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• Before coming to class, watch Episode 9 -- "Other Challenges and Methods" -- of Cosmic Distance Ladder.
There are promising newer (but less well proven) methods of determining large distances without relying on the lower rungs of the cosmic distance ladder. Two of them are treated in the following assignments. (In the Additional Resources below, there is an extensive discussion of differences between the results given by these newer methods and those from the cosmic distance ladder.)
• Read this article about one of the latest methods of measuring the rate of cosmic expansion "directly" (not from the lower rungs of the cosmic distance ladder) using gravitational lensing.
• A second method relies on detecting sound waves that traveled through the very early universe due to trapped photons of light -- trapped by free electrons before most electrons joined atomic nuclei to form neutral atoms. Watch this video on these sound waves, which are called baryon acoustic oscillations (BAOs):
• Watch this silent video about baryon acoustic oscillations (BAOs). It shows how BAOs are detected by measuring distances between randomly selected pairs of galaxies and graphing the frequencies are which different pair-distances occur. This shows us the size of the BAO signal at any selected red-shift. I will explain what you are seeing during class.
• Submit your questions, comments, and/or suggestions on these assignments in the form at the bottom of this (and every) page at That Star, How Far?
Questions to To Think About
• Red shifts are measured by comparing the observed wavelength of a spectral line with the actual emitted wavelength. Think about how you might find and identify a specific spectral line that has been dramatically red-shifted. If the hydrogen 656 nm red line is shifted completely out of the visible spectrum into, say, the microwave region (as is true with CMB radiation), how do you know it is that same hydrogen line?
• The Hubble constant H represents what is called the recession velocityof a distant object, or the speed at which an object at a specific distance is receding from us. In other words, it connects red-shift with distance. All methods of measuring the Hubble constant arrive at a value in the neighborhood of
70 kilometers per second per megaparsec, or 70 (km/s)/Mpc.
Can you express in words what 70 (km/s)/Mpc means? (Ask yourself: km/s of what? Mpc of what?) (Answer: km/s of recession velocity of an object; Mpc of distance to that object.)
• Why might astronomers often think of, and report distances to, objects as z, the redshift, rather than in parsecs or light-years?
Additional Resources
• Illustrating the Cosmic Distance Ladder: a clear and information-packed summary of the methods we have studied. Click to enlarge.
Standard candles, as you know by now, are objects whose inherent brightness (luminosity) is known, and so its apparent brightness (flux) reveals its distance by way of the flux-luminosity equation.
A standard ruler is an object whose actualsize is known, and so its apparent size on the sky reveals its distance, by a simple inverse relationship: the smaller the apparent size of the standard ruler, the farther away it is.
• Here are animations of the density of baryons, dark matter, photons, and neutrinos before and after the universe became transparent (about 500 million years). This video shows separately the movement of these four components of the early universe.
• Among the videos I could find on this subject, here's the most thorough, logical, and clear explanation of BAOs in today's sky and in the CMB radiation, and how they make a standard ruler on the sky.
• And here's a discussion of the the current crisis in measuring the Hubble constant: the disagreement of older and newer methods on the precise value of the constant. They agree pretty well, but not as well as they should. Is something missing in our model of the cosmos?
