Why is the dis­tant Uni­verse opaque?


Most of the mat­ter in the Uni­verse is in deep space. When we think of a deep pic­ture of the Uni­verse we think of gal­ax­ies, but that’s not ac­tu­ally where you’ll find most of the stuff in the Uni­verse. In­stead, most of it is stored in an in­ter­ga­lac­tic net­work of fil­a­ments that con­nects the gal­ax­ies.

Most of the stuff in these fil­a­ments is hy­dro­gen gas and nowa­days most of it is ionised: elec­trons have been sep­a­rated from pro­tons. Ionised gas does not ab­sorb ul­tra­vi­o­let (UV) light so the Uni­verse to­day is pretty trans­par­ent to UV light.

But if you go back far enough in time the Uni­verse was com­pletely opaque be­cause it was filled with neu­tral, un-ionised gas. As stars formed their ra­di­a­tion ionised the gas in a process called reion­i­sa­tion. Patches of neu­tral hy­dro­gen in deep space can be iden­ti­fied by be­ing partly opaque to ul­tra­vi­o­let light since their atoms are very ef­fec­tive at ab­sorb­ing it.

What my re­search has un­cov­ered is a patch around a bil­lion years af­ter the Big Bang, when this tran­si­tion from a mostly neu­tral Uni­verse to a highly ionised Uni­verse may have only just hap­pened. We use quasars as back­ground lights to study the gas in deep space. To­wards one par­tic­u­lar quasar there is a 500 mil­lion lightyear-long stretch where the ul­tra­vi­o­let light from the quasar was com­pletely ab­sorbed, sug­gest­ing that some­thing was ab­sorb­ing the light. Mean­while, re­gions to­wards other quasars still show quite a lot of UV trans­mis­sion. We wanted to know why there was such a large dif­fer­ence.

A sur­pris­ing lack of gal­ax­ies

We thought that the re­gion could ei­ther be low den­sity with the lit­tle gas there is be­ing pre­dom­i­nantly neu­tral; or it could be a cold re­gion in the Uni­verse. If it is a re­gion with lit­tle gas there should be few gal­ax­ies there be­cause the two go to­gether; but if it is a cold re­gion, it should be full of gas and gal­ax­ies that have had plenty of time to cool af­ter go­ing through this ion­i­sa­tion process. So we started to look for gal­ax­ies in the re­gion. To do that we used a pow­er­ful cam­era on the Subaru Tele­scope on Mauna Kea, Hawaii, us­ing fil­ters at mul­ti­ple wave­lengths let us count the gal­ax­ies in the opaque re­gion. To our sur­prise we found very few. If we’d done this ex­per­i­ment a lit­tle bit later in cos­mic time, we would def­i­nitely have found a dif­fer­ent re­sult, be­cause long af­ter the gal­ax­ies form their ul­tra­vi­o­let light floods the Uni­verse and ionises the hy­dro­gen more or less equally ev­ery­where. That means if you have a par­tic­u­lar patch that is more opaque than oth­ers at these later times, it’s be­cause it has more mat­ter in it block­ing out the light.

Dark but not dense

But here we see the op­po­site. It’s the dark­est place in the Uni­verse at that time, but it’s also one of the least dense. We may be see­ing this patch at a time just af­ter the gal­ax­ies ap­peared and ionised the gas. Be­fore reion­i­sa­tion the gas and deep space would have been en­tirely neu­tral and there­fore com­pletely opaque to ul­tra­vi­o­let light.

We don’t know ex­actly when this process of reion­i­sa­tion hap­pened but we think that it was com­pleted not long be­fore the pe­riod we’re talk­ing about. The gas in this re­gion may still have some neu­tral por­tion left. Al­ter­na­tively, the ul­tra­vi­o­let ra­di­a­tion here may be weaker than it is else­where, per­haps be­cause the ra­di­a­tion has a dif­fi­cult time trav­el­ling even short dis­tances in this par­tic­u­lar patch of space.

The test of whether we’re see­ing reion­i­sa­tion end­ing or some­thing that oc­curs long af­ter ion­i­sa­tion will be when we look at the op­po­site type of re­gion – one that has plenty of trans­mis­sion.

We’d like to look at one of these very trans­par­ent re­gions. We have good spec­tra on them and I sus­pect that we will be go­ing af­ter more of these re­gions in the spring us­ing the Subaru tele­scope.

Most of the ‘stuff’ in the Uni­verse is stored in an in­ter­ga­lac­tic net­work of fil­a­ments that con­nects the gal­ax­ies

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