{"id":15,"date":"2009-10-08T15:54:02","date_gmt":"2009-10-08T13:54:02","guid":{"rendered":"http:\/\/astronomy.lamost.org\/blog\/?p=15"},"modified":"2009-10-08T16:00:05","modified_gmt":"2009-10-08T14:00:05","slug":"lyman-alpha-systems-and-cosmology-lyman-alpha-forest","status":"publish","type":"post","link":"http:\/\/astronomy.lamost.org\/blog\/?p=15","title":{"rendered":"Lyman alpha systems and cosmology- Lyman alpha forest"},"content":{"rendered":"<p><a href=\"http:\/\/astro.berkeley.edu\/~jcohn\/lya.html\">http:\/\/astro.berkeley.edu\/~jcohn\/lya.html<\/a><\/p>\n<p><a href=\"http:\/\/astro.berkeley.edu\/~jcohn\/lya.html\"><\/a>1 why forest<br \/>\nNeutral hydrogen atoms in their lowest state will interact with <strong>whatever light<\/strong> has been<strong> redshifted <\/strong>to a wavelength of 1216 angstroms when it reaches them. The rest of the light will keep travelling to us.<\/p>\n<p>2 why absorption lines<\/p>\n<p>if you shine a light with wavelength 1216 at a bunch of neutral hydrogen atoms in their ground state, the atoms will absorb the light, using it to boost the electron to a higher energy state. If there are<strong> a lot of neutral hydrogen atoms<\/strong> in their ground state, they will absorb more and more of the light. So if you look at the light you receive, intensity as a function of wavelength, you will see a dip in the intensity at 1216 angstroms, depending on the amount of neutral hydrogen present in its ground state. The amount of light absorbed (<strong>&#8216;optical depth&#8217;<\/strong>) is proportional to the probability that the hydrogen will absorb the photon (<strong>cross section<\/strong>) times the<strong> number of hydrogen atoms <\/strong>along its path.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>http:\/\/astro.berkeley.edu\/~jcohn\/lya.html 1 why forest &#8230;<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[10],"tags":[],"class_list":["post-15","post","type-post","status-publish","format-standard","hentry","category-lyman-alpha"],"_links":{"self":[{"href":"http:\/\/astronomy.lamost.org\/blog\/index.php?rest_route=\/wp\/v2\/posts\/15","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/astronomy.lamost.org\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/astronomy.lamost.org\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/astronomy.lamost.org\/blog\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/astronomy.lamost.org\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=15"}],"version-history":[{"count":2,"href":"http:\/\/astronomy.lamost.org\/blog\/index.php?rest_route=\/wp\/v2\/posts\/15\/revisions"}],"predecessor-version":[{"id":16,"href":"http:\/\/astronomy.lamost.org\/blog\/index.php?rest_route=\/wp\/v2\/posts\/15\/revisions\/16"}],"wp:attachment":[{"href":"http:\/\/astronomy.lamost.org\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=15"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/astronomy.lamost.org\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=15"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/astronomy.lamost.org\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=15"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}