The Heat History of the Universe
Ben - Climatologists and geologists have developed a number of ways to estimate the temperature on Earth going back a very long way on Earth's history but astronomers would like to know the historic temperature of the entire universe, going back 13 billion years. George Becker, a fellow at the Kavli Institute for Cosmology uses the light from quasars to read off the climatological history of the universe.
George - The broad question is really broad and it's how did galaxies form in the early universe and then how did these other things sort of materialise out of the wash of gas and dark matter that we live in. What fills up most of the universe is this very, very thin network of dark matter and gas that we call the intergalactic medium. If you were able to see it, it would look like a cobweb or like a sponge. And what we believe happens is that over time, the small variations that were present right after the Big Bang gave rise, over time, by the way they collapsed through gravity, to this network of material and within that network, galaxies. So, this is a system that comprises all of the matter of the universe and out of which galaxies form. And it's dynamic, it changes with time. One of the things that happens is that it goes from being electronically neutral to being ionised. Neutral means that it's similar to the gas that's in the air in this room. The gas is made up of atoms and those atoms have electrons and the electrons are attached. But as you go out into space that changes and especially as you go out into the regions between galaxies, where if you find an atom, chances are very good, it won't have its electron attached. It would've been knocked off a long time ago by some energetic photon. And that's the way the universe is today. It's spread out in this network of material that is very, very highly ionised. And the question I'm addressing is, how did it become that way? How did the ionisation happen?
Ben - I guess that because we know that in order to become ionised, they'd have to interact with photons at some point, we can sort of use the ions themselves to get an idea of the history of the universe.
George - Yes, that's exactly right. If you knew how this ionisation process happened then you would know when it was for example that galaxies had gotten far enough along, enough had formed to produce enough photons to do the job. Similarly, quasars are also important in this mix because it turns out that all this gas that's out there is primarily one of two elements. It's either hydrogen and hydrogen is far and away the more abundant part of this gas, and then there's also helium. We believe that galaxies are responsible for ionising the hydrogen, but it takes quasars to ionise the helium and the reason for that is that helium atoms have two protons and they're just a little bit better at holding on to their electrons, and you need higher energy photons with enough oomph to knock those electrons off. So we believe that it was quasars that ionised helium and that's actually the stage of this ionisation process that I was looking at.
Ben - I assume we can't directly observe these fairly rare ions themselves and instead we have to use light coming from distant sources. In your case you're looking at quasars but how can you actually do that? What observations do you need to make?
George - That's right. The quasars are playing two roles here. Number one, they're providing the photons that we think is important for the physics and number two, they're serving as background sources of light. So what we do is we look at these quasars and the light from the quasars has travelled through the intergalactic medium - all this gas - and the gas has absorbed part of the light from those objects. In fact, every time the light goes through a cloud of gas, it loses a few photons. And so, by the time it reaches us, it reads like ticker tape or like a bar code in the information about where the gas is, and what it's made of is all encoded in the quasar light itself.
Ben - That must be quite a difficult task. For example, how do you tell the difference between a cloud of gas that's a very long way away but very large or one that's quite close to you but quite small?
George - The inverse does us a great favour here by expanding. When the photons were emitted, that was a long time ago, and the universe was much, much more crowded in the sense that any two points we're just a lot closer together than they are today. So you have to imagine, the quasar gives off some photons and those photons begin to travel towards us, and along the way, the universe expands and expands. The photons also expand. Only when the photon is a very special length or a very special frequency that they can be absorbed that they will come to a cloud of gas, and get absorbed by it. The expansion of the universe stretches out the quasar light and allows us to read off by the wavelength that we've observed from the ground where it was or rather by how much the universe had stretched out by the time the photon had been absorbed. It all sounds a little bit convoluted but the end result is that we get the light from the quasar and there are series of absorption features that we can read off, and the redder an absorption feature is, the further back it was absorbed.
Ben - So using this technique, reading the quasar barcode as it were, what have you been able to learn about the universe?
George - Yeah. There is an interesting climatological history to the universe and you can start way back at the Big Bang where it had been extremely hot. We're talking millions of degrees but shortly after the Big Bang, things actually do get quite cold. The universe will get down to just a few tens or maybe few hundreds of Kelvin. Then when the first galaxies light up, we don't know because we haven't been able to measure temperatures that far back, but what we believe is that when galaxies light up and the re-ionisation of hydrogen happens, that you'll get somewhere up to the range of say, 20,000 Kelvin. It might be considerably higher or a bit lower. Then the universe will start to cool down again until this helium re-ionisation that we were looking at occurs. And there, the theoretical expectation was that you'd get a boost again of say, another 10,000 Kelvin. So you might have cooled down the 10,000 Kelvin - which sounds hot, it's hotter than the surface of the Sun - and up to say, 20,000 Kelvin, and after that the universe starts to cool down again because it doesn't have this source of heating any longer, and then sort of strange things happen in the very recent past where these sheets of gas that are collapsing start to shock and heat in very interesting ways, but that's a different process.
Ben - So just looking at this ionised gas, you can really infer quite a lot about galaxy formation, about galaxy progression, and not just about the history of the gas itself.
George - We're right at the beginning of being able to make those kinds of inferences. This process with helium really involves the quasars - the quasars live in galaxies and we know that the formation of quasars which are black holes that are swallowing up the insides of galaxies and giving off tremendous amounts of radiation, those black holes are intimately connected with the development of the galaxies. They reside in it somehow. So putting together how quasars form, how black holes form, how galaxies interact with their surroundings, what we call the intergalactic medium, it's all part of getting the complete picture right.
Ben - George Becker from the Kavli Institute for Cosmology here in Cambridge, explaining how understanding re-ionisation can help to fill the gaps in the history of the universe.