I read 3 papers! Three beautiful, science-packed, revolutionary and mind-blowing papers! Ok, maybe not – but trust me, after being so busy with things like measuring redshifts, fixing codes and mountains of admin/conference organising, almost any science is pure beauty for the old brain.
But one of these papers did tap into something I’m quite interested in, and it’s related to the Cosmic Microwave Background (CMB). I’ve briefly mentioned the CMB before, but here’s a more decent introduction: when we say CMB we are talking about radiation that was created when the Universe was very very young - around 300,000 years old. At that time the Universe was hot and radiation (or photons) were the dominant component of the Universe. Because it was so hot at that time, the Universe was actually opaque – matter was ionized, meaning electrons were bobbling about not really being attached to any nuclei because of the high temperature. What this means is that photons could not get very far without bumping into something – they could not travel in a straight line for any decent sort of time, and were perpetually scattered around. That’s essentially what opaque means. However, something special happened around 300,000 years into the Universe’s lifetime, and that was a decrease in temperature that allowed these electrons to settle into atoms, effectively setting the photons free. We call this the time of last-scattering.
These photons were then free to travel unhindered. They were travelling then, and they are still travelling now! Straight into our telescopes, carrying information from around 13 billion years ago, virtually unaltered! This, for Cosmologists, is like Christmas – almost too good to be true. Anyway, I say virtually because some things affect these photons as they travel along. A big one is the fact that the Universe is expanding, and this causes their frequency to change. This is the reason why we see them in the Microwave part of the spectrum today – they were a lot hotter 13 billion years ago. Had we lived earlier on during the lifetime of the Universe, the CMB would have peaked in the visible band and the sky would be rather pretty! (of course, whether a planetary system with life could even exist then is another matter) But the Universe, in spite of being mostly empty, still has a lot of stuff around. You know, galaxies and things. And as these photons go through expanding regions with more matter (clusters) or less matter (voids) they see their frequency slightly changed.
Now, what I didn’t say and should have done is that not all CMB photons are the same. They are all very similar, but depending on the density of the region of space they were in at the time of last-scattering, today they are either a little bit hotter than the average, or a little bit colder. It’s a very small little – around 1 part in 10,000! But as I said before, a good enough experiment is able to pick up these tiny differences. When we look at the sky, we obviously only see the CMB photons travelling towards our telescope (they are travelling uniformly in every direction), but these tiny differences are projected in the sky in what is now a very familiar pattern, and that I need to show time and time again because it really is beauty:
What you’re seeing here is the whole sky, as seen in the microwave band (after having radiation from our own galaxy cleverly removed). Red patches are slightly hotter photons, blue patches are slightly colder photons. These patches tell you about the density distribution in the early Universe. It’s these fluctuations that seed the density fluctuations that give birth to stars and galaxies and you and I, but I’ll leave that to another time.
Right now, I want to focus on a small patch of these fluctuations. In you look at the bottom right corner rim, around 4:30pm if that map was a clock (oooohhhhhh!! That’s an idea! Can I have a CMB clock anyone?), you’ll find a cold patch that has been nick named The Cold Spot (one day I’ll write a rant about how Astronomers, being a rather clever and creative bunch of people, are rubbish at coming up with good names for things. mm.. maybe I just have.). Now, by eye the Cold Spot doesn’t look any different from other cold spots around the map. However, statistically, and given our model for the early Universe (which describes things pretty well, including the distribution of galaxies we see today), a spot of that shape, size and temperature has a very low probability to exist. ‘Very low’ here means something around 0.1% to 5%, depending on different estimates. This is slightly uncomfortable and Astronomers have spent a significant amount of time studying the Cold Spot.
There are a few options here:
1) The Cold Spot was formed at the last-scattering.
2) The Cold Spot was formed during the photon’s path to us.
3) The Cold Spot is an instrumental artifact.
4) The Cold Spot is a data-reduction artifact.
There are papers studying all of the above. 3) and 4) seem unlikely at this stage but should not yet be discarded. Having data from another experiment like Planck, with a whole new reduction pipeline, will help. 1) is fascinating because it could potentially mean there is something amiss from our early-Universe theory. But the paper that prompted this ridiculously long post actually focuses on 2).
