ABSTRACT
Where did our universe come from? When and how did it begin? What was there before it existed? What lies beyond it? Is our universe eternal, or does it have an end? How can we even begin to grapple with such questions? The cosmologist does! - and comes up with some astonishing answers. I would like to share some of these answers, using illustrations from art, and analogies from everyday objects.
A JOURNEY THROUGH SPACE AND TIME: A Brief History of Our Universe
Prof. Suketu Bhavsar
Department of Physics & Astronomy
University of Kentucky
Inquiry into our origins involve among the most fascinating questions that we as humans grapplewith. We yearn to know where we came from and how all this began. Or ancestors and those before them, from time immemorial have wondered about the universe and our place in it. This is evident from paintings and artifacts that we find today - as an example Stonehenge.
In the last 500 years, beginning with Copernicus, we have begun to understand our place in this vast universe; and in the last 50 years or so we have begun to understand the universe itself, how it began and even how it may end.
What do we mean when we say "universe"? There is a very simple answer to this question; it is everything there is! Since it includes EVERYTHING, every physical thing you can think of is in the universe. You may ask, but what is beyond it? It turns out, there is NO beyond. This may be worrisome to some of you and I will come back to this question. Let us first put ourselves in the context of the rest of the universe.
The universe is immense. Now big numbers don't mean anything unless we can visualize them. So, let us do just that. Let me give you a scaled down model of our galaxy which you can visualize.
The Sun --- 1 mm (a grain of sand)
Earth's orbit --- 10 inches
Pluto --- 30 feet
This is our solar system.....what about the nearest star?
Alpha Centauri --- 50 miles
Center of our galaxy - 10 times the distance to the Moon
Instead of a scaled model, astronomers often talk about distances in light travel time. You may have heard the term light year. It is not a time but a distance, the distance light travels in one year - which is enormous. This provides a convenient way to grasp the immensity of the universe. Light travels faster than anything else. A beam of light could go 7 1/2 times around the world in one second. We can ask how long it would take light to reach us from various objects, traveling at this fantastic speed:
slide 1
slide 2 (scales of distance)
We are seeing these objects as they were, this long ago! We are seeing the past. The photons left these objects that long ago.
Now, I had asked "what our universe is" and answered "everything". This includes many many many things, too many. Let me ask another question. What is in this lecture hall? You will say people, chairs, carpet walls.....but we could go on and on because the chair is made of many things and so on down to describing the atoms. This is really unnecessary to describe this hall. In describing anything we must pick the appropriate "building blocks" and leave out the details.
What is the primary building block that is of consequence for the universe as a whole? It turns out that galaxies are the building blocks of our universe. We can forget about details at smaller scales. Not to say that these details aren't interesting, but they are of very little consequence in understanding the grand design. Let us look at these building blocks in more detail.
Here is the most famous galaxy of all...our galaxy the milky way
Tucson, Phoenix, Tree, Jupiter, Andromeda - nearest galaxy
When we look at Andromeda through telescopes we see this (M31) spectacular view
There are other galaxies too. We didn't always know this. Objects in our galaxy and other galaxies were just fuzzy patches in the sky. Now with large telescopes we can tell the galaxies from the non galaxies. Here are two objects that are not galaxies, (crab nebula), (Eskimo nebula)
But as I mentioned there are other galaxies too, like ours and Andromeda. In fact there are 100 billion other galaxies in the visible universe....that's a lot of building blocks.
Galaxies have about 100 billion stars, that's how many grains of sand would fill this hall. Since there are 30 million sec in a year, it would take a life time to count to a billion. If someone started counting the stars in our galaxy at the time of Christ, they would just be finishing..... and there are 100 billion such galaxies. There are more stars in the visible universe than all the grains of sand on all the beaches of the world. - The galaxies then are the building blocks. The galaxies are not randomly distributed, the occur in groups, clusters, and the universe is rich in structure. (Virgo cluster) (Hercules cluster) (SW cat.)
