What has physics got to say about time?

We can debate the matter endless within the realms of philosophy but what do current cosmological observations support?
25 January 2016

Interview with 

Dr Robertto Trotta, Imperial College London


In the quest to understand whether there was indeed a beginning of time, or not,Questions in the stars Graihagh Jackson needed to find out more about which theory match the current cosmological observations support. Roberto Trotta took Graihagh on a tour of the universe with lasagne...

Roberto - We know the universe is expanding with time because if we look around us with a telescope and we look at distance galaxies, we see most of those distant galaxies moving away from us because the very fabric of space time is being stretched in all directions at the same time.  But then, if this is the case now, if we wind back that movie and we look at it going backwards in time, then you'll see that all galaxies, if we go back in time, were closes and closer together in the past.  And if we go back further enough, all galaxies were heaped up in one point, and that point we call the big bang, time zero, a singularity.

Graihagh - So were there any theories that might suggest what was before this singularity or how this singularity came to be?

Roberto - Speculative theories, yes.  There are different ways of thinking about how the big bang came about.  One of the possibilities is that the big bang wasn't actually a big bang but merrily was the bouncing off of a previous phase in the history of the universe.  So, instead of thinking of time beginning with the big bang, we can think that it was more of the same.  A previous universe would be re-collapsing in to a big crunch, which is sort of the reverse of a big bang, and then when it gets to be close to singularity, it would bounce back out again and that we would experience as our own big bang.  So, this is a picture of a cyclic universe that forever bounces.  This picture is actually in doubt today because we now know that our universe is not going to re-collapse in the future, it is actually going to expand forever at an ever accelerating rate that's...

Graihagh - How do we know that?  Sorry.  How do we know that our universe is going to re-collapse?

Roberto - That's because of dark energy.  70% of our universe is made of an unknown type of force, or energy that we call dark energy whose main impact on the universe is to make the expansion of the universe accelerate with time.  So, not only the universe is growing with time today, it's growing at an ever accelerating speed and will not re-collapse.  There will not be a big crunch that will end our universe.  In the far future, the end of our universe will be a state of darkness where all the galaxies will have turned off, all the matter will have been sucked into black holes, and the universe will be a very cold, very dark, very boring place.

Graihagh - A slightly scary space.  It sounds like something out of a scifi movie.

Roberto - Yes, but it's about 20 billion years in the futures, so there is nothing to worry about for our immediate futures.

Graihagh - So that theory's in doubt.  So what other theories might there be that have more support?

Roberto - Some ideas are rather different.  They postulate the existence of parallel universes effectively.  So, our universe is made of three space dimensions and one time dimension - so it's a four dimensional universe that we experience, but what if there were additional dimensions that we cannot actually penetrate ourselves but that exist perhaps just a fraction of a millimetre away from our reality.  A very scifi concept that we can actually make a little bit more visible and a little bit more tangible with a simple analogy using some lasagne, actually.  So why don't we go over and put the lasagne in the oven and then we'll have a little bit of an analogy, and a tasty one to the early universe to what the speculative parallel universes might look like.

Graihagh - Great I'm hungry already.

Roberto - Let's give our parallel universes four minutes.

Graihagh - So while this is quickly heating up.  This is to demonstrate the various parallel universes.  You've got layers of vegetable - because this is a vegetarian lasagne I notice - and then you've got the layers of pasta and cheese.  So what signifies what here?

Roberto - Okay.  So we have a lasagne exactly made of layers of pasta and the layers of pasta are going to symbolise and represent different parallel universes, if you like.  So, our own universe will be one of the layers and then separated by something, which in this case is the vegetable filling, you'll have other layers which are just universes just like us but they are separate from us across a fifth dimensions and that's the dimension where the stuffing presides - cosmologists would call it the bulk.  And then you have all these parallel universes stuck on top of each other - and we'll call the parallel universes which are the lasagne layers, the pasta layers, we call them brains.

Graihagh - Ahh, okay, okay, I see, I see. So when this comes out - I'm quite intrigued - what are you going to do with it?

Roberto - We're just going to look at it and we're just going to try to make this big bang happen as mashing together those two parallel universes and see what happens.

Graihagh - So we're going to make an explosion with the lasagne?

