assertion that is at the foundation of modern cosmology.MORE
For astronomers no observational challenge is more pervasive and persistent than measuring distances; unlike here on Earth one cannot use ordinary rulers to measure the separation between astronomical objects. Yet distance measurements are vitally important to our understanding of essentially all astronomical objects and systems. For hundreds of years astronomers have struggled to find creative ways to reliably measure astronomical distances. Those efforts continue even now.
In the early years of the 20th century astronomers’ efforts focussed on finding objects of known intrinsic brightness that could be readily identified and used to infer their distance by measuring their apparent brightness. Such objects are commonly known to astronomers as standard candles. Harvard astronomer Henrietta Levitt discovered an important type of standard candle known as a cepheid—variable stars whose periodic brightness variations are related to their intrinsic brightness by the so-called Leavitt’s law. At Mt. Wilson astronomers Edwin Hubble and Walter Baade studied and used cepheid standard candles to estimate the distances to external galaxies. Hubble in particular observationally established cosmic expansion and what is known as the Hubble-Lemaître law—the founding cornerstone of modern “Big Bang” cosmology.
Following on from Hubble’s work, astronomer Allan Sandage dedicated most of his professional career to studying cosmic expansion. The expansion rate is parameterized by the so-called Hubble constant (H0), and in 1958 Sandage presented results obtained with the Hale Telescope—his estimate of H0 of 75 (km/s)/Mpc is remarkably close to the modern consensus value. Sandage also used the Hale to expand the range over which H0 was measured: at greater distances and along multiple sight lines, testing what astronomers call the linearity and isotropy of cosmic expansion. Sandage’s results with the Hale helped build evidence and support for the broad adoption of Big Bang cosmology in the astronomical community.
Astronomers’ preoccupation with measuring distance continues today. In the late 1990s two separate teams used a type of supernova standard candle to establish that at great distances cosmic expansion even accelerates. This discovery underpins the modern view that the universe’s matter-energy density is dominated by a so-called “dark energy” that drives this large-scale acceleration. For this remarkable acceleration discovery astronomers Saul Perlmutter, Adam Reiss, and Brian Schmidt were awarded the 2011 Nobel Prize in Physics. Closer to home, modern detailed study of the same cepheid standard candles used by Baade and Hubble decades ago suggest a subtle discrepancy in cosmic expansion in the local universe compared with measurements at the largest accessible scales. This discrepancy may be a sign that our Big Bang cosmology needs to be refined. Additional study is needed to assess the correctness of these results and humankind’s understanding of the universe at large scale.