Cosmic microwave background (CMB) also called cosmic microwave radiation(CMBR) is electromagnetic radiation as a remnant from an early stage of the universe in Big Bang cosmology. The discovery of CMB is landmark evidence of the Big Bang origin of the universe.

Before we go any further I would recommend taking a look at this article on the Big Bang Theory once. (Please do that)

Cosmic microwave background

Outer space looks black right?

What would be your answer? A big YES ??

Of course, I mean, except the stars and the planets which glow like fireflies in the dark canvas of the night sky everything does seem dark. But does everything actually are, the way they look? I mean does the existence of something actually depend on whether we can see it or not?

Maybe not.

What do you mean by “seeing” something?

Cosmic microwave background

Well, basically the receptors in your eye are catching up the light that is coming from some object either by reflecting from it or by getting, directly emitted from it.

But that doesn’t mean, that if we don’t see something it doesn’t exist in reality. In fact, we are surrounded by invisible fields and signals. We can only see a small part of the electromagnetic spectrum as visible light.

So let me ask you once more the same question, is outer space really dark?

If you are using an optical telescope then you will definitely see dark space beyond the stars and planets. But if you use a radio telescope and point it out in the dark patch of the sky you will be picking up a static noise and no matter whichever direction you point your antenna at, you will be picking that same static noise. This noise also makes up for the static salt and pepper in your TV when it doesn’t pick up channels.

Cosmic microwave background

No matter how you orient your dish, there is this constant underlying microwave band static that is always in the darkness of space emitting the same pattern over an over, even accounting for all the other possible interferences.

Now since we picked up this static from every direction we look, it would seem like it was coming from a source that exists literally everywhere on the sky, but the problem is we don’t know any source anywhere that would emit this observed pattern of microwave emission.

So where is this coming from?

Cosmic microwave background

What if I told you the source of this static which we call the Cosmic Microwave Background(CMB) is the process that formed the first atoms in the universe almost 13.7 billion years ago. Also if I told you that the source of CMB caused all of the space to look orange in color for several hundreds of thousands of years.

That’s right, the universe used to be orange in color!

To understand how this could be true, we need to take a detour.

Have you ever observed hot metal? Like the heating element in your toaster or a metal rod that has just been wielded? Then you must have noticed that they glow with a faint red light,

Cosmic microwave background

and that isn’t any ambient light that is being reflected of it, it’s the light being emitted by the heating element itself. If you were to analyze that glow with instruments you would realize that the heating element isn’t just emitting pale red light but also emitting electromagnetic waves of all wavelength at very specific proportions of intensity that trace out a graph which is very close to this,

Cosmic microwave background

This emission graph is the called the thermal spectrum or the blackbody spectrum.

So everything has a temperature, therefore, has a thermal spectrum and emits electromagnetic radiation of all wavelengths at specific proportions of intensity. This includes you, me the food you eat, the sun, and everything in this universe.

In fact, it is called the thermal spectrum because the light is generated by the random motion of the particles in the material, those random motions are themselves the reflection of temperature.

Cosmic microwave background

Now if you go really low in temperature down to 2.7K, the same thermal spectrum shifts into microwave band and exactly matches the CMB.

Therefore CMB is the most mathematically accurate thermal spectrum that has ever been observed.

The problem is space is pretty much empty there is nothing really in there that has such a specific temperature of 2.7K, so why does CMB look like a thermal spectrum at all?

To answer this and see why space was orange in color we need to go back in time to about 400,000 years after the big bang.

During that era, a soup of charged particles with a temperature of several thousand degrees permeated all of space. At this temperature, it was too hot for electrons and protons to even coalesce into atoms, let alone stars, planets, and galaxies. This ionized soup is called plasma and just like the heating elements of our toaster,

Cosmic microwave background

it was also emitting a thermal distribution of electromagnetic waves.

But because there were no neutral atoms yet, the light from the plasma emitted just couldn’t travel far. As the photons tried going ahead they struck with the free electrons on their way and got deviated just like a pinball hits obstacles. And this process of deviating the photon kept on going as the space was filled with these free electrons.

Cosmic microwave background

As a result, the photons couldn’t travel much.

So the visibility in this era was very limited.

This moment was like flash bulbs continuously glowing in space, but the light was being blocked out. But as the plasma cooled, its temperature eventually dropped below the 3000-degree Celsius mark when neutral atoms could finally form with no more free electrons to redirect them.

The universe became for the very first time transparent.

The light that the plasma had emitted then just before it neutralized was like one last blow, one final flash of infinite orange bulbs going off at every point in the universe more or less simultaneously.

Now the light could free stream through the universe forever without any obstruction until it hits something solid.

Before, during and after this event space was expanding and that’s what thinned down the plasma and made it cool in the first place, but as we know expanding space stretches the wavelength of free running light through a process called cosmological red-shift.

So over the course of few million years that orangy thermal spectrum of light was red-shifted to longer and longer wavelengths becoming red and eventually infra-red.

So to human eyes, the sky eventually turned dark. If you put another 13.7 billion years of space expansion, all that light was red-shifted into the microwave band to become what we today perceive as the CMB.

