In the space between galaxies: dark matter and interstellar dust

Every time we open a new win­dow into the cos­mos, that is, on cer­tain col­ours or regions of the elec­tro­mag­net­ic spec­trum in the sky, we dis­cov­er new objects and stars. This is because we have access to a whole range of wavelengths that are totally inac­cess­ible from Earth’s sur­face. For example, the first time we sent a satel­lite to observe the Uni­verse using X‑rays, in the 1960s, we dis­covered spots in the sky of an unknown ori­gin. We later learned that they were due to plasma heated to sev­er­al mil­lion degrees. These spots indic­ated the pres­ence of ‘mega­cit­ies’ in the Uni­verse: regions inhab­ited by hun­dreds of galaxies.

This plasma is the mater­i­al in which galax­ies float, and it is com­pletely invis­ible from the ground. When we study this source of radi­ation, we dis­cov­er sev­er­al things. First, that the tem­per­at­ure of this gas is dir­ectly related to the mass that is con­tained in the mega­city. And second, that it con­tains ten times more mass than any­thing that radi­ates vis­ible light. As such it is evid­ence for the exist­ence of some form of dark mat­ter in the Universe.

Thanks to X‑ray astro­nomy, we also dis­covered that iron exists between galax­ies. Accord­ing to cur­rent know­ledge, this ele­ment can only be cre­ated dur­ing the explo­sion of a star, and not­ably in the core of the most massive stars. How­ever, there are no stars between galax­ies. This obser­va­tion there­fore provides evid­ence that galax­ies must be los­ing some of their mat­ter in the form of ‘galactic winds’. These winds come from the explo­sions of stars in the interi­or of galax­ies, which pro­ject their mat­ter far out from them­selves and thus feed­ing the plasma between galax­ies with their iron atoms.

The oth­er extreme of the elec­tro­mag­net­ic spec­trum, the infrared, is anoth­er area that is vir­tu­ally inac­cess­ible from the ground and for which we must send satel­lites into space. IRAS (Infrared Astro­nom­ic­al Satel­lite), an Amer­ic­an satel­lite launched in 1985, was the first to observe the uni­verse at infrared wavelengths. It made an aston­ish­ing dis­cov­ery: what appeared as ‘holes’ in the sky were in fact the densest and most con­cen­trated regions of mat­ter in the Milky Way, so-called giant molecu­lar clouds com­posed of atoms, molecules and dust grains. In short, places where new gen­er­a­tions of stars are born.

This inter­stel­lar dust cre­ates regions that absorb star­light, mak­ing them appear opaque. It is heated to a tem­per­at­ure of about 40 degrees above abso­lute zero (-230 °C) and radi­ates in the infrared. In our labor­at­ory at the CEA, we developed infrared detect­ors that allow us to cre­ate the cam­era for the ISO (Infrared Space Obser­vat­ory), a European satel­lite launched in 1995. Obser­va­tions from the ISO showed that there are regions in which stars were born in the Milky Way that had escaped detec­tion. Fur­ther ana­lyses revealed that, in fact, through­out the his­tory of the Uni­verse, most of the births of news stars have eluded us.

This satel­lite was fol­lowed by Spitzer (Space Infrared Tele­scope Facil­ity), an Amer­ic­an satel­lite, in 2003 and by Her­schel, from Europe, in 2009. Again, our labor­at­ory built one of the most import­ant cam­er­as on Herschel.

Space tele­scopes are unique in that all the elec­tron­ic com­pon­ents on board must with­stand the harsh con­di­tions of the cos­mos. For one, they must be res­ist­ant to cos­mic rays. They must also be robust to vibra­tions. Even the screws used in these satel­lites are spe­cially designed to with­stand the cold of space. It is there­fore a whole new type of tech­no­logy that we are con­stantly devel­op­ing and improving.

This quest for high-per­form­ance mater­i­als for space is driv­ing research into new mater­i­als and detec­tion sys­tems. For example, when we observed objects that are extremely faint, we real­ised that we had to devel­op cam­er­as cap­able of cap­tur­ing only a few photons (particles of light). In every­day life, there was no reas­on to do this, oth­er than to observe the stars in the Uni­verse – it was essential.

We real­ised that such detect­ors can be use­ful else­where; when we have a small aper­ture, as is the case in our mobile phones, we need to have detect­ors cap­able of col­lect­ing very little light and, des­pite this, pro­duce a very good image. So, a good part of the optics and detect­ors found in phones and oth­er devices today have benefited from space exploration.

What has sur­prised me most is that when I study the Uni­verse, from the earli­est times to the present day, what I see seems to be in total con­tra­dic­tion with what we learn in phys­ics – that the second prin­ciple of ther­mo­dy­nam­ics leads to an increase in entropy, or dis­order. Some say that this is the logic­al con­sequence of the fact that the nat­ur­al evol­u­tion of the Uni­verse is to go towards more and more dis­order. In real­ity, what we observe is that entropy increases not through dis­order, but through the pro­duc­tion of light. It is much more effi­cient for mat­ter to organ­ise itself by form­ing com­plex struc­tures, which in turn will pro­duce entropy in the form of light, than to be dis­ordered. My book « La plus belle ruse de la lumière » tells the story of how the his­tory of our Uni­verse is based on the struc­tur­ing of mat­ter into such increas­ingly com­plex structures.