The universe has no shame in revealing its age. There are numerous routes that it offers us to take to find out how much time has elapsed since the Big Bang until this present moment. We estimate that 13.4 billion years have passed, with an uncertainty of 200 million.
A margin of error of hundreds of millions of years is no small thing. However, that inaccuracy is narrowing thanks to increasingly precise cosmic chronometers .
To know the age of the universe we take advantage of the fact that it is expanding, something we have known for almost 100 years. This expansion produces phenomena with dizzying numbers. For example, our galaxy neighbor, the Sagittarius A* black hole , is moving away at 80,000 km/s from its distant cousins OJ287 . It basically happens with almost all black holes in the universe. They are moving apart from each other at the rate that their host galaxies are. However, the veracity of the scientific conclusions is supported by the repetition of the experiments. And that is something that the universe does not allow.
HOW TO MEASURE TIME SINCE THE BIG BANG
To compensate for this lack, we compared different data sources. In this way we managed to adjust our cosmic chronometers well. But how do we manage to measure the time elapsed since the Big Bang?
Our fundamental data is the Hubble factor. It is a quantity that represents the percentage growth of the universe averaged over time. Let’s imagine that we can measure that growth itself and also at what rate it has occurred. Combining both we obtain the time elapsed in that evolution. That is, we have a cosmic stopwatch.
But let’s put it in everyday terms. A revolutionary cosmetic promises to make eyelashes twice as long in 60 days. So if we apply it and see that our eyelashes have grown by 50%, a month will have passed. Nope?
No, maybe not. If we have not applied the product constantly on a daily basis, growth will have slowed down. We thus deduce that measuring time based on size change can lead to error. We need to know well what has happened day by day. This is what we call controlling the experiment. So is that also a bad method to measure the age of the universe?
WHEN THE UNIVERSE WAS YOUNGER THAN EARTH
In 1947 G. Gamow used Hubble data to estimate the age of the universe to be 2.5 billion years. Shortly after , geologists dated the age of the Earth to 4.5 billion years. How could the universe be younger than our planet?
Obviously the estimate of the age of the universe was incorrect. The problem was that it was not well understood what it was made of. But it was known that expansion normally decreases the density of the components of the universe. And according to their nature they go at different rates.
In the early ages of the universe radiation dominated. As the radiation dilutes very quickly, it was replaced by dark matter, since its density gets smaller more slowly. All of this follows the dictates of Einstein’s equations. The nature of both radiation and dark matter cause the universe to slow down. That means that, although in those stages there was also expansion, its pace was getting smaller and smaller.
But this clashed with the evidence. The rate of expansion of the universe was increasing.
THE ARRIVAL OF DARK ENERGY
There was a new component claiming prominence: dark energy . By one of those magical coincidences, the effects of the various stages of the universe are offset. In other words, the original delay in the rate of expansion has been eaten up by the current acceleration. Therefore, it is sensible to guess how old the universe is directly from the Hubble factor.
We reiterate that in this type of work it is necessary to measure increases in scale in nothing less than the universe itself. To do this, we take advantage of the fact that the expansion widens the wavelength that reaches us from the stars. The corresponding effect is the so-called redshift. This is done, for example, in spectroscopy using extensive catalogs with patterns of intensities and wavelengths. In this way, objects that are practically identical to each other but at different depths in the universe are identified.
It is important to keep in mind that the farther away they are comparatively, their light will have undergone more stretching. For example, the red light that reaches us from the farthest known galaxy, GN-z11, is ultraviolet in origin .
THE BASIS OF COSMIC CHRONOMETERS
By calculating the redshift of a galaxy, we estimate the expansion that has occurred since the moment each light ray was emitted. And then the calculation is repeated with an identical galaxy and the results are compared. The next step is to average that expansion difference over the corresponding time interval. Precisely that temporary window will be the difference in the travel time of the light depending on whether it comes from one galaxy or another. That is equivalent to obtaining the difference between the ages of the galaxies.
Thus, a technique is forged that is emerging strongly, that of the so-called cosmic chronometers. With this brilliant idea (pun intended) it is expected to be able to arbitrate the dispute over the values of the Hubble factor between the measurements of the local and the deep universe.
A SHORTCUT TO KNOW THE AGE OF EACH STAR
Since galaxies have hundreds of billions of stars, you have to be a little careful. To obtain the ages of the galaxies, the ages of their stars must be demographically averaged. And, curiously, we do it respecting your data protection law. This is not because we want to, but because we can’t do it any other way. You can guess the impossibility of finding out the age of each star individually. Fortunately, a providential trick makes the task easier. It consists of successfully using a very specific signal of change in intensity of light emitted at 4,000 angstroms. It is produced by the presence of metals heating the galaxy and allows rounding the technique of cosmic chronometers.
In fact, we not only estimate the current Hubble factor in this way, but also for earlier times. Combining this with relativistic cosmology we refine our understanding of dark energy. And the wheel keeps turning giving us answers about the components of the universe.
We currently have only a modest number of such cosmic chronometers, but exquisitely accurate nonetheless. However there are high hopes of scratching off a few more from future missions. That would allow to build a powerful and informative catalog. The promising experiments I am referring to are EUCLID and Nancy Roman . Undoubtedly, they will improve the prospects for cosmic chronometers to position themselves as key pieces to measure not only the Hubble factor but also the evolution of the universe itself.
These advances will fatten our arrogance to tackle the biggest puzzle of all. How was the universe formed? We don’t know, but we can reaffirm what Maxwell said: “Totally conscious ignorance is a prelude to any real advance in knowledge.”
Ruth Lazkoz , Professor of Theoretical Physics, University of the Basque Country / Euskal Herriko Unibertsitatea