Some 13.8 billion years ago, the universe began in a big bang – or, at least, that is what we think happened. Astrophysicist Jo Dunkley is at the forefront of efforts to work out exactly what took place in the immediate aftermath of that moment of cosmic creation. And a new telescope might just help her answer this question once and for all.
The issue with the big bang is that we can’t see it directly. The best we can do is look at the cosmic microwave background (CMB), often called the afterglow of the big bang. Faintly daubed across the whole sky, this radiation is all that is left of the first light that could travel in the universe. Subtle patterns in this light fit with the well-established idea that the big bang was followed by a period known as inflation, when the universe expanded at a rip-roaring pace. But it has never been proven.
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Dunkley, who is based at Princeton University, thinks that observing the CMB in finer detail than ever before will clinch the deal, specifically by helping us see patterns imprinted by gravitational waves from the dawn of time. To glimpse these, she plans to use the Simons Observatory, a purpose-built telescope in Chile that is on the cusp of switching on.
As she prepares for this opportunity, Dunkley spoke to Â鶹´«Ã½ about her hopes of finding new evidence of inflation, what gravitational waves could tell us about the early universe, and the other major mysteries in modern cosmology this observatory could help us resolve.
Jonathan O’Callaghan: Let’s start right at the beginning: what is the problem with understanding the dawn of time?
Jo Dunkley: We have this scenario that seems to make sense – after the big bang, the universe had an extremely rapid initial expansion, for a fraction of a second, that took a tiny, lumpy region of space and smoothed it out to produce the vast cosmos that we see now. We call that period inflation, but we don’t actually have definitive proof that this is truly what happened. If it did, what was the underlying physics like? What was around back then? It is by no means a done and dusted theory.
So the cosmic microwave background isn’t proof of inflation?
I wouldn’t say so. The CMB is this snapshot of the universe as it was when it was nearly 400,000 years old. We can extrapolate backwards and infer what could have happened to produce the patterns we see in the CMB. Certainly, inflation makes predictions that match the CMB. But there are other scenarios you could imagine that could produce those patterns. We’re not making any arguments that the CMB isn’t real – this is really an image of the early universe – but we’re still trying to find out if this is telling us what definitively happened in the first fraction of a second.
What are the alternatives to inflation?
There are models that propose the idea of bouncing cosmology, where you have a universe before the big bang that shrinks down to almost nothing, then expands again to form our current universe in a way that didn’t involve inflation. That could potentially be a more cyclic universe. There are loads of other flavours of inflation as well, where the main variable is the nature of the energy that drove it.
What is the current favoured model of inflation?
Before we had any of the particles we are familiar with now – protons, neutrons, light and so on – we think the universe was permeated by a different kind of energy. We call it the inflaton field, but we really don’t know exactly what it was. The energy stored in that field drove this exponentially fast growth of space at the beginning of time. It did so until the inflaton field decayed and we started forming the particles we know and the universe evolved into the form we have now. The very appealing property of inflation is that the seeds of all of the cosmic structures we see around us now, like the Milky Way and galaxy clusters, were put in during inflation as tiny quantum fluctuations.
Are the patterns we see in the CMB linked to these quantum fluctuations?
Yes, absolutely. We think these quantum fluctuations were put in at time zero, in other words right at the instant of the big bang. They would have then evolved and grown: gravity would have started to pull things together and made them larger. If you run the universe forward 400,000 years from the big bang, the prediction of what you would expect to see matches beautifully with what we see in the CMB.
If the CMB doesn’t yet prove that inflation is what really happened, can we get that proof somehow?
Yes, there are additional pieces of information that we’re trying to get out of the CMB with new data. My colleagues and I have been doing it with the Atacama Cosmology Telescope (ACT) in Chile and we’re about to do this more with the Simons Observatory. One of the key ideas is to do with gravitational waves – that is, ripples in the fabric of space-time – that could have been created by the super-quick inflation expansion.