All that we know about the universe, we know because light has traveled that immense distance from those galaxies to us. There are photons that have traveled for millions and even billions of years, to finally end up on the retina of our eye, or on the emulsion of a photographic plate.
The astronomer has devised clever ways to learn from the light that comes to us from these places. One very important technique is called spectroscopy. Here is what happens when light passes through a prism
Light is a wave, waves have characteristic wavelengths, and in the case of light each different wavelength corresponds to a different color of light. Red is long wavelength light and blue is short. The other colors are in between. Light from a star, and therefore a galaxy, has certain wavelengths missing from the spectrum. This is called an absorption spectrum. In fact a spectrum also helps tell us what materials the stars are made up of.
You all know a particular property that waves possess, when the source of the wave is moving towards or away from us. The wavelength gets modified. It has a name (but that is unimportant, except to give proper credit), the Doppler effect. Waves stretch out or get squeezed together, which changes their wavelength. Stretching out is a shift towards longer wavelengths and being squeezed is a shift that towards shorter wavelengths. For light, when the source moves away the spectrum is red shifted, and when the source moves towards us it is blue shifted.
What was seen when the spectra of galaxies were taken and analyzed? Let me show you some actual spectra. The spectra of galaxies showed a remarkable feature, except for the nearest few, all galaxies show red-shifted spectra! This means all these galaxies are moving away from us. Also, the farther galaxies have larger redshifts! Which means, the further away a galaxy is from us the faster it is moving away. This expansion of galaxies, away from us was pointed out by one of the most famous American astronomers, a Kentuckian, Edwin Hubble. Hubble went further, he pointed out that when the velocities are plotted on one axis and the distances to the galaxies on the other a relationship is seen. Such a graph is called a Hubble diagram,
There is a constant ratio between a galaxy's speed and its distance from us. A rate of 15 Km/sec for every million light years away. A galaxy 10 million ly away moves at 150 Km/sec, one at 20 million ly at 300 Km/sec and so on. The entire universe of galaxies is expanding! Why? -- What is the simplest model for this kind of observation? - - If there was a terrific explosion, a "big bang" that flung everything out, then things that were flung out faster would now be further away. If galaxy A was flung out at twice the speed of galaxy B, then A would be a t twice the distance.....That is exactly what we see in the Hubble relationship.
Suppose we trace everything back to its starting point. We know their speeds, we know how far they are, we can trace all the galaxies back to the time they were all on top of each other and what do we get? We get the time since the big bang! We get the age of the universe! From the simple observation of measuring red shifts! We get a time of about 15-20 billion years!
Some of you might have noticed a very disturbing thing here. Everything is expanding away from US. Why should our galaxy be so special, why is everything going away from us? Do we have bad breath? 500 years ago Copernicus took us out of this self indulging attitude, and now we are thrust right back into it. We are at the center of the universe!! Makes one feel a little uneasy?! To understand how this can be, without letting our egos get inflated, an analogy might be helpful
Let me now say something about the fate of our universe. Gravity is always attractive and is slowing down our expanding universe (in fact the age we calculated is therefore an upper limit, actually our universe is less than 20 billion years old). Will it keep expanding, or will it someday stop and re-collapse? These two possible fates are shown in the next two slides. What will actually happen depends on how much matter there is in the universe. If there is enough matter the expansion will reverse itself and the universe will re-collapse - the big crunch - otherwise the universe will keep expanding forever. These possibilities are shown graphically.
Well, what is the answer, how much matter is there? I will keep you in suspense for the time being. ......
The next question I want to pose is: Is the universe finite or infinite? A finite universe, we would imagine, has a boundary, and an infinite one goes on forever.
Newton had concluded that the universe had to be infinite, because a finite universe would, because of gravity, collapse into one big clump. The infinite universe on the other hand would collapse into an infinite number of small lumps scattered all over space. Since Newton did not know how to deal with an infinite universe, he made no further progress.