Roberto - Yes, a messy one, I'm afraid. Ahh, here we go - they are ready.  So now we are going to cut the lasagne in the middle to reveal nicely our parallel universes.  Yes we can see there are about four or five parallel universes and so I'm just picking out the top layer of the lasagne, so this brain here is going to be our universe.  If a peel back the top layer of the lasagne I'm going to reveal another parallel universe - just below it that's our other lasagne pasta sheet.  And now the idea is that if we have gravity leaking through the two lasagne layers, the two pasta layers will be mashed together they will be attracted on to the other and when they hit, like this, they're going to splash all the sauce out and this is going to be the big bang effectively.  We describe it as the birth of our university, but in reality what it is just two pasta layers in the high dimensional universe smashing together.

Graihagh - So is that how each parallel universe could be created?  This sort of seeping of gravity through and and that means there could be, what, infinite big bangs for equal parallel universes?

Roberto - That's right, some people describe them as a gas of brains.  So as many brain universes as atoms in a room.

Graihagh - And these other universes, are they like ours?

Roberto - We don't know - that's a very good question.  They could be very different from our own universe.  They could have different laws of physics in them or different properties.  Certainly that's a matter of speculation and there are some ideas that actually say those other universes ought to be very different from ours.

Graihagh - Is there any way that we'd ever be able to tell or is there any evidence that we might be able to gather that might support this theory?

Roberto - Well, I have another little experiment prepared in the kitchen that might actually help us to understand how we might be able to tell if not of existence of parallel universes of this kind, existence of other universes that have potentially mashed into our own universe.

So chocolate balls; one of them is going to be our universe and another one is going to represent another universe.  Now, what we are going to do - I've got some double cream here... I'm going to take the spherical balls and dip them...

Graihagh - Not drop them...

Roberto - Not drop them... Somehow I was under the impression that the double cream would make them float...

Graihagh - I think you might have to whisk it together first.

Roberto - Well yes.  Very good point... Perfect.  Now I've coated the two universes in cream - it's all white.  So this coat of white cream now represents the cosmic microarray background.  That is to say the relic radiation of the big bang.  That's a luminous echo of the big bang itself that we can pick up with our telescopes and satellites and we can observe, today, on Earth 13.7 billion years later.  I also sprinkle them with powder chocolate, and the chocolate powder here represents the little irregularities in the relic radiation from the big bang coming from quantum fluctuations at the singularity itself.  Now I'm going to now carefully smash those universes together and so, you can see here on the side where they have touched, there is a little bit of a spot in the cream that's been left over....

Graihagh - Oh yes, just like a lighter patch on those...

Roberto - Yes, that's right, that's right - again here.

Graihagh - Where the sort of chocolate powders rubbed off, you've got the cream underneath and that's what's left, right?

Roberto - Yes, that's right.  Exactly right.  And this is a sort of a bruise that's left over from the earlier collision of those different universes.  Later on, we can still potentially pick up these bruises that has been generated by the collision very early on near the singularity, and if we can do so, then perhaps, just perhaps, we can find evidence that that collision happened in the very beginning of time.

Graihagh - How might we be able to detect that tiny little white spot of cream?

Roberto - That's a challenging task because, of course, there are many, many other effects that will imprints different kinds of spots and spot patterns onto this luminous echo, so it's a statistical problem effectively.  It's about analysing those spots that we see in the sky looking for the potential bruise left over by a collision early on but, of course, we don't know exactly what we're looking for.  We don't know how many bruises there are, what size they are, which position they are in the sky.  All of those things are completely unknown so it's really like searching for needle in a haystack.  We know what a signature could look like but then there are many different possibilities for it so we have to look for all of them in a statistical fashion and see if something comes up, and in this case, it could be a hint that something like that has happened in the past.

Graihagh - I'm picking up from you that perhaps then the favoured theory might be the parallel universe scenario because it's actually something that we can potentially test and see?

Roberto - In terms of testability it's promising in that if it leaves signatures that we can pick up, even just in principle, maybe not today, not with today's technology but if it can be proved in principle, then it becomes a testable theory and then it becomes an interesting one in the sense that we can actually go out and see if we can either reject it or possibly confirm it.  I think it's actually quite impressive to be able to say that we can now reconstruct with a high degree of fidelity the history of the universe from today 13.7 billion years after the big bang to the very beginning - some 10 to the minus 32 second after the big bang with very high accuracy.  So all that remains to be tested is the tiny sliver of time - 10 to the minus 32 seconds after the big bang.  And so, in a sense, the job of scientists at the forefront of research is always that of pushing back the limit of the unknown and so now we are really hitting, the very hard, very deep, very fundamental questions.


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