Nov 202017
 

Today is the birthday (1889) of Edwin Powell Hubble an amazingly influential U.S. astronomer who is probably known chiefly these days outside of astrophysics for the telescope named after him. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology and is regarded in scientific circles as one of the most important astronomers of all time. Hubble discovered that many objects previously thought to be clouds of dust and gas and classified as “nebulae” were actually galaxies beyond the Milky Way. Before Hubble astronomers thought that the Milky Way was the universe. After Hubble the universe was an awful lot bigger and more complex than ever conjectured. Not only that, his calculations directly implied that the universe is expanding.

Hubble was born in Marshfield, Missouri, and moved to Wheaton, Illinois, in 1900. In his youth he was noted more for his athletic prowess than his intellectual abilities, although he did earn good grades in every subject except for spelling. Hubble was skilled in baseball, American football, and basketball, and he ran track in both high school and college. He played a variety of positions on the basketball court from center to shooting guard. In fact, Hubble even led the University of Chicago’s basketball team to their first conference title in 1907. He won seven first places and a third place in a single high school track and field meeting in 1906.

His studies at the University of Chicago were concentrated on law with some science leading to a bachelor of science degree in 1910. Subsequently he spent three years at Queen’s College, Oxford as one of the university’s first Rhodes Scholars, initially studying jurisprudence instead of science (as a promise to his dying father), and later added literature and Spanish. His father died in the winter of 1913, while Edwin was still in England, and in the summer of 1913, Edwin returned to care for his mother, two sisters, and younger brother, as did his brother William.

Hubble did not have the motivation to practice law. Instead, he got a job teaching Spanish, physics and mathematics at New Albany High School in New Albany, Indiana, where he also coached the boys’ basketball team. After a year of high-school teaching, which he did not like very much but where he was liked by all the students, he entered graduate school with the help of his former professor from the University of Chicago to study astronomy at the university’s Yerkes Observatory. He received his PhD in 1917 with a dissertation, “Photographic Investigations of Faint Nebulae.” He was on track. Note to fathers: don’t hobble your sons with your own desires for them.  Let them choose their own paths.

After the United States declared war on Germany in 1917, Hubble rushed to complete his PhD dissertation so he could join the military. Hubble volunteered for the United States Army and was assigned to the newly created 86th Division, where he served in 2nd Battalion, 343 Infantry Regiment. He rose to the rank of lieutenant colonel, and was found fit for overseas duty on July 9, 1918, but the 86th Division never saw combat. After the end of the war, Hubble spent a year in Cambridge University, where he renewed his studies of astronomy. In 1919, Hubble was offered a staff position at the Carnegie Institution for Science’s Mount Wilson Observatory, near Pasadena, California, by George Ellery Hale, the founder and director of the observatory. Hubble remained on staff at Mount Wilson until his death in 1953.

Edwin Hubble’s arrival at Mount Wilson Observatory, California in 1919 coincided with the completion of the 100-inch (2.5 m) Hooker Telescope, then the world’s largest. At that time, the prevailing view of the cosmos was that the universe consisted entirely of the Milky Way Galaxy. Using the Hooker Telescope at Mt. Wilson, Hubble identified Cepheid variables (a kind of star used to calculate stellar distances) in several spiral nebulae, including the Andromeda Nebula and Triangulum. His observations, made in 1922–1923, proved conclusively that these nebulae were much too distant to be part of the Milky Way and were, in fact, entire galaxies outside our own, suspected by researchers at least as early as 1755 when Immanuel Kant published General History of Nature and Theory of the Heavens. This idea had been opposed by many in the astronomy establishment of the time, in particular by Harvard University-based Harlow Shapley. Despite the opposition, Hubble, then only 35, had his findings first published in the New York Times on November 23, 1924, and then more formally presented in the form of an academic paper at the January 1, 1925 meeting of the American Astronomical Society.

Hubble’s findings fundamentally changed the scientific view of the universe. Let me repeat that: Hubble’s findings fundamentally changed the scientific view of the universe. Although some of his more renowned colleagues simply scoffed at his results, Hubble ended up publishing his findings on nebulae. Hubble also devised the most commonly used system for classifying galaxies, grouping them according to their appearance in photographic images. He arranged the different groups of galaxies in what became known as the Hubble sequence.

In 1929, Hubble examined the relation between distance and redshift of galaxies. Combining his measurements of galaxy distances with measurements of the redshifts of the galaxies by Vesto Slipher, and by his assistant Milton L. Humason, he found a roughly linear relation between the distances of the galaxies and their redshifts, a discovery that later became known as Hubble’s law (v = Ho d where: v = velocity of a galaxy, in km/s. Ho = Hubble Constant, measured in km/s/Mpc).

