Does ‘dark matter’ really matter?

Originally published 22 November 1993

The hunt is on. The scent of the prey is in the air. The hunters are work­ing them­selves into a fren­zy of excitement.

The quar­ry? Dark mat­ter. The unseen sub­stance that sup­pos­ed­ly con­sti­tutes as much as 95 per­cent of the uni­verse, the hid­den mass that will deter­mine the uni­verse’s ulti­mate fate.

There is glo­ry to be gar­nered. Tro­phies. The prize is so great that the com­pe­ti­tion is stiff to get one’s name before the pub­lic first, impressed upon the minds and mem­o­ries of the Nobel selec­tion committee.

Imag­ine! A great dark con­ti­nent of mat­ter, con­sti­tut­ing most of what exists in the cos­mos, wait­ing to be dis­cov­ered. The ulti­mate ter­ra incog­ni­ta. The Foun­tain of Youth, the Sev­en Cities of Cibo­la, and the Source of the Nile — all rolled into one.

The exis­tence of this hid­den stuff was first guessed by astronomer Fritz Zwicky in the ear­ly 1930s. He mea­sured the rel­a­tive veloc­i­ties of galax­ies in the Coma clus­ter of galax­ies, and he esti­mat­ed the total mass of the galax­ies in the clus­ter by adding up all of their lumi­nous mat­ter. The prob­lem was this: The galax­ies are mov­ing so fast that they should long ago have escaped the grav­i­ta­tion­al attrac­tion of the sys­tem. In the time since the galax­ies formed, more than 10 bil­lion years ago, the mem­bers of the clus­ter should have dis­persed. Yet the clus­ter is appar­ent­ly sta­ble. Some­thing non-lumi­nous is hold­ing it together.

Some­thing with mass. Some­thing with lots of mass.

In recent years, more and more grav­i­ta­tion­al evi­dence has been added to Zwick­y’s obser­va­tions to sug­gest that galax­ies and clumps of galax­ies con­tain vast­ly more mat­ter than is appar­ent from the light of stars.

What is this miss­ing stuff?

Around our cor­ner of the cos­mos, most of the known mat­ter is con­cen­trat­ed in our star, the sun; every­thing else — plan­ets, moons, aster­oids, comets, mete­oric dust — is just a drop in the buck­et. What­ev­er hid­den mat­ter is, it is cer­tain­ly not the kind of famil­iar objects that pop­u­late our imme­di­ate environment.

A num­ber of can­di­dates have been sug­gest­ed. Among the most eager­ly sought:

• MACHOs, or “mas­sive com­pact halo objects,” huge num­bers of plan­et-like objects too small to have ignit­ed as stars, or burned-out cores of stars, form­ing a vast dark halo around galaxies.
• WIMPs, or “weak­ly inter­act­ing mas­sive par­ti­cles,” sub­atom­ic par­ti­cles fill­ing the uni­verse, that were cre­at­ed in myr­i­ad num­bers in the Big Bang, but which are almost impos­si­ble to detect because they inter­act so weak­ly with ordi­nary matter.

The search is on. Armed with tele­scopes, satel­lites, com­put­ers, and exquis­ite­ly-sen­si­tive par­ti­cle detec­tors, a slew of clever astronomers and physi­cists are each hop­ing to become the Christo­pher Colum­bus of dark matter.

If dark mat­ter can be proven to exist, in the amounts pre­dict­ed, then we will gain con­fi­dence in our under­stand­ing of the uni­verse. If dark mat­ter remains elu­sive, then our present under­stand­ing of nature may be seri­ous­ly flawed. The rid­dle is well worth solv­ing; the search is of pro­found importance.

But tune up your hype detectors.

Treat skep­ti­cal­ly recent claims by sev­er­al groups of astronomers to have detect­ed MACHOs, based on vari­a­tions in the bright­ness of stars or quasars. The premise of these claims is that invis­i­ble MACHOs pass­ing between us and dis­tant lumi­nous objects briefly focus the light and cause blips in bright­ness. The blips have been bal­ly­hooed in the media as The Dis­cov­ery of Dark Mat­ter. But stars and quasars may vary in bright­ness in ways we do not yet ful­ly under­stand, and a few blips hard­ly con­sti­tutes proof.

Take any claim of dis­cov­ery of “the Holy Grail of cos­mol­o­gy” with a grain of salt. In fact, keep handy a whole bowl of salt.

You’ll hear that detec­tion of MACHOs or WIMPs con­sti­tutes a “new Coper­ni­can Rev­o­lu­tion,” or even the “ulti­mate Coper­ni­can Rev­o­lu­tion.” Coper­ni­cus dis­placed us from the cen­ter of the cos­mos, say the seek­ers of dark mat­ter; con­fir­ma­tion of dark mat­ter will bump us even fur­ther from prominence.

“How much fur­ther can we fall?” asks astro­physi­cist Lawrence Krauss. If dark mat­ter exists in the quan­ti­ty sought, then we are “an insignif­i­cant bit of “noise’ — a cos­mic after­thought,” he says.

Dish up the salt! The mere fact that most of what has weight present­ly eludes our sens­es hard­ly con­sti­tutes a rev­o­lu­tion on the same scale as that of Copernicus.

Most sci­en­tists, and many philoso­phers and the­olo­gians, have long since accept­ed our spa­tial and tem­po­ral insignif­i­cance in the uni­verse. Whether life is a cos­mic after­thought, as Krauss claims, or a cos­mic fore­thought remains an open sci­en­tif­ic ques­tion, but detec­tion of dark mat­ter will do lit­tle to change our under­stand­ing of ourselves.

The entire bio­mass of Earth is neg­li­gi­ble com­pared to the mass of the plan­et, just as the mass of the plan­et is neg­li­gi­ble com­pared to the mass of our star. If all lumi­nous mat­ter in the uni­verse turns out to be only the tip of the grav­i­ta­tion­al ice­berg, then that will hard­ly alter our view of our­selves. In that sense, at least, we have no fur­ther yet to fall.

Far more sig­nif­i­cant for our cos­mic sense of self would be detec­tion of an intel­li­gent sig­nal from space, from a real E.T., either macho or wimp­ish. Know­ing with cer­tain­ty that the flame of intel­li­gence burns bright­ly among the stars would indeed be a rev­o­lu­tion to rival that of Coper­ni­cus. Not a fall, but a restora­tion to centrality.