These gravitational waves squeeze and stretch space in a particular way; they squeeze in one direction and stretch in another. That action should polarise the light of the CMB. If the light is unpolarised, you have the same amount of energy vibrating in all directions, but polarised light has a preferred direction. This is similar to what’s happening in polarising sunglasses, which cut out some of the light. So, we’re looking for this very, very faint polarised signal in the CMB that could only come from gravitational waves.
I guess we haven’t seen any sign of this so far. What are our chances of finding it soon?
The current best attempt to see this signal came from the BICEP-Keck telescopes at the South Pole, which measure the CMB polarisation – but no, those have not provided a bona fide detection so far. But we are very excited about the Simons Observatory switching on soon, as the anticipated sensitivity from these two programmes is comparable.
The trouble is, it’s a really tiny signal. One of the big challenges is that we have to look through the Milky Way to see out. Also, the signal is mixed up with light that’s produced in the galaxy from other sources. Water vapour in the atmosphere is a problem too, as it’s really hard to separate out the signal we want from the signature of water – even though we go to some of the driest places on Earth to do these measurements. When the Simons Observatory is working, we will have measurements from both Chile and the South Pole, where observing conditions are quite different, and that will help us tease out the signal we want from all the noise.
We’re looking for variations in what we can think of as the temperature of the CMB that are billionths of a fraction of a degree.
Jo Dunkley
What makes the Simons Observatory so good for potentially spotting this?
We’re looking for variations in what we can think of as the temperature of the CMB that are billionths of a fraction of a degree. The polarised signal is a very subtle departure from something uniform. The Simons Observatory will help because it has tens of thousands of detectors, which is 10 times more than previous generations of telescopes such as the ACT.
When will the observatory be operational?
It’s undergoing final testing right now, so we hope within the next few months. It’s actually a set of four telescopes all on the same site in Chile. The initial configuration is three smaller [0.4-metre] telescopes that are really targeting this gravitational wave signature. And then we have a large, 6-metre aperture telescope, and this will look at the CMB in higher resolution. That’s set to be operational in 2025.
What other scientific mysteries will these new telescopes help us tackle?
There are three big problems in cosmology. One is: did inflation happen, which we’ve talked about. Then there are the mysteries around dark matter and dark energy. Probing each of these three problems is a goal for the observatory.
In addition, there are two big possible tensions in cosmology today, both of which are related to the ongoing expansion of the universe. First, there is the sigma-8 tension, which is the observation that vast cosmic structures like clusters of galaxies may not have grown as big as we would have predicted based on the CMB and our simple model of the universe, called lambda-CDM. Then there is the Hubble tension. The CMB and lambda-CDM model suggest the universe should be growing at a slower rate today than some observations of local galaxies indicate. In other words, the universe may be growing too fast.
The larger telescope at the Simons Observatory should be able to weigh in on these two tensions related to the expansion of the universe. It should also make a better map of dark matter.
It is amazing that all this could be extracted from the faint glow of the CMB…
There’s so much richness of information in the CMB. I remember a few years ago people thinking we had measured the CMB, and that was it, done. Then we realised how much there is to know, not only in its polarisation, but as a backlight through the whole universe. It just keeps producing more exciting stuff.
Do you think we are on the cusp of solving some of these key problems in cosmology, like the Hubble tension?
I think we’ll know more in a few years. We’ll never know the answers to all our questions, but I hope we figure some out. I also hope that something unexpected happens. Usually in science when we have something we don’t understand, something we didn’t expect comes along to fix it.
These are big questions you are trying to answer. But why do they matter?
We want to know where we came from. How did we come to be here on Earth? To me, it’s this idea of trying to fill in the gaps of our big story of where we came from. It might not change our daily lives, but the knowledge of our longer history is enriching.
NewScientist Live
Hear more from Jo Dunkley at this year's Â鶹´«Ã½ Live. On 13 October, she will be describing the quest to understand the big bang and identify the fundamental ingredients of the universe, including elusive dark matter.
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