Cosmology of the physical universe

[Jackalope]

I'm finding the usual 'atheist vs theist deathmatch" threads a bit tiresome these days, and thought I'd start a thread for sharing information and discussing aspects of physical cosmology. While metaphysical and theological cosmology are certainly valid topics at AF, let's keep this thread's scope to physical cosmology please (feel free to start your own thread on other-than-physical cosmology).

I'll start with a short summary of the Cosmological Principle. I'll periodically add to this thread in easily digestible portions. Feel free to add your own comments and questions.

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The Cosmological Principle

Digested from the Wikipedia article on the Cosmological Principle and other sources.

Issac Newton is credited with first clearly asserting the principle, usually stated as:

"Viewed on a sufficiently large scale, the properties of the Universe are the same for all observers."

The principle contains three implied qualifiers:

1. "Observers" - means any observer at any location in the Universe, not just on Earth.
   
2. "Looks the same" - refers not to appearance of physical structures, but rather the effects of physical laws in observable phenomena.
   
3. Variation in physical structures can be overlooked - provided that said variation does not imperil the uniformity of conclusions drawn from observation.
   

Furthermore, the principle has two testable consequences:

1. Homogeneity - the same observational evidence is available to all observers. In other words, what is observable by us is a fair sample.
   
2. Isotropy - the same observational evidence is available in any direction. That is to say that the same physical laws apply throughout the universe.
   

Current observational evidence supports both homogeneity and isotropy (in the interests of keeping this post from being TL;DR it will be left as a point for discussion). Past discoveries that at first appeared to contradict homogeneity or isotropy have been resolved by later discoveries (also left as a point for future discussion).

[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.

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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.

[Jackalope]

I have several more posts along these lines planned out, aimed at an audience that is unfamiliar with the observational aspects of cosmology, intended to be a starting point for learning.

If you think these posts are useful to that audience, kudos this post. If there's enough interest, I'll continue.

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Observational evidence - Measuring Redshift

Another reasonable to question to ask in light of Hubble's discovery of the relationship between distance and redshift is how is that redshift measured?

Astronomers use a technique called spectroscopy to measure the spectrum of light emitted (or reflected) by an object. Such measurements can tell us a lot about an object's composition.

Based on Earthbound observations, we know what the emission spectrum of various elements and compounds are. For example, the emission spectrum of hydrogen looks like this:



The emission spectrum of iron looks like this:



Each element and compound has a distinct spectrum that can be measured via spectroscopy. It seems apparent that emission sources with mixed composition should have mixed spectra, this has been observationally confirmed experimentally. If we can obtain a spectrum of an object, we can determine what elements and compounds it consists of.

A spectrum that is red- or blue- shifted will have the same distinct spectral lines as a baseline sample, but they will be shifted either towards the red or blue end of the spectrum, according to their velocity relative to the observer. It is the pattern of the spectral emission (or absorption) lines, and not their location on the electromagnetic spectrum that determines their composition. Once the composition is determined, the amount of spectral shift is apparent.