WHAT I REALLY WANT TO KNOW IS…
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