The CMB Energy Paradox

What if the oldest light in the universe reveals a hidden paradox? Discover the Cosmic Microwave Background, why it reaches us as microwaves, and how cosmic expansion raises surprising questions about energy, spacetime, and the nature of our ever-changing universe.

Introduction

Do you know when you look into the clear sky in the night, you are looking in the past! Even if you look at the moon, you are watching it before 1.3 seconds. But the question is how? The Light coming from the the stars take time to reach to us and as a result we are looking at their past. Like if the sun, suppose disappeared somehow, we would notice it around after eight minutes because light takes eight minutes to reach earth. The fact is that there are billions of stars in our galaxy and then there are the other stars in different galaxies which are very much far away from us and the light from those stars take millions of years of time to reach us. So can there be the earliest light of the universe that we can detect? the place where the light finally moved freely after 380000 years? Yes, there exists the ancient light known as the CMB.

What is Cosmic Microwave Background?

Cosmic Microwave Background or CMB is basically the earliest light of the universe that reaches us as the microwave radiation and that is why there is “Microwave” in its name. The question arises but why do we receive the earliest known light as a microwave? I will get to this Question later on but for now lets just get into the detail of CMB.

In the previous blog, I mentioned that the first atoms formed after 380000 years when the electrons formed (electrons formed from pure energy but under less temperature compared to the temperature needed for the formation of the protons and neutrons) attached to the nuclei to create the neutral atoms. After that the light could finally travel freely, don’t you think that light didn’t existed before the formation of atoms, light existed but was scattered by the particles of the hot plasma. One more thing is that light is made up photons, which simply are massless particles of light.

Now i will come to the question that why do we receive the CMB in the form of microwaves? why not as the visible light?

Expansion of Space & Its Effects on Light

Light is both Electromagnetic wave and a photon, explained through wave particle duality but for now we will consider light as a wave for more easy understanding. So when light finally could move freely after atoms formed, the universe as we know was also expanding since the 13.8 billion years ago and was expanding during when the light was travelling. Over a long time, as space itself was expanding and as a result the wavelength of the light also started to increase. But how? let me make this very clear, the light has wavelength, its the distance from one crest to another crest of light and most of us know that the wavelength of a wave is inversely proportional to the frequency. so if the wavelength increases the frequency decreases. Same is the case here, the frequency therefore decreased. the microwaves come with lower frequency and higher wavelength.

But it isn’t making any sense how can the wavelength change, give us an analogy…okay alright lets understand this by an analogy

Imagine two nearby mountains, and you are at a distance looking at them and you can clearly see that the two mountains have some distance in between them and you can see their peaks. Now lets take the distance between those mountains as the wavelength and here comes the hypothetical scenario, suddenly the distance between the mountains started increasing not because the mountains are moving.. the mountains are at their fixed original positions. what increases is the distance between the mountains.. it just increased automatically out of nothing. as a result the wavelength of mountains increased. Same applies in the context of light, the space between the two crests or peaks increased. this is the real expansion that’s going on since the beginning of universe. Scientists then call dark energy as the cause the acceleration of space and no doubt we will come to this in the upcoming blog.

This phenomena is called cosmological redshift and this is the reason we receive earliest light as microwave

Is Energy is Really Conserved?

The topic above I was explaining to you touches the core idea of energy conservation. We all have heard from the childhood, seniors and even most of the teachers saying this so called sentence, “Energy can neither be created nor destroyed” but this is wrong energy isn’t conserved, yeah you heard it right. Energy is conserved at small scales but not at the cosmic scale.

As the light converted into the microwave, during this process frequency decreased and frequency is nothing the energy, so in contrast the energy of light disappeared into nothing. I know it seems weird as it challenges our classical thinking but it is what it is!

Another way to think of it is that earliest light had a temperature of 3000k and the CMB we receive today as a temperature of 2.7k, around 2997k energy is lost today but the catch is there is no recipient for this energy.

Imagine measuring distances with a ruler whose markings are continuously changing. If someone asks, “Where did the missing centimeters go?” the question may not make sense because the measuring system itself is changing.

It sounds strange because it conflicts with our everyday experience, but it is a direct consequence of how general relativity describes the universe.

Conclusion

This blog is was about understanding the light was receive today, why and how we receive it? I really hope you guys would have got the idea of What really the CMB is! Important thing is that this blog challenged your classical thinking by diving deep into the secrets of spacetime. As i said this is just the beginning there is much more explore. I will meet you in the next blog till then, STAY INTERESTED AND STAY CURIOUS!