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Skip to 0 minutes and 20 seconds Planck was a European satellite designed to study the cosmic microwave background radiation. Here we’re going to talk to a couple of the people who had a lot to do with designing and analysing the Planck data. Let’s go.

Skip to 0 minutes and 36 seconds Jean-Loup, can you give us an idea of how Planck started out? Oh, it started a long time ago. In fact, I had the idea of participating in measurement of the cosmological background for many years, and I proposed several experiments which were not selected. And in 1992, what came about was that a low temperature physicist in Grenoble, Alain Benoit, devised a scheme to cool detectors at a very, very low temperature. 1/10 of a degree above absolute zero. So in ‘92, we made this proposal to the European Agency, to CNES to start with, and then the European Space Agency, to build a satellite around that idea, to measure with unprecedented accuracy the cosmological background.

Skip to 1 minute and 24 seconds -Attention pour le decompte final-

Skip to 1 minute and 30 seconds -Dix, neuf, huit, sept, six, cinq, quatre, trois, deux, un, top- So what was the launch and operations like for Planck? Well, the launch was in 2009, which was 16, 17 years after the start I described. And of course, when you have worked on that, not only myself, many people, 400 scientists on the Planck collaboration, plus many, many engineers in the industry, and so on. And when it’s on top of this big rocket, huge rocket, and you imagine that it could fail, and everything would just blow out, and so on– It’s stressful. It’s extremely stressful.

Skip to 2 minutes and 21 seconds And so for the 10 minutes it lasted until it reached the velocity to be basically shot directly to the Lagrange point, at 1.5 million kilometres from the earth, outside, I was really following each step I knew of separation of various stages and so on. Well, with boosters falling off and things like that. And the stuff which could have looked, by looking at that, when the boosters, there is kind of an explosion there. And it’s just because it’s an explosive bolt that you see. And so I didn’t want to be horrified by seeing something like that and not knowing what it was.

Skip to 3 minutes and 3 seconds Michel. Hi, Ken. How are you? Fine, you? So you’re the Deputy Instrument Scientist for the High Frequency Instrument on Planck. Yes. Could you explain to us how you, exactly, detect microwave light? So the photons are first captured by a telescope, which will focus light on the detectors. And the detectors are thermal detectors, which are called bolometers. So the light is absorbed by an absorber, and this absorber is heated up because of this absorption. And you measure just the heating caused by this absorption by using a thermometer which is a simple resistor, which depends on the temperature.

Skip to 3 minutes and 45 seconds So these detectors are very sensitive, because when you cool these detectors to very low temperature, say, 0.1 Kelvin, which corresponds to 0.1 degrees above absolute zero, you can reach a very high sensitivity. And for Planck, especially for the low-frequency channel of the HFI instrument, we are able to see the fluctuation of the incoming photon flux of the CMB itself.

Skip to 4 minutes and 13 seconds So who made the dilution part of the cooling system? So this part has been made by a French group, especially in Grenoble, associated to an industry, Air Liquide, they have produced this dilution system which is able to be used without gravity, especially in space. And this is really a very nice system that is a new system that is being used on HFI. How exactly do you get these detectors at such a cold temperature? So in the technical languages, we are using what we call the cryogenic chain. So we have a chain of a few thermal systems that allow to cool the detectors to such a low temperature.

Skip to 4 minutes and 55 seconds The first system is just a passive one, so by just using the telescope, looking at the cold sky, you are able to cool the telescope in the first cryogenic stage, to a temperature which is 40 or 50 Kelvin, so 40 degrees above the 0, the absolute zero. And that’s just because it’s in space? Yes, exactly. Just passive. So it’s free. You have just to put your telescope on the correct configuration. So looking at the cold sky. And after that, what you need, to go below this temperature, is to have an active system. So the first stage is a system that allows to cool the low frequency instruments detectors, at 20 K(elvin).

Skip to 5 minutes and 32 seconds So the first stage, the first active stage is what we call the absorbtion cooler. It allows to cool the low frequency instrument detectors to 20K. And in fact, it’s a system that has been made by our US colleagues, from JPL especially. So it’s based on an extension, a Joule-Thomson extension of hydrogen. And after that, you have another stage to cool the first part of HFI to 4K, which has been produced by the UK. And it’s a Joule-Thomson of helium. And the last part is by using two isotopes of helium, helium three and helium four.

