Joseph Silk, astronomer
If it is finite, what is its size?
Big bangs on Earth, and the 'dark energy' problem
"We may never know whether the Universe is finite or infinite" - an interview with Joseph Silk, astronomer
02-May-2001
Joseph Silk, a 58 year old British astronomer, currently works at the University of Oxford (United Kingdom). His curriculum vitae includes positions in some of the most prestigious universities in the world, as well as many awards. Most of his scientific research is related to the Cosmic Microwave Background and cosmology in general. He is author or co-author of more than 300 papers in refereed journals, as well as of many popular articles and books such as "The Left Hand of Creation", "Cosmic Enigmas" and "A Short History of the Universe". In this interview he clarifies basic concepts such as the concept of 'flatness', and explains why we may never get to know whether the Universe is finite or infinite.
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British astronomer Joseph Silk | |
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Q: The term Big Bang suggests that the Universe began with an explosion. But cosmologists often reject the concept of an explosion, and even the word itself. Why?
A: Cosmologists don't like the term explosion because it conveys the idea of sound, like a 'bang!', and it doesn't make any sense to think of that. But apart from that the word explosion is valid. I think that the simplest description of how the Universe originated is an explosion, in the sense that it began from a very small volume and increased very rapidly. That is usually what you mean by explosion.
Q: Is the Universe finite or infinite?
A: We don't know. The expanding Universe theory says that the Universe could expand forever [that corresponds to a 'flat' Universe]. And that is probably the model of the Universe that we feel closest to now. But it could also be finite, because it could be that the Universe has a very large volume now, but finite, and that that volume will increase, so only in the infinite future will it actually be infinite.
Q: It sounds like a game of words.
A: No. We do not know whether the Universe is finite or not. To give you an example, imagine the geometry of the Universe in two dimensions as a plane. It is flat, and a plane is normally infinite. But you can take a sheet of paper [an 'infinite' sheet of paper] and you can roll it up and make a cylinder, and you can roll the cylinder again and make a torus [a torus is like the shape of a doughnut]. The surface of the torus is also spatially flat, but it is finite. So you have two possibilities for a flat Universe: one infinite, like a plane, and one finite, like a torus, which is also flat.
The meaning of 'flat'
Q: For you physicists the term 'flat' seems to have a different meaning than for non-scientists. By 'flat' most people understand a flat surface, like a table, which necessarily has a certain width... But the Universe doesn't have a 'width', does it?
A: Flat is just a two-dimensional analogy. What we mean is that the Universe is 'Euclidean', meaning that parallel lines always run parallel, and that the angles of a triangle add up to 180o. Now, the two-dimensional equivalent to that is a plane, an infinite sheet of paper. On the surface of that plane you can draw parallel lines that will never meet. A curved geometry would be a sphere. If you draw parallel lines on a sphere, these lines will meet at a certain point, and if you draw a triangle its angles add up more than 180o. So the surface of the sphere is not flat. It's a finite space but it's not flat, while the surface of a torus is a flat space.
Q: Planck will measure the Cosmic Microwave Background (CMB), the first light that filled the Universe after the Big Bang, with the highest accuracy ever achieved. That light carries information about the geometry of the Universe, so it will soon be possible to confirm whether the Universe is 'flat' or not. What you say is that even with that knowledge in hand it won't be possible to find out whether the Universe is finite or infinite, right?
A: Even if with our Cosmic Microwave Background data we can prove that the Universe is flat, we still won't know whether it's finite or infinite.
If it is finite, what is its size?
Q: Then, how are we going to know whether the Universe is infinite?
A: With great difficulty. We may never know it. If the Universe is finite, that means that in a two-dimensional geometry it would be like a torus. Now, think about a torus. In such a Universe, light travelling on the surface of a torus can take two paths: it can go around the sides of the tube [J. Silk draws a spiral as he talks], but it can also go in a straight line. This mean that if the Universe is like a torus, light can have different ways to get to the same point. You can have a long way and a short way. And that would not be true on a plane. But a torus means that space is more complicated. It would mean that when you measure the CMB you will see strange patterns on the sky, because the light from far away would not have come to us in quite a straight line because of the topology of the Universe. So the hope would be, eventually, to look for those strange patterns on the sky.
