Nature Podcast：“失踪”的重子

又到了每周一次的 Nature Podcast 时间了！欢迎收听本周由Shamini Bundell  和 Nick Howe 带来的一周科学故事，本期播客片段里讨论了宇宙中的正常物质。欢迎前往iTunes或你喜欢的其他播客平台下载完整版，随时随地收听一周科研新鲜事。

音频文本：

Host: Shamini Bundell

Next up, reporter Adam Levy has been investigating a curious case.

Interviewer: Adam Levy

Today, I’d like to share a mystery of missing matter – missing baryons, to be precise. Whether you’ve heard of them or not, believe me, you have a very familiar relationship with baryons. The most famous are protons and neutrons, which make up the majority of visible matter in the Universe, including the majority of the Sun, the Earth and, yes, your very own body. But the question is, just how much baryonic matter – how much proton and neutron stuff – is out there in the Universe? Physicists know how much should exist in the Universe. They know this by studying properties of the Big Bang. You can get an answer by peering at the Big Bang’s afterglow, also known as the cosmic microwave background, as well as the relative amount of the lightest atoms created by the Big Bang. But when you try to directly measure baryons by actually working out how much you can see in the Universe, things don’t seem to add up.

Interviewee: Jean-Pierre Macquart

So, roughly half has been missing, which is a bit of an embarrassment.

Interviewer: Adam Levy

This is astrophysicist Jean-Pierre Macquart of Curtin University in Perth, Australia. So, we know how many baryons – in other words, protons and neutrons – should be in the Universe based on the Big Bang, but when we look, we can’t find a hefty chunk of it. Well, Jean-Pierre has been hunting for the hidden stuff using fast radio bursts as a probe. I gave him a call and asked for a quick refresher on what fast radio bursts actually are.

Interviewee: Jean-Pierre Macquart

Well, no one really knows exactly what they are, but all we care about for the purposes of this kind of physics is that they’re very bright lights and they’re very impulsive. So, they last about a millisecond and they’re so bright that we can detect them over the other side of the Universe. We can use them as like a cosmological Swiss Army knife. They’re so impulsive – their radiation – that it’s highly susceptible to effects that occur when they travel through this cosmic gas, even when it’s very diffuse, and that process is dispersion. It’s the same kind of process that occurs when you shine sunlight through a prism and it disperses the radiation out in towards different colours of the spectrum. So, as it travels through this intergalactic gas, the longer wavelengths, the redder wavelengths get delayed more than the bluer wavelengths. So, when you actually observe this with a radio telescope, you don’t find that the pulse all arrives at once. It actually dribbles in.

Interviewer: Adam Levy

So, you’re looking at these fast radio bursts to see how they disperse. I mean how difficult of a task is that to actually do, to firstly, track them down and to get that data in sufficient detail to get answer?

Interviewee: Jean-Pierre Macquart

It’s been relatively easy to detect the dispersion that’s associated with the bursts themselves. What’s been the stumbling block is getting the precise position of these things to actually point at an optical telescope and go and measure the red shift and hence the distance to that thing, which you need to do if you want to figure out the density of this baryonic matter in the Universe. So, that’s been the key problem.

Interviewer: Adam Levy

And how did you overcome that issue so you could actually not only see these fast radio bursts but understand how far away they are?

Interviewee: Jean-Pierre Macquart

Well, this involves technology on the Australian SKA Pathfinder, which enable us to see 30 square degrees of the sky all at once, so it means you can capture these fast radio bursts in sufficient numbers. And so, once you’ve detected one of these bursts, you have a buffer inside the telescope and you say, ‘Aha, we’ve detected the burst, now please dump all of the high-resolution data from that burst’, and that enables us to go back after the fact and to triangulate on the position on the precisely.

Interviewer: Adam Levy

Now that you have this data and you crunched the numbers, what answer do you actually get?

Interviewee: Jean-Pierre Macquart

Well, it’s both a relief and exciting that the number we get is actually very close to the number that you expect for the density of baryonic matter in the Universe.

Interviewer: Adam Levy

Were you expecting this in your heart of hearts? Were you expecting that you’d get the same answer predicted by the theory?

Interviewee: Jean-Pierre Macquart

I’ve learnt not to expect anything in science, but what was more surprising to me was that these fast radio bursts are much better cosmological probes than we had dared to think, and that was the real surprise.

Interviewer: Adam Levy

Is this now case closed for the missing protons and neutrons in the Universe or do you think this result will be at all controversial for the community?

Interviewee: Jean-Pierre Macquart

It won’t be controversial but it is but the beginning because we’ve said, ‘Okay, now we know that those baryons are there, we can account for them.’ What we haven’t done is say where they are, so are they distributed completely diffusely throughout the Universe or do they hang around large groups of galaxies? Now that we know the gas is there, where exactly is it and what’s it doing? And this is critical because you want to know how all of these galaxies that you see about you in the Universe are forming. There are violent processes in those galaxy star formations, black hole spewing jets out, and they’re throwing that baryonic matter back out into intergalactic space, whereas there’s also cool baryonic matter raining down on these galaxies to form next generational stars. So, it’s a whole ecosystem here, and we have many more puzzles to resolve from this point onwards, but now we’ve got a tool to do it.

Host: Shamini Bundell

That was Jean-Pierre Macquart of Curtin University in Australia. We’ll put a link to his paper in the show notes.

《自然》论文：A census of baryons in the Universe from localized fast radio bursts

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