In my last post I introduced something known as the integrated Sachs-Wolfe (ISW) effect. You'll probably get more out of today's post if you've read that one. However, I've tried to make today's post as self-contained as possible, so don't fret if you're new to the blog or have forgotten things over the last six weeks.
Put most simply, the ISW effect is the very subtle heating and cooling of light as it travels through structures in the universe. In the standard model for the universe's history this ISW effect grows with time and is most significant when dark energy starts to dominate the universe late in its history. The effect occurs because the energy gained or lost by light as it climbs into or falls out of structures becomes smaller with time. Therefore light receives an overall change in energy when it travels through these structures.
Unfortunately, the ISW effect is tiny. It will happen to any light travelling anywhere through the universe, but it is really, really tiny. This means that, for almost every light source in the universe (galaxies, stars, supernovae, etc), we just don't know the initial light source well enough to be able to tell if it has changed by the tiny amount we expect from the ISW effect. But, there is one source for which we have a very clear, very precise prediction. This is the cosmic microwave background, or CMB (note: I introduced the CMB in this post). As regular readers of the blog might be starting to appreciate, the CMB is more or less every cosmologist's favourite data source.
Unfortunately even the CMB has tiny fluctuations in it. These arise because the source of the CMB, a plasma of hydrogen that once permeated the entire universe, was not uniform (I explained the shape of the fluctuations in the CMB in an earlier post). And, most unfortunately, even these tiny fluctuations, fluctuations so small that Nobel prizes were awarded for their detection, are
Alas! So it seems that we can't even see the ISW effect in the CMB?
The thing is, we know the source of the ISW effect. It is caused by structures in the universe. If we could see those structures then we wouldn't just know that there is an ISW effect, we would also know on which points of the sky it should be bigger and on which points of the sky it should be smaller. This is crucial. At any point of the sky where we expect the ISW effect to make the CMB hotter/smaller there is no reason we should also expect the CMB itself to start off hotter/smaller. So, while we can't measure the ISW effect directly we can ask whether on average the CMB is hotter where we expect a hot ISW effect and whether on average the CMB is colder where we expect a cold ISW effect.
But can we see structures in the universe? Well, yes and no. We can of course see some things in the universe (or the sky would be dark at night!). But, unfortunately, most of the stuff in the universe can't be seen at all. The common name for this unseeable stuff is dark matter, but a much more appropriate name would be transparent matter. This is because dark matter isn't really “dark” at all, instead it's completely and utterly invisible. It doesn't emit light and any light emitted by something else goes right through it. Thankfully, dark matter does interact gravitationally (in fact, that's how we know it's there!). Now, gravity causes matter to attract matter. As a result, all the visible things in the universe tend to follow the dark matter around. If there is more dark matter somewhere, there is a greater probability that we will see stuff there, if there is less dark matter, there's a smaller probability we will see stuff. So that's the 'yes' and the 'no' of whether we can see the structures of the universe. No, we can't see every structure in the universe, but yes, we can see something else that on average follows these structures around.
And that's it. Attempting to detect the ISW effect is as simple as looking at the CMB and checking whether on average it is hotter in patches of the sky that have more luminous matter behind them and colder on patches where there is less luminous matter.
The results of such measurements comes next... (Now continued here)