Let me come back to this lecture hall. What IS the most important physical thing in this lecture hall? The people, the chairs without which you would have been long gone, the projector, the screen? What makes this a lecture hall? It is the space in this hall! Also, what about the time that allows us to be in it from event to event? Einstein realized that space and time were key ingredients to understanding our universe. (Where Newton had failed, Einstein would now succeed, help us comprehend the infinite.. and even the finite universe. Let us see how).
In this new view space and time are entities, with properties. The big bang was not just a humongous explosion flinging all matter out INTO space. It was the explosion of space itself. It was the explosion OF space AT the creation of time. Space and time began at the big bang. We must not think of space as the sterile arena in which the drama of the universe happens but the very fabric of the universe which is stretching as the universe expands.
Also, all the space there is, is the extent of EVERYTHING, there is no outside, there is no boundary. How can this be? How can we imagine all of SPACE starting out somewhere? How can there be no beyond? (well if you think about it, the concepts SOMEWHERE?? BEYOND?? have meaning only IN the context of space, no space these concepts loose their meaning!) To grasp all this we will have to turn to models and analogies.
Remember the raisin bread? The raisins were not moving, the dough was expanding! But there was a boundary to the raisin bread. I want something with no boundary. In fact I have something like that right here ...... a BALLOON ... just like the raisin bread analogy, the dots are expanding away from one another, the dots are not moving at all, but it is the very fabric of the balloon which is expanding. Also the balloon is an example of a finite space WITH NO BOUNDARY! For this we have to imagine ourselves as perfectly flat, 2-dimentional creatures on the surface of the balloon.
Think of a perfectly flat ant on the balloon. The balloon IS the ant's universe. It is a 2-D universe. This universe is finite, but it has no boundary! It has no beyond, for the flat ant, even though it is a finite universe. We are used to thinking of finite things as having boundaries. The surface of the balloon is a curved two dimensional space. There are flat surfaces and curved surfaces, we can imagine both, but it is impossible for us to imagine a curved volume. Space which is not nothing, but has properties, like its geometry, can be flat or curved too, and like the ant we are creatures confined to this space. In some curved spaces, analogous to the balloon for the ant, if we went far enough we would return to our origin. Geometrical constructs in such a space would be different. For example some constructions impossible in our universe, would be possible in a curved one. Such spaces are called non-Euclidean spaces.
It is hard to imagine being in such a space, but let me show you some art that has always fascinated me, and actually illustrates some of these strange concepts. The artist is M. C. Escher. The first is an etching titled waterfall, where the water comes back to fall again and again. In ascending and descending we have a staircase that never ends, but is finite. An impossible challenge perhaps for an aspiring architect, well impossible in a flat 3-D space. Though we deny the physical existence of the drawings as whole structures that can exist, locally each part of the picture is correct, can you find fault with any part of it? It is a situation that could occur in a non-Euclidean space.
It turns out that there are two kinds of generic curved spaces. Positively curved spaces (called Reimanian) and a negatively curved spaces (called Lobachevskian). Again, we can only show 2-D analogies, a saddle and a sphere respectively.
Well, OK enough of this no-Euclidean geometry, what is our universe like? Is it the 3-D equivalent of a saddle or a balloon? Remember what started all this was the simple question, is our universe finite or infinite. Unfortunately, we are not in the situation where we can step out of our universe and look at IT. Like the boy in this woodcut. This is analogous to the 2-D ant that is lifted off the balloon and allowed to have a look at the balloon. We are in the situation of the boy, in one of Escher's drawings, Art gallery. He is looking at the picture in the art gallery, but the gallery and he are part of the picture. We are in the situation of being IN the space, in this universe that we want to get a perspective on. How can we do that. How can the ant know weather it is on a balloon or a flat rubber sheet, or a saddle shaped one?
