Candles in the Sky; Happy Birthday, Universe.

It is fascinating to think how much our understanding of the cosmos has changed in a matter of just a century. Not too long ago, our understanding of the universe was that of a vast and empty space that was static, eternal, and composed of just one galaxy, the Milky Way. This understanding was in stark contrast to the origin stories advocated by the leading or “aspiring-to-lead” religions of the world. Almost all major religions of the world had held, until then and still do, that the universe was created by an omnipotent, conscious “Being”, which once humans came into existence, began answering private prayers and ordered and punished indiscriminately if things weren’t done the right way. This religious picture, oftentimes, holds the existence of some sort of “rhythm” or “vibration” that governs the functioning of the universe. The religious scholars, depending on their specific affinities, propounded varied timeframes for the creation of the universe and the earth, ranging from 6 days to 15 X 10^10 (150 billion) years.

Until the 1920s, astrophysics or cosmology did not have much by way of empirical/ experimental evidence to explain either the nature or the age of the universe. Consequently, religious scholars tended to have unbridled field days. However, whereas religion revelled in obscurity and mysticism, science marched on towards the true nature of the universe. In this short essay, we will try to understand how the age of the universe was determined using the principles of physics. As you may conclude, this essay also narrates a story of toil and resolve, which doesn’t need you to simply “believe” what the Books say or to impose meaningless meanings to the duration of 1 day on divine calendars. ???

In the year 1925, the great astronomer Edwin Hubble, with incomparable help from the findings of the underappreciated astronomer Henrietta Leavitt, made a discovery that was to change our understanding of the magnitude of the universe for ever. Using the celebrated Mount Wilson 100-inch Hooker telescope, Hubble discovered a certain type of star, called a Cepheid star, inside celestial bodies known back then as “nebulae”.

When we look at the night sky, we see stars that seem to be of varied sizes. This may be because they are indeed of different sizes, or perhaps because those that look smaller are farther. Perspective, pure and simple. We would all have experienced that based on the perceived brightness of a light source, we can roughly predict how far it may be if we know the actual brightness of the source. ?When driving at night, we know how far a car travelling in our direction may be, because we instinctively know the innate brightness of the headlights. This happens because a relationship exists between the inherent brightness of the source, the perceived brightness, and its distance from us. This is important, so I will write it again. A relationship exists between the inherent brightness, the perceived brightness, and the distance of the light source from us. This concept of using knowledge of the inherent brightness of objects to determine their distance has revealed that the Milky Way is not the edge of the Universe and informed us about the age of the Universe. We shall see, how.

Cepheid stars are a type of stars whose brightness pulsates. These are also called “Standard Candles” as their inherent brightness is measurable. During a fixed period, the brightness of Cepheid stars dims, increases, dims again, and so on. While working at the Harvard College Observatory, Henrietta Levitt, discovered a relationship between the brightness of Cepheid stars and the period of their pulsation. Cepheid stars that pulsate at the same speed, have the same maximum brightness. This implied that using the period of their pulsation, one could evaluate their “absolute” brightness. Once their absolute brightness became known, using their “perceived” brightness on a sufficiently powerful telescope, the determination of their distance was possible. This was done in a manner similar to how, when stuck at a traffic light, by looking at the traffic signal, we instinctively know how far the signal is and this, how long the traffic cue is. ?One may ask, “So, what”? Let’s see.

Why Hubble’s discovery of a Cepheid star was so earth-shattering is because of how faint the light from this star was. The faintness of this star meant that it was present really, really far. Using mathematical formulae, Hubble definitively demonstrated that the Cepheid star he discovered was present in a “nebula” that was much too distant to have been inside the Milky Way. This was revolutionary as it showed for the first time that stars existed beyond our own Galaxy, and, more importantly, suggested that the Universe was much larger than had been previously thought. This humble “nebula” later came to be known as Andromeda. This is the galaxy nearest to ours. The seeds of the expanse of the Universe had been sown and it opened the gates for all subsequent research that aimed at estimating the size and age of the universe.

To show that the Universe exists much beyond the Milky Way, as radical as it may be, still did nothing to tell us how it began and when it began. Hubble’s discovery that the Universe is wider than the Milky Way did something that was more remarkable than the offering of a demonstrable proof of the expanse of the Universe. It showed the Universe is expanding. And the implication of this is that if it is expanding, it must have had a starting point from where it began expanding. And, consequently, it had to have a beginning. But to estimate when it began, one had to estimate the speed of the expansion of the Universe. How do we do determine the speed of the expanding Universe?

Around the same time, another astronomer, Vesto Slipher, had looked at light from distant stars using special kinds of telescopes that break down the light into individual spectral components (wavelength). This is a bit like the disintegration of light into seven colours of unique wavelength by a prism. The nature of this disintegrated light reflected the elemental composition of these stars, and it turned out that almost all stars have around about the same elemental composition. However, Hubble’s observations indicated that while the pattern of these spectra was similar, the exact position of the spectra was a little shifted in wavelength making them appear redder. Anyone who has studied high school science would remember that that the wavelength of red light is longer than that of other lights of the visible spectrum. The shift in the wavelength of these spectral lines was the key to unlocking the speed of the expanding universe.

Drawing on day-to-day experience, we must have noted that when a source of sound, like an ambulance moves towards us, it sounds louder. When it speeds away from us, the sound is fainter. This phenomenon is called the “Doppler effect”. The reason why the ambulance sounds fainter when it moves away from us is because relative to us, the wavelength of the sound waves becomes stretched. When the ambulance approaches us, relative to us, the wavelength of the sound waves constricts. A similar thing was happening to the stars that Hubble observed. The lights appeared redder suggested that just as an ambulance moving away from us, the stars were moving away. This phenomenon of starlight appearing redder, called “Redshift”, allows us to determine the speed with which celestial objects move away from us. The extent of “Redshift” of the wavelength is directly related to the speed of the receding objects; the faster the recession, the greater the Redshift.

And that, it turned out, was that. Once Standard Candles could be spotted, using their perceived brightness, we could determine their actual distance “D”. On the other hand, by measuring their Redshift, one could determine their speed “S” with which they were moving away from our galaxy. Again, as any high school physics student will tell you, these two elements are all that are needed to determine time. Using the formulae, T = D/S, we could estimate the time taken by the star to have traversed the distance “D” based on the speed “S”. Turning the clocks back, T would also have been the time before which the star would have been in the proximity of our galaxy or even our planet.

Our ability to spot distant objects in the universe has spectacularly enhanced in the past 3 decades. This has allowed us to spot galaxies that would take even light a few billion years to travel. By the using the concept of spotting Standard Candles in these distant galaxies and by measuring the Redshift of the light emitted by these Standard Candles, we have managed to calculate the speed with which our Universe has been expanded. What it has also allowed us to do is to have allowed us to calculate the age of the Universe to be a precise 13.72 billion years.

The computation of the age of the Universe using evidence rooted in the observable world is remarkable on several fronts. Each of these very diverse, depending on our individual predilections. To some, it would manifest the boundless strength of the human mind. To some, it would mean that method to compute something seemingly incomprehensible exists, freeing us of the fuzziness and dogmatic suspension of the natural forces imposed by religious thinking. To some others, it would offer a means of investigating how something so majestic could be formed in the absence of any deity and on timescales where 1 day means 1 day. ?