What Lit Up The Universe?

Scientists using quasars - massive black holes - to understand the origin of light in our universe
01 September 2014

Interview with 

Andrew Pontzen, University College London


You might think that space is pretty dark, but in fact it's bathed with light from all The first stars in the Universe turn on at about 400 million years after the Big Bang.kinds of sources - from stars like our sun and other more exotic celestial objects such as quasars. Quasars are distant objects powered by black holes a billion times as massive as our sun. These powerful dynamos have fascinated and perplexed astronomers since their discovery half a century ago. Now, a team of space scientists, led by researchers at University College London, have published a paper predicting how to study this light from objects billions of light years away, giving us a view back in time to the early universe. Kat Arney spoke to lead researcher cosmologist Dr Andrew Pontzen to find out more about the work, and why it's so important to map out the light in the universe.

Andrew -   What we're trying to understand is the way that light is distributed through the universe, which sounds like kind of odd thing to be a problem because of course, we use light all the time. To learn about the universe, we see light, that's what we're picking up with our telescopes when we look at the universe.  But this whole other side of light which is, that it's transporting a lot of energy around in the universe, in the same way say, the biosphere here on Earth is powered by light coming from the sun.  The universe is kind of powered by light generated in stars, in galaxies, in more exotic things like quasars.  So the question is really, how much of that energy is being generated in total and where is it coming from because it's not necessarily stuff we can see directly.  When we start trying to find out what's lighting up the universe, we are measuring it in a limited patch.  Although, when I say limited, we are talking about millions of light years across.  So, these are extremely large scales nonetheless.

Kat -   So basically, in this study, what were you trying to do?

Andrew -   We're trying to make predictions or we call them 'forecasts' for what we're going to learn from future studies and in particular, from one particular study called the dark energy spectroscopic instrument or DESI for short, which will come online in, we think, 2018.  And we're making predictions for what that can tell us about what's generating the energy or most light in the universe.  What this telescope is actually going to be measuring is light coming from quasars.  They're incredibly bright sources and they're actually made by gas sort of spiralling into a black hole.  And though you might think a black hole is dark, it's as the gas is falling in, that it generates a lot of light.  And then we're using that light to tell us something about what's in the universe, what's in between that very bright, actually, very distant light and us.  That kind of changes the light as the light comes through the universe.  It picks up information about what's in between.

Kat -   It's almost like seeing a very, very distant lighthouse and looking at the shadows and the things that are in the way in between that and you.

Andrew -   It's exactly like that, yeah.

Kat -   And I guess, does that tell you a bit about the sorts of things you should be looking out for when you do get the data from this telescope.

Andrew -   Yes.  It tells us about how should we process the data coming from this telescope to really get everything.  One of the big things that's changing in astronomy is, it's no longer about one person going up a mountain and taking a picture of one object.  We now get catalogues of millions of objects and then we have to work out how do you piece together all that massive amount of data.  What this work is really about, is showing if you process that data in the right way then you're going to get this handle on what's generating all the energy in the universe.

Kat -   So, what do we know about some of the light sources in the universe and what are you trying to find out about them?

Andrew -   Well, we know about the brightest light sources in the universe.  If you're flying on a clear night and you look down at the Earth below then what will immediately be quite obvious to you is, if there's a big city underneath, you'll see a bright glow coming from that city.  But it might be that actually, there are a lot of towns surrounding the city or reaching far out into the countryside which themselves, they only have a few lights, only a few streetlights per town.  So, they appear much dimmer than the city does.  But on the other hand, there are many, many more towns than there are cities.  So, if you're able to get a really good bird's eye view of this, you might be able to tell actually there's more light in total being generated by these many, many tiny towns compared to maybe just one big city.

Kat -   And if light is energy then that could actually be a much more important energy source in the universe.

Andrew -   Exactly.  So, the universe doesn't care where its light is coming from and it may be that these tiny towns or, as they actually are in the universe, very small galaxies are generating huge amounts of energy.  We just don't see it directly because we tend to just pick out the big cities, or in the universe, it's known as quasars or really large galaxies.

Kat -   Does it kind of blow your mind, thinking that you're studying something that is so far away?

Andrew -   Yeah.  When you're researching this stuff, you tend to kind of pull these two things apart a bit because it turns out, you can analyse all this with maths and it all goes into bits and numbers on scraps of paper, on computers and so on.  But then occasionally, you do just stop and think, "Wow!  We're talking about - as you're saying - thousands of millions of light years across."  Those really are just massive scales and it's amazing that we have technology now that can actually start to tell us about those kind of scales.

Kat -   When you look up at the night sky, you're thinking, out there somewhere is a quasar that they can look at with my telescope.

Andrew -   Absolutely.  In fact, if anything, you look up at the night sky now and you think, "This is all small fry."  I mean, the universe is so much bigger than what we can see unaided.

Dave -   Kat Arney, speaking to Dr. Andrew Pontzen from the University College London.


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