Little Bang Posted October 12, 2010 Report Posted October 12, 2010 Anyone care to make a guess as to what the frequency of the CMBR was at 10^6 years after the BB? Quote
coldcreation Posted October 12, 2010 Report Posted October 12, 2010 Anyone care to make a guess as to what the frequency of the CMBR was at 10^6 years after the BB? Just out of curiosity, why do you ask? CC Quote
Little Bang Posted October 12, 2010 Author Report Posted October 12, 2010 CC, if I answer it defeats the purpose of my question. Quote
Little Bang Posted October 13, 2010 Author Report Posted October 13, 2010 I'm not wanting an exact answer just a guess. Quote
Erasmus00 Posted October 13, 2010 Report Posted October 13, 2010 All numbers pulled from my brain, so anyone who wants to should check this. The CMB decoupled from matter in the universe at around 400,000 years after the big bang. Around then, the temperature would have been about 3000 K. Wien's law says the peak wavelength of the radiation would be at something like 2.9 *10^(-3) m*K / T. This means the wavelength of the radiation would be about a micrometer. This is a frequency of about 3*10^(14) Hz. Quote
modest Posted October 13, 2010 Report Posted October 13, 2010 Anyone care to make a guess as to what the frequency of the CMBR was at 10^6 years after the BB? I might be able to answer that. The temperature of a relativistic gas scales with the inverse of the scale factor (T∝1/a). Subbing the definition of cosmic redshift would give T1=T0(1+z). 10^6 years after the big bang z is about equal to 600 (you can use ned wright's cosmology calculator for that). Using T1=T0(1+z) means the temperature was about 600 times greater then. The temp of the CMB today is 2.725 K making it about 1,635 K then. By Wien’s displacement law the peak wavelength today is .00106 meters and .00000177 meters then. So, I think the answer to your question is 1.77 x 10-6 meters... or, I guess you asked the frequency... 1.69 x 1014 hz. (I think that is infrared—not quite visible) ~modest EDIT: I say, Will, we posted within 60 seconds of each other :) Quote
Little Bang Posted October 14, 2010 Author Report Posted October 14, 2010 Ok, If the source frequency of light is equal to the observed frequency, no matter how far away the source, why wouldn't the CMBR frequency be the same? Quote
Qfwfq Posted October 14, 2010 Report Posted October 14, 2010 Ok, If the source frequency of light is equal to the observed frequency, no matter how far away the source, why wouldn't the CMBR frequency be the same?But redshift goes thermodynamically hand in hand with cooling. Quote
Little Bang Posted October 14, 2010 Author Report Posted October 14, 2010 So when we measure the expansion of the Universe we take this red shift due to distance into account? Quote
modest Posted October 14, 2010 Report Posted October 14, 2010 Ok, If the source frequency of light is equal to the observed frequency, no matter how far away the source... The source frequency isn't (and shouldn't be) equal to the observed frequency because it takes light time to get from place to place. Redshift is the ratio between emitted frequency and observed frequency, so any time there is a redshift there is a change in frequency between emission and observation. The universe has expanded 1292 times larger than it was when the CMBR was emitted. This means that the wavelength of CMB radiation has stretched 1292 times larger (the wavelength is 1292 times larger as observed now) than it was when emitted. The frequency is likewise 1292 times smaller. ...why wouldn't the CMBR frequency be the same? As time goes on the CMB is redshifted more and more. The light that we observe from z = 1 is about 7.7 billion years old. Back then the universe was half its current size, so the wavelength of the CMBR has doubled since then. Also, any light emitted at z = 1 has doubled in wavelength since it was emitted. Any light wandering around the universe for the past 7.7 billion years has doubled in wavelength over that time. Light has tripled in wavelength since z = 2 (10.3 billion years ago) and quadrupled in wavelength since z = 3 (11.5 billion years ago). As the universe expands, the wavelength of light in the universe expands with it. So when we measure the expansion of the Universe we take this red shift due to distance into account? Yes, but it isn't distance causing it—it's time. Here on earth (or anywhere in the universe) 7.7 billion years ago the CMBR had half its current wavelength and twice its current frequency. ~modest Quote
Little Bang Posted October 15, 2010 Author Report Posted October 15, 2010 So if I'm looking at a galaxy one billion LYS away and I want to know if it is coming at me or moving away, I pick some frequency emitted by it and check it's red or blue shift? Quote
modest Posted October 15, 2010 Report Posted October 15, 2010 Yeah, redshift is moving away and blueshift is towards. In a homogeneous and isotropic universe the former is expansion and the latter is collapse. ~modest Quote
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