How long has it been since the Big Bang?  (and how do we measure it)

Estimates have a margin of error of 200 million years. But this inaccuracy is being reduced thanks to increasingly precise cosmic chronometers.


The universe has no shame in revealing its age. It offers us many routes to know how much time has passed since the Big Bang until the 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 mean feat. However, that inaccuracy is reduced thanks to increasingly precise cosmic chronometers.

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To know the age of the universe, we take advantage of the fact that it is expanding, which we have known for almost 100 years.

This expansion produces phenomena with dizzying figures. For example, our neighboring galaxy, the Sagittarius A* black hole, is moving 80,000 km/s away from its distant cousins ​​OJ287.

This basically happens with almost all black holes in the universe. They are moving away from each other at the same rate as their host galaxies. However, the veracity of scientific conclusions is supported by the repetition of experiments. And this is something that the universe does not allow.


How to measure the time elapsed since the Big Bang

To fill this gap, we compared different data sources. In this way, we managed to set our cosmic chronometers well. But how do we manage to measure the time elapsed since the Big Bang?

Our basic data are hubble factor. It is a quantity that represents the percentage of growth of the universe on average over time. Imagine that we could measure this growth itself and also how quickly it happened. By combining the two, we get the time elapsed in this evolution. In other words, we have a cosmic chronometer.

But let’s say it on a daily basis. A revolutionary cosmetic promises to make eyelashes twice as long in 60 days. So if we apply it and we see that our eyelashes have grown by 50%, a month will have passed. Nope?

No, maybe not. If we haven’t applied the product consistently daily, growth will have slowed. So we deduce that measurement time based on size change may cause errors. We need to know what happened on a day-to-day basis. This is called controlling the experience. So is this 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 at 2.5 billion years. Soon after, geologists dated the age of the Earth at 4.5 billion years. How could the universe be younger than our planet?

Obviously, the estimate of the age of the universe was wrong. The problem was that we didn’t quite understand what it was made of. But we knew that expansion normally decreases the density of the components of the universe. And according to their nature they go at different rhythms.

In the early ages of the universe, radiation dominated. As the radiation dilutes very quickly, was replaced by dark matter, because its density decreases more slowly. All this follows the precepts of Einstein’s equations. The nature of radiation and dark matter slows down the universe. This means that, although at these stages there was also expansion, its pace became smaller and smaller.

But that contradicted the evidence. The expansion rate of the universe was increasing.

NASA, ESA, MJ Jee and H. Ford et al. (John Hopkins)

The arrival of dark energy

There was a new component claiming importance: dark energy. By one of these magical coincidences, the effects of the different stages of the universe are offset. In other words, the initial delay in the rate of expansion has been absorbed by the current acceleration. Therefore, it makes sense to guess the age of the universe directly from the Hubble factor.

We repeat that in this type of work it is necessary to measure increases in scale in nothing less than the universe itself. For this, we take advantage of expansion widens the wavelength reaching us from the stars. The corresponding effect is called red shift. This is done, for example, in spectroscopy using extended catalogs with patterns of intensities and wavelengths. In this way, objects virtually identical to each other but at different depths in the universe are identified.

It is important to pay attention to the more they are relatively distant, the more their light will have undergone stretching. For example, the red light reaching us from the most distant known galaxy, GN-z11, is of ultraviolet origin.

The basis of cosmic chronometers

By calculating the redshift of a galaxy, we estimate the expansion that has occurred since the time 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 this expansion difference over the corresponding time interval. It is precisely this temporary window that will be the difference in travel time of light depending on whether it comes from one galaxy or another. This is equivalent to getting the difference between the ages of the galaxies.

Thus is forged a technique which emerges strongly, that of the so-called cosmic chronometers. With this brilliant idea (pun intended), one expects to be able to arbitrate the dispute over the values ​​of the Hubble factor between the measurements of the local universe and the deep universe.


A shortcut to find out 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 galaxies, it is necessary to establish a demographic average of the ages of their stars. And, interestingly, we do so in compliance with your data protection law. It’s not because we want to, but because we can’t help it. You can guess the impossibility of knowing the age of each star individually.

Fortunately, a providential trick makes the task easier. It consists in successfully using a very specific signal for the change in intensity of the light emitted at 4,000 angstroms. It is produced by the presence of metals heating the galaxy and helps to round off the technique of cosmic chronometers.

In fact, we not only estimate the current Hubble factor in this way, but also for earlier times. By 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 these cosmic chronometers, but nevertheless of exquisite precision. However there are high hopes of scratching some more from future quests.

This would build a powerful and informative catalog. The promising experiments to which I refer are EUCLID and Nancy Roman. Undoubtedly, they will improve the chances of cosmic chronometers positioning themselves as centerpieces for measuring 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 did the universe form? We don’t know, but we can reaffirm what Maxwell said: “Fully conscious ignorance is a prelude to any real advance in knowledge.”

*This article originally appeared on The Conversation. You can read the original version here.

Ruth Lazkoz it’s businesstheoretical physics teacher in the University of the Basque Country, Euskal Herriko University.

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