Sunday, May 24, 2009

Hubble's Constant and the Expansion of the Universe

We discussed earlier how Henrietta Leavitt discovered how to use Cepheid variable stars as one of astronomy’s most precise “standard candles” to measure vast distances. Edwin Hubble built on Leavitt’s discovery to make some of the most important scientific advances of the 20th century.

Before Hubble’s work, astronomers thought our galaxy, the Milky Way, was the entire universe – they really didn’t have a concept of galaxies being separate structures within a larger universe.

Starting in 1919, Hubble began measuring the period and apparent brightness of numerous Cepheid variables in various so-called “nebulae” that were then thought to be fuzzy patches with “lots” of stars. In 1925, Hubble announced his findings. Using Leavitt’s work, he found that some of these “nebulae” were immensely farther from us than the rest of the Milky Way. And to be as bright as they appear to us, they must have billions of stars – they must themselves be separate galaxies, some comparable or even larger than our galaxy. Almost overnight, our concept of the size of the universe was literally blown away – its vastness was far beyond anyone’s wildest dreams.

In 1929, Hubble went even further. He combined his galaxy distance measurements with “redshift” measurements of the same galaxies. Redshifts are changes in the frequency of starlight that allow us to determine a star’s (or a galaxy’s) velocity. What Hubble discovered was that almost every galaxy was moving away from us, and for distant galaxies, their recessional velocity was proportional to their distance from Earth. In math, this is written v = H d, where d is the galaxy’s distance away, v is its recessional velocity, and H is a number we call the Hubble “constant.”

Based on Hubble Space Telescope data published by NASA two weeks ago, we now know H = 51,000 mph per million light-years, to a precision of 5%. (A light-year is the distance light travels in one year, about 6 trillion miles.) This means a galaxy 10 million light-years away is moving away from us at 510,000 mph, and a galaxy 100 million light-years away is moving 5.1 million mph.

The value of H has an enormous impact on the evolution of the universe. For nearly a century, measuring H has been one of the most important and most difficult tasks in astronomy and the source of great controversy. Edwin Hubble’s first measurements were about 7 times too high and had a very large measurement uncertainty. Even as recently as the 1990’s, some astronomers believed H was twice as large as what others thought. Using the space telescope named in his honor, Hubble’s constant has been measured with great precision.

1 comment:

  1. Could it be that the galaxies are actually not traveling faster the farther away they are but rather we are seeing them earlier on when they still had greater velocity due to the Big Bang. So the farther the galaxy is from us we are seeing a galaxy with more of its initial velocity.