Top Document: [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9) Previous Document: Table of Contents Next Document: I.02. Why do astronomers favor the Big Bang model of the Universe? See reader questions & answers on this topic! - Help others by sharing your knowledge There are three key facts we know about the properties of the Universe: galaxies recede, there's a faint microwave glow coming from all directions in the sky, and the Universe is mostly hydrogen and helium. In 1929 Edwin Hubble published a claim that the radial velocities of galaxies are proportional to their distance. His claim was based on the measurement of the galaxies' redshifts and estimates of their distances. The redshift is a measure of how much the wavelength of a spectral line has been shifted from the value measured in laboratories; if assumed to occur because of the Doppler effect, the redshift of a galaxy is then a measure of its radial velocity. His estimates of the galaxies' distances was based on the brightness of a particular kind of star (a pulsating star known as a Cepheid). The constant of proportionality in Hubble's relationship (v = H * d, where v is a velocity and d is a distance) is known as Hubble's parameter or Hubble's constant. Hubble's initial estimate was that the Hubble parameter is 464 km/s/Mpc (in other words, a galaxy 1 Mpc = 3 million light years away would have a velocity of 464 km/s). We know now that Hubble didn't realize that there are two kinds of Cepheid stars. Various estimates of the Hubble parameter today are between 50--100 km/s/Mpc. Hubble also measured the number of galaxies in different directions and at different brightness in the sky. He found approximately the same number of faint galaxies in all directions (though there is a large excess of bright galaxies in the northern sky). When a distribution is the same in all directions, it is isotropic. When Hubble looked for galaxies four times fainter than a particular brightness, he found approximately 8 times more galaxies than he found that were brighter than this cutoff. A brightness 4 times smaller implies a doubled distance. In turn, doubling the distance means one is looking into a volume that is 8 times larger. This result indicates that the Universe is close to homogeneous or it has a uniform density on large scales. (Of course, the Universe is not really homogeneous and isotropic, because it contains dense regions like the Earth. However, if you take a large enough box, you will find about the same number of galaxies in it, no matter where you place the box. So, it's a reasonable approximation to take the Universe to be homogeneous and isotropic.) Surveys of very large regions confirm this tendency toward homogeneity and isotropy on the scales larger than about 300 million light years. The case for an isotropic and homogeneous Universe became much stronger after Penzias & Wilson announced the discovery of the Cosmic Microwave Background in 1965. They observed an excess brightness at a wavelength of 7.5 cm, equivalent to the radiation from a blackbody with a temperature of 3.7+/-1 degrees Kelvin. (The Kelvin temperature scale has degrees of the same size as the Celsius scale, but it is referenced at absolute zero, so the freezing point of water is 273.15 K.) A blackbody radiator is an object that absorbs any radiation that hits it and has a constant temperature. Since then, many astronomers have measured the intensity of the CMB at different wavelengths. Currently the best information on the spectrum of the CMB comes from the FIRAS instrument on the COBE satellite. The COBE data are consistent with the radiation from a blackbody with T = 2.728 K. (In effect, we're sitting in an oven with a temperature of 2.728 K.) The temperature of the CMB is almost the same all over the sky. Over the distance from which the CMB travels to us, the Universe must be exceedingly close to homogeneous and isotropic. These observations have been combined into the so-called Cosmological Principle: The Universe is *homogeneous* and *isotropic*. If the Universe is expanding---as the recession of galaxies suggests---and it is at some temperature today, then in the past galaxies would have been closer together and the Universe would have been hotter. If one continues to extrapolate backward in time, one reaches a time when the temperature would be about that of a star's interior (millions of degrees; galaxies at this time would have been so close that they would not retain their form as we see them today). If the temperature was about that of a star's interior, then fusion should have been occurring. The majority of the Universe is hydrogen and helium. Using the known rate of expansion of the Universe, one can figure out how long fusion would have been occurred. From that one predicts that, starting with pure hydrogen, about 25% of it would have been fused to form deuterium (heavy hydrogen), helium (both helium-4 and helium-3), and lithium; the bulk of the fusion products would helium-4. Observations of very old stars and very distant gas show that the abundance of hydrogen and helium is about 75% to 25%. User Contributions:Top Document: [sci.astro] Cosmology (Astronomy Frequently Asked Questions) (9/9) Previous Document: Table of Contents Next Document: I.02. Why do astronomers favor the Big Bang model of the Universe? Part0 - Part1 - Part2 - Part3 - Part4 - Part5 - Part6 - Part7 - Part8 - Single Page [ Usenet FAQs | Web FAQs | Documents | RFC Index ] Send corrections/additions to the FAQ Maintainer: jlazio@patriot.net
Last Update March 27 2014 @ 02:11 PM
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