The Visible Universe and a puzzle
Our universe is something that has been endlessly fascinating for mankind. Its origin, its history, what it’s like today and its future- we have asked these questions and have looked for answers through the lens of science and philosophy. Today we know the answers to some of them — although by no means easy to appreciate, they nonetheless keep our curiosity burning bright as ever. Young or old, the universe is a constant source of wonder for the curious mind.
One such question is about the age of our universe — just how old is it? Thanks to popular science, a lot of us today are acquainted with the currently accepted answer in the science community. The universe began about 13.8 billion years ago with a ‘Big Bang’ (a misleading term since there wasn’t really any ‘Bang’ or explosion! It simply refers to an early stage of the universe that was very dense and very hot). The Big Bang, in fact, marks the beginning of time itself. There is no “before the Big Bang” since “before” refers to an instant in time and time itself did not exist before Big Bang! As for where it happened, it wasn’t a single point in space. It was everywhere. At all points in space! Crazy, right?
A lot of us probably also know that according to the Big Bang theory, the universe is expanding and the farther the objects are from us, the greater is their speed of recession. Furthermore, Einstein’s relativity theory tells us that we’re bound by the speed of light- nothing can travel faster than light in free space and nothing other than light ( or, to be more precise, a photon) can even travel at the speed of light. What does this have to do with our expanding universe? Well, if we’re really limited by the speed of light, then there has to be a “horizon” for our universe beyond which we just can’t see! Which means, beyond this “horizon”, the objects are receding away at speeds greater than the speed of light itself! But, wait! Didn’t I just say that nothing other than light can travel at the speed of light? Well, it so happens that while an object can indeed never move faster than the speed of light locally (i.e. at the object’s location), there is no rule that says they can’t go faster than light relative to a distant observer (here, us). Mindboggling, isn’t it?
The next question that logically follows is how far into our universe can we then see? The answer seemingly follows from our knowledge of the universe’s age: If the universe is 13.8 billion years old as they say, we should only be able to see as far as 13.8 billion light-years, since that’s the amount of distance light will travel in that time. However, the accepted answer happens to be a much bigger number: 46 billion light-years, in all directions!
“How is that possible?” you’re probably going to throw your hands up in despair and ask. How can the universe be only 13.8 billion years old but at the same time allow us to see farther than the distance light would travel in that time? How do we make sense of this?
Well, we do know that our universe is full of stars and galaxies and all sorts of stuff and that they began to form pretty early after its birth. Now, taking away the time it took for the universe to cool down and galaxies and stars to form, we argue that the light emitted from the oldest stuff must have traveled for almost 13.8 billion years now and therefore we should be able to see only about 13.8 billion light-years back.
However, this line of reasoning misses out on a crucial aspect: the objects emitting light and receiving it (i.e.us) have also been moving relative to one another. In fact, when the light was emitted, the object was much, much closer to us — about perhaps only 4–5 billion light-years away. What if it so happens that the stuff radiating this light was moving close to the speed of light?
Special Relativity tells us in that case, we might be able to see twice as far as in the first case! If their light is only reaching us now, the objects are likely as far as 27.6 billion light-years away from us now given that they receded away at close to the speed of light. However, we’re obviously still missing something as the number is nowhere near 46 billion light-years. What can we have possibly left out?
Why, it is but the most important aspect of our today’s universe: that it is expanding, or to be more precise, that our space itself is expanding!
When cosmologists look at a distant galaxy and find its spectrum to be shifted more towards the “red” i.e. longer wavelengths, they’d tell you that it’s not just because the galaxy is moving away from us but also because the fabric of space itself is stretching out. The light waves traveling through this space would therefore also undergo a stretch in their wavelengths which would make them “redder”.
“But wait!” you ask, “How can we actually tell them apart? How do we know there are two different effects (galaxy motion and expansion of space) involved in the first place? After all, aren’t we only measuring the change in wavelength?”
Good question! It turns out the redshift is related to the observed brightness of the galaxy, which in turn, depends upon its distance from us. Now if the universe were truly a non-expanding one, we could have observed only up to twice the universe’s age in light-years, as argued earlier i.e. 27.6 billion light-years. But in fact, we have already observed galaxies that are farther away than that!
So, how far can we really see? Well, there is still one final element that needs to be accounted for: dark energy. If our universe had no dark energy, only stars, galaxies, and the remnant cosmic microwave radiation, the limit would still only come to 41.4 billion light-years. But thanks to the dark energy, our visibility limit gets extended to an even larger number- 46 billion light-years, as claimed. This implies that the edge-to-edge extent of our visible universe is about 92 billion light-years!! A number arrived at with the help of matter, radiation, and dark energy, all obeying the laws of Einstein’s general relativity. In fact, Einstein’s theory pretty much guarantees that if the universe’s constituents are what we say they are and they obey general relativity, this has to be the answer.
That in itself is something to marvel at.
