RE: Cosmology of the physical universe
September 8, 2012 at 12:41 am
(This post was last modified: September 8, 2012 at 1:30 am by Jackalope.)
Observational evidence - Redshift
To understand the standard model of inflationary cosmology, it's important to grasp the observational evidence, one element of which is observed redshift of distant galaxies.
"Redshift" is a phenomenon associated with the Doppler Effect, first explained in the mid 19th century with respect to sound waves. The effect applies to light waves as well, with light-producing objects moving away from the observer appearing to have the wavelength of light shifted towards the red portion of the spectrum (longer wavelengths), and those moving towards being shifted to the blue portion (shorter wavelength). The degree to which the spectrum is shifted is proportional to it's velocity relative to the observer.
While redshift was observed in stars earlier, it is the work of Edwin Hubble that is of interest to cosmology. In 1929 he formulated the Redshift Distance Law based on observations of "spiral nebulae" or galaxies (some of which were known in antiquity, however up until a few years prior were not known to be outside of our own galaxy). Hubble determined that the degree of observed redshift in distant galaxies correlated to their distance which implied that the farther the distance, the faster the galaxy in question was receding from us. Given that the speed of light was thought to be constant, the inescapable conclusion was that the universe itself was expanding in all directions (*).
(*) There exist observational exceptions on smaller cosmological scales which later discoveries explain, however on the large scales that cosmology is concerned with the rule holds true.
Observational evidence - Determining Distance
One reasonable question that could be raised from Edwin Hubble's discovery of the relationship between observed redshift and distance is how is distance to cosmic objects which are millions or more light years away measured?
It's a fair question. Methods used to measure distances to relatively close stars such as the parallax method as measured from Earth aren't useful over such distances - even in the modern era, the best direct parallax measurements (obtained with the Hipparcos satellite launched in 1989) are only accurate out to about 1600 light years, while a great distance, in galactic terms is not far at all. At cosmological distances, it's a miniscule distance.
One method that does work over intergalactic distances is the use of standard candles, which are objects of known luminosity. As luminosity scales inversely to distance, measuring the observed brightness of a distant standard candle gives a useful approximation of the object's distance. Two (but not the only) standard candles useful at intergalactic scales are Cepheid variable stars and Type 1a supernovae.
Other methods used to measure the distance of distant galaxies are briefly described in Wikipedia's article on Cosmological Distance.
With statistically sufficient observational data (e.g. from standard candles) correlated with measured redshift, it became clear that under the Cosmological Principle, redshift alone could be used to measure distances to galaxies where more direct methods could not be used. It's important to note that the validity of using of redshift to measure distances on cosmological scales has been independently confirmed with multiple sets of independent data and methods.
To understand the standard model of inflationary cosmology, it's important to grasp the observational evidence, one element of which is observed redshift of distant galaxies.
"Redshift" is a phenomenon associated with the Doppler Effect, first explained in the mid 19th century with respect to sound waves. The effect applies to light waves as well, with light-producing objects moving away from the observer appearing to have the wavelength of light shifted towards the red portion of the spectrum (longer wavelengths), and those moving towards being shifted to the blue portion (shorter wavelength). The degree to which the spectrum is shifted is proportional to it's velocity relative to the observer.
While redshift was observed in stars earlier, it is the work of Edwin Hubble that is of interest to cosmology. In 1929 he formulated the Redshift Distance Law based on observations of "spiral nebulae" or galaxies (some of which were known in antiquity, however up until a few years prior were not known to be outside of our own galaxy). Hubble determined that the degree of observed redshift in distant galaxies correlated to their distance which implied that the farther the distance, the faster the galaxy in question was receding from us. Given that the speed of light was thought to be constant, the inescapable conclusion was that the universe itself was expanding in all directions (*).
(*) There exist observational exceptions on smaller cosmological scales which later discoveries explain, however on the large scales that cosmology is concerned with the rule holds true.
Observational evidence - Determining Distance
One reasonable question that could be raised from Edwin Hubble's discovery of the relationship between observed redshift and distance is how is distance to cosmic objects which are millions or more light years away measured?
It's a fair question. Methods used to measure distances to relatively close stars such as the parallax method as measured from Earth aren't useful over such distances - even in the modern era, the best direct parallax measurements (obtained with the Hipparcos satellite launched in 1989) are only accurate out to about 1600 light years, while a great distance, in galactic terms is not far at all. At cosmological distances, it's a miniscule distance.
One method that does work over intergalactic distances is the use of standard candles, which are objects of known luminosity. As luminosity scales inversely to distance, measuring the observed brightness of a distant standard candle gives a useful approximation of the object's distance. Two (but not the only) standard candles useful at intergalactic scales are Cepheid variable stars and Type 1a supernovae.
Other methods used to measure the distance of distant galaxies are briefly described in Wikipedia's article on Cosmological Distance.
With statistically sufficient observational data (e.g. from standard candles) correlated with measured redshift, it became clear that under the Cosmological Principle, redshift alone could be used to measure distances to galaxies where more direct methods could not be used. It's important to note that the validity of using of redshift to measure distances on cosmological scales has been independently confirmed with multiple sets of independent data and methods.
