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Lecture 1

PHYS 242 Lecture 1: PHYS242_Lecture_13-15

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Queen's University
PHYS 242
Wolfgang Rau

39 ENPHPHYS 242 Fall 2014 L 13 Cosmic Microwave Background Shortly after the Big Bang the Universe was very dense and hot. Since then it has expanded and cooled down. However, we still can measure the thermal radiation from the Big Bang which today has a temperature of ~2.7 K (0 K 273.15 C is the lowest possible temperature, absolute zero; a difference of 1 K is the same as a difference of 1 C ). The typical wavelength of thermal radiation of this temrature is about 1 mm (which corresponds to a frequency of 310 Hz). This radiation is called Cosmic Microwave Background radiation (CMB) and is very homogeneous across the whole sky. Only when we measure very precisely we find a dipole which means in one direction the temperature (or correspondingly the frequency) is slightly higher than the average, and slightly lower (by the same amount) in the opposite direction. The reason for this shift is our velocity in the cosmic frame: the Earth moves about the Sun, the Sun orbits the Milky Way and the Milky Way moves towards a group of galaxies in our cosmic neighbourhood. Solving the Doppler Shift formula (29) and taking the measured frequency shift (~0.12 ) we find for our velocity : 1 1 0.12 360 kms 1 1 In 2006 the Nobel Prize in physics honoured the COBE experiment which did a high precision measurement of the CMB (way beyond the here mentioned dipole) and with its successors provided an observational test of cosmological models with unprecedented precision. In Figure 23 you see the temperature distribution across the sky as measured by COBE. More recently the fluctuations in the CMB and also its polarization have been measured with much more precision by the WMAP, the Planck satellite and a ground based microwave telescope in Antarctica, called BICEP. Figure 23: Temperature map of the sky as measured by the COBE satelite (red is warm, blue is cold; the refers to the maximum temperature difference; data are from four years of measurement). In the upper panel we clearly see the dipol discussed in the text. The lower panel is based on the same data, but now the dipole has been removed which allows us to see much finer fluctuations. As indicated in the picture the difference between the hottest and coldest spot in the lower panel is only 18 K. The hot band in the middle is our milkyway (picture taken from http:aether.lbl.govwwwprojects cobeCOBE_HomeDMR_Images.html ). W. Rau
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