π Space
How Space telescopes unravel the mysteries of the cosmos

In the space between galaxies: dark matter and interstellar dust

Isabelle Dumé, Science journalist
On November 17th, 2021 |
3 mins reading time
In the space between galaxies: dark matter and interstellar dust
David Elbaz
David Elbaz
Astrophysicist and Scientific Director of the CEA Astrophysics Department
Key takeaways
  • Space telescopes give us the opportunity to collect crucial data about the Universe, which are unobtainable and invisible from Earth.
  • Those observations provide us with more knowledge on the composition of space; for example, we now know that galaxies float in ‘plasma’, and that they can lose some of their own matter through galactic winds.
  • New ways of observing space can drive technical innovation. And some of them can be useful to everyday objects on Earth, such as our mobile phones.

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.

Using the electromagnetic spectrum to observe space

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.

Advances in space materials research benefit applications on Earth

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.

« La plus belle ruse de la lumière »

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.