Many people are curious about the universe they live in. Many of them are afraid to ask obvious questions in case they are accused of asking stupid questions. When it comes to matters of science there are no stupid questions; it’s the simple questions that reveal the most interesting findings. Here I want to pose some of the most obvious questions that people might ask. These questions, it turns out, are not simple but reveal some of the deepest and most complex properties of our universe. Here I am reviewing some of the questions that people ask together with my understanding of how science answers them. Remember that I could be wrong in my explanations but I think I’m mostly right.
One of the first questions people ask is ‘how old is the universe?’ This is a very non-stupid question. Scientists and philosophers have been staring at the night sky for centuries and more, contemplating this very question. Now we have an answer that has been a long time coming. As best we can tell our universe is about 13.8 billion years old.
Your next question should be, ‘how do you know that?’ That too is not a stupid question. Our first clue came from the work of the famous American astronomer, Edwin Hubble and the Belgian astronomer Georges Lemaitre. Hubble found that distant galaxies were moving away from each other at a speed that increased with distance, Hubble’s Law. That being the case, turning the clock backward would find all of the galaxies in one place at one time. This finding was initially greatly resisted by other astronomers who insisted that the universe was static and infinite in time and space. It was one of them, Fred Hoyle, who coined the phrase Big Bang as a derogatory epithet. The phrase stuck as did the expansion theory with its conclusion of a finite age to the universe.
Modern estimates of the age of the universe depend less on Hubble’s Law and more on the precise measurement of the Cosmic Background Radiation (CMB).
As the universe expanded after the Big Bang, the universe cooled from the almost unimaginable temperatures that it started with. That cooling continues to this day. By measuring the temperature of the CMB today we can estimate for how long the initial fireball has been cooling, leading to the most accurate estimate of the age of the universe of 13.8 billion years.
The next question people might ask is, ‘how big is the universe?’ That too is not a stupid question and it has multiple answers. We can see up to 13.8 billion years away, almost back to the beginning of the universe, but it has expanded since then. Because of this expansion, what was 13.8 billion light-years away is now estimated to be 46 billion light-years away. Thus the diameter of the observable universe is estimated to be 93 billion light-years and still expanding.
There is no sure answer to the question, ‘what existed before the Big Bang?’ There have been many attempts to answer this question with no completely convincing theory. These attempts continue today with some claims that the CMB shows signs of ‘bruising’ caused by contact with an adjacent universe together with other indications of a life before the Big Bang. Steven Hawking and James Hartle suggested in a famous scientific paper that the evolution of the universe was shaped like a shuttlecock, with no beginning. They claimed that asking what came before the Big Bang was like asking what was south of the South Pole.
This answer is very unsatisfactory since it avoids an answer. This drives scientists to strive even harder to find a theory that explains the facts and is satisfying.
Perhaps the next question people ask is, ‘what will happen to the universe over time?’ Again there is no clear answer, it all depends on the nature of space and time. We once thought that the future of the universe depended on how much matter (stuff) was in it. It’s a bit like throwing a baseball straight up into the air. Since you can’t throw it fast enough it will fall back down to you. The faster you throw it the higher it will go before slowing down, coming to a stop, and falling back down for you to catch. Imagine if you could throw it really fast, fast enough that it would never slow down enough to come to a stop; it would be gone forever. There are three conditions in this baseball throwing challenge; not fast enough; just fast enough; and more than fast enough. In the first condition, the ball falls back to the thrower. In the second the ball slows to a stop at infinity and stays there. In the third condition, the ball is still moving away when it gets infinitely distant from the thrower. Using Newton’s gravity, these conditions were labeled, elliptic, parabolic, and hyperbolic.
Getting back to the universe. Depending on how much stuff is in the universe, we once thought it could fly apart forever, stop moving at infinite distance, or fall back together into a big crunch. By estimating how much mass we could see in the universe it looked like it would fly apart forever.
Then we discovered Dark Matter, and later Dark Energy. When we look at the planets in our solar system as they orbit the sun we notice that those farther away take longer to complete an orbit.
The closer planets complete their orbits more quickly. When astronomers looked at stars orbiting the centers of spiral galaxies they got a shock; the outer stars were all orbiting at more or less the same speed. This behavior, and that of orbiting galaxies in galactic clusters, did not obey the rules of gravity as we knew them.
One way to make this orbital behavior fit was to propose that these galaxies contained much more matter than we could see; Dark Matter.
To confirm the Dark Matter theory, astronomers started to look for other signs of its existence. One major confirmation was when they saw that individual galaxies could act as a gravitational lens to distort the view of galaxies farther away. When astronomers computed how much gravity was needed to do this it was much more than was visible in the galaxy that formed the lens.
In addition, when astronomers plotted the positions of galactic clusters in a three-dimensional map of the universe, they discovered that they were distributed in strings and sheets with voids between them much like Swiss cheese or a sponge. It seems that it was Dark Matter that formed the structure of the universe around which ordinary matter such as stars and galaxies coalesced.
Astronomers then calculated how much Dark Matter was in the universe and got another shock. It seems that Dark Matter accounts for about 85% of the matter in the universe while ordinary matter like stars and galaxies, accounts for the other 15%.
However, even adding the Dark Matter estimate did not provide enough stuff to slow down the expansion enough to stop the universe from expanding forever. Another shock came when astronomers checked on the expansion rate of the universe using more powerful and more accurate techniques than before; the expansion was accelerating.
According to the usual theory of gravity, Einstein’s General Relativity, the expansion rate must slow down over time due to the pull of the totality of matter in the universe; but it wasn’t. It seemed that another force was at work, much like gravity but this time a repulsive force rather than an attractive one. This force was named Dark Energy, mainly because no one knows what it is or how it works. Most people have heard of Einstein’s most famous equation, E-mc2, and understand that it means that mass can turn into energy and vice versa, but it also means that energy can act just like mass to cause a gravitational pull. In the case of Dark Energy, it seems to cause a change in the overall force of gravity in the universe over time to accelerate the expansion. When astronomers added up the total energy in the universe from Dark Energy, Dark Matter, and ordinary matter, they found that Dark Energy contributed an estimated 69% of it, with Dark Matter contributing 26% and ordinary matter only 5%. It seems then that 95% of our universe is made up of stuff we can neither see nor understand as yet. It also seems that the Dark Energy contribution sends our universe on a one-way journey, expanding forever until all the stars go out and it becomes a cold and dark, and lonely place. We shall be long gone by then.
