Spacetime and Symmetry.
This article is a part of a series of articles on Cosmology. This is part 1/4.
In the last series on time, I talked exclusively on time and symmetry in the first three articles. Now, lets focus on symmetry. While reading about time, you must have been wondering- what caused the arrow of time, to go in the direction it does? This is where Cosmology comes in.
Cosmology, Symmetry and the Laws of the Universe
When asked about which discovery of modern science is the most important in his mind, Richard Feynman answered, “The universe is made up of atoms.” When offered a second choice, he and his colleagues answered, “Symmetry underlies the laws of the universe.” This is indeed true, and is a prime force which pushes physicists to understand the arrow of time. But first things first, what is Cosmology? Cosmology is the field of physics which focuses on how the universe came to be and how it has evolved. A powerful weapon to deal with such a foe is provided collectively by laws of the universe and physics.
All the laws of the universe are applicable everywhere, at all locations in our spacetime. This basically means that, if an athlete is practicing for the Olympic games in London, mastering all the positions for a long jump, he can do, by the classical laws of physics, the same perfect long jump in Tokyo, where the games are held.
Such symmetry is present in all laws that govern our space and time. Isn’t it amazing, that one can find out, say the size of the universe, using some of Einstein’s equations(we will tackle this issue in the next articles.), just as you can find out the trajectory of a ball high in the air!
The Cosmic Microwave Background Radiation
If you could see light of some longer wavelengths, you could see the microwave oven burst into action as you heat up your leftover pasta for a midday snack, and a beautiful uniformity in space when you look up. This is the Cosmic Microwave Background Radiation, and is perhaps the most important finding of cosmology. This discovery is so fantastic, it has made the CMBR the most frequently observed object ever. Indeed one may be amazed by the sheer presence of it. But to someone who does not know about it, it is just another thing that you can’t see. But its more than that. You see in the picture, the CMBR tells us another thing about this realm we live in. It is surprisingly uniform. We are solely, and this is of importance, are talking about temperature here, i.e we are talking about the temperature of the CMBR. The CMBR is a cool remnant of the big bang, and the photons that are there, are whose temperature we are reading. The blue represents cooler regions, while the red hotter regions. But, if there are so many blue and red spots, how can we say that it is symmetric? Accurate Satellite observations suggest that the temperature of the CMBR in one part of space agrees to the temperature in another part of space to one thousands of a degree. It is lke saying that if it is 85.0000 degrees in Tokyo, the temperature in New York and Paris would be between 84.9999 to 85.0001 degrees. So the red spots are hotter by just some fractions of a degree. This is the overall, hidden symmetry. This may seem insignificant to you, but this symmetry reveals two important things.
First, it reveals that the universe, even though is irregular on the galactic scales, with many galaxies, is highly symmetrical throughout. This contributes to the fact that universe, as a whole unit, has evolved uniformly. Every part of the universe has cooled down from an unfathomable amount of heat, to the one which has a general temperature of 2.7 degrees, flat. So, it shows us that universe, although highly heterogeneous in internal structure, is highly homogeneous as a whole. It is like saying that the organisms in the kingdom plantae are highly diverse, but all of them have the inherit characters of a plant.
Secondly, it tells us about the age of the universe. Remember from the loaf of bread analogy, that relativity allows each observer to make different observations, cut it at different angles and therefore have a different premonition of time. So you might ask, when I said that the universe is about 14 billion (13.8 billion to be exact) years old, one might be tempted to ask- By which clock? Which clock shows that the universe is approximately 14 billion years old? The answer is that by every human, animal, alien’s clocks! Just as archeologists and historians study relics of the past to figure out how things were back then, we study the CMBR, since it is the only known discovered relic of the big bang. And since it is highly uniform, everyone will more or less see what we see when we look at it, making all the clocks synchronised. Thus, as verified by precise observations, the universe is indeed 13.8 billion years old.
Expanding Space
Now, let us turn away from the CMBR for sometime, and shift our attention to the expanding universe. From the Gojou article(read it here), you can see that the universe is expanding. But, the sun is at a constant distance from the earth, the crew of the International Space Station has to travel the same distance every time they go or come. So how is that the universe is expanding? The answer to this question is that the universe is expanding, but on scales far larger. The scale of Galaxy Clusters. This is what led Hubble to discover that universe is expanding in the first place. To grasp this better, think of a balloon, a spacetime balloon(a frequently used analogy) with pennies stuck on it as cosmic clusters. When you inflate the balloon, the pennies rush away from each other, and each Lincon sees what the other sees, the Lincons around it rushing away from it but he is where he is, glued to a point, and the surface of the balloon increases distance. Note that nothing happens to the pennies, they stay the same, because of the internal gravitation of the atoms that make them hold them together. Similarly, the internal gravitation of these clusters hold them together, and so the distance to the sun from the earth remains the same. Similarly, just like the pennies, you can sticks clocks on the balloon, and get a knack for what happens to time in an expanding universe. This also shows symmetry. But how? Won’t the clocks that move relative to each other show different times, as Einstein taught us in Special Relativity? The answer is no, and here is the straightforward answer. The clocks are not moving at all, they are glued to a fixed point on the balloon. It is the balloon that swells up, and increases the distance. And since each clock sees what it’s counterparts see, all clocks tick time at the exact same rate.
Subtle Feature of an Expanding Universe
As observations have shown, Galaxy Superclusters are rushing away from us at a pace faster than that of light. But how is that? Didn’t Einstein in special relativity say that nothing can move faster than the speed of light? The answer is that the expansion perfectly is in coordination with special relativity. It is because special relativity describes an object in motion through space, not the motion of space itself. We cannot observe or maybe even mathematically predict motion of space. Thus, the expansion does not go toe to toe with special relativity. It also, sadly means that millions of years afterwards, there will be a point of time, when the sky in night will be dark, no galaxies can be found there. This can also mean that we won’t be able to study the phenomena that may occur, or the children won’t be able to see stars at night, because the speed of expansion will drive out these objects out of the visible/observable universe.
This is it for this article. I will leave you with a sneak peek of the next article of the series, The shape of the Unified Universe.
Thank you so much for reading! I hope you enjoyed!