Well, a smart ant can do some experiments. It could for example start walking and see if it comes back to where it started. What if it doesn't, it might conclude that it is on a saddle, but maybe the balloon is just too big to go around. There is another way. Spaces have extrinsic properties, (their appearance when we look at them etc.) but they also have distinct intrinsic properties, and it is possible, from just intrinsic properties, to determine the geometry of the space that one is in. here are illustrates the two kinds of geometric properties, extrinsic and intrinsic that an object can have.
Here are some intrinsic properties of positive and negative surfaces. (geometries), (Escher: angels and Demons), (Escher: smaller and smaller). Positively curved space is finite (like the balloon), and negatively curved space is infinite (like an infinite number of saddles glued together - which can't be done even embedded in 3-D). Einstein's General Theory of Relativity results in an amazing consequence: how space is curved depends on how much matter is in it. If you remember, something else depended on the amount of matter too....(? ) the fate of the universe also depended on how much matter was in it. Einstein's theory of General Relativity is a theory of gravity, and also of space and time. The theory describes the origin and fate of the universe! An amazing consequence is that the fate of the universe and its geometry are intricately connected.
A universe that will collapse, that has an end, is also positively curved and therefore finite. Cosmologists call it a closed universe. A universe that will expand for ever, an eternal universe, is also infinite in spatial extent, called an open universe.
I had left you in suspense before. What kind of an universe is ours? Now I will give you the answer. We don't know. It is one of the principal questions of modern cosmology. We don't know because we are so near the critical boundary between the two.
There still remain unanswered questions. One such question is why is the universe expanding at all? In fact, where did the matter that is expanding even come from? Well one hour is a brief time to tell the history of 20 billion years. In fact it is the first trillionth of a trillionth of a second of the universe's life that would take hours to describe in all its detail. But it is the things that happened this early on and even earlier, that determined to a great extent the fate of the universe. Let me tackle these questions then, in the time that we have left.
Besides the observed expansion of the universe, another observation holds an equally important place in the history of cosmology. It is the discovery of the 3o k microwave background, made by this odd looking telescope. What it discovered was that the universe is filled for the most part with radiation, light that has been red-shifted to microwave wavelengths. Extrapolating back to early times indicates that the universe was tremendously hot when just a few seconds old.
Finally a philosophical questions: What happened before the beginning? Well there are several answers to this, and again each is connected to a certain level of our understanding and leaves us with a somewhat deeper insight and a further question. One answer is that since time began at the beginning, it doesn't make sense to ask what happened before then, because "before" has meaning only in the context of time. Thus we should really rephrase our question. This is the deeper insight. But, you may say, this is all semantics (it is not...) you know what I mean, what happened before the beginning. Well here is the simplistic answer for an open and a closed universe: there existed another universe, or universes, all information about which has been lost at the singularity (1 and 2)
Following up on the deeper insight, Stephen Hawking has suggested that time as we experience it and measure it may be just a pseudo parameter. Maybe something physical that depends on time never gets to zero. Thus the zero in time is analogous to the kind of singularity we experience with our system of defining positions on earth with latitudes and longitudes. They work everywhere on the globe but at the poles the system has some difficulty. What is the longitude at the North pole? What happened before the beginning would be analogous to asking what is north of the north pole?
It is a meaningless question in the context of our definition of North and assignment of a North pole. The Earth is no different at this place than it is anywhere else, nothing weird happens to it. It is the way we make measurements in this particular system. This has led Hawking to make the statement that the boundary conditions of the universe may be that there is no boundary.
We have built the frame-work of the universe. We have the grand design. But of course the interior design is just as important, and we have not touched on questions about that. Where did the matter and energy in the universe come from? How did the structures like galaxies and stars form? How are they arranged? Why are they distributed like they are? We have many of these answers too, but they will be left as the subject of another talk, sometime... somewhere.
Randall Splinter
