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Discovery of CMB

How was CMB discovered, and why is it so important? Prof. Goto will answer these questions in today's video.
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Hello everyone today we are going to talk about the cosmic microwave background CMB. There’s a lot of cosmological information in this CMB, so let’s take a look. In 1965, Arno Penzias and Robert Wilson was building this radio-antenna at the Bell Laboratories Satellite Telecommunications, and this is a microwave telescope. Wavelengths six, seven centimeters or so, the, this was our purpose was to hold it. One is this communication purpose, and then the other is to observe the universe as a telescope. And then, when they’re observing, they noticed a white noise, there’s a noise coming from everywhere. Whichever direction they point the telescope, there’s always a noise, about three Kelvin of the temperature.
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And then they thought, okay, there’s, something is wrong with their telescope, and they try to get rid of this noise as much as possible. They first thought this may be coming from New York City, because New York City is a big city, there may be a noise from there. But this, the noise is coming from everywhere, not particularly from New York City. And there are also birds living inside this telescope and so they tried to get rid of the birds and then they captured the birds and then moved the birds to the other city and then let them go, but they are birds, so they flew back to the telescope.
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So they had a hard time to try to get rid of the birds, but even they get rid of the birds the noise was still there. So, okay, they get rid of all the possibilities, so they are trying to write a paper on reporting this unknown white noise. They didn’t understand what this noise was, but they’re trying to write a paper. And then, when they are trying to report this white noise, Robert Dicke at Princeton University heard about this news and he said, “Boys, we’ve been scooped.” So Robert Dicke, he’s a theorist in Princeton, and then he was, theoretically actually predicted the existence of cosmic microwave background from the Big Bang the remnants of the Big Bang.
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And he himself was trying to build an antenna to receive, detect this cosmic microwave background. And then while he was preparing his own telescope, he heard the news from Arnold and then Penzias, and then so he knew “Oh, they detected the cosmic microwave background.” So, Robert Dicke called Penzias and Wilson and then told them that “congratulations you detected cosmic microwave background.” Now the result, so Penzias and Wilson wrote a paper about this detection of the cosmic microwave background, and then Robert Dicke wrote a paper about theoretical interpretation, and then in 1978, Penzias and Wilson received the Nobel Physics Prize for the discovery of this cosmic microwave background.
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Um, so this cosmic microwave background is the remnant of the Big Bang fireball. At the time of the Big Bang, the universe was a big fireball. Now with the expansion of the universe, this fireball temperature cooled down, and now it becomes 2.73 Kelvin, and then of course this universe was, the whole universe was a fireball, so this remnant of the fireball is coming from any direction. Now- now in the microwave background so it’s called cosmic microwave background. If I look at the history of the cosmic microwave background, this is very interesting. In 1946, this is 20 years before Penzias and Wilson’s detection.
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George Gamow, he’s famous for the Big Bang theory, already predicted there will be something like a cosmic microwave background, um, in 1946. That’s 20 years before, that’s amazing. And also, another interesting story is 1941, Andrew McKellar, he’s another astronomer, reported the paper that, reported in a paper that effective temperature of interstellar gas is 2.3 Kelvin. So he was measuring the temperature of the gas in the universe. And then by doing so-called spectroscopy, and then how to see how molecules are excited in these gasses you can tell the temperature, some molecules are easy to excite, some molecules are not, so you can measure temperature and he measured 2.3 Kelvin. This is actually was the temperature of the CMB.
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At that time Andrew McKellar didn’t know this is the temperature of the CMB, but because CMB is filling the universe it’s everywhere in the universe, so nothing can be colder than CMB. So the gas cloud, the temperature, the lowest temperature can be, it’s 2.73 Kelvin, the temperature of the CMB because if the gas is cooler than CMB is going to heat the gas up. Right, so, so this was, actually, um, measuring the temperature of the CMB. Okay then, CMB, detecting CMB is also evidence of the Big Bang. Here’s a brief schematic history of the universe.
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The universe started with a hot Big Bang, and it quickly expanded so at that time the universe was a fireball, thousands of Kelvins of degrees, thousands of Kelvins temperature. Then, with the expansion of the universe this fireball cooled down, and then nowadays it cooled down to 2.3, 73 Kelvin, so we can detect the CMB in the microwave wavelengths. So, detecting CMB in a backwards is the evidence of the hot Big Bang theory.
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On the CMB is coming from the last so-called the last scattering surface. So here’s the last scattering surface where CMB is coming from, and before that the universe was a hot fireball called the Big Bang. And at that time the um, universe was ionized, so there was an ionized plasma gas. In a sense, um, um, protons and electrons are ionized and they’re flying freely, and if there are a lot of free electrons in the universe, that light cannot go straight, because light is going to hit the electron and there’s a Thompson scattering and the light is bended to the, another direction, so here the light cannot go straight.
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But, with the expansion of the universe the universe cooled down, and then here, at the time at the scat- something we call recombination, the electron and a proton got together, temperature is low enough, and got together to form a hydrogen atom. And at that time, there are no more free electrons in the universe so the light can go straight. And then this scattering light goes straight to us, and that is the cosmic microwave background. And the CMB, one important feature of the CMB is a perfect black body radiation in the universe. You might be familiar with these kind of figures where CMB has a small mid-Kelvin, micro-Kelvin level of fluctuation.
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But, if you look at the degree level, CMB is perfectly uniform temperature of 2.73 Kelvin. If you look at here, this da- this light curve, that curve, is a black body radiation of 2.7 Kelvin, and these data points are measurements from the COBE satellite with error bars 400 times larger, and you see those data points are perfectly matching the black body radiation. In a modern laboratory we cannot create a perfect black body as CMB. The reason why CMB’s so perfect black body is at this time, Big Bang, the time of the fireball, the universe is so small, and there was enough time to mix those, uh, temperatures, so that’s why CMB is a perfect black body.
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In summary, the cosmic microwave background discovered in 1965 is a remnant of the Big Bang fireball. And it’s the most perfect black body in the universe. And it’s coming from the last scattering surface, where universe was recombined and became neutral.

When cosmic microwave background (CMB) was first discovered, no one knew that it was actually CMB!

Many scientists have dealt with studying CMB in its infant stages of research. Due to the lack of advanced equipment during that time, CMB was not so well-understood before. Now, we have finally concluded that CMB is actually one of the biggest evidences of the Big Bang. Prof. Goto will discuss these more in today’s video.

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