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Q: Will Planck be able to see those patterns?
A: In principle, yes. If the Universe is like a torus you can see something. If the Universe were finite it would be 100 times larger than the horizon, which is the distance the light has travelled since the Big Bang. That would correspond to the size of the `doughnut' of the torus. We could in principle be able to measure that with Planck. On the other hand, if the Universe was truly infinite then we would see no signal at all from this peculiar thing. What we could really say in that case is that the Universe is larger than a certain size. But if it was finite it could be measurable.
Q: What would the size of the Universe be if it was finite?
A: It could be as large as 100 times the horizon. That means that the Universe would be as much as a 100 thousand million parsecs [about 300 thousand million light years] if we could measure the topology.
Q: How do you make that estimation?
A: Because if it was much larger the signal from the torus would be so weak that you would never see it. Only the fact that you can measure it imposes a limit.
Q: Why don't cosmologists talk very much about the size of the Universe? It's not a frequent topic in the workshops.
A: Because it is something quite new.
Q: But it is a simple question...
A: Yes.
Before everything started
Q: Many cosmologists today seem to agree with the idea that the Big Bang started with an 'inflation', a short period during which the Universe expanded enormously at very high speed. But what happened even before that period?
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A: Maybe long before inflation there was a Universe that was collapsing near a singularity, which then inflated again... so there was already a history before the Big Bang. Some people think there was a 'pre-Big Bang'.
Q: How can that be tested?
A: One possibility is that the pre-Big Bang, if there was such a place, would have made lots of entropy [entropy is used to describe how disordered the Universe is]. And the Universe we live in does have huge amounts of entropy. That's one theory.
Q: What do you think?
A: Maybe.
Q: Some people think that those theories are outside our observational world...
A: That's because we have no understanding of how to change from collapsing to expanding. There's no physical way to explain that transition. Some people believe that they have predictions of just this, of the pre-Big Bang. So it's a respectable theory.
Big bangs on Earth, and the 'dark energy' problem
Q: A different topic. It is often said that particle accelerators reproduce the conditions at the Big Bang. Is it correct?
A: Particle physicists are trying to reproduce not the conditions at the Big Bang, but a fraction of a second later. That is a lot, a big difference.
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Q: One last question. One of the greatest problems in cosmology now seems to be the fact that the Universe is accelerating, possibly due to the action of a (not yet measured) 'dark energy' - some call it 'quintessence'. What do you think about it?
A: I am not going to work on this. I hope it [the 'dark energy'] will go away. It has come and gone over the years. It all remains to be confirmed.
Q: But the acceleration of the Universe is a result of independent observations, like those by the Supernova Project...
A: Yes, many people believe it... But we don't understand yet how a supernova explodes.
This interview with Joseph Silk was conducted in January 2001 at ESA's European Space Research and Technology Centre (ESTEC) in Noordwijk, the Netherlands, during an international workshop to discuss the scientific programme of the Planck satellite.
About Planck
The Planck satellite, one of ESA's main missions for this decade, has been designed to help answer key questions for humankind: how the Universe came to be and how it will evolve. To fulfil its mission Planck will examine with unprecedented sensitivity and angular resolution, the small anisotropies of the first light that filled the Universe after the Big Bang, the so-called Cosmic Microwave Background radiation. It will carry two arrays of highly sensitive detectors that are now being built by more than 40 institutes, most of them European and some from the United States. Planck will be launched in 2007 together with ESA's far-infrared space telescope, Herschel. They will separate shortly after launch and will be operated independently from different orbits around the second Lagrangian Point located about 1.5 million kilometres away from Earth.
Contact Person
Jan Tauber, Planck project scientist
Tel: +31 71 565 5342
E-mail: Jan.Tauber
esa.int