The Age And Size Of The Universe (Part 1)

HUBBLE’S LAW; v = H₀ x d
For the purposes of discussing the size and age of the universe, we use the Big Bang model. It is primarily about expansion of energy. If that is the case, what does it mean to say the “age” of the universe is about 14 billion years? This brings us to Hubble’s law. Hubble’s law says that the far way a galaxy is, the faster it is moving away from us. As expressed mathematically above, “v” is the recessive velocity of galaxy, “H₀” is Hubble’s constant and “d” is distance. A simple law yet impregnated with implications for how we do think about the Universe. What does it mean? Well, ours is a Universe of light. Light is both a message and messenger for astronomers. It is a message because it is an informative spectrum. It is a messenger because with its finite speed, it has to travel from one place to another.

Like archeologists digging out the earth for the fossils of living organisms and ancient artifacts, cosmologists do so as well. In scanning the heavens for fossils of exploded and exploding ancient stars that fill the Universe, Hubble realized that most of the lights were red-shifted in his spectrum measurement. This red-shifting phenomenon is similar to the everyday Doppler effect of sound in our lorry stations and markets. Stand by the roadside. The horn of all stationary vehicles sounds the same. However, the horns of incoming vehicles sounds louder as it approaches you and begins to fade increasingly as it moves further and further away from you. That is the Doppler effect.

Instead of sound astronomers use light. Analogously, taking our galaxy as you by the roadside and other galaxies as the vehicles with light as their horns, he found out that their lights were red-shifted. This means they are moving away from us like the vehicle moving away from you. This also means the universe is not static as Einstein thought. It is expanding. But expanding into what? Nothing really. Spacetime itself is stretching making the galaxies within it recede speedily away from one another. Since motion is only meaningful in the framework of relativity theory, every galaxy then is moving away from the other. In that case, roll everything back and logically they will all find themselves at a point.

The extremity of that point is mathematically called the singularity. The Big Bang model was born, the “Observable universe” was born too. Observable universe because light is the medium via which we observe the universe. Since light is a messenger travelling from one place to another, Hubble’s law means that light from other galaxies can reach us given enough time. In fact, just as every illumination of our eye took off eight minutes ago from the sun before reaching us on earth, so it is that every light we receive from other galaxies took some time to reach us.

So it is possible that there are lights from other receding galaxies that has not reached us yet and even few moments into the expansion itself. Also depending on the luminosity of any light reaching us, we can deduce how far away the star it took off from is as the inverse square of law of luminosity. This tell us whether it part or outside our Milky Way.

As a message, the illumination means we are looking back into the past of the Observable universe, in fact the entire Universe itself in principle. Since Hubble’s law then is about the rate of expansion of the Universe, the reciprocal of its constant gives us an estimate of the age of the Universe if the rate of expansion remains constant throughout.

Therefore, to say that the Universe is about 14 billion years old is like saying the farthest we can look into the past since the expansion via the light reaching us now took that time to reach us on this Earth in this Solar system. Also rearranging mathematically to get the distance covered implies that we cannot contemplate the vastness of the Universe in its entirety because not all lights had had time to reach us.

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