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What is the Cosmic Microwave Background?

May 22, 2017
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What is the Cosmic Microwave Background?

The Cosmic Microwave Background, or CMB, is a radiation that fills the universe. We can detect it in every direction.

So, this radiation started around 400,000 years after the Universe began. That may sound like a long time on human timescales, but it really is the blink of an eye when compared to the age of the Universe.

The universe is around 13.7 billion years old. Before this time, it was so hot and dense, not even simple atoms could form without instantly being ripped apart into their constituent protons and electrons by the intense radiation. The Universe was made of a “plasma”, or ionized gas, which is what the surface of the Sun is made of.

Ever since the Big Bang, the Universe has been cooling and expanding. Scientists call this expansion, the dark energy. By around 400,000 years through its life, it was cool enough (though still around 3000 Celsius) for the simplest atoms to form, and it became transparent. The light from this time has been traveling through space ever since. We can detect it all around us from here on Earth or in space. We can measure the afterglow of the Big Bang.

The expansion of the Universe has stretched out the CMB radiation by around 1000 times. Thus, making it look much cooler. So instead of seeing the afterglow at 3000 degrees, we see it at just 3° above absolute zero, or 3 Kelvin (-270o C).

History of the CMB Measurements

In the 1960s, Arno Penzias and Robert Wilson made the first measurements of the CMB. They confirmed that CMB was there and that it was all around, however they could not see it in details. They saw a constant signal which washed out their view of the galaxy. The CMB is so bright at millimeter-wavelengths that if you de-tune an old analog TV to show the snow-like static, a few percent of the signal your TV is picking up will have come from the start of the Universe.

In the 1990s, a satellite called COBE measured the CMB over the whole sky. It helped establish several things. Firstly, the CMB is almost completely uniform, with an almost constant temperature over the whole sky. However, it is not completely constant. There were 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!

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.

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