The universe is not ashamed to reveal its age. There are numerous paths that allow us to find out how much time has passed since the Big Bang to the present day.
It is estimated that, since then, 13.4 billion years have passed, with a margin of error of 200 million years.
A range of uncertainty spanning hundreds of millions of years is no small feat. However, this imprecision is diminishing, thanks to increasingly accurate cosmic timers.
To know the exact age of the universe, we take advantage of the fact that it is expanding, something we’ve known for almost a century.
This expansion produces phenomena with gigantic numbers. For example, an object close to our galaxy, the Sagittarius A* black hole, is moving away at 80,000 km/s from one of its distant cousins, OJ287.
This basically happens to almost every black hole in the universe. They are moving away from each other at the same speed as their host galaxies.
However, confidence in scientific results depends on repeating experiments. And that is something the universe does not allow.
How to measure time since the Big Bang
To compensate for this impossibility of repeating the experiments, we compared different data sources. In this way, we were able to fine-tune our cosmic timers.
But, after all, how can we measure the time elapsed since the Big Bang?
Our fundamental data is the Hubble factor. This is an amount of data that represents the average percentage growth of the universe over time. Let’s imagine that we can measure this growth itself and also at what rate it occurred. Combining the two factors, we obtain the elapsed time in this evolution. In other words, we have a cosmic timer at hand.
But let’s put this explanation in everyday terms. A revolutionary cosmetic product promises to make a person’s eyelashes twice as long in just 60 days. Following this logic, if we apply the substance and notice that our eyelashes have grown by 50%, that means that a month will have passed since the beginning of the application, correct?
The answer, however, may not be that simple. If we don’t apply the product daily steadily, the eyelash growth rate will slow down. We thus deduce that measurement time based on size change may lead to errors.
We need to know what happened on a day-to-day basis to understand this transformation. This is what we call controlling the experiment. But is this also a bad method for measuring the age of the universe?
When the universe was younger than the earth
In 1947, physicist George Gamow used Hubble factor data to estimate the age of the universe at 2.5 billion years. Shortly afterward, geologists dated the Earth’s age at 4.5 billion. How could the universe be younger than our planet?
Obviously, the estimation of the age of the universe was wrong. The problem was that they didn’t quite understand how to make this calculation. But it was known that expansion normally decreases the density of the components of the universe. And, according to the nature of each of them, this process happens at different rates.
In the early ages of the universe, radiation dominated. As the radiation dissipates very quickly, it was replaced by dark matter, as the density of this compound decreases more slowly.
All of this follows what is described in Einstein’s equations. The nature of radiation and dark matter causes the universe to slow down. This means that, although in these stages there was also expansion, the pace was getting slower and slower.
But that notion clashed with evidence found in other experiments. In them, the expansion rate of the universe was increasing.
The arrival of dark energy
There was a new component claiming prominence in this process: dark energy.
By one of these magical coincidences, the effects of the different stages of the universe are offset. In other words, the original delay in the rate of expansion was offset by the current acceleration. Therefore, it is sensible to guess the age of the universe directly through the Hubble factor.
We reiterate that in this type of work it is necessary to measure scale increases in nothing less than the universe itself. To do this, we take advantage of the fact that expansion extends the length of electromagnetic waves that reach us from the stars.
The corresponding effect is 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 are identified, but different when considering the depths of the universe.
It’s important to keep in mind that the more distant these objects are comparatively, the more their light will have been stretched. For example, the red light that reaches us from the farthest known galaxy, GN-z11, is ultraviolet.
The basis of cosmic timers
By calculating the red light shift from a distant galaxy, we estimate the expansion that has taken place since the moment each ray of light was emitted. Then the calculation is repeated with an identical galaxy and the results are compared.
The next step is to average this expansion difference over the corresponding time interval. And that temporary window will precisely be the difference in light travel time, depending on whether it comes from one galaxy or another. This is equivalent to obtaining the difference between the ages of the galaxies.
Thus, a technique is forged that is emerging with force: the cosmic timers. With this brilliant idea, pardon the pun, it is hoped to be able to arbitrate the dispute over Hubble factor values ​​between measurements of the local universe and the deep universe.
A shortcut to knowing the age of each star
As galaxies have hundreds of billions of stars, you have to be a little careful.
To obtain the ages of galaxies and stars, a general demographic average must be used. And we do it not because we want to, but because we can’t do it any other way. It is very difficult to determine the age of each individual star.
Fortunately, a providential trick makes this task easier. It consists of successfully using a very specific signal of change in the intensity of light emitted at 4,000 angstroms. [uma unidade de medida de comprimento]. The technique depends on the presence of metals that heat the galaxy and makes it possible to round off the results obtained using cosmic timers.
In fact, we don’t just estimate the current Hubble factor in this way, but this also holds for earlier epochs. Combining this knowledge with relativistic cosmology, we refine our understanding of dark energy. And the wheel keeps turning and giving us answers about the components of the universe.
We currently have only a modest number of such cosmic timers. Even so, they are extremely accurate. However, there are high hopes to scale up these results in future missions.
This would allow building a powerful and informative catalogue. The promising experiments I am referring to are EUCLID and Nancy Roman, missions launched by the European Space Agency and NASA, respectively.
Undoubtedly, they will improve the prospects of cosmic timers to position themselves as key pieces that will be able to measure not only the Hubble factor, but also the evolution of the universe itself.
These advances will amplify our greed to tackle the biggest puzzle of all: how was the universe formed? For now, we don’t know. But we can restate what physicist James Clerk Maxwell said: “Fully conscious ignorance is a prelude to any real advance in knowledge.”
*This article was originally published on The Conversation. You can read the original version here.
This text was originally published here.
Ruth Lazkoz is Professor of Theoretical Physics at the University of the Basque Country – Euskal Herriko Unibertsitete.