Skip to 6 minutes and 10 seconds By using these two isotopes, by mixing them together, you get a cooling power that allows you to cool the detector to 100 milliKelvin, so it is 0.1 kelvin above the absolute zero. Okay. Planck can only look at one point on the sky at a time. So how does it actually cover the entire sky? To make a map of the full sky, we actually spin the telescope so that, at any one moment, it looks at a circle around the sky. And with this circle, how do we actually cover the full sky?

Skip to 6 minutes and 43 seconds Well, the point is, with our orbit around L2, the second Lagrange point, we start out scanning the sky like this, but after three months, the earth moves around the sun, and then we’re scanning a circle that’s perpendicular to it. So then six months later, we’ve actually covered the whole sky. After nine months, we’ve covered it 1 and 1/2 times, and after a full year, we’ve actually covered the sky twice. What you’re going to be seeing here is actually, we take this full sky, and it’s really hard to actually project a sphere onto a piece of paper. So what you’re seeing now is that we try and project the full sky onto a Mollweide projection.

Skip to 7 minutes and 26 seconds And that’s what this egg shaped oval is.

Skip to 7 minutes and 31 seconds Michel, once you have the data on the satellite, how do we get it here on earth? So after it has been measured by the detector, it is digitised inside the readout electronics, and after that, transmitted to the on board computer, and compressed. And after it has been compressed, it is being sent to the earth. So you have to know that Planck is located on the second Lagrangian point. So it’s quite far away from us. So you need to get the data compressed in order to get all the information on board of the satellite.

Skip to 8 minutes and 11 seconds Jean-Loup, where’s the Planck satellite now? The Planck satellite was at this very special point, which is a Lagrange point away from the Earth. But international agreement requires that we remove the satellite from this point, and so the satellite, now, after the end of the observation, has been de-orbited, as it’s called, well, the orbit on the Lagrange point is a pseudo-orbit. And now, it’s on a solar orbit, which means it’s basically following the earth, more or less, around its motion around the sun. OK, like the Earth. Yeah. And what’s the Plank working on now?

Skip to 8 minutes and 55 seconds So the Planck collaboration, of course, had the role, not only of building, and testing, and so on, the instruments, but also to analyse the data, and release them and does the first scientific analysis, which was done first in 2013, when we released temperature maps. But until now, we had a very hard work, which was to get to the noise for the polarisation data, which are typically 100 times weaker than the temperature data, and which contain very interesting information about inflation, especially. Inflation, which is this hypothesis about the beginning of the Big Bang, and reionisation, first object in the universe, and so on. So that was very important and difficult, because that required stability of the instrument over several seconds.

Skip to 9 minutes and 53 seconds And so for that, we only succeeded to clean all the instrumental effect that could affect the data only now. And so we are going to release these data in the fall, now. And what do you think that the legacy of Planck will be? The scientific legacy. What will it be remembered most for? So of course, now, the legacy of Planck is a cosmological parameters, six parameters which describe our universe, plus a few others, which are less essential. And that’s quoted by everybody who works in extragalactic astronomy or cosmology. But the legacy is the maps of the sky, in temperature. And in temperature, it’s going to be difficult.

Skip to 10 minutes and 41 seconds Basically, we are limited by fundamental, basically, the foreground of the Milky Way, is a limitation which, for temperature, is very difficult to do better than what Planck has done. So that will be a very, very long legacy. In polarisation, it will be a long legacy, because the next satellite will probably be 10 to 15 years away from us. But this will be a very, very important legacy for 10 to 15 years, which is this polarisation.

The Story of Planck

The Planck mission has achieved maps of the Cosmic Microwave Background to a level of details that had never been reached before. This has allowed to measure a certain number of quantities describing the early Universe, and its evolution since, with unprecedented precision. Your host on this video is Ken Ganga, a member of our lab and an active member of the Planck collaboration. And you will meet the head of the mission, Jean-Loup Puget, and Michel Piat, the Deputy Instrument Scientist for one of the two detectors on the mission: the High Frequency Instrument of HFI.

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