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.