Bremer et. al investigate the hypothesis that actually, the reason why we see a particularly cold spot in that region of the sky, is because there is a large void (a region of space with much less galaxies than the average) along that line of sight. The trick here is to note that the Universe is expanding – i.e., the shape and size of clusters and voids changes with time. The frequency (or temperature) of photons is affected by this change because the energy they loose or gain when they go into these structures is not completely recovered when they come out. There is a net effect, with is to gain a little bit of energy going through clusters, and loosing a little bit going through voids. There’s a little animation here that may make it clearer.
So Bremer and collaborators chose 5 regions of the sky, all inside the Cold Spot, and took redshift surveys in these 5 regions. This allowed them to see how the distribution of galaxies changed with distance between us and the region of last-scattering, and they looked for a deficit of galaxies that would be significant enough as to imprint the Cold Spot in the observable CMB. They did this by comparing these redshift distributions with those from other regions of the sky (outside the Cold Spot) and did not find any sign of such a deficit, or void. They could only look up to redshift of 1 (or around 7.8 billion years ago) because of instrumental limitations, and they didn’t have enough galaxies below redshift of 0.35 (around 3.8 billion years ago), but they covered a significant chunck of time when this effect is more likely to happen and did not find what they were looking for. What this means, is that they discarded at least some theories that could lead to point 2), although not all of them.
So the jury is still out on the Cold Spot. Personally, I’m sometimes tempted to add a 5th option:
5) The Cold Spot was formed at last-scattering, but its significance is being over-estimated.
But that, I’m afraid, is another post (I’m late for work!!).
M. N. Bremer, J. Silk, L. J. M. Davies, & M. D. Lehnert (2010). A redshift survey towards the CMB Cold Spot Submitted to MNRAS arXiv: 1004.1178v1
Earlier this year, on the 14th of May, the rocket Ariane V launched from the French Guiana with an astonishing precious cargo on board: the Hershel and the Planck satellites, two very ambitious European astronomy experiments, each with its own mission.
So you may remember that a while ago, Emma announced the release of the first images from Hershel, and last Thursday it was the turn of its sister mission’s first light to be set free to the public. Planck is out there to capture radiation which was created when the Universe was incredibly young, just a mere 340,000 years old or so – we call it the Cosmic Microwave Background radiation. If that sounds like a lot, remember that the Universe is around 13,500,000,000 years old today (give or take a few hundreds of thousands of years)! In human terms, it’s the equivalent of looking at a picture of myself 6 hours after my birth – prior to my first bath, even.
The difference between me and the Universe – or one of – is the fact that we can tell an awful lot about the content, geometry and evolution of the Universe from such an early on picture. Our understanding of Cosmology has in fact been shaped by a previous experiment that has been mapping the Cosmic Microwave Background since 2004 – NASA’s Wilkinson Microwave Anisotropy Probe. WMAP has answered many questions and raised tons more, both being the mark of a successful science experiment. Planck is like WMAP in its primary goal – which is to map the Cosmic Microwave Background with unprecedented detail and precision – but differs in just how well it can do it, which in turn broadens the scope of scientific questions it can answer. And of the ones it can raise!
So here you have it – a true, honest to heart picture of the Universe when it was less than 400,000 years old:
What you see is actually a combination of two pictures, so let me explain. The colourful strip that twists around the picture is the data that has come from Planck. The background, is an image of the full night sky, projected into two dimensions (like you would for a world map, pretty much) and with our own Milk Way going along the centre. The background is just there to give you a frame of reference – eventually Planck will map the whole sky, as that colourful strip extends to cover more and more of the sky. The reason why they look so different is because Planck is designed to pick up radiation in the microwave region of the electromagnetic spectrum, whereas the background picture is in visible light.
Planck will also look at each region of the sky multiple times, and each time it does it will improve the scientific value of the data. This is simply a preliminary picture, in the way of an example – but the data quality is excellent and all seems to be in place for a highly successful and smooth mission. We have to wait a while yet for the first science results to come out, but rest assured that we will cover them here on weareallinthegutter as they arrive.
These are good times, exciting times for science and cosmology – exciting times indeed!