This meant that the greater the distance between any two galaxies, the greater their relative speed of separation. If interpreted that way, Hubble’s measurements on 46 galaxies lead to a value for the Hubble Constant of 500 km/s/Mpc, which is much higher than the currently accepted value of 70 km/s/Mpc due to errors in their distance calibrations.

Yet the reason for the redshift remained unclear. In reality, Georges Lemaître, a Belgian Catholic priest and physicist, predicted, on theoretical grounds based on Einstein’s equations for General Relativity, the redshift-distance relation two years before the proposal of Hubble’s law. However, many cosmologists and astronomers (including Hubble himself) failed to recognize the work of Lemaître. Hubble remained doubtful about Lemaître’s interpretation for his entire life. In 1931 he wrote a letter to the Dutch cosmologist Willem de Sitter expressing his opinion on the theoretical interpretation of the redshift-distance relation:

Mr. Humason and I are both deeply sensible of your gracious appreciation of the papers on velocities and distances of nebulae. We use the term ‘apparent’ velocities to emphasize the empirical features of the correlation. The interpretation, we feel, should be left to you and the very few others who are competent to discuss the matter with authority.

Today, the “apparent velocities” in question are understood as an increase in proper distance that occurs due to the expansion of spacetime. Light traveling through stretching space will experience a Hubble-type redshift, a mechanism different from the Doppler effect (although the two mechanisms become equivalent descriptions related by a coordinate transformation for nearby galaxies). Basically, objects traveling away from an observer at high speed will be redshifted, that is, the spectrum of light from those objects will be shifted towards the red end of the spectrum. Objects traveling towards the observer will appear violet shifted. ALL stars in the universe appear red shifted to observers on earth, leading to the conclusion that the universe is expanding, as demonstrated in the raisin bread analogy.

In the 1930s, Hubble was involved in determining the distribution of galaxies and spatial curvature. These data seemed to indicate that the universe was flat and homogeneous, but there was a deviation from flatness at large redshifts. There were methodological problems with Hubble’s survey technique that showed a deviation from flatness at large redshifts, however. In particular, the technique did not account for changes in luminosity of galaxies due to galaxy evolution. Earlier, in 1917, Albert Einstein had found that his newly developed theory of General Relativity indicated that the universe must be either expanding or contracting. Unable to believe what his own equations were telling him, Einstein introduced a cosmological constant (a fudge factor) to the equations to avoid this “problem.” When Einstein learned of Hubble’s redshifts, he immediately realized that the expansion predicted by General Relativity must be real, and in later life he said that changing his equations was “the biggest blunder of [his] life.” In fact, Einstein apparently once visited Hubble and tried to convince him that the universe was expanding. In December 1941, Hubble reported to the American Association for the Advancement of Science that results from a six-year survey with the Mt. Wilson telescope did not support the expanding universe theory. Even great scientists make mistakes. These were the days well before the Big Bang theory although Hubble’s observations led in that direction. Until 1964, when the cosmic background radiation was discovered, astrophysicists were split between the Big Bang and the Steady State theories. Now the Big Bang is the prevailing model.

In Hubble’s day the Nobel Prize committee did not recognize work done in astronomy as part of physics and so did not award prizes to astronomers. Hubble spent much of the later part of his career attempting to have astronomy considered an area of physics, instead of being its own science, not least so that astronomers could be recognized by the Nobel committee. This campaign was unsuccessful in Hubble’s lifetime, but shortly after his death, the Nobel Prize Committee decided that astronomical work would be eligible for the physics prize. Sadly, the prize cannot be awarded posthumously.

Given that the raisin bread analogy is the common one for explaining the redshifts of galaxies in an expanding universe, we have to bake raisin bread today. It’s normally baked as a yeast bread, but can be made using baking powder, which I find more convenient. For my money, raisin bread is best served in toasted slices with lashings of butter.

Raisin Bread

Ingredients

3 cups all-purpose flour
½ cup white sugar
3 tsp baking powder
½ tsp baking soda
1 tsp salt
¾ tsp ground cinnamon
1 cup raisins
1 egg
¼ cup melted butter
1 cup milk

Instructions

Preheat oven to 350˚F/175˚C.

Grease a 9x5x3” loaf pan.

Sieve the flour, sugar, baking powder, baking soda, salt, and cinnamon into a mixing bowl. Add the raisins and stir thoroughly. Make a well in center.

In small bowl beat the egg until frothy. Mix in the melted butter and milk.

Pour the wet ingredients into the well in the dry ingredients. Stir the ingredients gently so they are just combined, but do not overmix. Scrape the dough into the greased loaf pan.

Bake for 1 hour.

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