Deciphering the cosmic microwave background with telescopes in Tibet

Many experiments are currently trying to get additional information on the cosmic microwave background, most commonly known as the CMB, an object that plays a key role in standard cosmology.

All these experiments are currently located in the southern hemisphere. But this will change in a close future.

Recently, China has indeed launched a new project aiming to build the first CMB experiment in the northern hemisphere.

This experiment, that will yield an extension of the set of ’Ali’ telescopes in Tibet, is what I will talk about today.

I will also of course briefly come back to what is the CMB in the context of standard cosmology.

It must be kept in mind that past CMB experiments have gathered a lot of information on the CMB, and the key element is now to get more precise data (including in particular more information on the polarization of the CMB).

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BUILDING TELESCOPE IN ALTITUDE?

Why building CMB-dedicated telescopes in Tibet? Well, the answer is pretty obvious: there are huge mountains there! After all, the Himalaya mountains consist of the roof of the world.

Altitude means in fact less noise due to the Earth’s atmosphere. Let me detail that point a little bit more.

In the CMB acronym, we have the letter ‘M’ that stands for microwave. The CMB is indeed made of photons whose wavelength is in the microwave range (the exact same photons that you could get in a microwave oven).

Their detection on the ground is heavily challenged by the presence of water vapor in the atmosphere. As a consequence, very few sites are candidate for any ground-based experiment targeting the CMB observation.

For instance, one has the South pole in Antartica, the Atacama in Chile, etc. And the Himalaya moutains.

More in particular, the Tibetan Plateau is a perfect place for building CMB telescopes! And this is what China will do. The two planned Ali CMB Polarization Telescopes are expected to be respectively located at 5250m height and 6000m height.

The amount of water in the atmosphere at that place of the world is actually small enough so that these experiments will provide an opportunity to observe the CMB from October to March each year, as found in this paper.

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BUT WHAT IS THE CMB?

I have so far only mentioned that the CMB was made of photons with a wavelength lying in the microwave range. It is now time to give more details on this.

The CMB story starts a few of hundreds of thousands of years after the big bang. The universe is already pretty expanded and cold, so that atomic nuclei and electrons are combined into atoms.

This is called the recombination epoch. Before this recombination, matter was made of positively charged atomic nuclei and negatively charged electrons. After this recombination, matter was made of neutral atoms.

The key property is the electric charge. Let us rephrase all of this.

Before recombination, photons will interact with a charged particle very often and thus not travel very far before getting something happening. The universe is said opaque to light and photons are really zigzagging from one charged particle to the next one.

After recombination, matter is neutral so that photons can travel long distances in the universe without meeting any electrically-charged object. The universe is said to be transparent.

As a result, the photons released at that time are still visible today. If they do not interact, they will indeed be observed as when they were created long long ago.

This is the so-called cosmic microwave background that was discovered in the 1960s by Penzias and Wilson.

On the picture shown above, on the left, we can see the cosmic microwave background as observed by the Planck satellite. There is a structure, and this structure contains a lot of information we can use to constrain not only cosmological models, but also dark matter.

In addition, the map of this structure has allowed physicists to extract plenty of pieces of information on the Universe, its composition and evolution.

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POLARIZATION

The next properties of the CMB photons that we are after consist of their polarization. Experiments in space, like WMAP or Planck, are rather limited with this respect and we really need to rely on ground-based experiments.

On the ground, it is a real piece of cake to tune telescopes to target regions of the sky where we know the astrophysical foregrounds to be small. This is another way to say clean signals.

Ali telescopes in Tibet will be capable to cover all possible clean sky regions of the northern hemisphere, and even some of those of the southern hemisphere. They form thus a nice complementary experiment to what is already available on the southern hemisphere!

But why is CMB polarization important?

In fact, it can provide important test of fundamental physics of the universe in the early days, in particular with respect to the gravitational waves emitted at that moment. This could be, in other words, the first quantum probe of gravity!

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SUMMARY AND REFERENCES

In this article, I have used the excuse of a new experiment whose building has been launched by China in Tibet to discuss the cosmic microwave background. This quantity, commonly known as the CMB, is the real topic of this post and I wanted to explain where it was coming from.

I also mentioned the complications of measuring it on the ground, and why having experiments on the ground (on top of having them in space) was important. I finally briefly detailed why measuring the polarization of the CMB photons would be important.

For more information, please have a look here. This is the article in which I have learned about the Tibet Ali telescopes. For more information on the CMB, the Wikipedia page is a good starter as well. There are nice references on this page too.

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