Video: Understanding the Cosmic Microwave Background

Cosmic microwave background radiation was emitted as _ radiation just after the big bang. It has been travelling through space since then and the expansion of the universe has caused the wavelength of the radiation to _. We now detect the background radiation as _ radiation.

03:24

Video Transcript

Cosmic microwave background radiation was emitted as blank radiation just after the big bang. It has been travelling through space since then and the expansion of the universe has caused the wavelength of the radiation to blank. We now detect the background radiation as blank radiation.

So this is the typical fill in the blanks kind of question with lots of information to decode. So let’s go through it sentence by sentence. The first one: cosmic microwave background radiation was emitted as blank radiation just after the big bang. Well, the big bang was the fairly energetic occurrence to say the least. And therefore, the radiation released after the big bang would be extremely high energy radiation. Now, when it comes to the electromagnetic spectrum, we know that the largest energy radiation possible is gamma radiation. Therefore, after the big bang, gamma radiation was emitted. And that’s gonna fill in our first blank.

So let’s move on to the second sentence. It’s been travelling through space since then and the expansion of the universe has caused the wavelength of the radiation to blank. So straight out of the big bang, all of this gamma radiation — this high energy radiation — was emitted. We know that gamma radiation as well as being high in energy has a very short wavelength. This is because we can recall the relationship between wavelength and energy of electromagnetic radiation. So we’ve got all this short wavelength, high energy gamma radiation flying about as the universe is expanding straight after the big bang.

Well, if the universe is expanding, then the short wavelength gamma radiation that we’ve got will also be expanded with the universe. Therefore, logically, the wavelength of this radiation will also increase. Remember by the way that the wavelength of a wave is defined as just the distance between two adjacent peaks or troughs or two adjacent equivalent points on the wave. So on the left, we had gamma rays which had very short wavelength. And on the right, We’ve got much longer wavelength waves.

This is caused directly by the expansion of the universe as we’ve just discussed. And so the wavelength of the radiation has increased. And that’s gonna be the answer to our second blank. It has been travelling through space since then. And the expansion of the universe has caused the wavelength of the radiation to increase.

Finally, the last sentence, we now detect the background radiation as blank radiation. Well, we’ve got a massive clue for this in the very first sentence of the question. We’ve been told that we’re looking at cosmic microwave background radiation.

And the reason that it is called cosmic microwave background radiation is because it comes from the cosmos — from the universe. It’s also microwaves and it’s background radiation. So it’s not from any celestial bodies such as stars or galaxies or anything like that. It’s just a radiation from outer space. And that is the answer to our final blank.

So interestingly, the expansion of the universe has turned all these gamma rays that were initially released straight after the big bang into microwaves. That’s how much the universe has expanded. We’ve gone from really high-energy, low-wavelength gamma rays to relatively low-energy, large-wavelength microwaves.

And all of this happened because of the expansion of the universe. Quite literally, space stretched out. And that resulted in these waves almost be pulled apart in a way until the wavelength increased so much. So anyway, our final answer is that the three blanks are filled by the words gamma, increase, and microwave.

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