Have you ever imaged what is like if we can catch the speed of light and see the boundary of the universe? Lots of people may say, "you can't do that. The universe is infinite!" Well, before Einstein, no one ever argued that speed of light is the ultimate limit of speed of the universe and dared to think that the universe has a boundary. In Einstein's Special Theory of Relativity, he showed that it will require infinite amount of energy for things with mass like us to reach the speed of light. Furthermore, he demonstrated that the space and time are interrelated in his General Theory of Relativity. According to recent study, astrophysicists, using Einstein's model of the universe, can show that there is a boundary for the universe. Then, people start to ask, "why can't we see the boundary of the universe?"
Well, to answer this question, I need to explain more about Einstein's General Theory of Relativity.
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| Figure 1. Demonstration of Curving Spacetime |
In the figure 1 on the right, it shows the main concept in
General Relativity, the wrapping spacetime. The space and time are two aspects of the same phenomenon; they combine to form the dynamic 4-dimensional spacetime.
Massive objects can curve spacetime, as shown in the figure 1. Imagining to have a massive body that is thousand times as massive as the Sun, the spacetime will be so curved and wrap around the body. Wow, pretty scary, isn't it? However, it is possible if we take account of all the bodies in the universe. Einstein predicted that the combined mass of all the bodies in the universe will curve the spacetime into a ball-shaped universe with extraordinarily high density. At this stage, the universe is called a singularity.
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| Figure 2. The Singularity and the Black Hole |
If you find my explanation is hard to understand, consider figure 2; it shows the similarity of a singularity and a black hole, which are both extremely dense bodies.
But, what is that telling us? It suggests that the universe has a beginning, where space, time, and everything do not exist. Since it has a beginning, then it must find a way to become the universe that we live in now. Later on, Georges Lemaître, a famous astrophysicist, proposed the Big Bang Model of the origin of the universe. It is the first time that people start to be aware that the universe may have a boundary. Oh my god, is this exciting?
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| Figure 3. Looking back in Time, the Universe |
But how can we know if there actually is a boundary, even Albert Einstein cannot always be right. Here comes another genius,
Stephen Hawking. He proposed the model in figure 3, in which if we look back in time the universe is contracting and comes to one point, the singularity. This theory leads to the expansion of the universe, which later refer to as the cosmic inflation. To prove this thesis, scientists start to build devices to detect the radiation left from the
Big Bang. Finally, in 1965, they detect this radiation, which later is called the cosmic background radiation.
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| Figure 4. The Model of Multiverse |
As time proceeds, scientists realizes that the expansion of the universe is accelerating. Wow, how does that happen? After calculating the mass of the galaxies, they find that there is something we can't see, the
dark matter and
dark energy, dominates the universe and exerts negative pressure on the universe to cause the
accelerating expansion of the universe. But what is dark matter and dark energy indeed, we have no idea...
Finally, let me introduce some interesting concept. A theory developed in the late 20th century, the string theory, which treats the fundamental composition of matter strings, suggests that there
may not only one universe. Oh, dear lord, there is more than one universe? In figure 4, each ball represents a universe; our universe is just one of them. This model is called the
Multiverse model, which also opens the threshold of
parallel universe.
After this adventure, do you find it interesting? Now, would you wonder what is like to be as fast as the speed of light and what looks like at the boundary of the universe? Come on, let's find out ourselves!!