All those atoms from the plasma managed to clunk together and finally become stars, galaxies and through a complicated process of cosmic recycling US.

So the CMB or more specifically the shape of its thermal spectrum was pretty compelling evidence that when it comes to the color of space then black is the new orange!!

This light of the cosmic microwave background tells us about the temperature of empty space which is accurately 2.75K.

However, the universe is not exactly 2.75K in every direction. If we look closely in the picture of the CMB we will find seemingly noticeable bumps in the picture.

There are tiny fluctuations, or ripples, in the temperature, at the level of just one part in 100,000. If the Earth’s surface were smooth to 1 part in 100,000 the highest mountain would be just 100 m tall!

Also, the fact that we are moving through the universe can be measured very precisely by looking at whats called the dipole anisotropy of the CMB. So one part of the sky is slightly hotter, about 3mK hotter and one side is colder by 3mK.

This means that we are moving at approximately 370km/Sec through the universe.

Lets now look at the history of the Cosmic Microwave Background

The cosmic microwave background was first predicted in 1948 by Ralph Alpher and Robert Herman.

Alpher and Herman were able to estimate the temperature of the cosmic microwave background to be 5 K, though two years later they re-estimated it at 28 K. This high estimate was due to a mis-estimate of the Hubble constant by Alfred Behr, which could not be replicated and was later abandoned for the earlier estimate.

In 1964, Arno Penzias and Robert Woodrow Wilson at the Crawford Hill location of Bell Telephone Laboratories in nearby Holmdel Township, New Jersey had built a Dicke radiometer that they intended to use for radio astronomy and satellite communication experiments.

At first, they thought that the noise in their radiometer was due to some other source or maybe because of their own fault in their instruments. Even after re-checking the fault they were unable to find the actual cause. They even went ahead to shoo away the pigeons and cleaned all their poop thinking that they might give this unwanted noise.

On 20 May 1964, they made their first measurement clearly showing the presence of the microwave background, with their instrument having an excess 4.2K antenna temperature which they could not account for.

After receiving a telephone call from Crawford Hill, Dicke said: “Boys, we’ve been scooped”.A meeting between the Princeton and Crawford Hill groups determined that the antenna temperature was indeed due to the microwave background.

Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery.

Over the past couple of decades, many experiments have measured the tiny fluctuations CMB, with accuracies gradually getting better and better.

These small fluctuations are there because of tiny variations in the density of the Universe immediately after the Big Bang.

Any regions which are slightly denser tend to attract more matter, and get even denser and attract even more material. This runaway process is what led to the formation of the first stars and galaxies.

The properties of the fluctuations have been used to help determine the age of the Universe, what it’s made of, and even how it might end. As the measurements get even better, our knowledge of the Universe increases.

Planck will be an important milestone in our understanding, measuring these fluctuations to incredible accuracy and in finer detail over the whole sky that has been possible in the past.

So far the Planck satellite has been most accurate in finding the detailed picture of the CMB.

The cosmic microwave background radiation and the cosmological redshift-distance relation are together regarded as the best available evidence for the Big Bang theory. Measurements of the cosmic microwave background have made the inflationary Big Bang theory the Standard Cosmological Model till date.

So when you look up at the sky past all the stars and galaxies and see the ancient light of the CMB, remember you are literally seeing the starting point of the universe.

Isn’t that a great realization?

So let’s keep looking up and deep into the heavens.

More articles will be coming soon on cosmic microwave background will be coming soon.

please leave a comment below or ask any question you want to regarding this article

I would highly recommend a few books that would really help you to know  in depth regarding the cosmos:

  4. A Brief History of the Universe: From Ancient Babylon to the Big Bang (Brief Histories)
  5. The Physics Book: From the Big Bang to Quantum Resurrection, 250 Milestones in the History of Physics (Sterling Milestones)
  7. Cosmos
  8. Relativity: The Special and the General Theory (Routledge Classics)
  9. Black Holes: The Reith Lectures
  10. The Oxford Companion to Cosmology (Oxford Quick Reference)

Cheers and Thank you for reading!!

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I am Mayukh Bagchi. 3rd year Undergraduate Engineering student at SRM University , physics and in general Astrophysics is my passion and I want to pursue the same. This blog is dedicated to serve the community with honest and clear insights to the cosmos.


Ken Kempen · July 22, 2018 at 6:24 am

I’m amazed at how much faster we are finding out more and more about our fascinating Universe obviously due to our incredible technological advances. I’ve been an avid amateur astronomer enthusiast. Thank you for the most educating and interesting articles you scientists write and share with us mere mortals,thank you.😎

    Mayukh · July 22, 2018 at 10:04 am

    Hey, Ken! Your most welcome!!! Well even I am not a scientist yet! But definitely want to be one! Thanks for sticking around and reading the article!

my response · August 5, 2018 at 2:59 pm

I just want to say I’m new to blogs and honestly liked this website. Almost certainly I’m likely to bookmark your blog . You actually come with terrific stories. Thanks a bunch for sharing your web site.

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