#insights

Header image: Taurus molecular cloud, seen by Herschel and Planck © ESA

The Planck mission of the European Space Agency (ESA), which I worked on for 7 years, is very appealing from a scientific, and especially engineering, point of view. Its aim is to make a highly accurate map of the cosmic microwave background, or CMB, fossil radiation from the early universe.

After the Big Bang and during the subsequent evolution of the early cosmos, there were states of highly concentrated plasma in which matter and energy were indistinguishable. The universe was expanding and cooling, but it was still very hot and dense. And so even though matter and radiation were starting to separate into different essences, the latter could not escape from the former (at each point in space, radiation, or more simply put, light, was trapped).

There are no traces from that epoch, since no information could get out: nothing separated from wherever it was, not even light, so an observer could not have seen what was happening beyond, since no radiation could escape to inform him (of course, that hypothetical observer would have been crushed and disintegrated into a thousand pieces in a fraction of a second).

There is a point in the expansion of the universe (the famous 300,000-year boundary) when the temperature drops below a critical value and radiation can escape. Basically, at that point, the universe has cooled sufficiently so that complex interactions between elemental particles allow all that energy in the form of trapped radiation to travel. As a result, radiation emerges in all directions from every point in the universe; you could say that the universe becomes transparent to light, which begins to travel endlessly.

Much of this radiation has not interacted with anything in the time since, as it continues to arrive from all directions. The subsequent evolution of the universe (it’s around 13 billion years old) means that all this radiation has cooled and its frequency spectrum has shifted. Now, it’s in the microwave band. “Light” is basically electromagnetic radiation of many types. From radio waves, like the ones we hear at home, to X-rays, ultraviolet, infrared, microwave, gamma rays… and visible light, which is just one type of radiation, with the peculiarity that the human eye has evolved to detect it out of evolutionary convenience. But if, for example, the Earth had an atmosphere that didn’t let it through, it wouldn’t do us any good to see it and we would probably have evolved to see gamma rays, or even microwaves! In short, light is only one type of radiation, and all radiation is the same, although in different wavelengths (it is worth clarifying that all electromagnetic radiation differs only in wavelength, although there is non-electromagnetic radiation: neutrinos, which are considered “radiation”, and gravitational waves).

A space cooling machine to capture primordial “light”

At this point, the CMB (again, the cosmic microwave background, the first fossil radiation in the universe) is found in the microwave band at a very cold temperature, about -270 degrees Celsius, or 3 degrees Kelvin. Zero Kelvin, i.e., -273 degrees Celsius, is what is called absolute zero: nothing can be colder than that temperature, since that is the temperature of zero energy (and in this universe there is no negative energy, as far as we know).

In order to detect something so cold, we need to have very cold detectors. Otherwise, we run the risk of contaminating the measurement with thermal noise. Planck is basically a “space cooling machine”, a marvel of technology whose detectors are at a temperature of thousandths of Kelvins.

So what is the purpose of recording the CMB at this point? What information does this primordial “light” contain? Scientists have always wondered why there are galaxies and planets and concentrations of matter in the cosmos. If everything starts from one point (the Big Bang) and evolves outward, why isn’t the universe simply a uniform distribution of matter, i.e., an atom here, another some distance away, then another, and so on? But it seems that the matter has grouped over time, which in a perfectly symmetrical expansion model would not have happened.

When the CMB is measured with extreme precision, we can see that it is not uniform, it doesn’t have the exact same temperature in all directions. There are very minor variations in it, depending on where it comes from. This means that already at that point (300,000 years), symmetry had been broken and some parts were slightly more energetic than others.These asymmetries are what give rise to the universe we see. It turns out that the process of associating and dissociating matter and energy during that period, of creating and destroying particles, is not perfectly symmetrical and omnidirectional. And all the current cosmic structures are based on the heterogeneous “structure” of that primitive universe.

So, by making a very precise map of the CMB, recording the slightest variations in its energy, and knowing very precisely where each measurement comes from, we can make and validate models of the elementary processes that took place before the universe became “transparent” to radiation, in the first moments after it was created. We can also validate how matter and energy evolved to give us the universe we have. And we can also try to understand what dark matter and energy are, which occupy 90% of the cosmos according to gravitational models, even though we know nothing about them, only that, hypothetically, they have to exist, but we don’t even know their nature or essence.

At Sener, we had the privilege of being responsible for the Planck pointing system (as well as its 30GHz and 44GHz amplifiers-detectors and the medium- and low-gain X-band TTC antennas), which helped scientists observe where they really wanted to observe with great precision in order to build that map of the CMB.